# Tutorial: HTTP/2 client

In this tutorial, we are going to write a very primitive HTTP/2 client. The complete source code, libevent-client.c, is attached at the end of this page. It also resides in the examples directory in the archive or repository.

This simple client takes a single HTTPS URI and retrieves the resource at the URI. The synopsis is:

    $ libevent-client HTTPS_URI

We use libevent in this tutorial to handle networking I/O. Please note that nghttp2 itself does not depend on libevent.

The client starts with some libevent and OpenSSL setup in the `main()` and `run()` functions. This setup isn’t specific to nghttp2, but one thing you should look at is setup of ALPN. Client tells application protocols that it supports to server via ALPN:

    static SSL_CTX *create_ssl_ctx(void) {
      SSL_CTX *ssl_ctx;
      ssl_ctx = SSL_CTX_new(SSLv23_client_method());
      if (!ssl_ctx) {
        errx(1, "Could not create SSL/TLS context: %s",
             ERR_error_string(ERR_get_error(), NULL));
      }
      SSL_CTX_set_options(ssl_ctx,
                          SSL_OP_ALL | SSL_OP_NO_SSLv2 | SSL_OP_NO_SSLv3 |
                              SSL_OP_NO_COMPRESSION |
                              SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION);

      SSL_CTX_set_alpn_protos(ssl_ctx, (const unsigned char *)"\x02h2", 3);

      return ssl_ctx;
    }

Here we see `SSL_CTX_get_alpn_protos()` function call. We instructs OpenSSL to notify the server that we support h2, ALPN identifier for HTTP/2.

The example client defines a couple of structs:

We define and use a `http2_session_data` structure to store data related to the HTTP/2 session:

    typedef struct {
      nghttp2_session *session;
      struct evdns_base *dnsbase;
      struct bufferevent *bev;
      http2_stream_data *stream_data;
    } http2_session_data;

Since this program only handles one URI, it uses only one stream. We store the single stream’s data in a `http2_stream_data` structure and the `stream_data` points to it. The `http2_stream_data` structure is defined as follows:

    typedef struct {
      /* The NULL-terminated URI string to retrieve. */
      const char *uri;
      /* Parsed result of the |uri| */
      urlparse_url *u;
      /* The authority portion of the |uri|, not NULL-terminated */
      char *authority;
      /* The path portion of the |uri|, including query, not
         NULL-terminated */
      char *path;
      /* The length of the |authority| */
      size_t authoritylen;
      /* The length of the |path| */
      size_t pathlen;
      /* The stream ID of this stream */
      int32_t stream_id;
    } http2_stream_data;

We create and initialize these structures in `create_http2_session_data()` and `create_http2_stream_data()` respectively.

`initiate_connection()` is called to start the connection to the remote server. It’s defined as:

    static void initiate_connection(struct event_base *evbase, SSL_CTX *ssl_ctx,
                                    const char *host, uint16_t port,
                                    http2_session_data *session_data) {
      int rv;
      struct bufferