# nghttp2.h

    /*
     * nghttp2 - HTTP/2 C Library
     *
     * Copyright (c) 2013, 2014 Tatsuhiro Tsujikawa
     *
     * Permission is hereby granted, free of charge, to any person obtaining
     * a copy of this software and associated documentation files (the
     * "Software"), to deal in the Software without restriction, including
     * without limitation the rights to use, copy, modify, merge, publish,
     * distribute, sublicense, and/or sell copies of the Software, and to
     * permit persons to whom the Software is furnished to do so, subject to
     * the following conditions:
     *
     * The above copyright notice and this permission notice shall be
     * included in all copies or substantial portions of the Software.
     *
     * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
     * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
     * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
     * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
     * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
     * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
     * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
     */
    #ifndef NGHTTP2_H
    #define NGHTTP2_H

    /* Define WIN32 when build target is Win32 API (borrowed from
       libcurl) */
    #if (defined(_WIN32) || defined(__WIN32__)) && !defined(WIN32)
    #  define WIN32
    #endif

    /* Compatibility for non-Clang compilers */
    #ifndef __has_declspec_attribute
    #  define __has_declspec_attribute(x) 0
    #endif

    #ifdef __cplusplus
    extern "C" {
    #endif

    #include <stdlib.h>
    #if defined(_MSC_VER) && (_MSC_VER < 1800)
    /* MSVC < 2013 does not have inttypes.h because it is not C99
       compliant.  See compiler macros and version number in
       https://sourceforge.net/p/predef/wiki/Compilers/ */
    #  include <stdint.h>
    #else /* !defined(_MSC_VER) || (_MSC_VER >= 1800) */
    #  include <inttypes.h>
    #endif /* !defined(_MSC_VER) || (_MSC_VER >= 1800) */
    #include <sys/types.h>
    #include <stdarg.h>
    #include <stddef.h>

    #include <nghttp2/nghttp2ver.h>

    #ifdef NGHTTP2_STATICLIB
    #  define NGHTTP2_EXTERN
    #elif defined(WIN32) ||                                                        \
      (__has_declspec_attribute(dllexport) && __has_declspec_attribute(dllimport))
    #  ifdef BUILDING_NGHTTP2
    #    define NGHTTP2_EXTERN __declspec(dllexport)
    #  else /* !BUILDING_NGHTTP2 */
    #    define NGHTTP2_EXTERN __declspec(dllimport)
    #  endif /* !BUILDING_NGHTTP2 */
    #else    /* !defined(WIN32) */
    #  ifdef BUILDING_NGHTTP2
    #    define NGHTTP2_EXTERN __attribute__((visibility("default")))
    #  else /* !BUILDING_NGHTTP2 */
    #    define NGHTTP2_EXTERN
    #  endif /* !BUILDING_NGHTTP2 */
    #endif   /* !defined(WIN32) */

    #ifdef BUILDING_NGHTTP2
    #  undef NGHTTP2_NO_SSIZE_T
    #endif /* BUILDING_NGHTTP2 */

    /**
     * @typedef
     *
     * :type:`nghttp2_ssize` is a signed counterpart of size_t.
     */
    typedef ptrdiff_t nghttp2_ssize;

    /**
     * @macro
     *
     * The protocol version identification string of this library
     * supports.  This identifier is used if HTTP/2 is used over TLS.
     */
    #define NGHTTP2_PROTO_VERSION_ID "h2"
    /**
     * @macro
     *
     * The length of :macro:`NGHTTP2_PROTO_VERSION_ID`.
     */
    #define NGHTTP2_PROTO_VERSION_ID_LEN 2

    /**
     * @macro
     *
     * The serialized form of ALPN protocol identifier this library
     * supports.  Notice that first byte is the length of following
     * protocol identifier.  This is the same wire format of `TLS ALPN
     * extension <https://tools.ietf.org/html/rfc7301>`_.  This is useful
     * to process incoming ALPN tokens in wire format.
     */
    #define NGHTTP2_PROTO_ALPN "\x2h2"

    /**
     * @macro
     *
     * The length of :macro:`NGHTTP2_PROTO_ALPN`.
     */
    #define NGHTTP2_PROTO_ALPN_LEN (sizeof(NGHTTP2_PROTO_ALPN) - 1)

    /**
     * @macro
     *
     * The protocol version identification string of this library
     * supports.  This identifier is used if HTTP/2 is used over cleartext
     * TCP.
     */
    #define NGHTTP2_CLEARTEXT_PROTO_VERSION_ID "h2c"

    /**
     * @macro
     *
     * The length of :macro:`NGHTTP2_CLEARTEXT_PROTO_VERSION_ID`.
     */
    #define NGHTTP2_CLEARTEXT_PROTO_VERSION_ID_LEN 3

    struct nghttp2_session;
    /**
     * @struct
     *
     * The primary structure to hold the resources needed for a HTTP/2
     * session.  The details of this structure are intentionally hidden
     * from the public API.
     */
    typedef struct nghttp2_session nghttp2_session;

    /**
     * @macro
     *
     * The age of :type:`nghttp2_info`
     */
    #define NGHTTP2_VERSION_AGE 1

    /**
     * @struct
     *
     * This struct is what `nghttp2_version()` returns.  It holds
     * information about the particular nghttp2 version.
     */
    typedef struct {
      /**
       * Age of this struct.  This instance of nghttp2 sets it to
       * :macro:`NGHTTP2_VERSION_AGE` but a future version may bump it and
       * add more struct fields at the bottom
       */
      int age;
      /**
       * the :macro:`NGHTTP2_VERSION_NUM` number (since age ==1)
       */
      int version_num;
      /**
       * points to the :macro:`NGHTTP2_VERSION` string (since age ==1)
       */
      const char *version_str;
      /**
       * points to the :macro:`NGHTTP2_PROTO_VERSION_ID` string this
       * instance implements (since age ==1)
       */
      const char *proto_str;
      /* -------- the above fields all exist when age == 1 */
    } nghttp2_info;

    /**
     * @macro
     *
     * .. warning::
     *
     *   Deprecated.  :rfc:`7540` priorities are deprecated by
     *   :rfc:`9113`.  Consider migrating to :rfc:`9218` extensible
     *   prioritization scheme.
     *
     * The default weight of stream dependency.
     */
    #define NGHTTP2_DEFAULT_WEIGHT 16

    /**
     * @macro
     *
     * .. warning::
     *
     *   Deprecated.  :rfc:`7540` priorities are deprecated by
     *   :rfc:`9113`.  Consider migrating to :rfc:`9218` extensible
     *   prioritization scheme.
     *
     * The maximum weight of stream dependency.
     */
    #define NGHTTP2_MAX_WEIGHT 256

    /**
     * @macro
     *
     * .. warning::
     *
     *   Deprecated.  :rfc:`7540` priorities are deprecated by
     *   :rfc:`9113`.  Consider migrating to :rfc:`9218` extensible
     *   prioritization scheme.
     *
     * The minimum weight of stream dependency.
     */
    #define NGHTTP2_MIN_WEIGHT 1

    /**
     * @macro
     *
     * The maximum window size
     */
    #define NGHTTP2_MAX_WINDOW_SIZE ((int32_t)((1U << 31) - 1))

    /**
     * @macro
     *
     * The initial window size for stream level flow control.
     */
    #define NGHTTP2_INITIAL_WINDOW_SIZE ((1 << 16) - 1)
    /**
     * @macro
     *
     * The initial window size for connection level flow control.
     */
    #define NGHTTP2_INITIAL_CONNECTION_WINDOW_SIZE ((1 << 16) - 1)

    /**
     * @macro
     *
     * The default header table size.
     */
    #define NGHTTP2_DEFAULT_HEADER_TABLE_SIZE (1 << 12)

    /**
     * @macro
     *
     * The client magic string, which is the first 24 bytes byte string of
     * client connection preface.
     */
    #define NGHTTP2_CLIENT_MAGIC "PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n"

    /**
     * @macro
     *
     * The length of :macro:`NGHTTP2_CLIENT_MAGIC`.
     */
    #define NGHTTP2_CLIENT_MAGIC_LEN 24

    /**
     * @macro
     *
     * The default max number of settings per SETTINGS frame
     */
    #define NGHTTP2_DEFAULT_MAX_SETTINGS 32

    /**
     * @enum
     *
     * Error codes used in this library.  The code range is [-999, -500],
     * inclusive. The following values are defined:
     */
    typedef enum {
      /**
       * Invalid argument passed.
       */
      NGHTTP2_ERR_INVALID_ARGUMENT = -501,
      /**
       * Out of buffer space.
       */
      NGHTTP2_ERR_BUFFER_ERROR = -502,
      /**
       * The specified protocol version is not supported.
       */
      NGHTTP2_ERR_UNSUPPORTED_VERSION = -503,
      /**
       * Used as a return value from :type:`nghttp2_send_callback2`,
       * :type:`nghttp2_recv_callback` and
       * :type:`nghttp2_send_data_callback` to indicate that the operation
       * would block.
       */
      NGHTTP2_ERR_WOULDBLOCK = -504,
      /**
       * General protocol error
       */
      NGHTTP2_ERR_PROTO = -505,
      /**
       * The frame is invalid.
       */
      NGHTTP2_ERR_INVALID_FRAME = -506,
      /**
       * The peer performed a shutdown on the connection.
       */
      NGHTTP2_ERR_EOF = -507,
      /**
       * Used as a return value from
       * :func:`nghttp2_data_source_read_callback2` to indicate that data
       * transfer is postponed.  See
       * :func:`nghttp2_data_source_read_callback2` for details.
       */
      NGHTTP2_ERR_DEFERRED = -508,
      /**
       * Stream ID has reached the maximum value.  Therefore no stream ID
       * is available.
       */
      NGHTTP2_ERR_STREAM_ID_NOT_AVAILABLE = -509,
      /**
       * The stream is already closed; or the stream ID is invalid.
       */
      NGHTTP2_ERR_STREAM_CLOSED = -510,
      /**
       * RST_STREAM has been added to the outbound queue.  The stream is
       * in closing state.
       */
      NGHTTP2_ERR_STREAM_CLOSING = -511,
      /**
       * The transmission is not allowed for this stream (e.g., a frame
       * with END_STREAM flag set has already sent).
       */
      NGHTTP2_ERR_STREAM_SHUT_WR = -512,
      /**
       * The stream ID is invalid.
       */
      NGHTTP2_ERR_INVALID_STREAM_ID = -513,
      /**
       * The state of the stream is not valid (e.g., DATA cannot be sent
       * to the stream if response HEADERS has not been sent).
       */
      NGHTTP2_ERR_INVALID_STREAM_STATE = -514,
      /**
       * Another DATA frame has already been deferred.
       */
      NGHTTP2_ERR_DEFERRED_DATA_EXIST = -515,
      /**
       * Starting new stream is not allowed (e.g., GOAWAY has been sent
       * and/or received).
       */
      NGHTTP2_ERR_START_STREAM_NOT_ALLOWED = -516,
      /**
       * GOAWAY has already been sent.
       */
      NGHTTP2_ERR_GOAWAY_ALREADY_SENT = -517,
      /**
       * The received frame contains the invalid header block (e.g., There
       * are duplicate header names; or the header names are not encoded
       * in US-ASCII character set and not lower cased; or the header name
       * is zero-length string; or the header value contains multiple
       * in-sequence NUL bytes).
       */
      NGHTTP2_ERR_INVALID_HEADER_BLOCK = -518,
      /**
       * Indicates that the context is not suitable to perform the
       * requested operation.
       */
      NGHTTP2_ERR_INVALID_STATE = -519,
      /**
       * The user callback function failed due to the temporal error.
       */
      NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE = -521,
      /**
       * The length of the frame is invalid, either too large or too small.
       */
      NGHTTP2_ERR_FRAME_SIZE_ERROR = -522,
      /**
       * Header block inflate/deflate error.
       */
      NGHTTP2_ERR_HEADER_COMP = -523,
      /**
       * Flow control error
       */
      NGHTTP2_ERR_FLOW_CONTROL = -524,
      /**
       * Insufficient buffer size given to function.
       */
      NGHTTP2_ERR_INSUFF_BUFSIZE = -525,
      /**
       * Callback was paused by the application
       */
      NGHTTP2_ERR_PAUSE = -526,
      /**
       * There are too many in-flight SETTING frame and no more
       * transmission of SETTINGS is allowed.
       */
      NGHTTP2_ERR_TOO_MANY_INFLIGHT_SETTINGS = -527,
      /**
       * The server push is disabled.
       */
      NGHTTP2_ERR_PUSH_DISABLED = -528,
      /**
       * DATA or HEADERS frame for a given stream has been already
       * submitted and has not been fully processed yet.  Application
       * should wait for the transmission of the previously submitted
       * frame before submitting another.
       */
      NGHTTP2_ERR_DATA_EXIST = -529,
      /**
       * The current session is closing due to a connection error or
       * `nghttp2_session_terminate_session()` is called.
       */
      NGHTTP2_ERR_SESSION_CLOSING = -530,
      /**
       * Invalid HTTP header field was received and stream is going to be
       * closed.
       */
      NGHTTP2_ERR_HTTP_HEADER = -531,
      /**
       * Violation in HTTP messaging rule.
       */
      NGHTTP2_ERR_HTTP_MESSAGING = -532,
      /**
       * Stream was refused.
       */
      NGHTTP2_ERR_REFUSED_STREAM = -533,
      /**
       * Unexpected internal error, but recovered.
       */
      NGHTTP2_ERR_INTERNAL = -534,
      /**
       * Indicates that a processing was canceled.
       */
      NGHTTP2_ERR_CANCEL = -535,
      /**
       * When a local endpoint expects to receive SETTINGS frame, it
       * receives an other type of frame.
       */
      NGHTTP2_ERR_SETTINGS_EXPECTED = -536,
      /**
       * When a local endpoint receives too many settings entries
       * in a single SETTINGS frame.
       */
      NGHTTP2_ERR_TOO_MANY_SETTINGS = -537,
      /**
       * The errors < :enum:`nghttp2_error.NGHTTP2_ERR_FATAL` mean that
       * the library is under unexpected condition and processing was
       * terminated (e.g., out of memory).  If application receives this
       * error code, it must stop using that :type:`nghttp2_session`
       * object and only allowed operation for that object is deallocate
       * it using `nghttp2_session_del()`.
       */
      NGHTTP2_ERR_FATAL = -900,
      /**
       * Out of memory.  This is a fatal error.
       */
      NGHTTP2_ERR_NOMEM = -901,
      /**
       * The user callback function failed.  This is a fatal error.
       */
      NGHTTP2_ERR_CALLBACK_FAILURE = -902,
      /**
       * Invalid client magic (see :macro:`NGHTTP2_CLIENT_MAGIC`) was
       * received and further processing is not possible.
       */
      NGHTTP2_ERR_BAD_CLIENT_MAGIC = -903,
      /**
       * Possible flooding by peer was detected in this HTTP/2 session.
       * Flooding is measured by how many PING and SETTINGS frames with
       * ACK flag set are queued for transmission.  These frames are
       * response for the peer initiated frames, and peer can cause memory
       * exhaustion on server side to send these frames forever and does
       * not read network.
       */
      NGHTTP2_ERR_FLOODED = -904,
      /**
       * When a local endpoint receives too many CONTINUATION frames
       * following a HEADER frame.
       */
      NGHTTP2_ERR_TOO_MANY_CONTINUATIONS = -905,
    } nghttp2_error;

    /**
     * @struct
     *
     * The object representing single contiguous buffer.
     */
    typedef struct {
      /**
       * The pointer to the buffer.
       */
      uint8_t *base;
      /**
       * The length of the buffer.
       */
      size_t len;
    } nghttp2_vec;

    struct nghttp2_rcbuf;

    /**
     * @struct
     *
     * The object representing reference counted buffer.  The details of
     * this structure are intentionally hidden from the public API.
     */
    typedef struct nghttp2_rcbuf nghttp2_rcbuf;

    /**
     * @function
     *
     * Increments the reference count of |rcbuf| by 1.
     */
    NGHTTP2_EXTERN void nghttp2_rcbuf_incref(nghttp2_rcbuf *rcbuf);

    /**
     * @function
     *
     * Decrements the reference count of |rcbuf| by 1.  If the reference
     * count becomes zero, the object pointed by |rcbuf| will be freed.
     * In this case, application must not use |rcbuf| again.
     */
    NGHTTP2_EXTERN void nghttp2_rcbuf_decref(nghttp2_rcbuf *rcbuf);

    /**
     * @function
     *
     * Returns the underlying buffer managed by |rcbuf|.
     */
    NGHTTP2_EXTERN nghttp2_vec nghttp2_rcbuf_get_buf(nghttp2_rcbuf *rcbuf);

    /**
     * @function
     *
     * Returns nonzero if the underlying buffer is statically allocated,
     * and 0 otherwise. This can be useful for language bindings that wish
     * to avoid creating duplicate strings for these buffers.
     */
    NGHTTP2_EXTERN int nghttp2_rcbuf_is_static(const nghttp2_rcbuf *rcbuf);

    /**
     * @enum
     *
     * The flags for header field name/value pair.
     */
    typedef enum {
      /**
       * No flag set.
       */
      NGHTTP2_NV_FLAG_NONE = 0,
      /**
       * Indicates that this name/value pair must not be indexed ("Literal
       * Header Field never Indexed" representation must be used in HPACK
       * encoding).  Other implementation calls this bit as "sensitive".
       */
      NGHTTP2_NV_FLAG_NO_INDEX = 0x01,
      /**
       * This flag is set solely by application.  If this flag is set, the
       * library does not make a copy of header field name.  This could
       * improve performance.
       */
      NGHTTP2_NV_FLAG_NO_COPY_NAME = 0x02,
      /**
       * This flag is set solely by application.  If this flag is set, the
       * library does not make a copy of header field value.  This could
       * improve performance.
       */
      NGHTTP2_NV_FLAG_NO_COPY_VALUE = 0x04
    } nghttp2_nv_flag;

    /**
     * @struct
     *
     * The name/value pair, which mainly used to represent header fields.
     */
    typedef struct {
      /**
       * The |name| byte string.  If this struct is presented from library
       * (e.g., :type:`nghttp2_on_frame_recv_callback`), |name| is
       * guaranteed to be NULL-terminated.  For some callbacks
       * (:type:`nghttp2_before_frame_send_callback`,
       * :type:`nghttp2_on_frame_send_callback`, and
       * :type:`nghttp2_on_frame_not_send_callback`), it may not be
       * NULL-terminated if header field is passed from application with
       * the flag :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_NAME`).
       * When application is constructing this struct, |name| is not
       * required to be NULL-terminated.
       */
      uint8_t *name;
      /**
       * The |value| byte string.  If this struct is presented from
       * library (e.g., :type:`nghttp2_on_frame_recv_callback`), |value|
       * is guaranteed to be NULL-terminated.  For some callbacks
       * (:type:`nghttp2_before_frame_send_callback`,
       * :type:`nghttp2_on_frame_send_callback`, and
       * :type:`nghttp2_on_frame_not_send_callback`), it may not be
       * NULL-terminated if header field is passed from application with
       * the flag :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_VALUE`).
       * When application is constructing this struct, |value| is not
       * required to be NULL-terminated.
       */
      uint8_t *value;
      /**
       * The length of the |name|, excluding terminating NULL.
       */
      size_t namelen;
      /**
       * The length of the |value|, excluding terminating NULL.
       */
      size_t valuelen;
      /**
       * Bitwise OR of one or more of :type:`nghttp2_nv_flag`.
       */
      uint8_t flags;
    } nghttp2_nv;

    /**
     * @enum
     *
     * The frame types in HTTP/2 specification.
     */
    typedef enum {
      /**
       * The DATA frame.
       */
      NGHTTP2_DATA = 0,
      /**
       * The HEADERS frame.
       */
      NGHTTP2_HEADERS = 0x01,
      /**
       * The PRIORITY frame.
       */
      NGHTTP2_PRIORITY = 0x02,
      /**
       * The RST_STREAM frame.
       */
      NGHTTP2_RST_STREAM = 0x03,
      /**
       * The SETTINGS frame.
       */
      NGHTTP2_SETTINGS = 0x04,
      /**
       * The PUSH_PROMISE frame.
       */
      NGHTTP2_PUSH_PROMISE = 0x05,
      /**
       * The PING frame.
       */
      NGHTTP2_PING = 0x06,
      /**
       * The GOAWAY frame.
       */
      NGHTTP2_GOAWAY = 0x07,
      /**
       * The WINDOW_UPDATE frame.
       */
      NGHTTP2_WINDOW_UPDATE = 0x08,
      /**
       * The CONTINUATION frame.  This frame type won't be passed to any
       * callbacks because the library processes this frame type and its
       * preceding HEADERS/PUSH_PROMISE as a single frame.
       */
      NGHTTP2_CONTINUATION = 0x09,
      /**
       * The ALTSVC frame, which is defined in `RFC 7383
       * <https://tools.ietf.org/html/rfc7838#section-4>`_.
       */
      NGHTTP2_ALTSVC = 0x0A,
      /**
       * The ORIGIN frame, which is defined by `RFC 8336
       * <https://tools.ietf.org/html/rfc8336>`_.
       */
      NGHTTP2_ORIGIN = 0x0C,
      /**
       * The PRIORITY_UPDATE frame, which is defined by :rfc:`9218`.
       */
      NGHTTP2_PRIORITY_UPDATE = 0x10
    } nghttp2_frame_type;

    /**
     * @enum
     *
     * The flags for HTTP/2 frames.  This enum defines all flags for all
     * frames.
     */
    typedef enum {
      /**
       * No flag set.
       */
      NGHTTP2_FLAG_NONE = 0,
      /**
       * The END_STREAM flag.
       */
      NGHTTP2_FLAG_END_STREAM = 0x01,
      /**
       * The END_HEADERS flag.
       */
      NGHTTP2_FLAG_END_HEADERS = 0x04,
      /**
       * The ACK flag.
       */
      NGHTTP2_FLAG_ACK = 0x01,
      /**
       * The PADDED flag.
       */
      NGHTTP2_FLAG_PADDED = 0x08,
      /**
       * The PRIORITY flag.
       */
      NGHTTP2_FLAG_PRIORITY = 0x20
    } nghttp2_flag;

    /**
     * @enum
     * The SETTINGS ID.
     */
    typedef enum {
      /**
       * SETTINGS_HEADER_TABLE_SIZE
       */
      NGHTTP2_SETTINGS_HEADER_TABLE_SIZE = 0x01,
      /**
       * SETTINGS_ENABLE_PUSH
       */
      NGHTTP2_SETTINGS_ENABLE_PUSH = 0x02,
      /**
       * SETTINGS_MAX_CONCURRENT_STREAMS
       */
      NGHTTP2_SETTINGS_MAX_CONCURRENT_STREAMS = 0x03,
      /**
       * SETTINGS_INITIAL_WINDOW_SIZE
       */
      NGHTTP2_SETTINGS_INITIAL_WINDOW_SIZE = 0x04,
      /**
       * SETTINGS_MAX_FRAME_SIZE
       */
      NGHTTP2_SETTINGS_MAX_FRAME_SIZE = 0x05,
      /**
       * SETTINGS_MAX_HEADER_LIST_SIZE
       */
      NGHTTP2_SETTINGS_MAX_HEADER_LIST_SIZE = 0x06,
      /**
       * SETTINGS_ENABLE_CONNECT_PROTOCOL
       * (`RFC 8441 <https://tools.ietf.org/html/rfc8441>`_)
       */
      NGHTTP2_SETTINGS_ENABLE_CONNECT_PROTOCOL = 0x08,
      /**
       * SETTINGS_NO_RFC7540_PRIORITIES (:rfc:`9218`)
       */
      NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES = 0x09
    } nghttp2_settings_id;
    /* Note: If we add SETTINGS, update the capacity of
       NGHTTP2_INBOUND_NUM_IV as well */

    /**
     * @macro
     *
     * .. warning::
     *
     *   Deprecated.  The initial max concurrent streams is 0xFFFFFFFFU.
     *
     * Default maximum number of incoming concurrent streams.  Use
     * `nghttp2_submit_settings()` with
     * :enum:`nghttp2_settings_id.NGHTTP2_SETTINGS_MAX_CONCURRENT_STREAMS`
     * to change the maximum number of incoming concurrent streams.
     *
     * .. note::
     *
     *   The maximum number of outgoing concurrent streams is 100 by
     *   default.
     */
    #define NGHTTP2_INITIAL_MAX_CONCURRENT_STREAMS ((1U << 31) - 1)

    /**
     * @enum
     * The status codes for the RST_STREAM and GOAWAY frames.
     */
    typedef enum {
      /**
       * No errors.
       */
      NGHTTP2_NO_ERROR = 0x00,
      /**
       * PROTOCOL_ERROR
       */
      NGHTTP2_PROTOCOL_ERROR = 0x01,
      /**
       * INTERNAL_ERROR
       */
      NGHTTP2_INTERNAL_ERROR = 0x02,
      /**
       * FLOW_CONTROL_ERROR
       */
      NGHTTP2_FLOW_CONTROL_ERROR = 0x03,
      /**
       * SETTINGS_TIMEOUT
       */
      NGHTTP2_SETTINGS_TIMEOUT = 0x04,
      /**
       * STREAM_CLOSED
       */
      NGHTTP2_STREAM_CLOSED = 0x05,
      /**
       * FRAME_SIZE_ERROR
       */
      NGHTTP2_FRAME_SIZE_ERROR = 0x06,
      /**
       * REFUSED_STREAM
       */
      NGHTTP2_REFUSED_STREAM = 0x07,
      /**
       * CANCEL
       */
      NGHTTP2_CANCEL = 0x08,
      /**
       * COMPRESSION_ERROR
       */
      NGHTTP2_COMPRESSION_ERROR = 0x09,
      /**
       * CONNECT_ERROR
       */
      NGHTTP2_CONNECT_ERROR = 0x0A,
      /**
       * ENHANCE_YOUR_CALM
       */
      NGHTTP2_ENHANCE_YOUR_CALM = 0x0B,
      /**
       * INADEQUATE_SECURITY
       */
      NGHTTP2_INADEQUATE_SECURITY = 0x0C,
      /**
       * HTTP_1_1_REQUIRED
       */
      NGHTTP2_HTTP_1_1_REQUIRED = 0x0D
    } nghttp2_error_code;

    /**
     * @struct
     * The frame header.
     */
    typedef struct {
      /**
       * The length field of this frame, excluding frame header.
       */
      size_t length;
      /**
       * The stream identifier (aka, stream ID)
       */
      int32_t stream_id;
      /**
       * The type of this frame.  See `nghttp2_frame_type`.
       */
      uint8_t type;
      /**
       * The flags.
       */
      uint8_t flags;
      /**
       * Reserved bit in frame header.  Currently, this is always set to 0
       * and application should not expect something useful in here.
       */
      uint8_t reserved;
    } nghttp2_frame_hd;

    /**
     * @union
     *
     * This union represents the some kind of data source passed to
     * :type:`nghttp2_data_source_read_callback2`.
     */
    typedef union {
      /**
       * The integer field, suitable for a file descriptor.
       */
      int fd;
      /**
       * The pointer to an arbitrary object.
       */
      void *ptr;
    } nghttp2_data_source;

    /**
     * @enum
     *
     * The flags used to set in |data_flags| output parameter in
     * :type:`nghttp2_data_source_read_callback2`.
     */
    typedef enum {
      /**
       * No flag set.
       */
      NGHTTP2_DATA_FLAG_NONE = 0,
      /**
       * Indicates EOF was sensed.
       */
      NGHTTP2_DATA_FLAG_EOF = 0x01,
      /**
       * Indicates that END_STREAM flag must not be set even if
       * NGHTTP2_DATA_FLAG_EOF is set.  Usually this flag is used to send
       * trailer fields with `nghttp2_submit_request2()` or
       * `nghttp2_submit_response2()`.
       */
      NGHTTP2_DATA_FLAG_NO_END_STREAM = 0x02,
      /**
       * Indicates that application will send complete DATA frame in
       * :type:`nghttp2_send_data_callback`.
       */
      NGHTTP2_DATA_FLAG_NO_COPY = 0x04
    } nghttp2_data_flag;

    #ifndef NGHTTP2_NO_SSIZE_T
    /**
     * @functypedef
     *
     * .. warning::
     *
     *   Deprecated.  Use :type:`nghttp2_data_source_read_callback2`
     *   instead.
     *
     * Callback function invoked when the library wants to read data from
     * the |source|.  The read data is sent in the stream |stream_id|.
     * The implementation of this function must read at most |length|
     * bytes of data from |source| (or possibly other places) and store
     * them in |buf| and return number of data stored in |buf|.  If EOF is
     * reached, set :enum:`nghttp2_data_flag.NGHTTP2_DATA_FLAG_EOF` flag
     * in |*data_flags|.
     *
     * Sometime it is desirable to avoid copying data into |buf| and let
     * application to send data directly.  To achieve this, set
     * :enum:`nghttp2_data_flag.NGHTTP2_DATA_FLAG_NO_COPY` to
     * |*data_flags| (and possibly other flags, just like when we do
     * copy), and return the number of bytes to send without copying data
     * into |buf|.  The library, seeing
     * :enum:`nghttp2_data_flag.NGHTTP2_DATA_FLAG_NO_COPY`, will invoke
     * :type:`nghttp2_send_data_callback`.  The application must send
     * complete DATA frame in that callback.
     *
     * If this callback is set by `nghttp2_submit_request()`,
     * `nghttp2_submit_response()` or `nghttp2_submit_headers()` and
     * `nghttp2_submit_data()` with flag parameter
     * :enum:`nghttp2_flag.NGHTTP2_FLAG_END_STREAM` set, and
     * :enum:`nghttp2_data_flag.NGHTTP2_DATA_FLAG_EOF` flag is set to
     * |*data_flags|, DATA frame will have END_STREAM flag set.  Usually,
     * this is expected behaviour and all are fine.  One exception is send
     * trailer fields.  You cannot send trailer fields after sending frame
     * with END_STREAM set.  To avoid this problem, one can set
     * :enum:`nghttp2_data_flag.NGHTTP2_DATA_FLAG_NO_END_STREAM` along
     * with :enum:`nghttp2_data_flag.NGHTTP2_DATA_FLAG_EOF` to signal the
     * library not to set END_STREAM in DATA frame.  Then application can
     * use `nghttp2_submit_trailer()` to send trailer fields.
     * `nghttp2_submit_trailer()` can be called inside this callback.
     *
     * If the application wants to postpone DATA frames (e.g.,
     * asynchronous I/O, or reading data blocks for long time), it is
     * achieved by returning :enum:`nghttp2_error.NGHTTP2_ERR_DEFERRED`
     * without reading any data in this invocation.  The library removes
     * DATA frame from the outgoing queue temporarily.  To move back
     * deferred DATA frame to outgoing queue, call
     * `nghttp2_session_resume_data()`.
     *
     * By default, |length| is limited to 16KiB at maximum.  If peer
     * allows larger frames, application can enlarge transmission buffer
     * size.  See :type:`nghttp2_data_source_read_length_callback` for
     * more details.
     *
     * If the application just wants to return from
     * `nghttp2_session_send()` or `nghttp2_session_mem_send()` without
     * sending anything, return :enum:`nghttp2_error.NGHTTP2_ERR_PAUSE`.
     *
     * In case of error, there are 2 choices. Returning
     * :enum:`nghttp2_error.NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE` will
     * close the stream by issuing RST_STREAM with
     * :enum:`nghttp2_error_code.NGHTTP2_INTERNAL_ERROR`.  If a different
     * error code is desirable, use `nghttp2_submit_rst_stream()` with a
     * desired error code and then return
     * :enum:`nghttp2_error.NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE`.
     * Returning :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE` will
     * signal the entire session failure.
     */
    typedef ssize_t (*nghttp2_data_source_read_callback)(
      nghttp2_session *session, int32_t stream_id, uint8_t *buf, size_t length,
      uint32_t *data_flags, nghttp2_data_source *source, void *user_data);

    #endif /* NGHTTP2_NO_SSIZE_T */

    /**
     * @functypedef
     *
     * Callback function invoked when the library wants to read data from
     * the |source|.  The read data is sent in the stream |stream_id|.
     * The implementation of this function must read at most |length|
     * bytes of data from |source| (or possibly other places) and store
     * them in |buf| and return number of data stored in |buf|.  If EOF is
     * reached, set :enum:`nghttp2_data_flag.NGHTTP2_DATA_FLAG_EOF` flag
     * in |*data_flags|.
     *
     * Sometime it is desirable to avoid copying data into |buf| and let
     * application to send data directly.  To achieve this, set
     * :enum:`nghttp2_data_flag.NGHTTP2_DATA_FLAG_NO_COPY` to
     * |*data_flags| (and possibly other flags, just like when we do
     * copy), and return the number of bytes to send without copying data
     * into |buf|.  The library, seeing
     * :enum:`nghttp2_data_flag.NGHTTP2_DATA_FLAG_NO_COPY`, will invoke
     * :type:`nghttp2_send_data_callback`.  The application must send
     * complete DATA frame in that callback.
     *
     * If this callback is set by `nghttp2_submit_request2()`,
     * `nghttp2_submit_response2()` or `nghttp2_submit_headers()` and
     * `nghttp2_submit_data2()` with flag parameter
     * :enum:`nghttp2_flag.NGHTTP2_FLAG_END_STREAM` set, and
     * :enum:`nghttp2_data_flag.NGHTTP2_DATA_FLAG_EOF` flag is set to
     * |*data_flags|, DATA frame will have END_STREAM flag set.  Usually,
     * this is expected behaviour and all are fine.  One exception is send
     * trailer fields.  You cannot send trailer fields after sending frame
     * with END_STREAM set.  To avoid this problem, one can set
     * :enum:`nghttp2_data_flag.NGHTTP2_DATA_FLAG_NO_END_STREAM` along
     * with :enum:`nghttp2_data_flag.NGHTTP2_DATA_FLAG_EOF` to signal the
     * library not to set END_STREAM in DATA frame.  Then application can
     * use `nghttp2_submit_trailer()` to send trailer fields.
     * `nghttp2_submit_trailer()` can be called inside this callback.
     *
     * If the application wants to postpone DATA frames (e.g.,
     * asynchronous I/O, or reading data blocks for long time), it is
     * achieved by returning :enum:`nghttp2_error.NGHTTP2_ERR_DEFERRED`
     * without reading any data in this invocation.  The library removes
     * DATA frame from the outgoing queue temporarily.  To move back
     * deferred DATA frame to outgoing queue, call
     * `nghttp2_session_resume_data()`.
     *
     * By default, |length| is limited to 16KiB at maximum.  If peer
     * allows larger frames, application can enlarge transmission buffer
     * size.  See :type:`nghttp2_data_source_read_length_callback` for
     * more details.
     *
     * If the application just wants to return from
     * `nghttp2_session_send()` or `nghttp2_session_mem_send2()` without
     * sending anything, return :enum:`nghttp2_error.NGHTTP2_ERR_PAUSE`.
     *
     * In case of error, there are 2 choices. Returning
     * :enum:`nghttp2_error.NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE` will
     * close the stream by issuing RST_STREAM with
     * :enum:`nghttp2_error_code.NGHTTP2_INTERNAL_ERROR`.  If a different
     * error code is desirable, use `nghttp2_submit_rst_stream()` with a
     * desired error code and then return
     * :enum:`nghttp2_error.NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE`.
     * Returning :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE` will
     * signal the entire session failure.
     */
    typedef nghttp2_ssize (*nghttp2_data_source_read_callback2)(
      nghttp2_session *session, int32_t stream_id, uint8_t *buf, size_t length,
      uint32_t *data_flags, nghttp2_data_source *source, void *user_data);

    #ifndef NGHTTP2_NO_SSIZE_T
    /**
     * @struct
     *
     * .. warning::
     *
     *   Deprecated.  Use :type:`nghttp2_data_provider2` instead.
     *
     * This struct represents the data source and the way to read a chunk
     * of data from it.
     */
    typedef struct {
      /**
       * The data source.
       */
      nghttp2_data_source source;
      /**
       * The callback function to read a chunk of data from the |source|.
       */
      nghttp2_data_source_read_callback read_callback;
    } nghttp2_data_provider;

    #endif /* NGHTTP2_NO_SSIZE_T */

    /**
     * @struct
     *
     * This struct represents the data source and the way to read a chunk
     * of data from it.
     */
    typedef struct {
      /**
       * The data source.
       */
      nghttp2_data_source source;
      /**
       * The callback function to read a chunk of data from the |source|.
       */
      nghttp2_data_source_read_callback2 read_callback;
    } nghttp2_data_provider2;

    /**
     * @struct
     *
     * The DATA frame.  The received data is delivered via
     * :type:`nghttp2_on_data_chunk_recv_callback`.
     */
    typedef struct {
      nghttp2_frame_hd hd;
      /**
       * The length of the padding in this frame.  This includes PAD_HIGH
       * and PAD_LOW.
       */
      size_t padlen;
    } nghttp2_data;

    /**
     * @enum
     *
     * The category of HEADERS, which indicates the role of the frame.  In
     * HTTP/2 spec, request, response, push response and other arbitrary
     * headers (e.g., trailer fields) are all called just HEADERS.  To
     * give the application the role of incoming HEADERS frame, we define
     * several categories.
     */
    typedef enum {
      /**
       * The HEADERS frame is opening new stream, which is analogous to
       * SYN_STREAM in SPDY.
       */
      NGHTTP2_HCAT_REQUEST = 0,
      /**
       * The HEADERS frame is the first response headers, which is
       * analogous to SYN_REPLY in SPDY.
       */
      NGHTTP2_HCAT_RESPONSE = 1,
      /**
       * The HEADERS frame is the first headers sent against reserved
       * stream.
       */
      NGHTTP2_HCAT_PUSH_RESPONSE = 2,
      /**
       * The HEADERS frame which does not apply for the above categories,
       * which is analogous to HEADERS in SPDY.  If non-final response
       * (e.g., status 1xx) is used, final response HEADERS frame will be
       * categorized here.
       */
      NGHTTP2_HCAT_HEADERS = 3
    } nghttp2_headers_category;

    /**
     * @struct
     *
     * .. warning::
     *
     *   Deprecated.  :rfc:`7540` priorities are deprecated by
     *   :rfc:`9113`.  Consider migrating to :rfc:`9218` extensible
     *   prioritization scheme.
     *
     * The structure to specify stream dependency.
     */
    typedef struct {
      /**
       * The stream ID of the stream to depend on.  Specifying 0 makes
       * stream not depend any other stream.
       */
      int32_t stream_id;
      /**
       * The weight of this dependency.
       */
      int32_t weight;
      /**
       * nonzero means exclusive dependency
       */
      uint8_t exclusive;
    } nghttp2_priority_spec;

    /**
     * @struct
     *
     * The HEADERS frame.  It has the following members:
     */
    typedef struct {
      /**
       * The frame header.
       */
      nghttp2_frame_hd hd;
      /**
       * The length of the padding in this frame.  This includes PAD_HIGH
       * and PAD_LOW.
       */
      size_t padlen;
      /**
       * .. warning::
       *
       *   Deprecated.  :rfc:`7540` priorities are deprecated by
       *   :rfc:`9113`.  Consider migrating to :rfc:`9218` extensible
       *   prioritization scheme.
       *
       * The priority specification
       */
      nghttp2_priority_spec pri_spec;
      /**
       * The name/value pairs.
       */
      nghttp2_nv *nva;
      /**
       * The number of name/value pairs in |nva|.
       */
      size_t nvlen;
      /**
       * The category of this HEADERS frame.
       */
      nghttp2_headers_category cat;
    } nghttp2_headers;

    /**
     * @struct
     *
     * .. warning::
     *
     *   Deprecated.  :rfc:`7540` priorities are deprecated by
     *   :rfc:`9113`.  Consider migrating to :rfc:`9218` extensible
     *   prioritization scheme.
     *
     * The PRIORITY frame.  It has the following members:
     */
    typedef struct {
      /**
       * The frame header.
       */
      nghttp2_frame_hd hd;
      /**
       * The priority specification.
       */
      nghttp2_priority_spec pri_spec;
    } nghttp2_priority;

    /**
     * @struct
     *
     * The RST_STREAM frame.  It has the following members:
     */
    typedef struct {
      /**
       * The frame header.
       */
      nghttp2_frame_hd hd;
      /**
       * The error code.  See :type:`nghttp2_error_code`.
       */
      uint32_t error_code;
    } nghttp2_rst_stream;

    /**
     * @struct
     *
     * The SETTINGS ID/Value pair.  It has the following members:
     */
    typedef struct {
      /**
       * The SETTINGS ID.  See :type:`nghttp2_settings_id`.
       */
      int32_t settings_id;
      /**
       * The value of this entry.
       */
      uint32_t value;
    } nghttp2_settings_entry;

    /**
     * @struct
     *
     * The SETTINGS frame.  It has the following members:
     */
    typedef struct {
      /**
       * The frame header.
       */
      nghttp2_frame_hd hd;
      /**
       * The number of SETTINGS ID/Value pairs in |iv|.
       */
      size_t niv;
      /**
       * The pointer to the array of SETTINGS ID/Value pair.
       */
      nghttp2_settings_entry *iv;
    } nghttp2_settings;

    /**
     * @struct
     *
     * The PUSH_PROMISE frame.  It has the following members:
     */
    typedef struct {
      /**
       * The frame header.
       */
      nghttp2_frame_hd hd;
      /**
       * The length of the padding in this frame.  This includes PAD_HIGH
       * and PAD_LOW.
       */
      size_t padlen;
      /**
       * The name/value pairs.
       */
      nghttp2_nv *nva;
      /**
       * The number of name/value pairs in |nva|.
       */
      size_t nvlen;
      /**
       * The promised stream ID
       */
      int32_t promised_stream_id;
      /**
       * Reserved bit.  Currently this is always set to 0 and application
       * should not expect something useful in here.
       */
      uint8_t reserved;
    } nghttp2_push_promise;

    /**
     * @struct
     *
     * The PING frame.  It has the following members:
     */
    typedef struct {
      /**
       * The frame header.
       */
      nghttp2_frame_hd hd;
      /**
       * The opaque data
       */
      uint8_t opaque_data[8];
    } nghttp2_ping;

    /**
     * @struct
     *
     * The GOAWAY frame.  It has the following members:
     */
    typedef struct {
      /**
       * The frame header.
       */
      nghttp2_frame_hd hd;
      /**
       * The last stream stream ID.
       */
      int32_t last_stream_id;
      /**
       * The error code.  See :type:`nghttp2_error_code`.
       */
      uint32_t error_code;
      /**
       * The additional debug data
       */
      uint8_t *opaque_data;
      /**
       * The length of |opaque_data| member.
       */
      size_t opaque_data_len;
      /**
       * Reserved bit.  Currently this is always set to 0 and application
       * should not expect something useful in here.
       */
      uint8_t reserved;
    } nghttp2_goaway;

    /**
     * @struct
     *
     * The WINDOW_UPDATE frame.  It has the following members:
     */
    typedef struct {
      /**
       * The frame header.
       */
      nghttp2_frame_hd hd;
      /**
       * The window size increment.
       */
      int32_t window_size_increment;
      /**
       * Reserved bit.  Currently this is always set to 0 and application
       * should not expect something useful in here.
       */
      uint8_t reserved;
    } nghttp2_window_update;

    /**
     * @struct
     *
     * The extension frame.  It has following members:
     */
    typedef struct {
      /**
       * The frame header.
       */
      nghttp2_frame_hd hd;
      /**
       * The pointer to extension payload.  The exact pointer type is
       * determined by hd.type.
       *
       * Currently, no extension is supported.  This is a place holder for
       * the future extensions.
       */
      void *payload;
    } nghttp2_extension;

    /**
     * @union
     *
     * This union includes all frames to pass them to various function
     * calls as nghttp2_frame type.  The CONTINUATION frame is omitted
     * from here because the library deals with it internally.
     */
    typedef union {
      /**
       * The frame header, which is convenient to inspect frame header.
       */
      nghttp2_frame_hd hd;
      /**
       * The DATA frame.
       */
      nghttp2_data data;
      /**
       * The HEADERS frame.
       */
      nghttp2_headers headers;
      /**
       * The PRIORITY frame.
       */
      nghttp2_priority priority;
      /**
       * The RST_STREAM frame.
       */
      nghttp2_rst_stream rst_stream;
      /**
       * The SETTINGS frame.
       */
      nghttp2_settings settings;
      /**
       * The PUSH_PROMISE frame.
       */
      nghttp2_push_promise push_promise;
      /**
       * The PING frame.
       */
      nghttp2_ping ping;
      /**
       * The GOAWAY frame.
       */
      nghttp2_goaway goaway;
      /**
       * The WINDOW_UPDATE frame.
       */
      nghttp2_window_update window_update;
      /**
       * The extension frame.
       */
      nghttp2_extension ext;
    } nghttp2_frame;

    #ifndef NGHTTP2_NO_SSIZE_T
    /**
     * @functypedef
     *
     * .. warning::
     *
     *   Deprecated.  Use :type:`nghttp2_send_callback2` instead.
     *
     * Callback function invoked when |session| wants to send data to the
     * remote peer.  The implementation of this function must send at most
     * |length| bytes of data stored in |data|.  The |flags| is currently
     * not used and always 0. It must return the number of bytes sent if
     * it succeeds.  If it cannot send any single byte without blocking,
     * it must return :enum:`nghttp2_error.NGHTTP2_ERR_WOULDBLOCK`.  For
     * other errors, it must return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.  The
     * |user_data| pointer is the third argument passed in to the call to
     * `nghttp2_session_client_new()` or `nghttp2_session_server_new()`.
     *
     * This callback is required if the application uses
     * `nghttp2_session_send()` to send data to the remote endpoint.  If
     * the application uses solely `nghttp2_session_mem_send()` instead,
     * this callback function is unnecessary.
     *
     * To set this callback to :type:`nghttp2_session_callbacks`, use
     * `nghttp2_session_callbacks_set_send_callback()`.
     *
     * .. note::
     *
     *   The |length| may be very small.  If that is the case, and
     *   application disables Nagle algorithm (``TCP_NODELAY``), then just
     *   writing |data| to the network stack leads to very small packet,
     *   and it is very inefficient.  An application should be responsible
     *   to buffer up small chunks of data as necessary to avoid this
     *   situation.
     */
    typedef ssize_t (*nghttp2_send_callback)(nghttp2_session *session,
                                             const uint8_t *data, size_t length,
                                             int flags, void *user_data);

    #endif /* NGHTTP2_NO_SSIZE_T */

    /**
     * @functypedef
     *
     * Callback function invoked when |session| wants to send data to the
     * remote peer.  The implementation of this function must send at most
     * |length| bytes of data stored in |data|.  The |flags| is currently
     * not used and always 0. It must return the number of bytes sent if
     * it succeeds.  If it cannot send any single byte without blocking,
     * it must return :enum:`nghttp2_error.NGHTTP2_ERR_WOULDBLOCK`.  For
     * other errors, it must return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.  The
     * |user_data| pointer is the third argument passed in to the call to
     * `nghttp2_session_client_new()` or `nghttp2_session_server_new()`.
     *
     * This callback is required if the application uses
     * `nghttp2_session_send()` to send data to the remote endpoint.  If
     * the application uses solely `nghttp2_session_mem_send2()` instead,
     * this callback function is unnecessary.
     *
     * To set this callback to :type:`nghttp2_session_callbacks`, use
     * `nghttp2_session_callbacks_set_send_callback2()`.
     *
     * .. note::
     *
     *   The |length| may be very small.  If that is the case, and
     *   application disables Nagle algorithm (``TCP_NODELAY``), then just
     *   writing |data| to the network stack leads to very small packet,
     *   and it is very inefficient.  An application should be responsible
     *   to buffer up small chunks of data as necessary to avoid this
     *   situation.
     */
    typedef nghttp2_ssize (*nghttp2_send_callback2)(nghttp2_session *session,
                                                    const uint8_t *data,
                                                    size_t length, int flags,
                                                    void *user_data);

    /**
     * @functypedef
     *
     * Callback function invoked when
     * :enum:`nghttp2_data_flag.NGHTTP2_DATA_FLAG_NO_COPY` is used in
     * :type:`nghttp2_data_source_read_callback` to send complete DATA
     * frame.
     *
     * The |frame| is a DATA frame to send.  The |framehd| is the
     * serialized frame header (9 bytes). The |length| is the length of
     * application data to send (this does not include padding).  The
     * |source| is the same pointer passed to
     * :type:`nghttp2_data_source_read_callback`.
     *
     * The application first must send frame header |framehd| of length 9
     * bytes.  If ``frame->data.padlen > 0``, send 1 byte of value
     * ``frame->data.padlen - 1``.  Then send exactly |length| bytes of
     * application data.  Finally, if ``frame->data.padlen > 1``, send
     * ``frame->data.padlen - 1`` bytes of zero as padding.
     *
     * The application has to send complete DATA frame in this callback.
     * If all data were written successfully, return 0.
     *
     * If it cannot send any data at all, just return
     * :enum:`nghttp2_error.NGHTTP2_ERR_WOULDBLOCK`; the library will call
     * this callback with the same parameters later (It is recommended to
     * send complete DATA frame at once in this function to deal with
     * error; if partial frame data has already sent, it is impossible to
     * send another data in that state, and all we can do is tear down
     * connection).  When data is fully processed, but application wants
     * to make `nghttp2_session_mem_send2()` or `nghttp2_session_send()`
     * return immediately without processing next frames, return
     * :enum:`nghttp2_error.NGHTTP2_ERR_PAUSE`.  If application decided to
     * reset this stream, return
     * :enum:`nghttp2_error.NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE`, then
     * the library will send RST_STREAM with INTERNAL_ERROR as error code.
     * The application can also return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`, which will
     * result in connection closure.  Returning any other value is treated
     * as :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE` is returned.
     */
    typedef int (*nghttp2_send_data_callback)(nghttp2_session *session,
                                              nghttp2_frame *frame,
                                              const uint8_t *framehd, size_t length,
                                              nghttp2_data_source *source,
                                              void *user_data);

    #ifndef NGHTTP2_NO_SSIZE_T
    /**
     * @functypedef
     *
     * .. warning::
     *
     *   Deprecated.  Use :type:`nghttp2_recv_callback2` instead.
     *
     * Callback function invoked when |session| wants to receive data from
     * the remote peer.  The implementation of this function must read at
     * most |length| bytes of data and store it in |buf|.  The |flags| is
     * currently not used and always 0.  It must return the number of
     * bytes written in |buf| if it succeeds.  If it cannot read any
     * single byte without blocking, it must return
     * :enum:`nghttp2_error.NGHTTP2_ERR_WOULDBLOCK`.  If it gets EOF
     * before it reads any single byte, it must return
     * :enum:`nghttp2_error.NGHTTP2_ERR_EOF`.  For other errors, it must
     * return :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.
     * Returning 0 is treated as
     * :enum:`nghttp2_error.NGHTTP2_ERR_WOULDBLOCK`.  The |user_data|
     * pointer is the third argument passed in to the call to
     * `nghttp2_session_client_new()` or `nghttp2_session_server_new()`.
     *
     * This callback is required if the application uses
     * `nghttp2_session_recv()` to receive data from the remote endpoint.
     * If the application uses solely `nghttp2_session_mem_recv()`
     * instead, this callback function is unnecessary.
     *
     * To set this callback to :type:`nghttp2_session_callbacks`, use
     * `nghttp2_session_callbacks_set_recv_callback()`.
     */
    typedef ssize_t (*nghttp2_recv_callback)(nghttp2_session *session, uint8_t *buf,
                                             size_t length, int flags,
                                             void *user_data);

    #endif /* NGHTTP2_NO_SSIZE_T */

    /**
     * @functypedef
     *
     * Callback function invoked when |session| wants to receive data from
     * the remote peer.  The implementation of this function must read at
     * most |length| bytes of data and store it in |buf|.  The |flags| is
     * currently not used and always 0.  It must return the number of
     * bytes written in |buf| if it succeeds.  If it cannot read any
     * single byte without blocking, it must return
     * :enum:`nghttp2_error.NGHTTP2_ERR_WOULDBLOCK`.  If it gets EOF
     * before it reads any single byte, it must return
     * :enum:`nghttp2_error.NGHTTP2_ERR_EOF`.  For other errors, it must
     * return :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.
     * Returning 0 is treated as
     * :enum:`nghttp2_error.NGHTTP2_ERR_WOULDBLOCK`.  The |user_data|
     * pointer is the third argument passed in to the call to
     * `nghttp2_session_client_new()` or `nghttp2_session_server_new()`.
     *
     * This callback is required if the application uses
     * `nghttp2_session_recv()` to receive data from the remote endpoint.
     * If the application uses solely `nghttp2_session_mem_recv2()`
     * instead, this callback function is unnecessary.
     *
     * To set this callback to :type:`nghttp2_session_callbacks`, use
     * `nghttp2_session_callbacks_set_recv_callback2()`.
     */
    typedef nghttp2_ssize (*nghttp2_recv_callback2)(nghttp2_session *session,
                                                    uint8_t *buf, size_t length,
                                                    int flags, void *user_data);

    /**
     * @functypedef
     *
     * Callback function invoked by `nghttp2_session_recv()` and
     * `nghttp2_session_mem_recv2()` when a frame is received.  The
     * |user_data| pointer is the third argument passed in to the call to
     * `nghttp2_session_client_new()` or `nghttp2_session_server_new()`.
     *
     * If frame is HEADERS or PUSH_PROMISE, the ``nva`` and ``nvlen``
     * member of their data structure are always ``NULL`` and 0
     * respectively.  The header name/value pairs are emitted via
     * :type:`nghttp2_on_header_callback`.
     *
     * Only HEADERS and DATA frame can signal the end of incoming data.
     * If ``frame->hd.flags & NGHTTP2_FLAG_END_STREAM`` is nonzero, the
     * |frame| is the last frame from the remote peer in this stream.
     *
     * This callback won't be called for CONTINUATION frames.
     * HEADERS/PUSH_PROMISE + CONTINUATIONs are treated as single frame.
     *
     * The implementation of this function must return 0 if it succeeds.
     * If nonzero value is returned, it is treated as fatal error and
     * `nghttp2_session_recv()` and `nghttp2_session_mem_recv2()`
     * functions immediately return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.
     *
     * To set this callback to :type:`nghttp2_session_callbacks`, use
     * `nghttp2_session_callbacks_set_on_frame_recv_callback()`.
     */
    typedef int (*nghttp2_on_frame_recv_callback)(nghttp2_session *session,
                                                  const nghttp2_frame *frame,
                                                  void *user_data);

    /**
     * @functypedef
     *
     * Callback function invoked by `nghttp2_session_recv()` and
     * `nghttp2_session_mem_recv2()` when an invalid non-DATA frame is
     * received.  The error is indicated by the |lib_error_code|, which is
     * one of the values defined in :type:`nghttp2_error`.  When this
     * callback function is invoked, the library automatically submits
     * either RST_STREAM or GOAWAY frame.  The |user_data| pointer is the
     * third argument passed in to the call to
     * `nghttp2_session_client_new()` or `nghttp2_session_server_new()`.
     *
     * If frame is HEADERS or PUSH_PROMISE, the ``nva`` and ``nvlen``
     * member of their data structure are always ``NULL`` and 0
     * respectively.
     *
     * The implementation of this function must return 0 if it succeeds.
     * If nonzero is returned, it is treated as fatal error and
     * `nghttp2_session_recv()` and `nghttp2_session_mem_recv2()`
     * functions immediately return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.
     *
     * To set this callback to :type:`nghttp2_session_callbacks`, use
     * `nghttp2_session_callbacks_set_on_invalid_frame_recv_callback()`.
     */
    typedef int (*nghttp2_on_invalid_frame_recv_callback)(
      nghttp2_session *session, const nghttp2_frame *frame, int lib_error_code,
      void *user_data);

    /**
     * @functypedef
     *
     * Callback function invoked when a chunk of data in DATA frame is
     * received.  The |stream_id| is the stream ID this DATA frame belongs
     * to.  The |flags| is the flags of DATA frame which this data chunk
     * is contained.  ``(flags & NGHTTP2_FLAG_END_STREAM) != 0`` does not
     * necessarily mean this chunk of data is the last one in the stream.
     * You should use :type:`nghttp2_on_frame_recv_callback` to know all
     * data frames are received.  The |user_data| pointer is the third
     * argument passed in to the call to `nghttp2_session_client_new()` or
     * `nghttp2_session_server_new()`.
     *
     * If the application uses `nghttp2_session_mem_recv2()`, it can
     * return :enum:`nghttp2_error.NGHTTP2_ERR_PAUSE` to make
     * `nghttp2_session_mem_recv2()` return without processing further
     * input bytes.  The memory by pointed by the |data| is retained until
     * `nghttp2_session_mem_recv2()` or `nghttp2_session_recv()` is
     * called.  The application must retain the input bytes which was used
     * to produce the |data| parameter, because it may refer to the memory
     * region included in the input bytes.
     *
     * The implementation of this function must return 0 if it succeeds.
     * If nonzero is returned, it is treated as fatal error, and
     * `nghttp2_session_recv()` and `nghttp2_session_mem_recv2()`
     * functions immediately return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.
     *
     * To set this callback to :type:`nghttp2_session_callbacks`, use
     * `nghttp2_session_callbacks_set_on_data_chunk_recv_callback()`.
     */
    typedef int (*nghttp2_on_data_chunk_recv_callback)(nghttp2_session *session,
                                                       uint8_t flags,
                                                       int32_t stream_id,
                                                       const uint8_t *data,
                                                       size_t len, void *user_data);

    /**
     * @functypedef
     *
     * Callback function invoked just before the non-DATA frame |frame| is
     * sent.  The |user_data| pointer is the third argument passed in to
     * the call to `nghttp2_session_client_new()` or
     * `nghttp2_session_server_new()`.
     *
     * The implementation of this function must return 0 if it succeeds.
     * It can also return :enum:`nghttp2_error.NGHTTP2_ERR_CANCEL` to
     * cancel the transmission of the given frame.
     *
     * If there is a fatal error while executing this callback, the
     * implementation should return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`, which makes
     * `nghttp2_session_send()` and `nghttp2_session_mem_send2()`
     * functions immediately return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.
     *
     * If the other value is returned, it is treated as if
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE` is returned.
     * But the implementation should not rely on this since the library
     * may define new return value to extend its capability.
     *
     * To set this callback to :type:`nghttp2_session_callbacks`, use
     * `nghttp2_session_callbacks_set_before_frame_send_callback()`.
     */
    typedef int (*nghttp2_before_frame_send_callback)(nghttp2_session *session,
                                                      const nghttp2_frame *frame,
                                                      void *user_data);

    /**
     * @functypedef
     *
     * Callback function invoked after the frame |frame| is sent.  The
     * |user_data| pointer is the third argument passed in to the call to
     * `nghttp2_session_client_new()` or `nghttp2_session_server_new()`.
     *
     * The implementation of this function must return 0 if it succeeds.
     * If nonzero is returned, it is treated as fatal error and
     * `nghttp2_session_send()` and `nghttp2_session_mem_send2()`
     * functions immediately return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.
     *
     * To set this callback to :type:`nghttp2_session_callbacks`, use
     * `nghttp2_session_callbacks_set_on_frame_send_callback()`.
     */
    typedef int (*nghttp2_on_frame_send_callback)(nghttp2_session *session,
                                                  const nghttp2_frame *frame,
                                                  void *user_data);

    /**
     * @functypedef
     *
     * Callback function invoked after the non-DATA frame |frame| is not
     * sent because of the error.  The error is indicated by the
     * |lib_error_code|, which is one of the values defined in
     * :type:`nghttp2_error`.  The |user_data| pointer is the third
     * argument passed in to the call to `nghttp2_session_client_new()` or
     * `nghttp2_session_server_new()`.
     *
     * The implementation of this function must return 0 if it succeeds.
     * If nonzero is returned, it is treated as fatal error and
     * `nghttp2_session_send()` and `nghttp2_session_mem_send2()`
     * functions immediately return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.
     *
     * `nghttp2_session_get_stream_user_data()` can be used to get
     * associated data.
     *
     * To set this callback to :type:`nghttp2_session_callbacks`, use
     * `nghttp2_session_callbacks_set_on_frame_not_send_callback()`.
     */
    typedef int (*nghttp2_on_frame_not_send_callback)(nghttp2_session *session,
                                                      const nghttp2_frame *frame,
                                                      int lib_error_code,
                                                      void *user_data);

    /**
     * @functypedef
     *
     * Callback function invoked when the stream |stream_id| is closed.
     * The reason of closure is indicated by the |error_code|.  The
     * |error_code| is usually one of :enum:`nghttp2_error_code`, but that
     * is not guaranteed.  The stream_user_data, which was specified in
     * `nghttp2_submit_request2()` or `nghttp2_submit_headers()`, is still
     * available in this function.  The |user_data| pointer is the third
     * argument passed in to the call to `nghttp2_session_client_new()` or
     * `nghttp2_session_server_new()`.
     *
     * This function is also called for a stream in reserved state.
     *
     * The implementation of this function must return 0 if it succeeds.
     * If nonzero is returned, it is treated as fatal error and
     * `nghttp2_session_recv()`, `nghttp2_session_mem_recv2()`,
     * `nghttp2_session_send()`, and `nghttp2_session_mem_send2()`
     * functions immediately return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.
     *
     * To set this callback to :type:`nghttp2_session_callbacks`, use
     * `nghttp2_session_callbacks_set_on_stream_close_callback()`.
     */
    typedef int (*nghttp2_on_stream_close_callback)(nghttp2_session *session,
                                                    int32_t stream_id,
                                                    uint32_t error_code,
                                                    void *user_data);

    /**
     * @functypedef
     *
     * Callback function invoked when the reception of header block in
     * HEADERS or PUSH_PROMISE is started.  Each header name/value pair
     * will be emitted by :type:`nghttp2_on_header_callback`.
     *
     * The ``frame->hd.flags`` may not have
     * :enum:`nghttp2_flag.NGHTTP2_FLAG_END_HEADERS` flag set, which
     * indicates that one or more CONTINUATION frames are involved.  But
     * the application does not need to care about that because the header
     * name/value pairs are emitted transparently regardless of
     * CONTINUATION frames.
     *
     * The server applications probably create an object to store
     * information about new stream if ``frame->hd.type ==
     * NGHTTP2_HEADERS`` and ``frame->headers.cat ==
     * NGHTTP2_HCAT_REQUEST``.  If |session| is configured as server side,
     * ``frame->headers.cat`` is either ``NGHTTP2_HCAT_REQUEST``
     * containing request headers or ``NGHTTP2_HCAT_HEADERS`` containing
     * trailer fields and never get PUSH_PROMISE in this callback.
     *
     * For the client applications, ``frame->hd.type`` is either
     * ``NGHTTP2_HEADERS`` or ``NGHTTP2_PUSH_PROMISE``.  In case of
     * ``NGHTTP2_HEADERS``, ``frame->headers.cat ==
     * NGHTTP2_HCAT_RESPONSE`` means that it is the first response
     * headers, but it may be non-final response which is indicated by 1xx
     * status code.  In this case, there may be zero or more HEADERS frame
     * with ``frame->headers.cat == NGHTTP2_HCAT_HEADERS`` which has
     * non-final response code and finally client gets exactly one HEADERS
     * frame with ``frame->headers.cat == NGHTTP2_HCAT_HEADERS``
     * containing final response headers (non-1xx status code).  The
     * trailer fields also has ``frame->headers.cat ==
     * NGHTTP2_HCAT_HEADERS`` which does not contain any status code.
     *
     * Returning
     * :enum:`nghttp2_error.NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE` will
     * close the stream (promised stream if frame is PUSH_PROMISE) by
     * issuing RST_STREAM with
     * :enum:`nghttp2_error_code.NGHTTP2_INTERNAL_ERROR`.  In this case,
     * :type:`nghttp2_on_header_callback` and
     * :type:`nghttp2_on_frame_recv_callback` will not be invoked.  If a
     * different error code is desirable, use
     * `nghttp2_submit_rst_stream()` with a desired error code and then
     * return :enum:`nghttp2_error.NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE`.
     * Again, use ``frame->push_promise.promised_stream_id`` as stream_id
     * parameter in `nghttp2_submit_rst_stream()` if frame is
     * PUSH_PROMISE.
     *
     * The implementation of this function must return 0 if it succeeds.
     * It can return
     * :enum:`nghttp2_error.NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE` to
     * reset the stream (promised stream if frame is PUSH_PROMISE).  For
     * critical errors, it must return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.  If the other
     * value is returned, it is treated as if
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE` is returned.  If
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE` is returned,
     * `nghttp2_session_mem_recv2()` function will immediately return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.
     *
     * To set this callback to :type:`nghttp2_session_callbacks`, use
     * `nghttp2_session_callbacks_set_on_begin_headers_callback()`.
     */
    typedef int (*nghttp2_on_begin_headers_callback)(nghttp2_session *session,
                                                     const nghttp2_frame *frame,
                                                     void *user_data);

    /**
     * @functypedef
     *
     * Callback function invoked when a header name/value pair is received
     * for the |frame|.  The |name| of length |namelen| is header name.
     * The |value| of length |valuelen| is header value.  The |flags| is
     * bitwise OR of one or more of :type:`nghttp2_nv_flag`.
     *
     * If :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_INDEX` is set in
     * |flags|, the receiver must not index this name/value pair when
     * forwarding it to the next hop.  More specifically, "Literal Header
     * Field never Indexed" representation must be used in HPACK encoding.
     *
     * When this callback is invoked, ``frame->hd.type`` is either
     * :enum:`nghttp2_frame_type.NGHTTP2_HEADERS` or
     * :enum:`nghttp2_frame_type.NGHTTP2_PUSH_PROMISE`.  After all header
     * name/value pairs are processed with this callback, and no error has
     * been detected, :type:`nghttp2_on_frame_recv_callback` will be
     * invoked.  If there is an error in decompression,
     * :type:`nghttp2_on_frame_recv_callback` for the |frame| will not be
     * invoked.
     *
     * Both |name| and |value| are guaranteed to be NULL-terminated.  The
     * |namelen| and |valuelen| do not include terminal NULL.  If
     * `nghttp2_option_set_no_http_messaging()` is used with nonzero
     * value, NULL character may be included in |name| or |value| before
     * terminating NULL.
     *
     * Please note that unless `nghttp2_option_set_no_http_messaging()` is
     * used, nghttp2 library does perform validation against the |name|
     * and the |value| using `nghttp2_check_header_name()` and
     * `nghttp2_check_header_value()`.  In addition to this, nghttp2
     * performs validation based on HTTP Messaging rule, which is briefly
     * explained in :ref:`http-messaging` section.
     *
     * If the application uses `nghttp2_session_mem_recv2()`, it can
     * return :enum:`nghttp2_error.NGHTTP2_ERR_PAUSE` to make
     * `nghttp2_session_mem_recv2()` return without processing further
     * input bytes.  The memory pointed by |frame|, |name| and |value|
     * parameters are retained until `nghttp2_session_mem_recv2()` or
     * `nghttp2_session_recv()` is called.  The application must retain
     * the input bytes which was used to produce these parameters, because
     * it may refer to the memory region included in the input bytes.
     *
     * Returning
     * :enum:`nghttp2_error.NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE` will
     * close the stream (promised stream if frame is PUSH_PROMISE) by
     * issuing RST_STREAM with
     * :enum:`nghttp2_error_code.NGHTTP2_INTERNAL_ERROR`.  In this case,
     * :type:`nghttp2_on_header_callback` and
     * :type:`nghttp2_on_frame_recv_callback` will not be invoked.  If a
     * different error code is desirable, use
     * `nghttp2_submit_rst_stream()` with a desired error code and then
     * return :enum:`nghttp2_error.NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE`.
     * Again, use ``frame->push_promise.promised_stream_id`` as stream_id
     * parameter in `nghttp2_submit_rst_stream()` if frame is
     * PUSH_PROMISE.
     *
     * The implementation of this function must return 0 if it succeeds.
     * It may return :enum:`nghttp2_error.NGHTTP2_ERR_PAUSE` or
     * :enum:`nghttp2_error.NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE`.  For
     * other critical failures, it must return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.  If the other
     * nonzero value is returned, it is treated as
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.  If
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE` is returned,
     * `nghttp2_session_recv()` and `nghttp2_session_mem_recv2()`
     * functions immediately return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.
     *
     * To set this callback to :type:`nghttp2_session_callbacks`, use
     * `nghttp2_session_callbacks_set_on_header_callback()`.
     *
     * .. warning::
     *
     *   Application should properly limit the total buffer size to store
     *   incoming header fields.  Without it, peer may send large number
     *   of header fields or large header fields to cause out of memory in
     *   local endpoint.  Due to how HPACK works, peer can do this
     *   effectively without using much memory on their own.
     */
    typedef int (*nghttp2_on_header_callback)(nghttp2_session *session,
                                              const nghttp2_frame *frame,
                                              const uint8_t *name, size_t namelen,
                                              const uint8_t *value, size_t valuelen,
                                              uint8_t flags, void *user_data);

    /**
     * @functypedef
     *
     * Callback function invoked when a header name/value pair is received
     * for the |frame|.  The |name| is header name.  The |value| is header
     * value.  The |flags| is bitwise OR of one or more of
     * :type:`nghttp2_nv_flag`.
     *
     * This callback behaves like :type:`nghttp2_on_header_callback`,
     * except that |name| and |value| are stored in reference counted
     * buffer.  If application wishes to keep these references without
     * copying them, use `nghttp2_rcbuf_incref()` to increment their
     * reference count.  It is the application's responsibility to call
     * `nghttp2_rcbuf_decref()` if they called `nghttp2_rcbuf_incref()` so
     * as not to leak memory.  If the |session| is created by
     * `nghttp2_session_server_new3()` or `nghttp2_session_client_new3()`,
     * the function to free memory is the one belongs to the mem
     * parameter.  As long as this free function alives, |name| and
     * |value| can live after |session| was destroyed.
     */
    typedef int (*nghttp2_on_header_callback2)(nghttp2_session *session,
                                               const nghttp2_frame *frame,
                                               nghttp2_rcbuf *name,
                                               nghttp2_rcbuf *value, uint8_t flags,
                                               void *user_data);

    /**
     * @functypedef
     *
     * Callback function invoked when an invalid header name/value pair is
     * received for the |frame|.
     *
     * The parameter and behaviour are similar to
     * :type:`nghttp2_on_header_callback`.  The difference is that this
     * callback is only invoked when an invalid header name/value pair is
     * received which is treated as stream error if this callback returns
     * :enum:`nghttp2_error.NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE` and
     * :type:`nghttp2_on_invalid_header_callback2` is not set.  Only
     * invalid regular header field are passed to this callback.  In other
     * words, invalid pseudo header field is not passed to this callback.
     * Also header fields which includes upper cased latter are also
     * treated as error without passing them to this callback.
     *
     * This callback is only considered if HTTP messaging validation is
     * turned on (which is on by default, see
     * `nghttp2_option_set_no_http_messaging()`).
     *
     * With this callback, application inspects the incoming invalid
     * field, and it also can reset stream from this callback by returning
     * :enum:`nghttp2_error.NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE`.  By
     * default, the error code is
     * :enum:`nghttp2_error_code.NGHTTP2_PROTOCOL_ERROR`.  To change the
     * error code, call `nghttp2_submit_rst_stream()` with the error code
     * of choice in addition to returning
     * :enum:`nghttp2_error.NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE`.
     *
     * If 0 is returned, the header field is ignored, and the stream is
     * not reset.
     */
    typedef int (*nghttp2_on_invalid_header_callback)(
      nghttp2_session *session, const nghttp2_frame *frame, const uint8_t *name,
      size_t namelen, const uint8_t *value, size_t valuelen, uint8_t flags,
      void *user_data);

    /**
     * @functypedef
     *
     * Callback function invoked when an invalid header name/value pair is
     * received for the |frame|.
     *
     * The parameter and behaviour are similar to
     * :type:`nghttp2_on_header_callback2`.  The difference is that this
     * callback is only invoked when an invalid header name/value pair is
     * received which is silently ignored if neither this callback nor
     * :type:`nghttp2_on_invalid_header_callback` is set.  Only invalid
     * regular header field are passed to this callback.  In other words,
     * invalid pseudo header field is not passed to this callback.  Also
     * header fields which includes upper cased latter are also treated as
     * error without passing them to this callback.
     *
     * This callback is only considered if HTTP messaging validation is
     * turned on (which is on by default, see
     * `nghttp2_option_set_no_http_messaging()`).
     *
     * With this callback, application inspects the incoming invalid
     * field, and it also can reset stream from this callback by returning
     * :enum:`nghttp2_error.NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE`.  By
     * default, the error code is
     * :enum:`nghttp2_error_code.NGHTTP2_INTERNAL_ERROR`.  To change the
     * error code, call `nghttp2_submit_rst_stream()` with the error code
     * of choice in addition to returning
     * :enum:`nghttp2_error.NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE`.
     */
    typedef int (*nghttp2_on_invalid_header_callback2)(
      nghttp2_session *session, const nghttp2_frame *frame, nghttp2_rcbuf *name,
      nghttp2_rcbuf *value, uint8_t flags, void *user_data);

    #ifndef NGHTTP2_NO_SSIZE_T
    /**
     * @functypedef
     *
     * .. warning::
     *
     *   Deprecated.  Use :type:`nghttp2_select_padding_callback2`
     *   instead.
     *
     * Callback function invoked when the library asks application how
     * many padding bytes are required for the transmission of the
     * |frame|.  The application must choose the total length of payload
     * including padded bytes in range [frame->hd.length, max_payloadlen],
     * inclusive.  Choosing number not in this range will be treated as
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.  Returning
     * ``frame->hd.length`` means no padding is added.  Returning
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE` will make
     * `nghttp2_session_send()` and `nghttp2_session_mem_send()` functions
     * immediately return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.
     *
     * To set this callback to :type:`nghttp2_session_callbacks`, use
     * `nghttp2_session_callbacks_set_select_padding_callback()`.
     */
    typedef ssize_t (*nghttp2_select_padding_callback)(nghttp2_session *session,
                                                       const nghttp2_frame *frame,
                                                       size_t max_payloadlen,
                                                       void *user_data);

    #endif /* NGHTTP2_NO_SSIZE_T */

    /**
     * @functypedef
     *
     * Callback function invoked when the library asks application how
     * many padding bytes are required for the transmission of the
     * |frame|.  The application must choose the total length of payload
     * including padded bytes in range [frame->hd.length, max_payloadlen],
     * inclusive.  Choosing number not in this range will be treated as
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.  Returning
     * ``frame->hd.length`` means no padding is added.  Returning
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE` will make
     * `nghttp2_session_send()` and `nghttp2_session_mem_send2()`
     * functions immediately return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.
     *
     * To set this callback to :type:`nghttp2_session_callbacks`, use
     * `nghttp2_session_callbacks_set_select_padding_callback2()`.
     */
    typedef nghttp2_ssize (*nghttp2_select_padding_callback2)(
      nghttp2_session *session, const nghttp2_frame *frame, size_t max_payloadlen,
      void *user_data);

    #ifndef NGHTTP2_NO_SSIZE_T
    /**
     * @functypedef
     *
     * .. warning::
     *
     *   Deprecated.  Use
     *   :type:`nghttp2_data_source_read_length_callback2` instead.
     *
     * Callback function invoked when library wants to get max length of
     * data to send data to the remote peer.  The implementation of this
     * function should return a value in the following range.  [1,
     * min(|session_remote_window_size|, |stream_remote_window_size|,
     * |remote_max_frame_size|)].  If a value greater than this range is
     * returned than the max allow value will be used.  Returning a value
     * smaller than this range is treated as
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.  The
     * |frame_type| is provided for future extensibility and identifies
     * the type of frame (see :type:`nghttp2_frame_type`) for which to get
     * the length for.  Currently supported frame types are:
     * :enum:`nghttp2_frame_type.NGHTTP2_DATA`.
     *
     * This callback can be used to control the length in bytes for which
     * :type:`nghttp2_data_source_read_callback` is allowed to send to the
     * remote endpoint.  This callback is optional.  Returning
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE` will signal the
     * entire session failure.
     *
     * To set this callback to :type:`nghttp2_session_callbacks`, use
     * `nghttp2_session_callbacks_set_data_source_read_length_callback()`.
     */
    typedef ssize_t (*nghttp2_data_source_read_length_callback)(
      nghttp2_session *session, uint8_t frame_type, int32_t stream_id,
      int32_t session_remote_window_size, int32_t stream_remote_window_size,
      uint32_t remote_max_frame_size, void *user_data);

    #endif /* NGHTTP2_NO_SSIZE_T */

    /**
     * @functypedef
     *
     * Callback function invoked when library wants to get max length of
     * data to send data to the remote peer.  The implementation of this
     * function should return a value in the following range.  [1,
     * min(|session_remote_window_size|, |stream_remote_window_size|,
     * |remote_max_frame_size|)].  If a value greater than this range is
     * returned than the max allow value will be used.  Returning a value
     * smaller than this range is treated as
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.  The
     * |frame_type| is provided for future extensibility and identifies
     * the type of frame (see :type:`nghttp2_frame_type`) for which to get
     * the length for.  Currently supported frame types are:
     * :enum:`nghttp2_frame_type.NGHTTP2_DATA`.
     *
     * This callback can be used to control the length in bytes for which
     * :type:`nghttp2_data_source_read_callback` is allowed to send to the
     * remote endpoint.  This callback is optional.  Returning
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE` will signal the
     * entire session failure.
     *
     * To set this callback to :type:`nghttp2_session_callbacks`, use
     * `nghttp2_session_callbacks_set_data_source_read_length_callback2()`.
     */
    typedef nghttp2_ssize (*nghttp2_data_source_read_length_callback2)(
      nghttp2_session *session, uint8_t frame_type, int32_t stream_id,
      int32_t session_remote_window_size, int32_t stream_remote_window_size,
      uint32_t remote_max_frame_size, void *user_data);

    /**
     * @functypedef
     *
     * Callback function invoked when a frame header is received.  The
     * |hd| points to received frame header.
     *
     * Unlike :type:`nghttp2_on_frame_recv_callback`, this callback will
     * also be called when frame header of CONTINUATION frame is received.
     *
     * If both :type:`nghttp2_on_begin_frame_callback` and
     * :type:`nghttp2_on_begin_headers_callback` are set and HEADERS or
     * PUSH_PROMISE is received, :type:`nghttp2_on_begin_frame_callback`
     * will be called first.
     *
     * The implementation of this function must return 0 if it succeeds.
     * If nonzero value is returned, it is treated as fatal error and
     * `nghttp2_session_recv()` and `nghttp2_session_mem_recv2()`
     * functions immediately return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.
     *
     * To set this callback to :type:`nghttp2_session_callbacks`, use
     * `nghttp2_session_callbacks_set_on_begin_frame_callback()`.
     */
    typedef int (*nghttp2_on_begin_frame_callback)(nghttp2_session *session,
                                                   const nghttp2_frame_hd *hd,
                                                   void *user_data);

    /**
     * @functypedef
     *
     * Callback function invoked when chunk of extension frame payload is
     * received.  The |hd| points to frame header.  The received
     * chunk is |data| of length |len|.
     *
     * The implementation of this function must return 0 if it succeeds.
     *
     * To abort processing this extension frame, return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CANCEL`.
     *
     * If fatal error occurred, application should return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.  In this case,
     * `nghttp2_session_recv()` and `nghttp2_session_mem_recv2()`
     * functions immediately return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.  If the other
     * values are returned, currently they are treated as
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.
     */
    typedef int (*nghttp2_on_extension_chunk_recv_callback)(
      nghttp2_session *session, const nghttp2_frame_hd *hd, const uint8_t *data,
      size_t len, void *user_data);

    /**
     * @functypedef
     *
     * Callback function invoked when library asks the application to
     * unpack extension payload from its wire format.  The extension
     * payload has been passed to the application using
     * :type:`nghttp2_on_extension_chunk_recv_callback`.  The frame header
     * is already unpacked by the library and provided as |hd|.
     *
     * To receive extension frames, the application must tell desired
     * extension frame type to the library using
     * `nghttp2_option_set_user_recv_extension_type()`.
     *
     * The implementation of this function may store the pointer to the
     * created object as a result of unpacking in |*payload|, and returns
     * 0.  The pointer stored in |*payload| is opaque to the library, and
     * the library does not own its pointer.  |*payload| is initialized as
     * ``NULL``.  The |*payload| is available as ``frame->ext.payload`` in
     * :type:`nghttp2_on_frame_recv_callback`.  Therefore if application
     * can free that memory inside :type:`nghttp2_on_frame_recv_callback`
     * callback.  Of course, application has a liberty not to use
     * |*payload|, and do its own mechanism to process extension frames.
     *
     * To abort processing this extension frame, return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CANCEL`.
     *
     * If fatal error occurred, application should return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.  In this case,
     * `nghttp2_session_recv()` and `nghttp2_session_mem_recv2()`
     * functions immediately return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.  If the other
     * values are returned, currently they are treated as
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.
     */
    typedef int (*nghttp2_unpack_extension_callback)(nghttp2_session *session,
                                                     void **payload,
                                                     const nghttp2_frame_hd *hd,
                                                     void *user_data);

    #ifndef NGHTTP2_NO_SSIZE_T
    /**
     * @functypedef
     *
     * .. warning::
     *
     *   Deprecated.  Use :type:`nghttp2_pack_extension_callback2`
     *   instead.
     *
     * Callback function invoked when library asks the application to pack
     * extension payload in its wire format.  The frame header will be
     * packed by library.  Application must pack payload only.
     * ``frame->ext.payload`` is the object passed to
     * `nghttp2_submit_extension()` as payload parameter.  Application
     * must pack extension payload to the |buf| of its capacity |len|
     * bytes.  The |len| is at least 16KiB.
     *
     * The implementation of this function should return the number of
     * bytes written into |buf| when it succeeds.
     *
     * To abort processing this extension frame, return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CANCEL`, and
     * :type:`nghttp2_on_frame_not_send_callback` will be invoked.
     *
     * If fatal error occurred, application should return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.  In this case,
     * `nghttp2_session_send()` and `nghttp2_session_mem_send()` functions
     * immediately return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.  If the other
     * values are returned, currently they are treated as
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.  If the return
     * value is strictly larger than |len|, it is treated as
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.
     */
    typedef ssize_t (*nghttp2_pack_extension_callback)(nghttp2_session *session,
                                                       uint8_t *buf, size_t len,
                                                       const nghttp2_frame *frame,
                                                       void *user_data);

    #endif /* NGHTTP2_NO_SSIZE_T */

    /**
     * @functypedef
     *
     * Callback function invoked when library asks the application to pack
     * extension payload in its wire format.  The frame header will be
     * packed by library.  Application must pack payload only.
     * ``frame->ext.payload`` is the object passed to
     * `nghttp2_submit_extension()` as payload parameter.  Application
     * must pack extension payload to the |buf| of its capacity |len|
     * bytes.  The |len| is at least 16KiB.
     *
     * The implementation of this function should return the number of
     * bytes written into |buf| when it succeeds.
     *
     * To abort processing this extension frame, return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CANCEL`, and
     * :type:`nghttp2_on_frame_not_send_callback` will be invoked.
     *
     * If fatal error occurred, application should return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.  In this case,
     * `nghttp2_session_send()` and `nghttp2_session_mem_send2()`
     * functions immediately return
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.  If the other
     * values are returned, currently they are treated as
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.  If the return
     * value is strictly larger than |len|, it is treated as
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.
     */
    typedef nghttp2_ssize (*nghttp2_pack_extension_callback2)(
      nghttp2_session *session, uint8_t *buf, size_t len,
      const nghttp2_frame *frame, void *user_data);

    /**
     * @functypedef
     *
     * .. warning::
     *
     *   Deprecated.  Use :type:`nghttp2_error_callback2` instead.
     *
     * Callback function invoked when library provides the error message
     * intended for human consumption.  This callback is solely for
     * debugging purpose.  The |msg| is typically NULL-terminated string
     * of length |len|.  |len| does not include the sentinel NULL
     * character.
     *
     * The format of error message may change between nghttp2 library
     * versions.  The application should not depend on the particular
     * format.
     *
     * Normally, application should return 0 from this callback.  If fatal
     * error occurred while doing something in this callback, application
     * should return :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.
     * In this case, library will return immediately with return value
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.  Currently, if
     * nonzero value is returned from this callback, they are treated as
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`, but application
     * should not rely on this details.
     */
    typedef int (*nghttp2_error_callback)(nghttp2_session *session, const char *msg,
                                          size_t len, void *user_data);

    /**
     * @functypedef
     *
     * Callback function invoked when library provides the error code, and
     * message.  This callback is solely for debugging purpose.
     * |lib_error_code| is one of error code defined in
     * :enum:`nghttp2_error`.  The |msg| is typically NULL-terminated
     * string of length |len|, and intended for human consumption.  |len|
     * does not include the sentinel NULL character.
     *
     * The format of error message may change between nghttp2 library
     * versions.  The application should not depend on the particular
     * format.
     *
     * Normally, application should return 0 from this callback.  If fatal
     * error occurred while doing something in this callback, application
     * should return :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.
     * In this case, library will return immediately with return value
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`.  Currently, if
     * nonzero value is returned from this callback, they are treated as
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`, but application
     * should not rely on this details.
     */
    typedef int (*nghttp2_error_callback2)(nghttp2_session *session,
                                           int lib_error_code, const char *msg,
                                           size_t len, void *user_data);

    /**
     * @functypedef
     *
     * Callback function invoked when unpredictable data of |destlen|
     * bytes are needed.  The implementation must write unpredictable data
     * of |destlen| bytes into the buffer pointed by |dest|.
     */
    typedef void (*nghttp2_rand_callback)(uint8_t *dest, size_t destlen);

    struct nghttp2_session_callbacks;

    /**
     * @struct
     *
     * Callback functions for :type:`nghttp2_session`.  The details of
     * this structure are intentionally hidden from the public API.
     */
    typedef struct nghttp2_session_callbacks nghttp2_session_callbacks;

    /**
     * @function
     *
     * Initializes |*callbacks_ptr| with NULL values.
     *
     * The initialized object can be used when initializing multiple
     * :type:`nghttp2_session` objects.
     *
     * When the application finished using this object, it can use
     * `nghttp2_session_callbacks_del()` to free its memory.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     */
    NGHTTP2_EXTERN int
    nghttp2_session_callbacks_new(nghttp2_session_callbacks **callbacks_ptr);

    /**
     * @function
     *
     * Frees any resources allocated for |callbacks|.  If |callbacks| is
     * ``NULL``, this function does nothing.
     */
    NGHTTP2_EXTERN void
    nghttp2_session_callbacks_del(nghttp2_session_callbacks *callbacks);

    #ifndef NGHTTP2_NO_SSIZE_T
    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  Use `nghttp2_session_callbacks_set_send_callback2()`
     *   with :type:`nghttp2_send_callback2` instead.
     *
     * Sets callback function invoked when a session wants to send data to
     * the remote peer.  This callback is not necessary if the application
     * uses solely `nghttp2_session_mem_send()` to serialize data to
     * transmit.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_send_callback(
      nghttp2_session_callbacks *cbs, nghttp2_send_callback send_callback);

    #endif /* NGHTTP2_NO_SSIZE_T */

    /**
     * @function
     *
     * Sets callback function invoked when a session wants to send data to
     * the remote peer.  This callback is not necessary if the application
     * uses solely `nghttp2_session_mem_send2()` to serialize data to
     * transmit.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_send_callback2(
      nghttp2_session_callbacks *cbs, nghttp2_send_callback2 send_callback);

    #ifndef NGHTTP2_NO_SSIZE_T
    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  Use `nghttp2_session_callbacks_set_recv_callback2()`
     *   with :type:`nghttp2_recv_callback2` instead.
     *
     * Sets callback function invoked when the a session wants to receive
     * data from the remote peer.  This callback is not necessary if the
     * application uses solely `nghttp2_session_mem_recv()` to process
     * received data.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_recv_callback(
      nghttp2_session_callbacks *cbs, nghttp2_recv_callback recv_callback);

    #endif /* NGHTTP2_NO_SSIZE_T */

    /**
     * @function
     *
     * Sets callback function invoked when the a session wants to receive
     * data from the remote peer.  This callback is not necessary if the
     * application uses solely `nghttp2_session_mem_recv2()` to process
     * received data.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_recv_callback2(
      nghttp2_session_callbacks *cbs, nghttp2_recv_callback2 recv_callback);

    /**
     * @function
     *
     * Sets callback function invoked by `nghttp2_session_recv()` and
     * `nghttp2_session_mem_recv2()` when a frame is received.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_on_frame_recv_callback(
      nghttp2_session_callbacks *cbs,
      nghttp2_on_frame_recv_callback on_frame_recv_callback);

    /**
     * @function
     *
     * Sets callback function invoked by `nghttp2_session_recv()` and
     * `nghttp2_session_mem_recv2()` when an invalid non-DATA frame is
     * received.
     */
    NGHTTP2_EXTERN void
    nghttp2_session_callbacks_set_on_invalid_frame_recv_callback(
      nghttp2_session_callbacks *cbs,
      nghttp2_on_invalid_frame_recv_callback on_invalid_frame_recv_callback);

    /**
     * @function
     *
     * Sets callback function invoked when a chunk of data in DATA frame
     * is received.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_on_data_chunk_recv_callback(
      nghttp2_session_callbacks *cbs,
      nghttp2_on_data_chunk_recv_callback on_data_chunk_recv_callback);

    /**
     * @function
     *
     * Sets callback function invoked before a non-DATA frame is sent.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_before_frame_send_callback(
      nghttp2_session_callbacks *cbs,
      nghttp2_before_frame_send_callback before_frame_send_callback);

    /**
     * @function
     *
     * Sets callback function invoked after a frame is sent.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_on_frame_send_callback(
      nghttp2_session_callbacks *cbs,
      nghttp2_on_frame_send_callback on_frame_send_callback);

    /**
     * @function
     *
     * Sets callback function invoked when a non-DATA frame is not sent
     * because of an error.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_on_frame_not_send_callback(
      nghttp2_session_callbacks *cbs,
      nghttp2_on_frame_not_send_callback on_frame_not_send_callback);

    /**
     * @function
     *
     * Sets callback function invoked when the stream is closed.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_on_stream_close_callback(
      nghttp2_session_callbacks *cbs,
      nghttp2_on_stream_close_callback on_stream_close_callback);

    /**
     * @function
     *
     * Sets callback function invoked when the reception of header block
     * in HEADERS or PUSH_PROMISE is started.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_on_begin_headers_callback(
      nghttp2_session_callbacks *cbs,
      nghttp2_on_begin_headers_callback on_begin_headers_callback);

    /**
     * @function
     *
     * Sets callback function invoked when a header name/value pair is
     * received.  If both
     * `nghttp2_session_callbacks_set_on_header_callback()` and
     * `nghttp2_session_callbacks_set_on_header_callback2()` are used to
     * set callbacks, the latter has the precedence.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_on_header_callback(
      nghttp2_session_callbacks *cbs,
      nghttp2_on_header_callback on_header_callback);

    /**
     * @function
     *
     * Sets callback function invoked when a header name/value pair is
     * received.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_on_header_callback2(
      nghttp2_session_callbacks *cbs,
      nghttp2_on_header_callback2 on_header_callback2);

    /**
     * @function
     *
     * Sets callback function invoked when an invalid header name/value
     * pair is received.  If both
     * `nghttp2_session_callbacks_set_on_invalid_header_callback()` and
     * `nghttp2_session_callbacks_set_on_invalid_header_callback2()` are
     * used to set callbacks, the latter takes the precedence.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_on_invalid_header_callback(
      nghttp2_session_callbacks *cbs,
      nghttp2_on_invalid_header_callback on_invalid_header_callback);

    /**
     * @function
     *
     * Sets callback function invoked when an invalid header name/value
     * pair is received.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_on_invalid_header_callback2(
      nghttp2_session_callbacks *cbs,
      nghttp2_on_invalid_header_callback2 on_invalid_header_callback2);

    #ifndef NGHTTP2_NO_SSIZE_T
    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  Use
     *   `nghttp2_session_callbacks_set_select_padding_callback2()` with
     *   :type:`nghttp2_select_padding_callback2` instead.
     *
     * Sets callback function invoked when the library asks application
     * how many padding bytes are required for the transmission of the
     * given frame.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_select_padding_callback(
      nghttp2_session_callbacks *cbs,
      nghttp2_select_padding_callback select_padding_callback);

    #endif /* NGHTTP2_NO_SSIZE_T */

    /**
     * @function
     *
     * Sets callback function invoked when the library asks application
     * how many padding bytes are required for the transmission of the
     * given frame.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_select_padding_callback2(
      nghttp2_session_callbacks *cbs,
      nghttp2_select_padding_callback2 select_padding_callback);

    #ifndef NGHTTP2_NO_SSIZE_T
    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  Use
     *   `nghttp2_session_callbacks_set_data_source_read_length_callback2()`
     *   with :type:`nghttp2_data_source_read_length_callback2` instead.
     *
     * Sets callback function determine the length allowed in
     * :type:`nghttp2_data_source_read_callback`.
     */
    NGHTTP2_EXTERN void
    nghttp2_session_callbacks_set_data_source_read_length_callback(
      nghttp2_session_callbacks *cbs,
      nghttp2_data_source_read_length_callback data_source_read_length_callback);

    #endif /* NGHTTP2_NO_SSIZE_T */

    /**
     * @function
     *
     * Sets callback function determine the length allowed in
     * :type:`nghttp2_data_source_read_callback2`.
     */
    NGHTTP2_EXTERN void
    nghttp2_session_callbacks_set_data_source_read_length_callback2(
      nghttp2_session_callbacks *cbs,
      nghttp2_data_source_read_length_callback2 data_source_read_length_callback);

    /**
     * @function
     *
     * Sets callback function invoked when a frame header is received.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_on_begin_frame_callback(
      nghttp2_session_callbacks *cbs,
      nghttp2_on_begin_frame_callback on_begin_frame_callback);

    /**
     * @function
     *
     * Sets callback function invoked when
     * :enum:`nghttp2_data_flag.NGHTTP2_DATA_FLAG_NO_COPY` is used in
     * :type:`nghttp2_data_source_read_callback2` to avoid data copy.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_send_data_callback(
      nghttp2_session_callbacks *cbs,
      nghttp2_send_data_callback send_data_callback);

    #ifndef NGHTTP2_NO_SSIZE_T
    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  Use
     *   `nghttp2_session_callbacks_set_pack_extension_callback2()` with
     *   :type:`nghttp2_pack_extension_callback2` instead.
     *
     * Sets callback function invoked when the library asks the
     * application to pack extension frame payload in wire format.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_pack_extension_callback(
      nghttp2_session_callbacks *cbs,
      nghttp2_pack_extension_callback pack_extension_callback);

    #endif /* NGHTTP2_NO_SSIZE_T */

    /**
     * @function
     *
     * Sets callback function invoked when the library asks the
     * application to pack extension frame payload in wire format.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_pack_extension_callback2(
      nghttp2_session_callbacks *cbs,
      nghttp2_pack_extension_callback2 pack_extension_callback);

    /**
     * @function
     *
     * Sets callback function invoked when the library asks the
     * application to unpack extension frame payload from wire format.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_unpack_extension_callback(
      nghttp2_session_callbacks *cbs,
      nghttp2_unpack_extension_callback unpack_extension_callback);

    /**
     * @function
     *
     * Sets callback function invoked when chunk of extension frame
     * payload is received.
     */
    NGHTTP2_EXTERN void
    nghttp2_session_callbacks_set_on_extension_chunk_recv_callback(
      nghttp2_session_callbacks *cbs,
      nghttp2_on_extension_chunk_recv_callback on_extension_chunk_recv_callback);

    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  Use
     *   `nghttp2_session_callbacks_set_error_callback2()` with
     *   :type:`nghttp2_error_callback2` instead.
     *
     * Sets callback function invoked when library tells error message to
     * the application.
     *
     * If both :type:`nghttp2_error_callback` and
     * :type:`nghttp2_error_callback2` are set, the latter takes
     * precedence.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_error_callback(
      nghttp2_session_callbacks *cbs, nghttp2_error_callback error_callback);

    /**
     * @function
     *
     * Sets callback function invoked when library tells error code, and
     * message to the application.
     *
     * If both :type:`nghttp2_error_callback` and
     * :type:`nghttp2_error_callback2` are set, the latter takes
     * precedence.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_error_callback2(
      nghttp2_session_callbacks *cbs, nghttp2_error_callback2 error_callback2);

    /**
     * @function
     *
     * Sets callback function invoked when unpredictable data is needed.
     * Although this callback is optional due to the backward
     * compatibility, it is recommended to specify it to harden the
     * runtime behavior against suspicious activities of a remote
     * endpoint.
     */
    NGHTTP2_EXTERN void nghttp2_session_callbacks_set_rand_callback(
      nghttp2_session_callbacks *cbs, nghttp2_rand_callback rand_callback);

    /**
     * @functypedef
     *
     * Custom memory allocator to replace malloc().  The |mem_user_data|
     * is the mem_user_data member of :type:`nghttp2_mem` structure.
     */
    typedef void *(*nghttp2_malloc)(size_t size, void *mem_user_data);

    /**
     * @functypedef
     *
     * Custom memory allocator to replace free().  The |mem_user_data| is
     * the mem_user_data member of :type:`nghttp2_mem` structure.
     */
    typedef void (*nghttp2_free)(void *ptr, void *mem_user_data);

    /**
     * @functypedef
     *
     * Custom memory allocator to replace calloc().  The |mem_user_data|
     * is the mem_user_data member of :type:`nghttp2_mem` structure.
     */
    typedef void *(*nghttp2_calloc)(size_t nmemb, size_t size, void *mem_user_data);

    /**
     * @functypedef
     *
     * Custom memory allocator to replace realloc().  The |mem_user_data|
     * is the mem_user_data member of :type:`nghttp2_mem` structure.
     */
    typedef void *(*nghttp2_realloc)(void *ptr, size_t size, void *mem_user_data);

    /**
     * @struct
     *
     * Custom memory allocator functions and user defined pointer.  The
     * |mem_user_data| member is passed to each allocator function.  This
     * can be used, for example, to achieve per-session memory pool.
     *
     * In the following example code, ``my_malloc``, ``my_free``,
     * ``my_calloc`` and ``my_realloc`` are the replacement of the
     * standard allocators ``malloc``, ``free``, ``calloc`` and
     * ``realloc`` respectively::
     *
     *     void *my_malloc_cb(size_t size, void *mem_user_data) {
     *       return my_malloc(size);
     *     }
     *
     *     void my_free_cb(void *ptr, void *mem_user_data) { my_free(ptr); }
     *
     *     void *my_calloc_cb(size_t nmemb, size_t size, void *mem_user_data) {
     *       return my_calloc(nmemb, size);
     *     }
     *
     *     void *my_realloc_cb(void *ptr, size_t size, void *mem_user_data) {
     *       return my_realloc(ptr, size);
     *     }
     *
     *     void session_new() {
     *       nghttp2_session *session;
     *       nghttp2_session_callbacks *callbacks;
     *       nghttp2_mem mem = {NULL, my_malloc_cb, my_free_cb, my_calloc_cb,
     *                          my_realloc_cb};
     *
     *       ...
     *
     *       nghttp2_session_client_new3(&session, callbacks, NULL, NULL, &mem);
     *
     *       ...
     *     }
     */
    typedef struct {
      /**
       * An arbitrary user supplied data.  This is passed to each
       * allocator function.
       */
      void *mem_user_data;
      /**
       * Custom allocator function to replace malloc().
       */
      nghttp2_malloc malloc;
      /**
       * Custom allocator function to replace free().
       */
      nghttp2_free free;
      /**
       * Custom allocator function to replace calloc().
       */
      nghttp2_calloc calloc;
      /**
       * Custom allocator function to replace realloc().
       */
      nghttp2_realloc realloc;
    } nghttp2_mem;

    struct nghttp2_option;

    /**
     * @struct
     *
     * Configuration options for :type:`nghttp2_session`.  The details of
     * this structure are intentionally hidden from the public API.
     */
    typedef struct nghttp2_option nghttp2_option;

    /**
     * @function
     *
     * Initializes |*option_ptr| with default values.
     *
     * When the application finished using this object, it can use
     * `nghttp2_option_del()` to free its memory.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     */
    NGHTTP2_EXTERN int nghttp2_option_new(nghttp2_option **option_ptr);

    /**
     * @function
     *
     * Frees any resources allocated for |option|.  If |option| is
     * ``NULL``, this function does nothing.
     */
    NGHTTP2_EXTERN void nghttp2_option_del(nghttp2_option *option);

    /**
     * @function
     *
     * This option prevents the library from sending WINDOW_UPDATE for a
     * connection automatically.  If this option is set to nonzero, the
     * library won't send WINDOW_UPDATE for DATA until application calls
     * `nghttp2_session_consume()` to indicate the consumed amount of
     * data.  Don't use `nghttp2_submit_window_update()` for this purpose.
     * By default, this option is set to zero.
     */
    NGHTTP2_EXTERN void
    nghttp2_option_set_no_auto_window_update(nghttp2_option *option, int val);

    /**
     * @function
     *
     * This option sets the SETTINGS_MAX_CONCURRENT_STREAMS value of
     * remote endpoint as if it is received in SETTINGS frame.  Without
     * specifying this option, the maximum number of outgoing concurrent
     * streams is initially limited to 100 to avoid issues when the local
     * endpoint submits lots of requests before receiving initial SETTINGS
     * frame from the remote endpoint, since sending them at once to the
     * remote endpoint could lead to rejection of some of the requests.
     * This value will be overwritten when the local endpoint receives
     * initial SETTINGS frame from the remote endpoint, either to the
     * value advertised in SETTINGS_MAX_CONCURRENT_STREAMS or to the
     * default value (unlimited) if none was advertised.
     */
    NGHTTP2_EXTERN void
    nghttp2_option_set_peer_max_concurrent_streams(nghttp2_option *option,
                                                   uint32_t val);

    /**
     * @function
     *
     * By default, nghttp2 library, if configured as server, requires
     * first 24 bytes of client magic byte string (MAGIC).  In most cases,
     * this will simplify the implementation of server.  But sometimes
     * server may want to detect the application protocol based on first
     * few bytes on clear text communication.
     *
     * If this option is used with nonzero |val|, nghttp2 library does not
     * handle MAGIC.  It still checks following SETTINGS frame.  This
     * means that applications should deal with MAGIC by themselves.
     *
     * If this option is not used or used with zero value, if MAGIC does
     * not match :macro:`NGHTTP2_CLIENT_MAGIC`, `nghttp2_session_recv()`
     * and `nghttp2_session_mem_recv2()` will return error
     * :enum:`nghttp2_error.NGHTTP2_ERR_BAD_CLIENT_MAGIC`, which is fatal
     * error.
     */
    NGHTTP2_EXTERN void
    nghttp2_option_set_no_recv_client_magic(nghttp2_option *option, int val);

    /**
     * @function
     *
     * By default, nghttp2 library enforces subset of HTTP Messaging rules
     * described in `HTTP/2 specification, section 8
     * <https://tools.ietf.org/html/rfc7540#section-8>`_.  See
     * :ref:`http-messaging` section for details.  For those applications
     * who use nghttp2 library as non-HTTP use, give nonzero to |val| to
     * disable this enforcement.  Please note that disabling this feature
     * does not change the fundamental client and server model of HTTP.
     * That is, even if the validation is disabled, only client can send
     * requests.
     */
    NGHTTP2_EXTERN void nghttp2_option_set_no_http_messaging(nghttp2_option *option,
                                                             int val);

    /**
     * @function
     *
     * RFC 7540 does not enforce any limit on the number of incoming
     * reserved streams (in RFC 7540 terms, streams in reserved (remote)
     * state).  This only affects client side, since only server can push
     * streams.  Malicious server can push arbitrary number of streams,
     * and make client's memory exhausted.  This option can set the
     * maximum number of such incoming streams to avoid possible memory
     * exhaustion.  If this option is set, and pushed streams are
     * automatically closed on reception, without calling user provided
     * callback, if they exceed the given limit.  The default value is
     * 200.  If session is configured as server side, this option has no
     * effect.  Server can control the number of streams to push.
     */
    NGHTTP2_EXTERN void
    nghttp2_option_set_max_reserved_remote_streams(nghttp2_option *option,
                                                   uint32_t val);

    /**
     * @function
     *
     * Sets extension frame type the application is willing to handle with
     * user defined callbacks (see
     * :type:`nghttp2_on_extension_chunk_recv_callback` and
     * :type:`nghttp2_unpack_extension_callback`).  The |type| is
     * extension frame type, and must be strictly greater than 0x9.
     * Otherwise, this function does nothing.  The application can call
     * this function multiple times to set more than one frame type to
     * receive.  The application does not have to call this function if it
     * just sends extension frames.
     */
    NGHTTP2_EXTERN void
    nghttp2_option_set_user_recv_extension_type(nghttp2_option *option,
                                                uint8_t type);

    /**
     * @function
     *
     * Sets extension frame type the application is willing to receive
     * using builtin handler.  The |type| is the extension frame type to
     * receive, and must be strictly greater than 0x9.  Otherwise, this
     * function does nothing.  The application can call this function
     * multiple times to set more than one frame type to receive.  The
     * application does not have to call this function if it just sends
     * extension frames.
     *
     * If same frame type is passed to both
     * `nghttp2_option_set_builtin_recv_extension_type()` and
     * `nghttp2_option_set_user_recv_extension_type()`, the latter takes
     * precedence.
     */
    NGHTTP2_EXTERN void
    nghttp2_option_set_builtin_recv_extension_type(nghttp2_option *option,
                                                   uint8_t type);

    /**
     * @function
     *
     * This option prevents the library from sending PING frame with ACK
     * flag set automatically when PING frame without ACK flag set is
     * received.  If this option is set to nonzero, the library won't send
     * PING frame with ACK flag set in the response for incoming PING
     * frame.  The application can send PING frame with ACK flag set using
     * `nghttp2_submit_ping()` with :enum:`nghttp2_flag.NGHTTP2_FLAG_ACK`
     * as flags parameter.
     */
    NGHTTP2_EXTERN void nghttp2_option_set_no_auto_ping_ack(nghttp2_option *option,
                                                            int val);

    /**
     * @function
     *
     * This option sets the maximum length of header block (a set of
     * header fields per one HEADERS frame) to send.  The length of a
     * given set of header fields is calculated using
     * `nghttp2_hd_deflate_bound()`.  The default value is 64KiB.  If
     * application attempts to send header fields larger than this limit,
     * the transmission of the frame fails with error code
     * :enum:`nghttp2_error.NGHTTP2_ERR_FRAME_SIZE_ERROR`.
     */
    NGHTTP2_EXTERN void
    nghttp2_option_set_max_send_header_block_length(nghttp2_option *option,
                                                    size_t val);

    /**
     * @function
     *
     * This option sets the maximum dynamic table size for deflating
     * header fields.  The default value is 4KiB.  In HTTP/2, receiver of
     * deflated header block can specify maximum dynamic table size.  The
     * actual maximum size is the minimum of the size receiver specified
     * and this option value.
     */
    NGHTTP2_EXTERN void
    nghttp2_option_set_max_deflate_dynamic_table_size(nghttp2_option *option,
                                                      size_t val);

    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  Closed streams are not retained anymore.
     *
     * This function works as before, but it does not take any effect
     * against :type:`nghttp2_session`.
     */
    NGHTTP2_EXTERN void nghttp2_option_set_no_closed_streams(nghttp2_option *option,
                                                             int val);

    /**
     * @function
     *
     * This function sets the maximum number of outgoing SETTINGS ACK and
     * PING ACK frames retained in :type:`nghttp2_session` object.  If
     * more than those frames are retained, the peer is considered to be
     * misbehaving and session will be closed.  The default value is 1000.
     */
    NGHTTP2_EXTERN void nghttp2_option_set_max_outbound_ack(nghttp2_option *option,
                                                            size_t val);

    /**
     * @function
     *
     * This function sets the maximum number of SETTINGS entries per
     * SETTINGS frame that will be accepted. If more than those entries
     * are received, the peer is considered to be misbehaving and session
     * will be closed. The default value is 32.
     */
    NGHTTP2_EXTERN void nghttp2_option_set_max_settings(nghttp2_option *option,
                                                        size_t val);

    /**
     * @function
     *
     * .. warning::
     *    Deprecated.  :rfc:`7540` priorities have been removed.
     *
     * This function works as before, but it does not take any effect
     * against :type:`nghttp2_session`.
     */
    NGHTTP2_EXTERN void
    nghttp2_option_set_server_fallback_rfc7540_priorities(nghttp2_option *option,
                                                          int val);

    /**
     * @function
     *
     * This option, if set to nonzero, turns off RFC 9113 leading and
     * trailing white spaces validation against HTTP field value.  Some
     * important fields, such as HTTP/2 pseudo header fields, are
     * validated more strictly and this option does not apply to them.
     */
    NGHTTP2_EXTERN void
    nghttp2_option_set_no_rfc9113_leading_and_trailing_ws_validation(
      nghttp2_option *option, int val);

    /**
     * @function
     *
     * This function sets the rate limit for the incoming stream reset
     * (RST_STREAM frame).  It is server use only.  It is a token-bucket
     * based rate limiter.  |burst| specifies the number of tokens that is
     * initially available.  The maximum number of tokens is capped to
     * this value.  |rate| specifies the number of tokens that are
     * regenerated per second.  An incoming RST_STREAM consumes one token.
     * If there is no token available, GOAWAY is sent to tear down the
     * connection.  |burst| and |rate| default to 1000 and 33
     * respectively.
     */
    NGHTTP2_EXTERN void
    nghttp2_option_set_stream_reset_rate_limit(nghttp2_option *option,
                                               uint64_t burst, uint64_t rate);

    /**
     * @function
     *
     * This function sets the maximum number of CONTINUATION frames
     * following an incoming HEADER frame.  If more than those frames are
     * received, the remote endpoint is considered to be misbehaving and
     * session will be closed.  The default value is 8.
     */
    NGHTTP2_EXTERN void nghttp2_option_set_max_continuations(nghttp2_option *option,
                                                             size_t val);

    /**
     * @function
     *
     * This function sets the rate limit for the "glitches", the
     * suspicious activities from a remote endpoint.  It is a token-bucket
     * based rate limiter.  |burst| specifies the number of tokens that is
     * initially available.  The maximum number of tokens is capped to
     * this value.  |rate| specifies the number of tokens that are
     * regenerated per second.  When a suspicious activity is detected,
     * some amount of tokens are consumed.  If there is no token
     * available, GOAWAY is sent to tear down the connection.  |burst| and
     * |rate| default to 10000 and 330 respectively.
     */
    NGHTTP2_EXTERN void nghttp2_option_set_glitch_rate_limit(nghttp2_option *option,
                                                             uint64_t burst,
                                                             uint64_t rate);

    /**
     * @function
     *
     * Initializes |*session_ptr| for client use.  The all members of
     * |callbacks| are copied to |*session_ptr|.  Therefore |*session_ptr|
     * does not store |callbacks|.  The |user_data| is an arbitrary user
     * supplied data, which will be passed to the callback functions.
     *
     * The :type:`nghttp2_send_callback2` must be specified.  If the
     * application code uses `nghttp2_session_recv()`, the
     * :type:`nghttp2_recv_callback` must be specified.  The other members
     * of |callbacks| can be ``NULL``.
     *
     * If this function fails, |*session_ptr| is left untouched.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     */
    NGHTTP2_EXTERN int
    nghttp2_session_client_new(nghttp2_session **session_ptr,
                               const nghttp2_session_callbacks *callbacks,
                               void *user_data);

    /**
     * @function
     *
     * Initializes |*session_ptr| for server use.  The all members of
     * |callbacks| are copied to |*session_ptr|. Therefore |*session_ptr|
     * does not store |callbacks|.  The |user_data| is an arbitrary user
     * supplied data, which will be passed to the callback functions.
     *
     * The :type:`nghttp2_send_callback2` must be specified.  If the
     * application code uses `nghttp2_session_recv()`, the
     * :type:`nghttp2_recv_callback` must be specified.  The other members
     * of |callbacks| can be ``NULL``.
     *
     * If this function fails, |*session_ptr| is left untouched.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     */
    NGHTTP2_EXTERN int
    nghttp2_session_server_new(nghttp2_session **session_ptr,
                               const nghttp2_session_callbacks *callbacks,
                               void *user_data);

    /**
     * @function
     *
     * Like `nghttp2_session_client_new()`, but with additional options
     * specified in the |option|.
     *
     * The |option| can be ``NULL`` and the call is equivalent to
     * `nghttp2_session_client_new()`.
     *
     * This function does not take ownership |option|.  The application is
     * responsible for freeing |option| if it finishes using the object.
     *
     * The library code does not refer to |option| after this function
     * returns.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     */
    NGHTTP2_EXTERN int
    nghttp2_session_client_new2(nghttp2_session **session_ptr,
                                const nghttp2_session_callbacks *callbacks,
                                void *user_data, const nghttp2_option *option);

    /**
     * @function
     *
     * Like `nghttp2_session_server_new()`, but with additional options
     * specified in the |option|.
     *
     * The |option| can be ``NULL`` and the call is equivalent to
     * `nghttp2_session_server_new()`.
     *
     * This function does not take ownership |option|.  The application is
     * responsible for freeing |option| if it finishes using the object.
     *
     * The library code does not refer to |option| after this function
     * returns.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     */
    NGHTTP2_EXTERN int
    nghttp2_session_server_new2(nghttp2_session **session_ptr,
                                const nghttp2_session_callbacks *callbacks,
                                void *user_data, const nghttp2_option *option);

    /**
     * @function
     *
     * Like `nghttp2_session_client_new2()`, but with additional custom
     * memory allocator specified in the |mem|.
     *
     * The |mem| can be ``NULL`` and the call is equivalent to
     * `nghttp2_session_client_new2()`.
     *
     * This function does not take ownership |mem|.  The application is
     * responsible for freeing |mem|.
     *
     * The library code does not refer to |mem| pointer after this
     * function returns, so the application can safely free it.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     */
    NGHTTP2_EXTERN int nghttp2_session_client_new3(
      nghttp2_session **session_ptr, const nghttp2_session_callbacks *callbacks,
      void *user_data, const nghttp2_option *option, nghttp2_mem *mem);

    /**
     * @function
     *
     * Like `nghttp2_session_server_new2()`, but with additional custom
     * memory allocator specified in the |mem|.
     *
     * The |mem| can be ``NULL`` and the call is equivalent to
     * `nghttp2_session_server_new2()`.
     *
     * This function does not take ownership |mem|.  The application is
     * responsible for freeing |mem|.
     *
     * The library code does not refer to |mem| pointer after this
     * function returns, so the application can safely free it.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     */
    NGHTTP2_EXTERN int nghttp2_session_server_new3(
      nghttp2_session **session_ptr, const nghttp2_session_callbacks *callbacks,
      void *user_data, const nghttp2_option *option, nghttp2_mem *mem);

    /**
     * @function
     *
     * Frees any resources allocated for |session|.  If |session| is
     * ``NULL``, this function does nothing.
     */
    NGHTTP2_EXTERN void nghttp2_session_del(nghttp2_session *session);

    /**
     * @function
     *
     * Sends pending frames to the remote peer.
     *
     * This function retrieves the highest prioritized frame from the
     * outbound queue and sends it to the remote peer.  It does this as
     * many times as possible until the user callback
     * :type:`nghttp2_send_callback2` returns
     * :enum:`nghttp2_error.NGHTTP2_ERR_WOULDBLOCK`, the outbound queue
     * becomes empty or flow control is triggered (remote window size
     * becomes depleted or maximum number of concurrent streams is
     * reached).  This function calls several callback functions which are
     * passed when initializing the |session|.  Here is the simple time
     * chart which tells when each callback is invoked:
     *
     * 1. Get the next frame to send from outbound queue.
     *
     * 2. Prepare transmission of the frame.
     *
     * 3. If the control frame cannot be sent because some preconditions
     *    are not met (e.g., request HEADERS cannot be sent after GOAWAY),
     *    :type:`nghttp2_on_frame_not_send_callback` is invoked.  Abort
     *    the following steps.
     *
     * 4. If the frame is HEADERS, PUSH_PROMISE or DATA,
     *    :type:`nghttp2_select_padding_callback` is invoked.
     *
     * 5. If the frame is request HEADERS, the stream is opened here.
     *
     * 6. :type:`nghttp2_before_frame_send_callback` is invoked.
     *
     * 7. If :enum:`nghttp2_error.NGHTTP2_ERR_CANCEL` is returned from
     *    :type:`nghttp2_before_frame_send_callback`, the current frame
     *    transmission is canceled, and
     *    :type:`nghttp2_on_frame_not_send_callback` is invoked.  Abort
     *    the following steps.
     *
     * 8. :type:`nghttp2_send_callback2` is invoked one or more times to
     *    send the frame.
     *
     * 9. :type:`nghttp2_on_frame_send_callback` is invoked.
     *
     * 10. If the transmission of the frame triggers closure of the
     *     stream, the stream is closed and
     *     :type:`nghttp2_on_stream_close_callback` is invoked.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`
     *     The callback function failed.
     */
    NGHTTP2_EXTERN int nghttp2_session_send(nghttp2_session *session);

    #ifndef NGHTTP2_NO_SSIZE_T
    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  Use `nghttp2_session_mem_send2()` instead.
     *
     * Returns the serialized data to send.
     *
     * This function behaves like `nghttp2_session_send()` except that it
     * does not use :type:`nghttp2_send_callback` to transmit data.
     * Instead, it assigns the pointer to the serialized data to the
     * |*data_ptr| and returns its length.  The other callbacks are called
     * in the same way as they are in `nghttp2_session_send()`.
     *
     * If no data is available to send, this function returns 0.
     *
     * This function may not return all serialized data in one invocation.
     * To get all data, call this function repeatedly until it returns 0
     * or one of negative error codes.
     *
     * The assigned |*data_ptr| is valid until the next call of
     * `nghttp2_session_mem_send()` or `nghttp2_session_send()`.
     *
     * The caller must send all data before sending the next chunk of
     * data.
     *
     * This function returns the length of the data pointed by the
     * |*data_ptr| if it succeeds, or one of the following negative error
     * codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     *
     * .. note::
     *
     *   This function may produce very small byte string.  If that is the
     *   case, and application disables Nagle algorithm (``TCP_NODELAY``),
     *   then writing this small chunk leads to very small packet, and it
     *   is very inefficient.  An application should be responsible to
     *   buffer up small chunks of data as necessary to avoid this
     *   situation.
     */
    NGHTTP2_EXTERN ssize_t nghttp2_session_mem_send(nghttp2_session *session,
                                                    const uint8_t **data_ptr);

    #endif /* NGHTTP2_NO_SSIZE_T */

    /**
     * @function
     *
     * Returns the serialized data to send.
     *
     * This function behaves like `nghttp2_session_send()` except that it
     * does not use :type:`nghttp2_send_callback2` to transmit data.
     * Instead, it assigns the pointer to the serialized data to the
     * |*data_ptr| and returns its length.  The other callbacks are called
     * in the same way as they are in `nghttp2_session_send()`.
     *
     * If no data is available to send, this function returns 0.
     *
     * This function may not return all serialized data in one invocation.
     * To get all data, call this function repeatedly until it returns 0
     * or one of negative error codes.
     *
     * The assigned |*data_ptr| is valid until the next call of
     * `nghttp2_session_mem_send2()` or `nghttp2_session_send()`.
     *
     * The caller must send all data before sending the next chunk of
     * data.
     *
     * This function returns the length of the data pointed by the
     * |*data_ptr| if it succeeds, or one of the following negative error
     * codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     *
     * .. note::
     *
     *   This function may produce very small byte string.  If that is the
     *   case, and application disables Nagle algorithm (``TCP_NODELAY``),
     *   then writing this small chunk leads to very small packet, and it
     *   is very inefficient.  An application should be responsible to
     *   buffer up small chunks of data as necessary to avoid this
     *   situation.
     */
    NGHTTP2_EXTERN nghttp2_ssize
    nghttp2_session_mem_send2(nghttp2_session *session, const uint8_t **data_ptr);

    /**
     * @function
     *
     * Receives frames from the remote peer.
     *
     * This function receives as many frames as possible until the user
     * callback :type:`nghttp2_recv_callback` returns
     * :enum:`nghttp2_error.NGHTTP2_ERR_WOULDBLOCK`.  This function calls
     * several callback functions which are passed when initializing the
     * |session|.  Here is the simple time chart which tells when each
     * callback is invoked:
     *
     * 1. :type:`nghttp2_recv_callback` is invoked one or more times to
     *    receive frame header.
     *
     * 2. When frame header is received,
     *    :type:`nghttp2_on_begin_frame_callback` is invoked.
     *
     * 3. If the frame is DATA frame:
     *
     *    1. :type:`nghttp2_recv_callback` is invoked to receive DATA
     *       payload. For each chunk of data,
     *       :type:`nghttp2_on_data_chunk_recv_callback` is invoked.
     *
     *    2. If one DATA frame is completely received,
     *       :type:`nghttp2_on_frame_recv_callback` is invoked.  If the
     *       reception of the frame triggers the closure of the stream,
     *       :type:`nghttp2_on_stream_close_callback` is invoked.
     *
     * 4. If the frame is the control frame:
     *
     *    1. :type:`nghttp2_recv_callback` is invoked one or more times to
     *       receive whole frame.
     *
     *    2. If the received frame is valid, then following actions are
     *       taken.  If the frame is either HEADERS or PUSH_PROMISE,
     *       :type:`nghttp2_on_begin_headers_callback` is invoked.  Then
     *       :type:`nghttp2_on_header_callback` is invoked for each header
     *       name/value pair.  For invalid header field,
     *       :type:`nghttp2_on_invalid_header_callback` is called.  After
     *       all name/value pairs are emitted successfully,
     *       :type:`nghttp2_on_frame_recv_callback` is invoked.  For other
     *       frames, :type:`nghttp2_on_frame_recv_callback` is invoked.
     *       If the reception of the frame triggers the closure of the
     *       stream, :type:`nghttp2_on_stream_close_callback` is invoked.
     *
     *    3. If the received frame is unpacked but is interpreted as
     *       invalid, :type:`nghttp2_on_invalid_frame_recv_callback` is
     *       invoked.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_EOF`
     *     The remote peer did shutdown on the connection.
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`
     *     The callback function failed.
     * :enum:`nghttp2_error.NGHTTP2_ERR_BAD_CLIENT_MAGIC`
     *     Invalid client magic was detected.  This error only returns
     *     when |session| was configured as server and
     *     `nghttp2_option_set_no_recv_client_magic()` is not used with
     *     nonzero value.
     * :enum:`nghttp2_error.NGHTTP2_ERR_FLOODED`
     *     Flooding was detected in this HTTP/2 session, and it must be
     *     closed.  This is most likely caused by misbehaviour of peer.
     */
    NGHTTP2_EXTERN int nghttp2_session_recv(nghttp2_session *session);

    #ifndef NGHTTP2_NO_SSIZE_T
    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  Use `nghttp2_session_mem_recv2()` instead.
     *
     * Processes data |in| as an input from the remote endpoint.  The
     * |inlen| indicates the number of bytes to receive in the |in|.
     *
     * This function behaves like `nghttp2_session_recv()` except that it
     * does not use :type:`nghttp2_recv_callback` to receive data; the
     * |in| is the only data for the invocation of this function.  If all
     * bytes are processed, this function returns.  The other callbacks
     * are called in the same way as they are in `nghttp2_session_recv()`.
     *
     * In the current implementation, this function always tries to
     * processes |inlen| bytes of input data unless either an error occurs or
     * :enum:`nghttp2_error.NGHTTP2_ERR_PAUSE` is returned from
     * :type:`nghttp2_on_header_callback` or
     * :type:`nghttp2_on_data_chunk_recv_callback`.  If
     * :enum:`nghttp2_error.NGHTTP2_ERR_PAUSE` is used, the return value
     * includes the number of bytes which was used to produce the data or
     * frame for the callback.
     *
     * This function returns the number of processed bytes, or one of the
     * following negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`
     *     The callback function failed.
     * :enum:`nghttp2_error.NGHTTP2_ERR_BAD_CLIENT_MAGIC`
     *     Invalid client magic was detected.  This error only returns
     *     when |session| was configured as server and
     *     `nghttp2_option_set_no_recv_client_magic()` is not used with
     *     nonzero value.
     * :enum:`nghttp2_error.NGHTTP2_ERR_FLOODED`
     *     Flooding was detected in this HTTP/2 session, and it must be
     *     closed.  This is most likely caused by misbehaviour of peer.
     */
    NGHTTP2_EXTERN ssize_t nghttp2_session_mem_recv(nghttp2_session *session,
                                                    const uint8_t *in,
                                                    size_t inlen);

    #endif /* NGHTTP2_NO_SSIZE_T */

    /**
     * @function
     *
     * Processes data |in| as an input from the remote endpoint.  The
     * |inlen| indicates the number of bytes to receive in the |in|.
     *
     * This function behaves like `nghttp2_session_recv()` except that it
     * does not use :type:`nghttp2_recv_callback` to receive data; the
     * |in| is the only data for the invocation of this function.  If all
     * bytes are processed, this function returns.  The other callbacks
     * are called in the same way as they are in `nghttp2_session_recv()`.
     *
     * In the current implementation, this function always tries to
     * processes |inlen| bytes of input data unless either an error occurs or
     * :enum:`nghttp2_error.NGHTTP2_ERR_PAUSE` is returned from
     * :type:`nghttp2_on_header_callback` or
     * :type:`nghttp2_on_data_chunk_recv_callback`.  If
     * :enum:`nghttp2_error.NGHTTP2_ERR_PAUSE` is used, the return value
     * includes the number of bytes which was used to produce the data or
     * frame for the callback.
     *
     * This function returns the number of processed bytes, or one of the
     * following negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_CALLBACK_FAILURE`
     *     The callback function failed.
     * :enum:`nghttp2_error.NGHTTP2_ERR_BAD_CLIENT_MAGIC`
     *     Invalid client magic was detected.  This error only returns
     *     when |session| was configured as server and
     *     `nghttp2_option_set_no_recv_client_magic()` is not used with
     *     nonzero value.
     * :enum:`nghttp2_error.NGHTTP2_ERR_FLOODED`
     *     Flooding was detected in this HTTP/2 session, and it must be
     *     closed.  This is most likely caused by misbehaviour of peer.
     */
    NGHTTP2_EXTERN nghttp2_ssize nghttp2_session_mem_recv2(nghttp2_session *session,
                                                           const uint8_t *in,
                                                           size_t inlen);

    /**
     * @function
     *
     * Puts back previously deferred DATA frame in the stream |stream_id|
     * to the outbound queue.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     The stream does not exist; or no deferred data exist.
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     */
    NGHTTP2_EXTERN int nghttp2_session_resume_data(nghttp2_session *session,
                                                   int32_t stream_id);

    /**
     * @function
     *
     * Returns nonzero value if |session| wants to receive data from the
     * remote peer.
     *
     * If both `nghttp2_session_want_read()` and
     * `nghttp2_session_want_write()` return 0, the application should
     * drop the connection.
     */
    NGHTTP2_EXTERN int nghttp2_session_want_read(nghttp2_session *session);

    /**
     * @function
     *
     * Returns nonzero value if |session| wants to send data to the remote
     * peer.
     *
     * If both `nghttp2_session_want_read()` and
     * `nghttp2_session_want_write()` return 0, the application should
     * drop the connection.
     */
    NGHTTP2_EXTERN int nghttp2_session_want_write(nghttp2_session *session);

    /**
     * @function
     *
     * Returns stream_user_data for the stream |stream_id|.  The
     * stream_user_data is provided by `nghttp2_submit_request2()`,
     * `nghttp2_submit_headers()` or
     * `nghttp2_session_set_stream_user_data()`.  Unless it is set using
     * `nghttp2_session_set_stream_user_data()`, if the stream is
     * initiated by the remote endpoint, stream_user_data is always
     * ``NULL``.  If the stream does not exist, this function returns
     * ``NULL``.
     */
    NGHTTP2_EXTERN void *
    nghttp2_session_get_stream_user_data(nghttp2_session *session,
                                         int32_t stream_id);

    /**
     * @function
     *
     * Sets the |stream_user_data| to the stream denoted by the
     * |stream_id|.  If a stream user data is already set to the stream,
     * it is replaced with the |stream_user_data|.  It is valid to specify
     * ``NULL`` in the |stream_user_data|, which nullifies the associated
     * data pointer.
     *
     * It is valid to set the |stream_user_data| to the stream reserved by
     * PUSH_PROMISE frame.
     *
     * This function returns 0 if it succeeds, or one of following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     The stream does not exist
     */
    NGHTTP2_EXTERN int
    nghttp2_session_set_stream_user_data(nghttp2_session *session,
                                         int32_t stream_id, void *stream_user_data);

    /**
     * @function
     *
     * Sets |user_data| to |session|, overwriting the existing user data
     * specified in `nghttp2_session_client_new()`, or
     * `nghttp2_session_server_new()`.
     */
    NGHTTP2_EXTERN void nghttp2_session_set_user_data(nghttp2_session *session,
                                                      void *user_data);

    /**
     * @function
     *
     * Returns the number of frames in the outbound queue.  This does not
     * include the deferred DATA frames.
     */
    NGHTTP2_EXTERN size_t
    nghttp2_session_get_outbound_queue_size(nghttp2_session *session);

    /**
     * @function
     *
     * Returns the number of DATA payload in bytes received without
     * WINDOW_UPDATE transmission for the stream |stream_id|.  The local
     * (receive) window size can be adjusted by
     * `nghttp2_submit_window_update()`.  This function takes into account
     * that and returns effective data length.  In particular, if the
     * local window size is reduced by submitting negative
     * window_size_increment with `nghttp2_submit_window_update()`, this
     * function returns the number of bytes less than actually received.
     *
     * This function returns -1 if it fails.
     */
    NGHTTP2_EXTERN int32_t nghttp2_session_get_stream_effective_recv_data_length(
      nghttp2_session *session, int32_t stream_id);

    /**
     * @function
     *
     * Returns the local (receive) window size for the stream |stream_id|.
     * The local window size can be adjusted by
     * `nghttp2_submit_window_update()`.  This function takes into account
     * that and returns effective window size.
     *
     * This function does not take into account the amount of received
     * data from the remote endpoint.  Use
     * `nghttp2_session_get_stream_local_window_size()` to know the amount
     * of data the remote endpoint can send without receiving stream level
     * WINDOW_UPDATE frame.  Note that each stream is still subject to the
     * connection level flow control.
     *
     * This function returns -1 if it fails.
     */
    NGHTTP2_EXTERN int32_t nghttp2_session_get_stream_effective_local_window_size(
      nghttp2_session *session, int32_t stream_id);

    /**
     * @function
     *
     * Returns the amount of flow-controlled payload (e.g., DATA) that the
     * remote endpoint can send without receiving stream level
     * WINDOW_UPDATE frame.  It is also subject to the connection level
     * flow control.  So the actual amount of data to send is
     * min(`nghttp2_session_get_stream_local_window_size()`,
     * `nghttp2_session_get_local_window_size()`).
     *
     * This function returns -1 if it fails.
     */
    NGHTTP2_EXTERN int32_t nghttp2_session_get_stream_local_window_size(
      nghttp2_session *session, int32_t stream_id);

    /**
     * @function
     *
     * Returns the number of DATA payload in bytes received without
     * WINDOW_UPDATE transmission for a connection.  The local (receive)
     * window size can be adjusted by `nghttp2_submit_window_update()`.
     * This function takes into account that and returns effective data
     * length.  In particular, if the local window size is reduced by
     * submitting negative window_size_increment with
     * `nghttp2_submit_window_update()`, this function returns the number
     * of bytes less than actually received.
     *
     * This function returns -1 if it fails.
     */
    NGHTTP2_EXTERN int32_t
    nghttp2_session_get_effective_recv_data_length(nghttp2_session *session);

    /**
     * @function
     *
     * Returns the local (receive) window size for a connection.  The
     * local window size can be adjusted by
     * `nghttp2_submit_window_update()`.  This function takes into account
     * that and returns effective window size.
     *
     * This function does not take into account the amount of received
     * data from the remote endpoint.  Use
     * `nghttp2_session_get_local_window_size()` to know the amount of
     * data the remote endpoint can send without receiving
     * connection-level WINDOW_UPDATE frame.  Note that each stream is
     * still subject to the stream level flow control.
     *
     * This function returns -1 if it fails.
     */
    NGHTTP2_EXTERN int32_t
    nghttp2_session_get_effective_local_window_size(nghttp2_session *session);

    /**
     * @function
     *
     * Returns the amount of flow-controlled payload (e.g., DATA) that the
     * remote endpoint can send without receiving connection level
     * WINDOW_UPDATE frame.  Note that each stream is still subject to the
     * stream level flow control (see
     * `nghttp2_session_get_stream_local_window_size()`).
     *
     * This function returns -1 if it fails.
     */
    NGHTTP2_EXTERN int32_t
    nghttp2_session_get_local_window_size(nghttp2_session *session);

    /**
     * @function
     *
     * Returns the remote window size for a given stream |stream_id|.
     *
     * This is the amount of flow-controlled payload (e.g., DATA) that the
     * local endpoint can send without stream level WINDOW_UPDATE.  There
     * is also connection level flow control, so the effective size of
     * payload that the local endpoint can actually send is
     * min(`nghttp2_session_get_stream_remote_window_size()`,
     * `nghttp2_session_get_remote_window_size()`).
     *
     * This function returns -1 if it fails.
     */
    NGHTTP2_EXTERN int32_t nghttp2_session_get_stream_remote_window_size(
      nghttp2_session *session, int32_t stream_id);

    /**
     * @function
     *
     * Returns the remote window size for a connection.
     *
     * This function always succeeds.
     */
    NGHTTP2_EXTERN int32_t
    nghttp2_session_get_remote_window_size(nghttp2_session *session);

    /**
     * @function
     *
     * Returns 1 if local peer half closed the given stream |stream_id|.
     * Returns 0 if it did not.  Returns -1 if no such stream exists.
     */
    NGHTTP2_EXTERN int
    nghttp2_session_get_stream_local_close(nghttp2_session *session,
                                           int32_t stream_id);

    /**
     * @function
     *
     * Returns 1 if remote peer half closed the given stream |stream_id|.
     * Returns 0 if it did not.  Returns -1 if no such stream exists.
     */
    NGHTTP2_EXTERN int
    nghttp2_session_get_stream_remote_close(nghttp2_session *session,
                                            int32_t stream_id);

    /**
     * @function
     *
     * Returns the current dynamic table size of HPACK inflater, including
     * the overhead 32 bytes per entry described in RFC 7541.
     */
    NGHTTP2_EXTERN size_t
    nghttp2_session_get_hd_inflate_dynamic_table_size(nghttp2_session *session);

    /**
     * @function
     *
     * Returns the current dynamic table size of HPACK deflater including
     * the overhead 32 bytes per entry described in RFC 7541.
     */
    NGHTTP2_EXTERN size_t
    nghttp2_session_get_hd_deflate_dynamic_table_size(nghttp2_session *session);

    /**
     * @function
     *
     * Signals the session so that the connection should be terminated.
     *
     * The last stream ID is the minimum value between the stream ID of a
     * stream for which :type:`nghttp2_on_frame_recv_callback` was called
     * most recently and the last stream ID we have sent to the peer
     * previously.
     *
     * The |error_code| is the error code of this GOAWAY frame.  The
     * pre-defined error code is one of :enum:`nghttp2_error_code`.
     *
     * After the transmission, both `nghttp2_session_want_read()` and
     * `nghttp2_session_want_write()` return 0.
     *
     * This function should be called when the connection should be
     * terminated after sending GOAWAY.  If the remaining streams should
     * be processed after GOAWAY, use `nghttp2_submit_goaway()` instead.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     */
    NGHTTP2_EXTERN int nghttp2_session_terminate_session(nghttp2_session *session,
                                                         uint32_t error_code);

    /**
     * @function
     *
     * Signals the session so that the connection should be terminated.
     *
     * This function behaves like `nghttp2_session_terminate_session()`,
     * but the last stream ID can be specified by the application for fine
     * grained control of stream.  The HTTP/2 specification does not allow
     * last_stream_id to be increased.  So the actual value sent as
     * last_stream_id is the minimum value between the given
     * |last_stream_id| and the last_stream_id we have previously sent to
     * the peer.
     *
     * The |last_stream_id| is peer's stream ID or 0.  So if |session| is
     * initialized as client, |last_stream_id| must be even or 0.  If
     * |session| is initialized as server, |last_stream_id| must be odd or
     * 0.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     The |last_stream_id| is invalid.
     */
    NGHTTP2_EXTERN int nghttp2_session_terminate_session2(nghttp2_session *session,
                                                          int32_t last_stream_id,
                                                          uint32_t error_code);

    /**
     * @function
     *
     * Signals to the client that the server started graceful shutdown
     * procedure.
     *
     * This function is only usable for server.  If this function is
     * called with client side session, this function returns
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_STATE`.
     *
     * To gracefully shutdown HTTP/2 session, server should call this
     * function to send GOAWAY with last_stream_id (1u << 31) - 1.  And
     * after some delay (e.g., 1 RTT), send another GOAWAY with the stream
     * ID that the server has some processing using
     * `nghttp2_submit_goaway()`.  See also
     * `nghttp2_session_get_last_proc_stream_id()`.
     *
     * Unlike `nghttp2_submit_goaway()`, this function just sends GOAWAY
     * and does nothing more.  This is a mere indication to the client
     * that session shutdown is imminent.  The application should call
     * `nghttp2_submit_goaway()` with appropriate last_stream_id after
     * this call.
     *
     * If one or more GOAWAY frame have been already sent by either
     * `nghttp2_submit_goaway()` or `nghttp2_session_terminate_session()`,
     * this function has no effect.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_STATE`
     *     The |session| is initialized as client.
     */
    NGHTTP2_EXTERN int nghttp2_submit_shutdown_notice(nghttp2_session *session);

    /**
     * @function
     *
     * Returns the value of SETTINGS |id| notified by a remote endpoint.
     * The |id| must be one of values defined in
     * :enum:`nghttp2_settings_id`.
     */
    NGHTTP2_EXTERN uint32_t nghttp2_session_get_remote_settings(
      nghttp2_session *session, nghttp2_settings_id id);

    /**
     * @function
     *
     * Returns the value of SETTINGS |id| of local endpoint acknowledged
     * by the remote endpoint.  The |id| must be one of the values defined
     * in :enum:`nghttp2_settings_id`.
     */
    NGHTTP2_EXTERN uint32_t nghttp2_session_get_local_settings(
      nghttp2_session *session, nghttp2_settings_id id);

    /**
     * @function
     *
     * Tells the |session| that next stream ID is |next_stream_id|.  The
     * |next_stream_id| must be equal or greater than the value returned
     * by `nghttp2_session_get_next_stream_id()`.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     The |next_stream_id| is strictly less than the value
     *     `nghttp2_session_get_next_stream_id()` returns; or
     *     |next_stream_id| is invalid (e.g., even integer for client, or
     *     odd integer for server).
     */
    NGHTTP2_EXTERN int nghttp2_session_set_next_stream_id(nghttp2_session *session,
                                                          int32_t next_stream_id);

    /**
     * @function
     *
     * Returns the next outgoing stream ID.  Notice that return type is
     * uint32_t.  If we run out of stream ID for this session, this
     * function returns 1 << 31.
     */
    NGHTTP2_EXTERN uint32_t
    nghttp2_session_get_next_stream_id(nghttp2_session *session);

    /**
     * @function
     *
     * Tells the |session| that |size| bytes for a stream denoted by
     * |stream_id| were consumed by application and are ready to
     * WINDOW_UPDATE.  The consumed bytes are counted towards both
     * connection and stream level WINDOW_UPDATE (see
     * `nghttp2_session_consume_connection()` and
     * `nghttp2_session_consume_stream()` to update consumption
     * independently).  This function is intended to be used without
     * automatic window update (see
     * `nghttp2_option_set_no_auto_window_update()`).
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     The |stream_id| is 0.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_STATE`
     *     Automatic WINDOW_UPDATE is not disabled.
     */
    NGHTTP2_EXTERN int nghttp2_session_consume(nghttp2_session *session,
                                               int32_t stream_id, size_t size);

    /**
     * @function
     *
     * Like `nghttp2_session_consume()`, but this only tells library that
     * |size| bytes were consumed only for connection level.  Note that
     * HTTP/2 maintains connection and stream level flow control windows
     * independently.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_STATE`
     *     Automatic WINDOW_UPDATE is not disabled.
     */
    NGHTTP2_EXTERN int nghttp2_session_consume_connection(nghttp2_session *session,
                                                          size_t size);

    /**
     * @function
     *
     * Like `nghttp2_session_consume()`, but this only tells library that
     * |size| bytes were consumed only for stream denoted by |stream_id|.
     * Note that HTTP/2 maintains connection and stream level flow control
     * windows independently.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     The |stream_id| is 0.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_STATE`
     *     Automatic WINDOW_UPDATE is not disabled.
     */
    NGHTTP2_EXTERN int nghttp2_session_consume_stream(nghttp2_session *session,
                                                      int32_t stream_id,
                                                      size_t size);

    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  :rfc:`7540` priorities are deprecated by
     *   :rfc:`9113`.  Consider migrating to :rfc:`9218` extensible
     *   prioritization scheme.
     *
     * This function is noop.  It always returns 0.
     */
    NGHTTP2_EXTERN int
    nghttp2_session_change_stream_priority(nghttp2_session *session,
                                           int32_t stream_id,
                                           const nghttp2_priority_spec *pri_spec);

    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  :rfc:`7540` priorities are deprecated by
     *   :rfc:`9113`.  Consider migrating to :rfc:`9218` extensible
     *   prioritization scheme.
     *
     * This function is noop.  It always returns 0.
     */
    NGHTTP2_EXTERN int
    nghttp2_session_create_idle_stream(nghttp2_session *session, int32_t stream_id,
                                       const nghttp2_priority_spec *pri_spec);

    /**
     * @function
     *
     * .. warning::
     *
     *   This function is deprecated in favor of
     *   `nghttp2_session_upgrade2()`, because this function lacks the
     *   parameter to tell the library the request method used in the
     *   original HTTP request.  This information is required for client
     *   to validate actual response body length against content-length
     *   header field (see `nghttp2_option_set_no_http_messaging()`).  If
     *   HEAD is used in request, the length of response body must be 0
     *   regardless of value included in content-length header field.
     *
     * Performs post-process of HTTP Upgrade request.  This function can
     * be called from both client and server, but the behavior is very
     * different in each other.
     *
     * If called from client side, the |settings_payload| must be the
     * value sent in ``HTTP2-Settings`` header field and must be decoded
     * by base64url decoder.  The |settings_payloadlen| is the length of
     * |settings_payload|.  The |settings_payload| is unpacked and its
     * setting values will be submitted using `nghttp2_submit_settings()`.
     * This means that the client application code does not need to submit
     * SETTINGS by itself.  The stream with stream ID=1 is opened and the
     * |stream_user_data| is used for its stream_user_data.  The opened
     * stream becomes half-closed (local) state.
     *
     * If called from server side, the |settings_payload| must be the
     * value received in ``HTTP2-Settings`` header field and must be
     * decoded by base64url decoder.  The |settings_payloadlen| is the
     * length of |settings_payload|.  It is treated as if the SETTINGS
     * frame with that payload is received.  Thus, callback functions for
     * the reception of SETTINGS frame will be invoked.  The stream with
     * stream ID=1 is opened.  The |stream_user_data| is ignored.  The
     * opened stream becomes half-closed (remote).
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     The |settings_payload| is badly formed.
     * :enum:`nghttp2_error.NGHTTP2_ERR_PROTO`
     *     The stream ID 1 is already used or closed; or is not available.
     */
    NGHTTP2_EXTERN int nghttp2_session_upgrade(nghttp2_session *session,
                                               const uint8_t *settings_payload,
                                               size_t settings_payloadlen,
                                               void *stream_user_data);

    /**
     * @function
     *
     * Performs post-process of HTTP Upgrade request.  This function can
     * be called from both client and server, but the behavior is very
     * different in each other.
     *
     * If called from client side, the |settings_payload| must be the
     * value sent in ``HTTP2-Settings`` header field and must be decoded
     * by base64url decoder.  The |settings_payloadlen| is the length of
     * |settings_payload|.  The |settings_payload| is unpacked and its
     * setting values will be submitted using `nghttp2_submit_settings()`.
     * This means that the client application code does not need to submit
     * SETTINGS by itself.  The stream with stream ID=1 is opened and the
     * |stream_user_data| is used for its stream_user_data.  The opened
     * stream becomes half-closed (local) state.
     *
     * If called from server side, the |settings_payload| must be the
     * value received in ``HTTP2-Settings`` header field and must be
     * decoded by base64url decoder.  The |settings_payloadlen| is the
     * length of |settings_payload|.  It is treated as if the SETTINGS
     * frame with that payload is received.  Thus, callback functions for
     * the reception of SETTINGS frame will be invoked.  The stream with
     * stream ID=1 is opened.  The |stream_user_data| is ignored.  The
     * opened stream becomes half-closed (remote).
     *
     * If the request method is HEAD, pass nonzero value to
     * |head_request|.  Otherwise, pass 0.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     The |settings_payload| is badly formed.
     * :enum:`nghttp2_error.NGHTTP2_ERR_PROTO`
     *     The stream ID 1 is already used or closed; or is not available.
     */
    NGHTTP2_EXTERN int nghttp2_session_upgrade2(nghttp2_session *session,
                                                const uint8_t *settings_payload,
                                                size_t settings_payloadlen,
                                                int head_request,
                                                void *stream_user_data);

    #ifndef NGHTTP2_NO_SSIZE_T
    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  Use `nghttp2_pack_settings_payload2()` instead.
     *
     * Serializes the SETTINGS values |iv| in the |buf|.  The size of the
     * |buf| is specified by |buflen|.  The number of entries in the |iv|
     * array is given by |niv|.  The required space in |buf| for the |niv|
     * entries is ``6*niv`` bytes and if the given buffer is too small, an
     * error is returned.  This function is used mainly for creating a
     * SETTINGS payload to be sent with the ``HTTP2-Settings`` header
     * field in an HTTP Upgrade request.  The data written in |buf| is NOT
     * base64url encoded and the application is responsible for encoding.
     *
     * This function returns the number of bytes written in |buf|, or one
     * of the following negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     The |iv| contains duplicate settings ID or invalid value.
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_INSUFF_BUFSIZE`
     *     The provided |buflen| size is too small to hold the output.
     */
    NGHTTP2_EXTERN ssize_t nghttp2_pack_settings_payload(
      uint8_t *buf, size_t buflen, const nghttp2_settings_entry *iv, size_t niv);

    #endif /* NGHTTP2_NO_SSIZE_T */

    /**
     * @function
     *
     * Serializes the SETTINGS values |iv| in the |buf|.  The size of the
     * |buf| is specified by |buflen|.  The number of entries in the |iv|
     * array is given by |niv|.  The required space in |buf| for the |niv|
     * entries is ``6*niv`` bytes and if the given buffer is too small, an
     * error is returned.  This function is used mainly for creating a
     * SETTINGS payload to be sent with the ``HTTP2-Settings`` header
     * field in an HTTP Upgrade request.  The data written in |buf| is NOT
     * base64url encoded and the application is responsible for encoding.
     *
     * This function returns the number of bytes written in |buf|, or one
     * of the following negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     The |iv| contains duplicate settings ID or invalid value.
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_INSUFF_BUFSIZE`
     *     The provided |buflen| size is too small to hold the output.
     */
    NGHTTP2_EXTERN nghttp2_ssize nghttp2_pack_settings_payload2(
      uint8_t *buf, size_t buflen, const nghttp2_settings_entry *iv, size_t niv);

    /**
     * @function
     *
     * Returns string describing the |lib_error_code|.  The
     * |lib_error_code| must be one of the :enum:`nghttp2_error`.
     */
    NGHTTP2_EXTERN const char *nghttp2_strerror(int lib_error_code);

    /**
     * @function
     *
     * Returns string representation of HTTP/2 error code |error_code|
     * (e.g., ``PROTOCOL_ERROR`` is returned if ``error_code ==
     * NGHTTP2_PROTOCOL_ERROR``).  If string representation is unknown for
     * given |error_code|, this function returns string ``unknown``.
     */
    NGHTTP2_EXTERN const char *nghttp2_http2_strerror(uint32_t error_code);

    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  :rfc:`7540` priorities are deprecated by
     *   :rfc:`9113`.  Consider migrating to :rfc:`9218` extensible
     *   prioritization scheme.
     *
     * Initializes |pri_spec| with the |stream_id| of the stream to depend
     * on with |weight| and its exclusive flag.  If |exclusive| is
     * nonzero, exclusive flag is set.
     *
     * The |weight| must be in [:macro:`NGHTTP2_MIN_WEIGHT`,
     * :macro:`NGHTTP2_MAX_WEIGHT`], inclusive.
     */
    NGHTTP2_EXTERN void nghttp2_priority_spec_init(nghttp2_priority_spec *pri_spec,
                                                   int32_t stream_id,
                                                   int32_t weight, int exclusive);

    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  :rfc:`7540` priorities are deprecated by
     *   :rfc:`9113`.  Consider migrating to :rfc:`9218` extensible
     *   prioritization scheme.
     *
     * Initializes |pri_spec| with the default values.  The default values
     * are: stream_id = 0, weight = :macro:`NGHTTP2_DEFAULT_WEIGHT` and
     * exclusive = 0.
     */
    NGHTTP2_EXTERN void
    nghttp2_priority_spec_default_init(nghttp2_priority_spec *pri_spec);

    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  :rfc:`7540` priorities are deprecated by
     *   :rfc:`9113`.  Consider migrating to :rfc:`9218` extensible
     *   prioritization scheme.
     *
     * Returns nonzero if the |pri_spec| is filled with default values.
     */
    NGHTTP2_EXTERN int
    nghttp2_priority_spec_check_default(const nghttp2_priority_spec *pri_spec);

    #ifndef NGHTTP2_NO_SSIZE_T
    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  Use `nghttp2_submit_request2()` instead.
     *
     * Submits HEADERS frame and optionally one or more DATA frames.
     *
     * The |pri_spec| is ignored.
     *
     * The |nva| is an array of name/value pair :type:`nghttp2_nv` with
     * |nvlen| elements.  The application is responsible to include
     * required pseudo-header fields (header field whose name starts with
     * ":") in |nva| and must place pseudo-headers before regular header
     * fields.
     *
     * This function creates copies of all name/value pairs in |nva|.  It
     * also lower-cases all names in |nva|.  The order of elements in
     * |nva| is preserved.  For header fields with
     * :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_NAME` and
     * :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_VALUE` are set,
     * header field name and value are not copied respectively.  With
     * :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_NAME`, application
     * is responsible to pass header field name in lowercase.  The
     * application should maintain the references to them until
     * :type:`nghttp2_on_frame_send_callback` or
     * :type:`nghttp2_on_frame_not_send_callback` is called.
     *
     * HTTP/2 specification has requirement about header fields in the
     * request HEADERS.  See the specification for more details.
     *
     * If |data_prd| is not ``NULL``, it provides data which will be sent
     * in subsequent DATA frames.  In this case, a method that allows
     * request message bodies
     * (https://tools.ietf.org/html/rfc7231#section-4) must be specified
     * with ``:method`` key in |nva| (e.g. ``POST``).  This function does
     * not take ownership of the |data_prd|.  The function copies the
     * members of the |data_prd|.  If |data_prd| is ``NULL``, HEADERS have
     * END_STREAM set.  The |stream_user_data| is data associated to the
     * stream opened by this request and can be an arbitrary pointer,
     * which can be retrieved later by
     * `nghttp2_session_get_stream_user_data()`.
     *
     * This function returns assigned stream ID if it succeeds, or one of
     * the following negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_STREAM_ID_NOT_AVAILABLE`
     *     No stream ID is available because maximum stream ID was
     *     reached.
     * :enum:`nghttp2_error.NGHTTP2_ERR_PROTO`
     *     The |session| is server session.
     *
     * .. warning::
     *
     *   This function returns assigned stream ID if it succeeds.  But
     *   that stream is not created yet.  The application must not submit
     *   frame to that stream ID before
     *   :type:`nghttp2_before_frame_send_callback` is called for this
     *   frame.  This means `nghttp2_session_get_stream_user_data()` does
     *   not work before the callback.  But
     *   `nghttp2_session_set_stream_user_data()` handles this situation
     *   specially, and it can set data to a stream during this period.
     *
     */
    NGHTTP2_EXTERN int32_t nghttp2_submit_request(
      nghttp2_session *session, const nghttp2_priority_spec *pri_spec,
      const nghttp2_nv *nva, size_t nvlen, const nghttp2_data_provider *data_prd,
      void *stream_user_data);

    #endif /* NGHTTP2_NO_SSIZE_T */

    /**
     * @function
     *
     * Submits HEADERS frame and optionally one or more DATA frames.
     *
     * The |pri_spec| is ignored.
     *
     * The |nva| is an array of name/value pair :type:`nghttp2_nv` with
     * |nvlen| elements.  The application is responsible to include
     * required pseudo-header fields (header field whose name starts with
     * ":") in |nva| and must place pseudo-headers before regular header
     * fields.
     *
     * This function creates copies of all name/value pairs in |nva|.  It
     * also lower-cases all names in |nva|.  The order of elements in
     * |nva| is preserved.  For header fields with
     * :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_NAME` and
     * :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_VALUE` are set,
     * header field name and value are not copied respectively.  With
     * :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_NAME`, application
     * is responsible to pass header field name in lowercase.  The
     * application should maintain the references to them until
     * :type:`nghttp2_on_frame_send_callback` or
     * :type:`nghttp2_on_frame_not_send_callback` is called.
     *
     * HTTP/2 specification has requirement about header fields in the
     * request HEADERS.  See the specification for more details.
     *
     * If |data_prd| is not ``NULL``, it provides data which will be sent
     * in subsequent DATA frames.  In this case, a method that allows
     * request message bodies
     * (https://tools.ietf.org/html/rfc7231#section-4) must be specified
     * with ``:method`` key in |nva| (e.g. ``POST``).  This function does
     * not take ownership of the |data_prd|.  The function copies the
     * members of the |data_prd|.  If |data_prd| is ``NULL``, HEADERS have
     * END_STREAM set.  The |stream_user_data| is data associated to the
     * stream opened by this request and can be an arbitrary pointer,
     * which can be retrieved later by
     * `nghttp2_session_get_stream_user_data()`.
     *
     * This function returns assigned stream ID if it succeeds, or one of
     * the following negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_STREAM_ID_NOT_AVAILABLE`
     *     No stream ID is available because maximum stream ID was
     *     reached.
     * :enum:`nghttp2_error.NGHTTP2_ERR_PROTO`
     *     The |session| is server session.
     *
     * .. warning::
     *
     *   This function returns assigned stream ID if it succeeds.  But
     *   that stream is not created yet.  The application must not submit
     *   frame to that stream ID before
     *   :type:`nghttp2_before_frame_send_callback` is called for this
     *   frame.  This means `nghttp2_session_get_stream_user_data()` does
     *   not work before the callback.  But
     *   `nghttp2_session_set_stream_user_data()` handles this situation
     *   specially, and it can set data to a stream during this period.
     *
     */
    NGHTTP2_EXTERN int32_t nghttp2_submit_request2(
      nghttp2_session *session, const nghttp2_priority_spec *pri_spec,
      const nghttp2_nv *nva, size_t nvlen, const nghttp2_data_provider2 *data_prd,
      void *stream_user_data);

    #ifndef NGHTTP2_NO_SSIZE_T
    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  Use `nghttp2_submit_response2()` instead.
     *
     * Submits response HEADERS frame and optionally one or more DATA
     * frames against the stream |stream_id|.
     *
     * The |nva| is an array of name/value pair :type:`nghttp2_nv` with
     * |nvlen| elements.  The application is responsible to include
     * required pseudo-header fields (header field whose name starts with
     * ":") in |nva| and must place pseudo-headers before regular header
     * fields.
     *
     * This function creates copies of all name/value pairs in |nva|.  It
     * also lower-cases all names in |nva|.  The order of elements in
     * |nva| is preserved.  For header fields with
     * :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_NAME` and
     * :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_VALUE` are set,
     * header field name and value are not copied respectively.  With
     * :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_NAME`, application
     * is responsible to pass header field name in lowercase.  The
     * application should maintain the references to them until
     * :type:`nghttp2_on_frame_send_callback` or
     * :type:`nghttp2_on_frame_not_send_callback` is called.
     *
     * HTTP/2 specification has requirement about header fields in the
     * response HEADERS.  See the specification for more details.
     *
     * If |data_prd| is not ``NULL``, it provides data which will be sent
     * in subsequent DATA frames.  This function does not take ownership
     * of the |data_prd|.  The function copies the members of the
     * |data_prd|.  If |data_prd| is ``NULL``, HEADERS will have
     * END_STREAM flag set.
     *
     * This method can be used as normal HTTP response and push response.
     * When pushing a resource using this function, the |session| must be
     * configured using `nghttp2_session_server_new()` or its variants and
     * the target stream denoted by the |stream_id| must be reserved using
     * `nghttp2_submit_push_promise()`.
     *
     * To send non-final response headers (e.g., HTTP status 101), don't
     * use this function because this function half-closes the outbound
     * stream.  Instead, use `nghttp2_submit_headers()` for this purpose.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     The |stream_id| is 0.
     * :enum:`nghttp2_error.NGHTTP2_ERR_DATA_EXIST`
     *     DATA or HEADERS has been already submitted and not fully
     *     processed yet.  Normally, this does not happen, but when
     *     application wrongly calls `nghttp2_submit_response()` twice,
     *     this may happen.
     * :enum:`nghttp2_error.NGHTTP2_ERR_PROTO`
     *     The |session| is client session.
     *
     * .. warning::
     *
     *   Calling this function twice for the same stream ID may lead to
     *   program crash.  It is generally considered to a programming error
     *   to commit response twice.
     */
    NGHTTP2_EXTERN int
    nghttp2_submit_response(nghttp2_session *session, int32_t stream_id,
                            const nghttp2_nv *nva, size_t nvlen,
                            const nghttp2_data_provider *data_prd);

    #endif /* NGHTTP2_NO_SSIZE_T */

    /**
     * @function
     *
     * Submits response HEADERS frame and optionally one or more DATA
     * frames against the stream |stream_id|.
     *
     * The |nva| is an array of name/value pair :type:`nghttp2_nv` with
     * |nvlen| elements.  The application is responsible to include
     * required pseudo-header fields (header field whose name starts with
     * ":") in |nva| and must place pseudo-headers before regular header
     * fields.
     *
     * This function creates copies of all name/value pairs in |nva|.  It
     * also lower-cases all names in |nva|.  The order of elements in
     * |nva| is preserved.  For header fields with
     * :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_NAME` and
     * :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_VALUE` are set,
     * header field name and value are not copied respectively.  With
     * :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_NAME`, application
     * is responsible to pass header field name in lowercase.  The
     * application should maintain the references to them until
     * :type:`nghttp2_on_frame_send_callback` or
     * :type:`nghttp2_on_frame_not_send_callback` is called.
     *
     * HTTP/2 specification has requirement about header fields in the
     * response HEADERS.  See the specification for more details.
     *
     * If |data_prd| is not ``NULL``, it provides data which will be sent
     * in subsequent DATA frames.  This function does not take ownership
     * of the |data_prd|.  The function copies the members of the
     * |data_prd|.  If |data_prd| is ``NULL``, HEADERS will have
     * END_STREAM flag set.
     *
     * This method can be used as normal HTTP response and push response.
     * When pushing a resource using this function, the |session| must be
     * configured using `nghttp2_session_server_new()` or its variants and
     * the target stream denoted by the |stream_id| must be reserved using
     * `nghttp2_submit_push_promise()`.
     *
     * To send non-final response headers (e.g., HTTP status 101), don't
     * use this function because this function half-closes the outbound
     * stream.  Instead, use `nghttp2_submit_headers()` for this purpose.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     The |stream_id| is 0.
     * :enum:`nghttp2_error.NGHTTP2_ERR_DATA_EXIST`
     *     DATA or HEADERS has been already submitted and not fully
     *     processed yet.  Normally, this does not happen, but when
     *     application wrongly calls `nghttp2_submit_response2()` twice,
     *     this may happen.
     * :enum:`nghttp2_error.NGHTTP2_ERR_PROTO`
     *     The |session| is client session.
     *
     * .. warning::
     *
     *   Calling this function twice for the same stream ID may lead to
     *   program crash.  It is generally considered to a programming error
     *   to commit response twice.
     */
    NGHTTP2_EXTERN int
    nghttp2_submit_response2(nghttp2_session *session, int32_t stream_id,
                             const nghttp2_nv *nva, size_t nvlen,
                             const nghttp2_data_provider2 *data_prd);

    /**
     * @function
     *
     * Submits trailer fields HEADERS against the stream |stream_id|.
     *
     * The |nva| is an array of name/value pair :type:`nghttp2_nv` with
     * |nvlen| elements.  The application must not include pseudo-header
     * fields (headers whose names starts with ":") in |nva|.
     *
     * This function creates copies of all name/value pairs in |nva|.  It
     * also lower-cases all names in |nva|.  The order of elements in
     * |nva| is preserved.  For header fields with
     * :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_NAME` and
     * :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_VALUE` are set,
     * header field name and value are not copied respectively.  With
     * :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_NAME`, application
     * is responsible to pass header field name in lowercase.  The
     * application should maintain the references to them until
     * :type:`nghttp2_on_frame_send_callback` or
     * :type:`nghttp2_on_frame_not_send_callback` is called.
     *
     * For server, trailer fields must follow response HEADERS or response
     * DATA without END_STREAM flat set.  The library does not enforce
     * this requirement, and applications should do this for themselves.
     * If `nghttp2_submit_trailer()` is called before any response HEADERS
     * submission (usually by `nghttp2_submit_response2()`), the content
     * of |nva| will be sent as response headers, which will result in
     * error.
     *
     * This function has the same effect with `nghttp2_submit_headers()`,
     * with flags = :enum:`nghttp2_flag.NGHTTP2_FLAG_END_STREAM` and both
     * pri_spec and stream_user_data to NULL.
     *
     * To submit trailer fields after `nghttp2_submit_response2()` is
     * called, the application has to specify
     * :type:`nghttp2_data_provider2` to `nghttp2_submit_response2()`.
     * Inside of :type:`nghttp2_data_source_read_callback2`, when setting
     * :enum:`nghttp2_data_flag.NGHTTP2_DATA_FLAG_EOF`, also set
     * :enum:`nghttp2_data_flag.NGHTTP2_DATA_FLAG_NO_END_STREAM`.  After
     * that, the application can send trailer fields using
     * `nghttp2_submit_trailer()`.  `nghttp2_submit_trailer()` can be used
     * inside :type:`nghttp2_data_source_read_callback2`.
     *
     * This function returns 0 if it succeeds and |stream_id| is -1.
     * Otherwise, this function returns 0 if it succeeds, or one of the
     * following negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     The |stream_id| is 0.
     */
    NGHTTP2_EXTERN int nghttp2_submit_trailer(nghttp2_session *session,
                                              int32_t stream_id,
                                              const nghttp2_nv *nva, size_t nvlen);

    /**
     * @function
     *
     * Submits HEADERS frame. The |flags| is bitwise OR of the
     * following values:
     *
     * * :enum:`nghttp2_flag.NGHTTP2_FLAG_END_STREAM`
     *
     * If |flags| includes :enum:`nghttp2_flag.NGHTTP2_FLAG_END_STREAM`,
     * this frame has END_STREAM flag set.
     *
     * The library handles the CONTINUATION frame internally and it
     * correctly sets END_HEADERS to the last sequence of the PUSH_PROMISE
     * or CONTINUATION frame.
     *
     * If the |stream_id| is -1, this frame is assumed as request (i.e.,
     * request HEADERS frame which opens new stream).  In this case, the
     * assigned stream ID will be returned.  Otherwise, specify stream ID
     * in |stream_id|.
     *
     * The |pri_spec| is ignored.
     *
     * The |nva| is an array of name/value pair :type:`nghttp2_nv` with
     * |nvlen| elements.  The application is responsible to include
     * required pseudo-header fields (header field whose name starts with
     * ":") in |nva| and must place pseudo-headers before regular header
     * fields.
     *
     * This function creates copies of all name/value pairs in |nva|.  It
     * also lower-cases all names in |nva|.  The order of elements in
     * |nva| is preserved.  For header fields with
     * :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_NAME` and
     * :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_VALUE` are set,
     * header field name and value are not copied respectively.  With
     * :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_NAME`, application
     * is responsible to pass header field name in lowercase.  The
     * application should maintain the references to them until
     * :type:`nghttp2_on_frame_send_callback` or
     * :type:`nghttp2_on_frame_not_send_callback` is called.
     *
     * The |stream_user_data| is a pointer to an arbitrary data which is
     * associated to the stream this frame will open.  Therefore it is
     * only used if this frame opens streams, in other words, it changes
     * stream state from idle or reserved to open.
     *
     * This function is low-level in a sense that the application code can
     * specify flags directly.  For usual HTTP request,
     * `nghttp2_submit_request2()` is useful.  Likewise, for HTTP
     * response, prefer `nghttp2_submit_response2()`.
     *
     * This function returns newly assigned stream ID if it succeeds and
     * |stream_id| is -1.  Otherwise, this function returns 0 if it
     * succeeds, or one of the following negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_STREAM_ID_NOT_AVAILABLE`
     *     No stream ID is available because maximum stream ID was
     *     reached.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     The |stream_id| is 0.
     * :enum:`nghttp2_error.NGHTTP2_ERR_DATA_EXIST`
     *     DATA or HEADERS has been already submitted and not fully
     *     processed yet.  This happens if stream denoted by |stream_id|
     *     is in reserved state.
     * :enum:`nghttp2_error.NGHTTP2_ERR_PROTO`
     *     The |stream_id| is -1, and |session| is server session.
     *
     * .. warning::
     *
     *   This function returns assigned stream ID if it succeeds and
     *   |stream_id| is -1.  But that stream is not opened yet.  The
     *   application must not submit frame to that stream ID before
     *   :type:`nghttp2_before_frame_send_callback` is called for this
     *   frame.
     *
     */
    NGHTTP2_EXTERN int32_t nghttp2_submit_headers(
      nghttp2_session *session, uint8_t flags, int32_t stream_id,
      const nghttp2_priority_spec *pri_spec, const nghttp2_nv *nva, size_t nvlen,
      void *stream_user_data);

    #ifndef NGHTTP2_NO_SSIZE_T
    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  Use `nghttp2_submit_data2()` instead.
     *
     * Submits one or more DATA frames to the stream |stream_id|.  The
     * data to be sent are provided by |data_prd|.  If |flags| contains
     * :enum:`nghttp2_flag.NGHTTP2_FLAG_END_STREAM`, the last DATA frame
     * has END_STREAM flag set.
     *
     * This function does not take ownership of the |data_prd|.  The
     * function copies the members of the |data_prd|.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_DATA_EXIST`
     *     DATA or HEADERS has been already submitted and not fully
     *     processed yet.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     The |stream_id| is 0.
     * :enum:`nghttp2_error.NGHTTP2_ERR_STREAM_CLOSED`
     *     The stream was already closed; or the |stream_id| is invalid.
     *
     * .. note::
     *
     *   Currently, only one DATA or HEADERS is allowed for a stream at a
     *   time.  Submitting these frames more than once before first DATA
     *   or HEADERS is finished results in
     *   :enum:`nghttp2_error.NGHTTP2_ERR_DATA_EXIST` error code.  The
     *   earliest callback which tells that previous frame is done is
     *   :type:`nghttp2_on_frame_send_callback`.  In side that callback,
     *   new data can be submitted using `nghttp2_submit_data()`.  Of
     *   course, all data except for last one must not have
     *   :enum:`nghttp2_flag.NGHTTP2_FLAG_END_STREAM` flag set in |flags|.
     *   This sounds a bit complicated, and we recommend to use
     *   `nghttp2_submit_request()` and `nghttp2_submit_response()` to
     *   avoid this cascading issue.  The experience shows that for HTTP
     *   use, these two functions are enough to implement both client and
     *   server.
     */
    NGHTTP2_EXTERN int nghttp2_submit_data(nghttp2_session *session, uint8_t flags,
                                           int32_t stream_id,
                                           const nghttp2_data_provider *data_prd);

    #endif /* NGHTTP2_NO_SSIZE_T */

    /**
     * @function
     *
     * Submits one or more DATA frames to the stream |stream_id|.  The
     * data to be sent are provided by |data_prd|.  If |flags| contains
     * :enum:`nghttp2_flag.NGHTTP2_FLAG_END_STREAM`, the last DATA frame
     * has END_STREAM flag set.
     *
     * This function does not take ownership of the |data_prd|.  The
     * function copies the members of the |data_prd|.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_DATA_EXIST`
     *     DATA or HEADERS has been already submitted and not fully
     *     processed yet.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     The |stream_id| is 0.
     * :enum:`nghttp2_error.NGHTTP2_ERR_STREAM_CLOSED`
     *     The stream was already closed; or the |stream_id| is invalid.
     *
     * .. note::
     *
     *   Currently, only one DATA or HEADERS is allowed for a stream at a
     *   time.  Submitting these frames more than once before first DATA
     *   or HEADERS is finished results in
     *   :enum:`nghttp2_error.NGHTTP2_ERR_DATA_EXIST` error code.  The
     *   earliest callback which tells that previous frame is done is
     *   :type:`nghttp2_on_frame_send_callback`.  In side that callback,
     *   new data can be submitted using `nghttp2_submit_data2()`.  Of
     *   course, all data except for last one must not have
     *   :enum:`nghttp2_flag.NGHTTP2_FLAG_END_STREAM` flag set in |flags|.
     *   This sounds a bit complicated, and we recommend to use
     *   `nghttp2_submit_request2()` and `nghttp2_submit_response2()` to
     *   avoid this cascading issue.  The experience shows that for HTTP
     *   use, these two functions are enough to implement both client and
     *   server.
     */
    NGHTTP2_EXTERN int nghttp2_submit_data2(nghttp2_session *session, uint8_t flags,
                                            int32_t stream_id,
                                            const nghttp2_data_provider2 *data_prd);

    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  :rfc:`7540` priorities are deprecated by
     *   :rfc:`9113`.  Consider migrating to :rfc:`9218` extensible
     *   prioritization scheme.
     *
     * This function is noop.  It always returns 0.
     */
    NGHTTP2_EXTERN int
    nghttp2_submit_priority(nghttp2_session *session, uint8_t flags,
                            int32_t stream_id,
                            const nghttp2_priority_spec *pri_spec);

    /**
     * @macro
     *
     * :macro:`NGHTTP2_EXTPRI_DEFAULT_URGENCY` is the default urgency
     * level for :rfc:`9218` extensible priorities.
     */
    #define NGHTTP2_EXTPRI_DEFAULT_URGENCY 3

    /**
     * @macro
     *
     * :macro:`NGHTTP2_EXTPRI_URGENCY_HIGH` is the highest urgency level
     * for :rfc:`9218` extensible priorities.
     */
    #define NGHTTP2_EXTPRI_URGENCY_HIGH 0

    /**
     * @macro
     *
     * :macro:`NGHTTP2_EXTPRI_URGENCY_LOW` is the lowest urgency level for
     * :rfc:`9218` extensible priorities.
     */
    #define NGHTTP2_EXTPRI_URGENCY_LOW 7

    /**
     * @macro
     *
     * :macro:`NGHTTP2_EXTPRI_URGENCY_LEVELS` is the number of urgency
     * levels for :rfc:`9218` extensible priorities.
     */
    #define NGHTTP2_EXTPRI_URGENCY_LEVELS (NGHTTP2_EXTPRI_URGENCY_LOW + 1)

    /**
     * @struct
     *
     * :type:`nghttp2_extpri` is :rfc:`9218` extensible priorities
     * specification for a stream.
     */
    typedef struct nghttp2_extpri {
      /**
       * :member:`urgency` is the urgency of a stream, it must be in
       * [:macro:`NGHTTP2_EXTPRI_URGENCY_HIGH`,
       * :macro:`NGHTTP2_EXTPRI_URGENCY_LOW`], inclusive, and 0 is the
       * highest urgency.
       */
      uint32_t urgency;
      /**
       * :member:`inc` indicates that a content can be processed
       * incrementally or not.  If inc is 0, it cannot be processed
       * incrementally.  If inc is 1, it can be processed incrementally.
       * Other value is not permitted.
       */
      int inc;
    } nghttp2_extpri;

    /**
     * @function
     *
     * Submits RST_STREAM frame to cancel/reject the stream |stream_id|
     * with the error code |error_code|.
     *
     * The pre-defined error code is one of :enum:`nghttp2_error_code`.
     *
     * The |flags| is currently ignored and should be
     * :enum:`nghttp2_flag.NGHTTP2_FLAG_NONE`.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     The |stream_id| is 0.
     */
    NGHTTP2_EXTERN int nghttp2_submit_rst_stream(nghttp2_session *session,
                                                 uint8_t flags, int32_t stream_id,
                                                 uint32_t error_code);

    /**
     * @function
     *
     * Stores local settings and submits SETTINGS frame.  The |iv| is the
     * pointer to the array of :type:`nghttp2_settings_entry`.  The |niv|
     * indicates the number of :type:`nghttp2_settings_entry`.
     *
     * The |flags| is currently ignored and should be
     * :enum:`nghttp2_flag.NGHTTP2_FLAG_NONE`.
     *
     * This function does not take ownership of the |iv|.  This function
     * copies all the elements in the |iv|.
     *
     * While updating individual stream's local window size, if the window
     * size becomes strictly larger than NGHTTP2_MAX_WINDOW_SIZE,
     * RST_STREAM is issued against such a stream.
     *
     * SETTINGS with :enum:`nghttp2_flag.NGHTTP2_FLAG_ACK` is
     * automatically submitted by the library and application could not
     * send it at its will.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     The |iv| contains invalid value (e.g., initial window size
     *     strictly greater than (1 << 31) - 1.
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     */
    NGHTTP2_EXTERN int nghttp2_submit_settings(nghttp2_session *session,
                                               uint8_t flags,
                                               const nghttp2_settings_entry *iv,
                                               size_t niv);

    /**
     * @function
     *
     * Submits PUSH_PROMISE frame.
     *
     * The |flags| is currently ignored.  The library handles the
     * CONTINUATION frame internally and it correctly sets END_HEADERS to
     * the last sequence of the PUSH_PROMISE or CONTINUATION frame.
     *
     * The |stream_id| must be client initiated stream ID.
     *
     * The |nva| is an array of name/value pair :type:`nghttp2_nv` with
     * |nvlen| elements.  The application is responsible to include
     * required pseudo-header fields (header field whose name starts with
     * ":") in |nva| and must place pseudo-headers before regular header
     * fields.
     *
     * This function creates copies of all name/value pairs in |nva|.  It
     * also lower-cases all names in |nva|.  The order of elements in
     * |nva| is preserved.  For header fields with
     * :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_NAME` and
     * :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_VALUE` are set,
     * header field name and value are not copied respectively.  With
     * :enum:`nghttp2_nv_flag.NGHTTP2_NV_FLAG_NO_COPY_NAME`, application
     * is responsible to pass header field name in lowercase.  The
     * application should maintain the references to them until
     * :type:`nghttp2_on_frame_send_callback` or
     * :type:`nghttp2_on_frame_not_send_callback` is called.
     *
     * The |promised_stream_user_data| is a pointer to an arbitrary data
     * which is associated to the promised stream this frame will open and
     * make it in reserved state.  It is available using
     * `nghttp2_session_get_stream_user_data()`.  The application can
     * access it in :type:`nghttp2_before_frame_send_callback` and
     * :type:`nghttp2_on_frame_send_callback` of this frame.
     *
     * The client side is not allowed to use this function.
     *
     * To submit response headers and data, use
     * `nghttp2_submit_response2()`.
     *
     * This function returns assigned promised stream ID if it succeeds,
     * or one of the following negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_PROTO`
     *     This function was invoked when |session| is initialized as
     *     client.
     * :enum:`nghttp2_error.NGHTTP2_ERR_STREAM_ID_NOT_AVAILABLE`
     *     No stream ID is available because maximum stream ID was
     *     reached.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     The |stream_id| is 0; The |stream_id| does not designate stream
     *     that peer initiated.
     * :enum:`nghttp2_error.NGHTTP2_ERR_STREAM_CLOSED`
     *     The stream was already closed; or the |stream_id| is invalid.
     *
     * .. warning::
     *
     *   This function returns assigned promised stream ID if it succeeds.
     *   As of 1.16.0, stream object for pushed resource is created when
     *   this function succeeds.  In that case, the application can submit
     *   push response for the promised frame.
     *
     *   In 1.15.0 or prior versions, pushed stream is not opened yet when
     *   this function succeeds.  The application must not submit frame to
     *   that stream ID before :type:`nghttp2_before_frame_send_callback`
     *   is called for this frame.
     *
     */
    NGHTTP2_EXTERN int32_t nghttp2_submit_push_promise(
      nghttp2_session *session, uint8_t flags, int32_t stream_id,
      const nghttp2_nv *nva, size_t nvlen, void *promised_stream_user_data);

    /**
     * @function
     *
     * Submits PING frame.  You don't have to send PING back when you
     * received PING frame.  The library automatically submits PING frame
     * in this case.
     *
     * The |flags| is bitwise OR of 0 or more of the following value.
     *
     * * :enum:`nghttp2_flag.NGHTTP2_FLAG_ACK`
     *
     * Unless `nghttp2_option_set_no_auto_ping_ack()` is used, the |flags|
     * should be :enum:`nghttp2_flag.NGHTTP2_FLAG_NONE`.
     *
     * If the |opaque_data| is non ``NULL``, then it should point to the 8
     * bytes array of memory to specify opaque data to send with PING
     * frame.  If the |opaque_data| is ``NULL``, zero-cleared 8 bytes will
     * be sent as opaque data.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     */
    NGHTTP2_EXTERN int nghttp2_submit_ping(nghttp2_session *session, uint8_t flags,
                                           const uint8_t *opaque_data);

    /**
     * @function
     *
     * Submits GOAWAY frame with the last stream ID |last_stream_id| and
     * the error code |error_code|.
     *
     * The pre-defined error code is one of :enum:`nghttp2_error_code`.
     *
     * The |flags| is currently ignored and should be
     * :enum:`nghttp2_flag.NGHTTP2_FLAG_NONE`.
     *
     * The |last_stream_id| is peer's stream ID or 0.  So if |session| is
     * initialized as client, |last_stream_id| must be even or 0.  If
     * |session| is initialized as server, |last_stream_id| must be odd or
     * 0.
     *
     * The HTTP/2 specification says last_stream_id must not be increased
     * from the value previously sent.  So the actual value sent as
     * last_stream_id is the minimum value between the given
     * |last_stream_id| and the last_stream_id previously sent to the
     * peer.
     *
     * If the |opaque_data| is not ``NULL`` and |opaque_data_len| is not
     * zero, those data will be sent as additional debug data.  The
     * library makes a copy of the memory region pointed by |opaque_data|
     * with the length |opaque_data_len|, so the caller does not need to
     * keep this memory after the return of this function.  If the
     * |opaque_data_len| is 0, the |opaque_data| could be ``NULL``.
     *
     * After successful transmission of GOAWAY, following things happen.
     * All incoming streams having strictly more than |last_stream_id| are
     * closed.  All incoming HEADERS which starts new stream are simply
     * ignored.  After all active streams are handled, both
     * `nghttp2_session_want_read()` and `nghttp2_session_want_write()`
     * return 0 and the application can close session.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     The |opaque_data_len| is too large; the |last_stream_id| is
     *     invalid.
     */
    NGHTTP2_EXTERN int nghttp2_submit_goaway(nghttp2_session *session,
                                             uint8_t flags, int32_t last_stream_id,
                                             uint32_t error_code,
                                             const uint8_t *opaque_data,
                                             size_t opaque_data_len);

    /**
     * @function
     *
     * Returns the last stream ID of a stream for which
     * :type:`nghttp2_on_frame_recv_callback` was invoked most recently.
     * The returned value can be used as last_stream_id parameter for
     * `nghttp2_submit_goaway()` and
     * `nghttp2_session_terminate_session2()`.
     *
     * This function always succeeds.
     */
    NGHTTP2_EXTERN int32_t
    nghttp2_session_get_last_proc_stream_id(nghttp2_session *session);

    /**
     * @function
     *
     * Returns nonzero if new request can be sent from local endpoint.
     *
     * This function return 0 if request is not allowed for this session.
     * There are several reasons why request is not allowed.  Some of the
     * reasons are: session is server; stream ID has been spent; GOAWAY
     * has been sent or received.
     *
     * The application can call `nghttp2_submit_request2()` without
     * consulting this function.  In that case,
     * `nghttp2_submit_request2()` may return error.  Or, request is
     * failed to sent, and :type:`nghttp2_on_stream_close_callback` is
     * called.
     */
    NGHTTP2_EXTERN int
    nghttp2_session_check_request_allowed(nghttp2_session *session);

    /**
     * @function
     *
     * Returns nonzero if |session| is initialized as server side session.
     */
    NGHTTP2_EXTERN int
    nghttp2_session_check_server_session(nghttp2_session *session);

    /**
     * @function
     *
     * Submits WINDOW_UPDATE frame.
     *
     * The |flags| is currently ignored and should be
     * :enum:`nghttp2_flag.NGHTTP2_FLAG_NONE`.
     *
     * The |stream_id| is the stream ID to send this WINDOW_UPDATE.  To
     * send connection level WINDOW_UPDATE, specify 0 to |stream_id|.
     *
     * If the |window_size_increment| is positive, the WINDOW_UPDATE with
     * that value as window_size_increment is queued.  If the
     * |window_size_increment| is larger than the received bytes from the
     * remote endpoint, the local window size is increased by that
     * difference.  If the sole purpose is to increase the local window
     * size, consider to use `nghttp2_session_set_local_window_size()`.
     *
     * If the |window_size_increment| is negative, the local window size
     * is decreased by -|window_size_increment|.  If automatic
     * WINDOW_UPDATE is enabled
     * (`nghttp2_option_set_no_auto_window_update()`), and the library
     * decided that the WINDOW_UPDATE should be submitted, then
     * WINDOW_UPDATE is queued with the current received bytes count.  If
     * the sole purpose is to decrease the local window size, consider to
     * use `nghttp2_session_set_local_window_size()`.
     *
     * If the |window_size_increment| is 0, the function does nothing and
     * returns 0.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_FLOW_CONTROL`
     *     The local window size overflow or gets negative.
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     */
    NGHTTP2_EXTERN int nghttp2_submit_window_update(nghttp2_session *session,
                                                    uint8_t flags,
                                                    int32_t stream_id,
                                                    int32_t window_size_increment);

    /**
     * @function
     *
     * Set local window size (local endpoints's window size) to the given
     * |window_size| for the given stream denoted by |stream_id|.  To
     * change connection level window size, specify 0 to |stream_id|.  To
     * increase window size, this function may submit WINDOW_UPDATE frame
     * to transmission queue.
     *
     * The |flags| is currently ignored and should be
     * :enum:`nghttp2_flag.NGHTTP2_FLAG_NONE`.
     *
     * This sounds similar to `nghttp2_submit_window_update()`, but there
     * are 2 differences.  The first difference is that this function
     * takes the absolute value of window size to set, rather than the
     * delta.  To change the window size, this may be easier to use since
     * the application just declares the intended window size, rather than
     * calculating delta.  The second difference is that
     * `nghttp2_submit_window_update()` affects the received bytes count
     * which has not acked yet.  By the specification of
     * `nghttp2_submit_window_update()`, to strictly increase the local
     * window size, we have to submit delta including all received bytes
     * count, which might not be desirable in some cases.  On the other
     * hand, this function does not affect the received bytes count.  It
     * just sets the local window size to the given value.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     The |stream_id| is negative.
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     */
    NGHTTP2_EXTERN int
    nghttp2_session_set_local_window_size(nghttp2_session *session, uint8_t flags,
                                          int32_t stream_id, int32_t window_size);

    /**
     * @function
     *
     * Submits extension frame.
     *
     * Application can pass arbitrary frame flags and stream ID in |flags|
     * and |stream_id| respectively.  The |payload| is opaque pointer, and
     * it can be accessible though ``frame->ext.payload`` in
     * :type:`nghttp2_pack_extension_callback2`.  The library will not own
     * passed |payload| pointer.
     *
     * The application must set :type:`nghttp2_pack_extension_callback2`
     * using `nghttp2_session_callbacks_set_pack_extension_callback2()`.
     *
     * The application should retain the memory pointed by |payload| until
     * the transmission of extension frame is done (which is indicated by
     * :type:`nghttp2_on_frame_send_callback`), or transmission fails
     * (which is indicated by :type:`nghttp2_on_frame_not_send_callback`).
     * If application does not touch this memory region after packing it
     * into a wire format, application can free it inside
     * :type:`nghttp2_pack_extension_callback2`.
     *
     * The standard HTTP/2 frame cannot be sent with this function, so
     * |type| must be strictly grater than 0x9.  Otherwise, this function
     * will fail with error code
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_STATE`
     *     If :type:`nghttp2_pack_extension_callback2` is not set.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     If  |type| specifies  standard  HTTP/2 frame  type.  The  frame
     *     types  in the  rage [0x0,  0x9], both  inclusive, are  standard
     *     HTTP/2 frame type, and cannot be sent using this function.
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory
     */
    NGHTTP2_EXTERN int nghttp2_submit_extension(nghttp2_session *session,
                                                uint8_t type, uint8_t flags,
                                                int32_t stream_id, void *payload);

    /**
     * @struct
     *
     * The payload of ALTSVC frame.  ALTSVC frame is a non-critical
     * extension to HTTP/2.  If this frame is received, and
     * `nghttp2_option_set_user_recv_extension_type()` is not set, and
     * `nghttp2_option_set_builtin_recv_extension_type()` is set for
     * :enum:`nghttp2_frame_type.NGHTTP2_ALTSVC`,
     * ``nghttp2_extension.payload`` will point to this struct.
     *
     * It has the following members:
     */
    typedef struct {
      /**
       * The pointer to origin which this alternative service is
       * associated with.  This is not necessarily NULL-terminated.
       */
      uint8_t *origin;
      /**
       * The length of the |origin|.
       */
      size_t origin_len;
      /**
       * The pointer to Alt-Svc field value contained in ALTSVC frame.
       * This is not necessarily NULL-terminated.
       */
      uint8_t *field_value;
      /**
       * The length of the |field_value|.
       */
      size_t field_value_len;
    } nghttp2_ext_altsvc;

    /**
     * @function
     *
     * Submits ALTSVC frame.
     *
     * ALTSVC frame is a non-critical extension to HTTP/2, and defined in
     * `RFC 7383 <https://tools.ietf.org/html/rfc7838#section-4>`_.
     *
     * The |flags| is currently ignored and should be
     * :enum:`nghttp2_flag.NGHTTP2_FLAG_NONE`.
     *
     * The |origin| points to the origin this alternative service is
     * associated with.  The |origin_len| is the length of the origin.  If
     * |stream_id| is 0, the origin must be specified.  If |stream_id| is
     * not zero, the origin must be empty (in other words, |origin_len|
     * must be 0).
     *
     * The ALTSVC frame is only usable from server side.  If this function
     * is invoked with client side session, this function returns
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_STATE`.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_STATE`
     *     The function is called from client side session
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     The sum of |origin_len| and |field_value_len| is larger than
     *     16382; or |origin_len| is 0 while |stream_id| is 0; or
     *     |origin_len| is not 0 while |stream_id| is not 0.
     */
    NGHTTP2_EXTERN int nghttp2_submit_altsvc(nghttp2_session *session,
                                             uint8_t flags, int32_t stream_id,
                                             const uint8_t *origin,
                                             size_t origin_len,
                                             const uint8_t *field_value,
                                             size_t field_value_len);

    /**
     * @struct
     *
     * The single entry of an origin.
     */
    typedef struct {
      /**
       * The pointer to origin.  No validation is made against this field
       * by the library.  This is not necessarily NULL-terminated.
       */
      uint8_t *origin;
      /**
       * The length of the |origin|.
       */
      size_t origin_len;
    } nghttp2_origin_entry;

    /**
     * @struct
     *
     * The payload of ORIGIN frame.  ORIGIN frame is a non-critical
     * extension to HTTP/2 and defined by `RFC 8336
     * <https://tools.ietf.org/html/rfc8336>`_.
     *
     * If this frame is received, and
     * `nghttp2_option_set_user_recv_extension_type()` is not set, and
     * `nghttp2_option_set_builtin_recv_extension_type()` is set for
     * :enum:`nghttp2_frame_type.NGHTTP2_ORIGIN`,
     * ``nghttp2_extension.payload`` will point to this struct.
     *
     * It has the following members:
     */
    typedef struct {
      /**
       * The number of origins contained in |ov|.
       */
      size_t nov;
      /**
       * The pointer to the array of origins contained in ORIGIN frame.
       */
      nghttp2_origin_entry *ov;
    } nghttp2_ext_origin;

    /**
     * @function
     *
     * Submits ORIGIN frame.
     *
     * ORIGIN frame is a non-critical extension to HTTP/2 and defined by
     * `RFC 8336 <https://tools.ietf.org/html/rfc8336>`_.
     *
     * The |flags| is currently ignored and should be
     * :enum:`nghttp2_flag.NGHTTP2_FLAG_NONE`.
     *
     * The |ov| points to the array of origins.  The |nov| specifies the
     * number of origins included in |ov|.  This function creates copies
     * of all elements in |ov|.
     *
     * The ORIGIN frame is only usable by a server.  If this function is
     * invoked with client side session, this function returns
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_STATE`.
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_STATE`
     *     The function is called from client side session.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     There are too many origins, or an origin is too large to fit
     *     into a default frame payload.
     */
    NGHTTP2_EXTERN int nghttp2_submit_origin(nghttp2_session *session,
                                             uint8_t flags,
                                             const nghttp2_origin_entry *ov,
                                             size_t nov);

    /**
     * @struct
     *
     * The payload of PRIORITY_UPDATE frame.  PRIORITY_UPDATE frame is a
     * non-critical extension to HTTP/2.  If this frame is received, and
     * `nghttp2_option_set_user_recv_extension_type()` is not set, and
     * `nghttp2_option_set_builtin_recv_extension_type()` is set for
     * :enum:`nghttp2_frame_type.NGHTTP2_PRIORITY_UPDATE`,
     * ``nghttp2_extension.payload`` will point to this struct.
     *
     * It has the following members:
     */
    typedef struct {
      /**
       * The stream ID of the stream whose priority is updated.
       */
      int32_t stream_id;
      /**
       * The pointer to Priority field value.  It is not necessarily
       * NULL-terminated.
       */
      uint8_t *field_value;
      /**
       * The length of the :member:`field_value`.
       */
      size_t field_value_len;
    } nghttp2_ext_priority_update;

    /**
     * @function
     *
     * Submits PRIORITY_UPDATE frame.
     *
     * PRIORITY_UPDATE frame is a non-critical extension to HTTP/2, and
     * defined in :rfc:`9218#section-7.1`.
     *
     * The |flags| is currently ignored and should be
     * :enum:`nghttp2_flag.NGHTTP2_FLAG_NONE`.
     *
     * The |stream_id| is the ID of stream which is prioritized.  The
     * |field_value| points to the Priority field value.  The
     * |field_value_len| is the length of the Priority field value.
     *
     * If this function is called by server,
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_STATE` is returned.
     *
     * If
     * :enum:`nghttp2_settings_id.NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES`
     * of value of 0 is received by a remote endpoint (or it is omitted),
     * this function does nothing and returns 0.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_STATE`
     *     The function is called from server side session
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     The |field_value_len| is larger than 16380; or |stream_id| is
     *     0.
     */
    NGHTTP2_EXTERN int nghttp2_submit_priority_update(nghttp2_session *session,
                                                      uint8_t flags,
                                                      int32_t stream_id,
                                                      const uint8_t *field_value,
                                                      size_t field_value_len);

    /**
     * @function
     *
     * Changes the priority of the existing stream denoted by |stream_id|.
     * The new priority is |extpri|.  This function is meant to be used by
     * server for :rfc:`9218` extensible prioritization scheme.
     *
     * If |session| is initialized as client, this function returns
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_STATE`.  For client, use
     * `nghttp2_submit_priority_update()` instead.
     *
     * If :member:`extpri->urgency <nghttp2_extpri.urgency>` is out of
     * bound, it is set to :macro:`NGHTTP2_EXTPRI_URGENCY_LOW`.
     *
     * If |ignore_client_signal| is nonzero, server starts to ignore
     * client priority signals for this stream.
     *
     * If
     * :enum:`nghttp2_settings_id.NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES`
     * of value of 1 is not submitted via `nghttp2_submit_settings()`,
     * this function does nothing and returns 0.
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_STATE`
     *     The |session| is initialized as client.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     |stream_id| is zero; or a stream denoted by |stream_id| is not
     *     found.
     */
    NGHTTP2_EXTERN int nghttp2_session_change_extpri_stream_priority(
      nghttp2_session *session, int32_t stream_id, const nghttp2_extpri *extpri,
      int ignore_client_signal);

    /**
     * @function
     *
     * Stores the stream priority of the existing stream denoted by
     * |stream_id| in the object pointed by |extpri|.  This function is
     * meant to be used by server for :rfc:`9218` extensible
     * prioritization scheme.
     *
     * If |session| is initialized as client, this function returns
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_STATE`.
     *
     * If
     * :enum:`nghttp2_settings_id.NGHTTP2_SETTINGS_NO_RFC7540_PRIORITIES`
     * of value of 1 is not submitted via `nghttp2_submit_settings()`,
     * this function does nothing and returns 0.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_STATE`
     *     The |session| is initialized as client.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     |stream_id| is zero; or a stream denoted by |stream_id| is not
     *     found.
     */
    NGHTTP2_EXTERN int nghttp2_session_get_extpri_stream_priority(
      nghttp2_session *session, nghttp2_extpri *extpri, int32_t stream_id);

    /**
     * @function
     *
     * Parses Priority header field value pointed by |value| of length
     * |len|, and stores the result in the object pointed by |extpri|.
     * Priority header field is defined in :rfc:`9218`.
     *
     * This function does not initialize the object pointed by |extpri|
     * before storing the result.  It only assigns the values that the
     * parser correctly extracted to fields.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_ARGUMENT`
     *     Failed to parse the header field value.
     */
    NGHTTP2_EXTERN int nghttp2_extpri_parse_priority(nghttp2_extpri *extpri,
                                                     const uint8_t *value,
                                                     size_t len);

    /**
     * @function
     *
     * Compares ``lhs->name`` of length ``lhs->namelen`` bytes and
     * ``rhs->name`` of length ``rhs->namelen`` bytes.  Returns negative
     * integer if ``lhs->name`` is found to be less than ``rhs->name``; or
     * returns positive integer if ``lhs->name`` is found to be greater
     * than ``rhs->name``; or returns 0 otherwise.
     */
    NGHTTP2_EXTERN int nghttp2_nv_compare_name(const nghttp2_nv *lhs,
                                               const nghttp2_nv *rhs);

    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  Use `nghttp2_select_alpn` instead.
     *
     * A helper function for dealing with ALPN in server side.  The |in|
     * contains peer's protocol list in preferable order.  The format of
     * |in| is length-prefixed and not null-terminated.  For example,
     * ``h2`` and ``http/1.1`` stored in |in| like this::
     *
     *     in[0] = 2
     *     in[1..2] = "h2"
     *     in[3] = 8
     *     in[4..11] = "http/1.1"
     *     inlen = 12
     *
     * The selection algorithm is as follows:
     *
     * 1. If peer's list contains HTTP/2 protocol the library supports,
     *    it is selected and returns 1. The following step is not taken.
     *
     * 2. If peer's list contains ``http/1.1``, this function selects
     *    ``http/1.1`` and returns 0.  The following step is not taken.
     *
     * 3. This function selects nothing and returns -1 (So called
     *    non-overlap case).  In this case, |out| and |outlen| are left
     *    untouched.
     *
     * Selecting ``h2`` means that ``h2`` is written into |*out| and its
     * length (which is 2) is assigned to |*outlen|.
     *
     * For ALPN, refer to https://tools.ietf.org/html/rfc7301
     *
     * To use this method you should do something like::
     *
     *     static int alpn_select_proto_cb(SSL* ssl,
     *                                     const unsigned char **out,
     *                                     unsigned char *outlen,
     *                                     const unsigned char *in,
     *                                     unsigned int inlen,
     *                                     void *arg)
     *     {
     *         int rv;
     *         rv = nghttp2_select_next_protocol((unsigned char**)out, outlen,
     *                                           in, inlen);
     *         if (rv == -1) {
     *             return SSL_TLSEXT_ERR_NOACK;
     *         }
     *         if (rv == 1) {
     *             ((MyType*)arg)->http2_selected = 1;
     *         }
     *         return SSL_TLSEXT_ERR_OK;
     *     }
     *     ...
     *     SSL_CTX_set_alpn_select_cb(ssl_ctx, alpn_select_proto_cb, my_obj);
     *
     */
    NGHTTP2_EXTERN int nghttp2_select_next_protocol(unsigned char **out,
                                                    unsigned char *outlen,
                                                    const unsigned char *in,
                                                    unsigned int inlen);

    /**
     * @function
     *
     * A helper function for dealing with ALPN in server side.  The |in|
     * contains peer's protocol list in preferable order.  The format of
     * |in| is length-prefixed and not null-terminated.  For example,
     * ``h2`` and ``http/1.1`` stored in |in| like this::
     *
     *     in[0] = 2
     *     in[1..2] = "h2"
     *     in[3] = 8
     *     in[4..11] = "http/1.1"
     *     inlen = 12
     *
     * The selection algorithm is as follows:
     *
     * 1. If peer's list contains HTTP/2 protocol the library supports,
     *    it is selected and returns 1. The following step is not taken.
     *
     * 2. If peer's list contains ``http/1.1``, this function selects
     *    ``http/1.1`` and returns 0.  The following step is not taken.
     *
     * 3. This function selects nothing and returns -1 (So called
     *    non-overlap case).  In this case, |out| and |outlen| are left
     *    untouched.
     *
     * Selecting ``h2`` means that ``h2`` is written into |*out| and its
     * length (which is 2) is assigned to |*outlen|.
     *
     * For ALPN, refer to https://tools.ietf.org/html/rfc7301
     *
     * To use this method you should do something like::
     *
     *     static int alpn_select_proto_cb(SSL* ssl,
     *                                     const unsigned char **out,
     *                                     unsigned char *outlen,
     *                                     const unsigned char *in,
     *                                     unsigned int inlen,
     *                                     void *arg)
     *     {
     *         int rv;
     *         rv = nghttp2_select_alpn(out, outlen, in, inlen);
     *         if (rv == -1) {
     *             return SSL_TLSEXT_ERR_NOACK;
     *         }
     *         if (rv == 1) {
     *             ((MyType*)arg)->http2_selected = 1;
     *         }
     *         return SSL_TLSEXT_ERR_OK;
     *     }
     *     ...
     *     SSL_CTX_set_alpn_select_cb(ssl_ctx, alpn_select_proto_cb, my_obj);
     *
     */
    NGHTTP2_EXTERN int nghttp2_select_alpn(const unsigned char **out,
                                           unsigned char *outlen,
                                           const unsigned char *in,
                                           unsigned int inlen);

    /**
     * @function
     *
     * Returns a pointer to a nghttp2_info struct with version information
     * about the run-time library in use.  The |least_version| argument
     * can be set to a 24 bit numerical value for the least accepted
     * version number and if the condition is not met, this function will
     * return a ``NULL``.  Pass in 0 to skip the version checking.
     */
    NGHTTP2_EXTERN nghttp2_info *nghttp2_version(int least_version);

    /**
     * @function
     *
     * Returns nonzero if the :type:`nghttp2_error` library error code
     * |lib_error| is fatal.
     */
    NGHTTP2_EXTERN int nghttp2_is_fatal(int lib_error_code);

    /**
     * @function
     *
     * Returns nonzero if HTTP header field name |name| of length |len| is
     * valid according to http://tools.ietf.org/html/rfc7230#section-3.2
     *
     * Because this is a header field name in HTTP2, the upper cased alphabet
     * is treated as error.
     */
    NGHTTP2_EXTERN int nghttp2_check_header_name(const uint8_t *name, size_t len);

    /**
     * @function
     *
     * Returns nonzero if HTTP header field value |value| of length |len|
     * is valid according to
     * http://tools.ietf.org/html/rfc7230#section-3.2
     *
     * This function is considered obsolete, and application should
     * consider to use `nghttp2_check_header_value_rfc9113()` instead.
     */
    NGHTTP2_EXTERN int nghttp2_check_header_value(const uint8_t *value, size_t len);

    /**
     * @function
     *
     * Returns nonzero if HTTP header field value |value| of length |len|
     * is valid according to
     * http://tools.ietf.org/html/rfc7230#section-3.2, plus
     * https://datatracker.ietf.org/doc/html/rfc9113#section-8.2.1
     */
    NGHTTP2_EXTERN int nghttp2_check_header_value_rfc9113(const uint8_t *value,
                                                          size_t len);

    /**
     * @function
     *
     * Returns nonzero if the |value| which is supposed to be the value of
     * the :method header field is valid according to
     * https://datatracker.ietf.org/doc/html/rfc7231#section-4 and
     * https://datatracker.ietf.org/doc/html/rfc7230#section-3.2.6
     */
    NGHTTP2_EXTERN int nghttp2_check_method(const uint8_t *value, size_t len);

    /**
     * @function
     *
     * Returns nonzero if the |value| which is supposed to be the value of
     * the :path header field is valid according to
     * https://datatracker.ietf.org/doc/html/rfc7540#section-8.1.2.3
     *
     * |value| is valid if it merely consists of the allowed characters.
     * In particular, it does not check whether |value| follows the syntax
     * of path.  The allowed characters are all characters valid by
     * `nghttp2_check_header_value` minus SPC and HT.
     */
    NGHTTP2_EXTERN int nghttp2_check_path(const uint8_t *value, size_t len);

    /**
     * @function
     *
     * Returns nonzero if the |value| which is supposed to be the value of the
     * :authority or host header field is valid according to
     * https://tools.ietf.org/html/rfc3986#section-3.2
     *
     * Note that :authority and host field values are not authority.  They
     * do not include userinfo in RFC 3986, see
     * https://datatracker.ietf.org/doc/html/rfc3986#section-3.2.2, that
     * is, it does not include '@'.  This function treats '@' as a valid
     * character.
     *
     * |value| is valid if it merely consists of the allowed characters.
     * In particular, it does not check whether |value| follows the syntax
     * of authority.
     */
    NGHTTP2_EXTERN int nghttp2_check_authority(const uint8_t *value, size_t len);

    /* HPACK API */

    struct nghttp2_hd_deflater;

    /**
     * @struct
     *
     * HPACK deflater object.
     */
    typedef struct nghttp2_hd_deflater nghttp2_hd_deflater;

    /**
     * @function
     *
     * Initializes |*deflater_ptr| for deflating name/values pairs.
     *
     * The |max_deflate_dynamic_table_size| is the upper bound of header
     * table size the deflater will use.
     *
     * If this function fails, |*deflater_ptr| is left untouched.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     */
    NGHTTP2_EXTERN int
    nghttp2_hd_deflate_new(nghttp2_hd_deflater **deflater_ptr,
                           size_t max_deflate_dynamic_table_size);

    /**
     * @function
     *
     * Like `nghttp2_hd_deflate_new()`, but with additional custom memory
     * allocator specified in the |mem|.
     *
     * The |mem| can be ``NULL`` and the call is equivalent to
     * `nghttp2_hd_deflate_new()`.
     *
     * This function does not take ownership |mem|.  The application is
     * responsible for freeing |mem|.
     *
     * The library code does not refer to |mem| pointer after this
     * function returns, so the application can safely free it.
     */
    NGHTTP2_EXTERN int
    nghttp2_hd_deflate_new2(nghttp2_hd_deflater **deflater_ptr,
                            size_t max_deflate_dynamic_table_size,
                            nghttp2_mem *mem);

    /**
     * @function
     *
     * Deallocates any resources allocated for |deflater|.
     */
    NGHTTP2_EXTERN void nghttp2_hd_deflate_del(nghttp2_hd_deflater *deflater);

    /**
     * @function
     *
     * Changes header table size of the |deflater| to
     * |settings_max_dynamic_table_size| bytes.  This may trigger eviction
     * in the dynamic table.
     *
     * The |settings_max_dynamic_table_size| should be the value received
     * in SETTINGS_HEADER_TABLE_SIZE.
     *
     * The deflater never uses more memory than
     * ``max_deflate_dynamic_table_size`` bytes specified in
     * `nghttp2_hd_deflate_new()`.  Therefore, if
     * |settings_max_dynamic_table_size| >
     * ``max_deflate_dynamic_table_size``, resulting maximum table size
     * becomes ``max_deflate_dynamic_table_size``.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     */
    NGHTTP2_EXTERN int
    nghttp2_hd_deflate_change_table_size(nghttp2_hd_deflater *deflater,
                                         size_t settings_max_dynamic_table_size);

    #ifndef NGHTTP2_NO_SSIZE_T
    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  Use `nghttp2_hd_deflate_hd2()` instead.
     *
     * Deflates the |nva|, which has the |nvlen| name/value pairs, into
     * the |buf| of length |buflen|.
     *
     * If |buf| is not large enough to store the deflated header block,
     * this function fails with
     * :enum:`nghttp2_error.NGHTTP2_ERR_INSUFF_BUFSIZE`.  The caller
     * should use `nghttp2_hd_deflate_bound()` to know the upper bound of
     * buffer size required to deflate given header name/value pairs.
     *
     * Once this function fails, subsequent call of this function always
     * returns :enum:`nghttp2_error.NGHTTP2_ERR_HEADER_COMP`.
     *
     * After this function returns, it is safe to delete the |nva|.
     *
     * This function returns the number of bytes written to |buf| if it
     * succeeds, or one of the following negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_HEADER_COMP`
     *     Deflation process has failed.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INSUFF_BUFSIZE`
     *     The provided |buflen| size is too small to hold the output.
     */
    NGHTTP2_EXTERN ssize_t nghttp2_hd_deflate_hd(nghttp2_hd_deflater *deflater,
                                                 uint8_t *buf, size_t buflen,
                                                 const nghttp2_nv *nva,
                                                 size_t nvlen);

    #endif /* NGHTTP2_NO_SSIZE_T */

    /**
     * @function
     *
     * Deflates the |nva|, which has the |nvlen| name/value pairs, into
     * the |buf| of length |buflen|.
     *
     * If |buf| is not large enough to store the deflated header block,
     * this function fails with
     * :enum:`nghttp2_error.NGHTTP2_ERR_INSUFF_BUFSIZE`.  The caller
     * should use `nghttp2_hd_deflate_bound()` to know the upper bound of
     * buffer size required to deflate given header name/value pairs.
     *
     * Once this function fails, subsequent call of this function always
     * returns :enum:`nghttp2_error.NGHTTP2_ERR_HEADER_COMP`.
     *
     * After this function returns, it is safe to delete the |nva|.
     *
     * This function returns the number of bytes written to |buf| if it
     * succeeds, or one of the following negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_HEADER_COMP`
     *     Deflation process has failed.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INSUFF_BUFSIZE`
     *     The provided |buflen| size is too small to hold the output.
     */
    NGHTTP2_EXTERN nghttp2_ssize
    nghttp2_hd_deflate_hd2(nghttp2_hd_deflater *deflater, uint8_t *buf,
                           size_t buflen, const nghttp2_nv *nva, size_t nvlen);

    #ifndef NGHTTP2_NO_SSIZE_T
    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  Use `nghttp2_hd_deflate_hd_vec2()` instead.
     *
     * Deflates the |nva|, which has the |nvlen| name/value pairs, into
     * the |veclen| size of buf vector |vec|.  The each size of buffer
     * must be set in len field of :type:`nghttp2_vec`.  If and only if
     * one chunk is filled up completely, next chunk will be used.  If
     * |vec| is not large enough to store the deflated header block, this
     * function fails with
     * :enum:`nghttp2_error.NGHTTP2_ERR_INSUFF_BUFSIZE`.  The caller
     * should use `nghttp2_hd_deflate_bound()` to know the upper bound of
     * buffer size required to deflate given header name/value pairs.
     *
     * Once this function fails, subsequent call of this function always
     * returns :enum:`nghttp2_error.NGHTTP2_ERR_HEADER_COMP`.
     *
     * After this function returns, it is safe to delete the |nva|.
     *
     * This function returns the number of bytes written to |vec| if it
     * succeeds, or one of the following negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_HEADER_COMP`
     *     Deflation process has failed.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INSUFF_BUFSIZE`
     *     The provided |buflen| size is too small to hold the output.
     */
    NGHTTP2_EXTERN ssize_t nghttp2_hd_deflate_hd_vec(nghttp2_hd_deflater *deflater,
                                                     const nghttp2_vec *vec,
                                                     size_t veclen,
                                                     const nghttp2_nv *nva,
                                                     size_t nvlen);

    #endif /* NGHTTP2_NO_SSIZE_T */

    /**
     * @function
     *
     * Deflates the |nva|, which has the |nvlen| name/value pairs, into
     * the |veclen| size of buf vector |vec|.  The each size of buffer
     * must be set in len field of :type:`nghttp2_vec`.  If and only if
     * one chunk is filled up completely, next chunk will be used.  If
     * |vec| is not large enough to store the deflated header block, this
     * function fails with
     * :enum:`nghttp2_error.NGHTTP2_ERR_INSUFF_BUFSIZE`.  The caller
     * should use `nghttp2_hd_deflate_bound()` to know the upper bound of
     * buffer size required to deflate given header name/value pairs.
     *
     * Once this function fails, subsequent call of this function always
     * returns :enum:`nghttp2_error.NGHTTP2_ERR_HEADER_COMP`.
     *
     * After this function returns, it is safe to delete the |nva|.
     *
     * This function returns the number of bytes written to |vec| if it
     * succeeds, or one of the following negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_HEADER_COMP`
     *     Deflation process has failed.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INSUFF_BUFSIZE`
     *     The provided |buflen| size is too small to hold the output.
     */
    NGHTTP2_EXTERN nghttp2_ssize nghttp2_hd_deflate_hd_vec2(
      nghttp2_hd_deflater *deflater, const nghttp2_vec *vec, size_t veclen,
      const nghttp2_nv *nva, size_t nvlen);

    /**
     * @function
     *
     * Returns an upper bound on the compressed size after deflation of
     * |nva| of length |nvlen|.
     */
    NGHTTP2_EXTERN size_t nghttp2_hd_deflate_bound(nghttp2_hd_deflater *deflater,
                                                   const nghttp2_nv *nva,
                                                   size_t nvlen);

    /**
     * @function
     *
     * Returns the number of entries that header table of |deflater|
     * contains.  This is the sum of the number of static table and
     * dynamic table, so the return value is at least 61.
     */
    NGHTTP2_EXTERN
    size_t nghttp2_hd_deflate_get_num_table_entries(nghttp2_hd_deflater *deflater);

    /**
     * @function
     *
     * Returns the table entry denoted by |idx| from header table of
     * |deflater|.  The |idx| is 1-based, and idx=1 returns first entry of
     * static table.  idx=62 returns first entry of dynamic table if it
     * exists.  Specifying idx=0 is error, and this function returns NULL.
     * If |idx| is strictly greater than the number of entries the tables
     * contain, this function returns NULL.
     */
    NGHTTP2_EXTERN
    const nghttp2_nv *
    nghttp2_hd_deflate_get_table_entry(nghttp2_hd_deflater *deflater, size_t idx);

    /**
     * @function
     *
     * Returns the used dynamic table size, including the overhead 32
     * bytes per entry described in RFC 7541.
     */
    NGHTTP2_EXTERN
    size_t nghttp2_hd_deflate_get_dynamic_table_size(nghttp2_hd_deflater *deflater);

    /**
     * @function
     *
     * Returns the maximum dynamic table size.
     */
    NGHTTP2_EXTERN
    size_t
    nghttp2_hd_deflate_get_max_dynamic_table_size(nghttp2_hd_deflater *deflater);

    struct nghttp2_hd_inflater;

    /**
     * @struct
     *
     * HPACK inflater object.
     */
    typedef struct nghttp2_hd_inflater nghttp2_hd_inflater;

    /**
     * @function
     *
     * Initializes |*inflater_ptr| for inflating name/values pairs.
     *
     * If this function fails, |*inflater_ptr| is left untouched.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     */
    NGHTTP2_EXTERN int nghttp2_hd_inflate_new(nghttp2_hd_inflater **inflater_ptr);

    /**
     * @function
     *
     * Like `nghttp2_hd_inflate_new()`, but with additional custom memory
     * allocator specified in the |mem|.
     *
     * The |mem| can be ``NULL`` and the call is equivalent to
     * `nghttp2_hd_inflate_new()`.
     *
     * This function does not take ownership |mem|.  The application is
     * responsible for freeing |mem|.
     *
     * The library code does not refer to |mem| pointer after this
     * function returns, so the application can safely free it.
     */
    NGHTTP2_EXTERN int nghttp2_hd_inflate_new2(nghttp2_hd_inflater **inflater_ptr,
                                               nghttp2_mem *mem);

    /**
     * @function
     *
     * Deallocates any resources allocated for |inflater|.
     */
    NGHTTP2_EXTERN void nghttp2_hd_inflate_del(nghttp2_hd_inflater *inflater);

    /**
     * @function
     *
     * Changes header table size in the |inflater|.  This may trigger
     * eviction in the dynamic table.
     *
     * The |settings_max_dynamic_table_size| should be the value
     * transmitted in SETTINGS_HEADER_TABLE_SIZE.
     *
     * This function must not be called while header block is being
     * inflated.  In other words, this function must be called after
     * initialization of |inflater|, but before calling
     * `nghttp2_hd_inflate_hd3()`, or after
     * `nghttp2_hd_inflate_end_headers()`.  Otherwise,
     * `NGHTTP2_ERR_INVALID_STATE` was returned.
     *
     * This function returns 0 if it succeeds, or one of the following
     * negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_INVALID_STATE`
     *     The function is called while header block is being inflated.
     *     Probably, application missed to call
     *     `nghttp2_hd_inflate_end_headers()`.
     */
    NGHTTP2_EXTERN int
    nghttp2_hd_inflate_change_table_size(nghttp2_hd_inflater *inflater,
                                         size_t settings_max_dynamic_table_size);

    /**
     * @enum
     *
     * The flags for header inflation.
     */
    typedef enum {
      /**
       * No flag set.
       */
      NGHTTP2_HD_INFLATE_NONE = 0,
      /**
       * Indicates all headers were inflated.
       */
      NGHTTP2_HD_INFLATE_FINAL = 0x01,
      /**
       * Indicates a header was emitted.
       */
      NGHTTP2_HD_INFLATE_EMIT = 0x02
    } nghttp2_hd_inflate_flag;

    #ifndef NGHTTP2_NO_SSIZE_T
    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  Use `nghttp2_hd_inflate_hd2()` instead.
     *
     * Inflates name/value block stored in |in| with length |inlen|.  This
     * function performs decompression.  For each successful emission of
     * header name/value pair,
     * :enum:`nghttp2_hd_inflate_flag.NGHTTP2_HD_INFLATE_EMIT` is set in
     * |*inflate_flags| and name/value pair is assigned to the |nv_out|
     * and the function returns.  The caller must not free the members of
     * |nv_out|.
     *
     * The |nv_out| may include pointers to the memory region in the |in|.
     * The caller must retain the |in| while the |nv_out| is used.
     *
     * The application should call this function repeatedly until the
     * ``(*inflate_flags) & NGHTTP2_HD_INFLATE_FINAL`` is nonzero and
     * return value is non-negative.  This means the all input values are
     * processed successfully.  Then the application must call
     * `nghttp2_hd_inflate_end_headers()` to prepare for the next header
     * block input.
     *
     * The caller can feed complete compressed header block.  It also can
     * feed it in several chunks.  The caller must set |in_final| to
     * nonzero if the given input is the last block of the compressed
     * header.
     *
     * This function returns the number of bytes processed if it succeeds,
     * or one of the following negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_HEADER_COMP`
     *     Inflation process has failed.
     * :enum:`nghttp2_error.NGHTTP2_ERR_BUFFER_ERROR`
     *     The header field name or value is too large.
     *
     * Example follows::
     *
     *     int inflate_header_block(nghttp2_hd_inflater *hd_inflater,
     *                              uint8_t *in, size_t inlen, int final)
     *     {
     *         ssize_t rv;
     *
     *         for(;;) {
     *             nghttp2_nv nv;
     *             int inflate_flags = 0;
     *
     *             rv = nghttp2_hd_inflate_hd(hd_inflater, &nv, &inflate_flags,
     *                                        in, inlen, final);
     *
     *             if(rv < 0) {
     *                 fprintf(stderr, "inflate failed with error code %zd", rv);
     *                 return -1;
     *             }
     *
     *             in += rv;
     *             inlen -= rv;
     *
     *             if(inflate_flags & NGHTTP2_HD_INFLATE_EMIT) {
     *                 fwrite(nv.name, nv.namelen, 1, stderr);
     *                 fprintf(stderr, ": ");
     *                 fwrite(nv.value, nv.valuelen, 1, stderr);
     *                 fprintf(stderr, "\n");
     *             }
     *             if(inflate_flags & NGHTTP2_HD_INFLATE_FINAL) {
     *                 nghttp2_hd_inflate_end_headers(hd_inflater);
     *                 break;
     *             }
     *             if((inflate_flags & NGHTTP2_HD_INFLATE_EMIT) == 0 &&
     *                inlen == 0) {
     *                break;
     *             }
     *         }
     *
     *         return 0;
     *     }
     *
     */
    NGHTTP2_EXTERN ssize_t nghttp2_hd_inflate_hd(nghttp2_hd_inflater *inflater,
                                                 nghttp2_nv *nv_out,
                                                 int *inflate_flags, uint8_t *in,
                                                 size_t inlen, int in_final);

    #endif /* NGHTTP2_NO_SSIZE_T */

    #ifndef NGHTTP2_NO_SSIZE_T
    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  Use `nghttp2_hd_inflate_hd3()` instead.
     *
     * Inflates name/value block stored in |in| with length |inlen|.  This
     * function performs decompression.  For each successful emission of
     * header name/value pair,
     * :enum:`nghttp2_hd_inflate_flag.NGHTTP2_HD_INFLATE_EMIT` is set in
     * |*inflate_flags| and name/value pair is assigned to the |nv_out|
     * and the function returns.  The caller must not free the members of
     * |nv_out|.
     *
     * The |nv_out| may include pointers to the memory region in the |in|.
     * The caller must retain the |in| while the |nv_out| is used.
     *
     * The application should call this function repeatedly until the
     * ``(*inflate_flags) & NGHTTP2_HD_INFLATE_FINAL`` is nonzero and
     * return value is non-negative.  If that happens, all given input
     * data (|inlen| bytes) are processed successfully.  Then the
     * application must call `nghttp2_hd_inflate_end_headers()` to prepare
     * for the next header block input.
     *
     * In other words, if |in_final| is nonzero, and this function returns
     * |inlen|, you can assert that
     * :enum:`nghttp2_hd_inflate_final.NGHTTP2_HD_INFLATE_FINAL` is set in
     * |*inflate_flags|.
     *
     * The caller can feed complete compressed header block.  It also can
     * feed it in several chunks.  The caller must set |in_final| to
     * nonzero if the given input is the last block of the compressed
     * header.
     *
     * This function returns the number of bytes processed if it succeeds,
     * or one of the following negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_HEADER_COMP`
     *     Inflation process has failed.
     * :enum:`nghttp2_error.NGHTTP2_ERR_BUFFER_ERROR`
     *     The header field name or value is too large.
     *
     * Example follows::
     *
     *     int inflate_header_block(nghttp2_hd_inflater *hd_inflater,
     *                              uint8_t *in, size_t inlen, int final)
     *     {
     *         ssize_t rv;
     *
     *         for(;;) {
     *             nghttp2_nv nv;
     *             int inflate_flags = 0;
     *
     *             rv = nghttp2_hd_inflate_hd2(hd_inflater, &nv, &inflate_flags,
     *                                         in, inlen, final);
     *
     *             if(rv < 0) {
     *                 fprintf(stderr, "inflate failed with error code %zd", rv);
     *                 return -1;
     *             }
     *
     *             in += rv;
     *             inlen -= rv;
     *
     *             if(inflate_flags & NGHTTP2_HD_INFLATE_EMIT) {
     *                 fwrite(nv.name, nv.namelen, 1, stderr);
     *                 fprintf(stderr, ": ");
     *                 fwrite(nv.value, nv.valuelen, 1, stderr);
     *                 fprintf(stderr, "\n");
     *             }
     *             if(inflate_flags & NGHTTP2_HD_INFLATE_FINAL) {
     *                 nghttp2_hd_inflate_end_headers(hd_inflater);
     *                 break;
     *             }
     *             if((inflate_flags & NGHTTP2_HD_INFLATE_EMIT) == 0 &&
     *                inlen == 0) {
     *                break;
     *             }
     *         }
     *
     *         return 0;
     *     }
     *
     */
    NGHTTP2_EXTERN ssize_t nghttp2_hd_inflate_hd2(nghttp2_hd_inflater *inflater,
                                                  nghttp2_nv *nv_out,
                                                  int *inflate_flags,
                                                  const uint8_t *in, size_t inlen,
                                                  int in_final);

    #endif /* NGHTTP2_NO_SSIZE_T */

    /**
     * @function
     *
     * Inflates name/value block stored in |in| with length |inlen|.  This
     * function performs decompression.  For each successful emission of
     * header name/value pair,
     * :enum:`nghttp2_hd_inflate_flag.NGHTTP2_HD_INFLATE_EMIT` is set in
     * |*inflate_flags| and name/value pair is assigned to the |nv_out|
     * and the function returns.  The caller must not free the members of
     * |nv_out|.
     *
     * The |nv_out| may include pointers to the memory region in the |in|.
     * The caller must retain the |in| while the |nv_out| is used.
     *
     * The application should call this function repeatedly until the
     * ``(*inflate_flags) & NGHTTP2_HD_INFLATE_FINAL`` is nonzero and
     * return value is non-negative.  If that happens, all given input
     * data (|inlen| bytes) are processed successfully.  Then the
     * application must call `nghttp2_hd_inflate_end_headers()` to prepare
     * for the next header block input.
     *
     * In other words, if |in_final| is nonzero, and this function returns
     * |inlen|, you can assert that
     * :enum:`nghttp2_hd_inflate_final.NGHTTP2_HD_INFLATE_FINAL` is set in
     * |*inflate_flags|.
     *
     * The caller can feed complete compressed header block.  It also can
     * feed it in several chunks.  The caller must set |in_final| to
     * nonzero if the given input is the last block of the compressed
     * header.
     *
     * This function returns the number of bytes processed if it succeeds,
     * or one of the following negative error codes:
     *
     * :enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
     *     Out of memory.
     * :enum:`nghttp2_error.NGHTTP2_ERR_HEADER_COMP`
     *     Inflation process has failed.
     * :enum:`nghttp2_error.NGHTTP2_ERR_BUFFER_ERROR`
     *     The header field name or value is too large.
     *
     * Example follows::
     *
     *     int inflate_header_block(nghttp2_hd_inflater *hd_inflater,
     *                              uint8_t *in, size_t inlen, int final)
     *     {
     *         nghttp2_ssize rv;
     *
     *         for(;;) {
     *             nghttp2_nv nv;
     *             int inflate_flags = 0;
     *
     *             rv = nghttp2_hd_inflate_hd3(hd_inflater, &nv, &inflate_flags,
     *                                         in, inlen, final);
     *
     *             if(rv < 0) {
     *                 fprintf(stderr, "inflate failed with error code %td", rv);
     *                 return -1;
     *             }
     *
     *             in += rv;
     *             inlen -= rv;
     *
     *             if(inflate_flags & NGHTTP2_HD_INFLATE_EMIT) {
     *                 fwrite(nv.name, nv.namelen, 1, stderr);
     *                 fprintf(stderr, ": ");
     *                 fwrite(nv.value, nv.valuelen, 1, stderr);
     *                 fprintf(stderr, "\n");
     *             }
     *             if(inflate_flags & NGHTTP2_HD_INFLATE_FINAL) {
     *                 nghttp2_hd_inflate_end_headers(hd_inflater);
     *                 break;
     *             }
     *             if((inflate_flags & NGHTTP2_HD_INFLATE_EMIT) == 0 &&
     *                inlen == 0) {
     *                break;
     *             }
     *         }
     *
     *         return 0;
     *     }
     *
     */
    NGHTTP2_EXTERN nghttp2_ssize nghttp2_hd_inflate_hd3(
      nghttp2_hd_inflater *inflater, nghttp2_nv *nv_out, int *inflate_flags,
      const uint8_t *in, size_t inlen, int in_final);

    /**
     * @function
     *
     * Signals the end of decompression for one header block.
     *
     * This function returns 0 if it succeeds. Currently this function
     * always succeeds.
     */
    NGHTTP2_EXTERN int
    nghttp2_hd_inflate_end_headers(nghttp2_hd_inflater *inflater);

    /**
     * @function
     *
     * Returns the number of entries that header table of |inflater|
     * contains.  This is the sum of the number of static table and
     * dynamic table, so the return value is at least 61.
     */
    NGHTTP2_EXTERN
    size_t nghttp2_hd_inflate_get_num_table_entries(nghttp2_hd_inflater *inflater);

    /**
     * @function
     *
     * Returns the table entry denoted by |idx| from header table of
     * |inflater|.  The |idx| is 1-based, and idx=1 returns first entry of
     * static table.  idx=62 returns first entry of dynamic table if it
     * exists.  Specifying idx=0 is error, and this function returns NULL.
     * If |idx| is strictly greater than the number of entries the tables
     * contain, this function returns NULL.
     */
    NGHTTP2_EXTERN
    const nghttp2_nv *
    nghttp2_hd_inflate_get_table_entry(nghttp2_hd_inflater *inflater, size_t idx);

    /**
     * @function
     *
     * Returns the used dynamic table size, including the overhead 32
     * bytes per entry described in RFC 7541.
     */
    NGHTTP2_EXTERN
    size_t nghttp2_hd_inflate_get_dynamic_table_size(nghttp2_hd_inflater *inflater);

    /**
     * @function
     *
     * Returns the maximum dynamic table size.
     */
    NGHTTP2_EXTERN
    size_t
    nghttp2_hd_inflate_get_max_dynamic_table_size(nghttp2_hd_inflater *inflater);

    struct nghttp2_stream;

    /**
     * @struct
     *
     * The structure to represent HTTP/2 stream.  The details of this
     * structure are intentionally hidden from the public API.
     */
    typedef struct nghttp2_stream nghttp2_stream;

    /**
     * @function
     *
     * Returns pointer to :type:`nghttp2_stream` object denoted by
     * |stream_id|.  If stream was not found, returns NULL.
     *
     * Returns imaginary root stream (see
     * `nghttp2_session_get_root_stream()`) if 0 is given in |stream_id|.
     *
     * Unless |stream_id| == 0, the returned pointer is valid until next
     * call of `nghttp2_session_send()`, `nghttp2_session_mem_send2()`,
     * `nghttp2_session_recv()`, and `nghttp2_session_mem_recv2()`.
     */
    NGHTTP2_EXTERN nghttp2_stream *
    nghttp2_session_find_stream(nghttp2_session *session, int32_t stream_id);

    /**
     * @enum
     *
     * State of stream as described in RFC 7540.
     */
    typedef enum {
      /**
       * idle state.
       */
      NGHTTP2_STREAM_STATE_IDLE = 1,
      /**
       * open state.
       */
      NGHTTP2_STREAM_STATE_OPEN,
      /**
       * reserved (local) state.
       */
      NGHTTP2_STREAM_STATE_RESERVED_LOCAL,
      /**
       * reserved (remote) state.
       */
      NGHTTP2_STREAM_STATE_RESERVED_REMOTE,
      /**
       * half closed (local) state.
       */
      NGHTTP2_STREAM_STATE_HALF_CLOSED_LOCAL,
      /**
       * half closed (remote) state.
       */
      NGHTTP2_STREAM_STATE_HALF_CLOSED_REMOTE,
      /**
       * closed state.
       */
      NGHTTP2_STREAM_STATE_CLOSED
    } nghttp2_stream_proto_state;

    /**
     * @function
     *
     * Returns state of |stream|.  The root stream retrieved by
     * `nghttp2_session_get_root_stream()` will have stream state
     * :enum:`nghttp2_stream_proto_state.NGHTTP2_STREAM_STATE_IDLE`.
     */
    NGHTTP2_EXTERN nghttp2_stream_proto_state
    nghttp2_stream_get_state(nghttp2_stream *stream);

    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  :rfc:`7540` priorities are deprecated by
     *   :rfc:`9113`.  Consider migrating to :rfc:`9218` extensible
     *   prioritization scheme.
     *
     * Returns root of dependency tree, which is imaginary stream with
     * stream ID 0.  The returned pointer is valid until |session| is
     * freed by `nghttp2_session_del()`.
     */
    NGHTTP2_EXTERN nghttp2_stream *
    nghttp2_session_get_root_stream(nghttp2_session *session);

    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  :rfc:`7540` priorities are deprecated by
     *   :rfc:`9113`.  Consider migrating to :rfc:`9218` extensible
     *   prioritization scheme.
     *
     * This function always returns NULL.
     */
    NGHTTP2_EXTERN nghttp2_stream *
    nghttp2_stream_get_parent(nghttp2_stream *stream);

    NGHTTP2_EXTERN int32_t nghttp2_stream_get_stream_id(nghttp2_stream *stream);

    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  :rfc:`7540` priorities are deprecated by
     *   :rfc:`9113`.  Consider migrating to :rfc:`9218` extensible
     *   prioritization scheme.
     *
     * This function always returns NULL.
     */
    NGHTTP2_EXTERN nghttp2_stream *
    nghttp2_stream_get_next_sibling(nghttp2_stream *stream);

    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  :rfc:`7540` priorities are deprecated by
     *   :rfc:`9113`.  Consider migrating to :rfc:`9218` extensible
     *   prioritization scheme.
     *
     * This function always returns NULL.
     */
    NGHTTP2_EXTERN nghttp2_stream *
    nghttp2_stream_get_previous_sibling(nghttp2_stream *stream);

    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  :rfc:`7540` priorities are deprecated by
     *   :rfc:`9113`.  Consider migrating to :rfc:`9218` extensible
     *   prioritization scheme.
     *
     * This function always returns NULL.
     */
    NGHTTP2_EXTERN nghttp2_stream *
    nghttp2_stream_get_first_child(nghttp2_stream *stream);

    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  :rfc:`7540` priorities are deprecated by
     *   :rfc:`9113`.  Consider migrating to :rfc:`9218` extensible
     *   prioritization scheme.
     *
     * This function always returns :macro:`NGHTTP2_DEFAULT_WEIGHT`.
     */
    NGHTTP2_EXTERN int32_t nghttp2_stream_get_weight(nghttp2_stream *stream);

    /**
     * @function
     *
     * .. warning::
     *
     *   Deprecated.  :rfc:`7540` priorities are deprecated by
     *   :rfc:`9113`.  Consider migrating to :rfc:`9218` extensible
     *   prioritization scheme.
     *
     * This function always returns 0.
     */
    NGHTTP2_EXTERN int32_t
    nghttp2_stream_get_sum_dependency_weight(nghttp2_stream *stream);

    /**
     * @functypedef
     *
     * Callback function invoked when the library outputs debug logging.
     * The function is called with arguments suitable for ``vfprintf(3)``
     *
     * The debug output is only enabled if the library is built with
     * ``DEBUGBUILD`` macro defined.
     */
    typedef void (*nghttp2_debug_vprintf_callback)(const char *format,
                                                   va_list args);

    /**
     * @function
     *
     * Sets a debug output callback called by the library when built with
     * ``DEBUGBUILD`` macro defined.  If this option is not used, debug
     * log is written into standard error output.
     *
     * For builds without ``DEBUGBUILD`` macro defined, this function is
     * noop.
     *
     * Note that building with ``DEBUGBUILD`` may cause significant
     * performance penalty to libnghttp2 because of extra processing.  It
     * should be used for debugging purpose only.
     *
     * .. Warning::
     *
     *   Building with ``DEBUGBUILD`` may cause significant performance
     *   penalty to libnghttp2 because of extra processing.  It should be
     *   used for debugging purpose only.  We write this two times because
     *   this is important.
     */
    NGHTTP2_EXTERN void nghttp2_set_debug_vprintf_callback(
      nghttp2_debug_vprintf_callback debug_vprintf_callback);

    #ifdef __cplusplus
    }
    #endif

    #endif /* NGHTTP2_H */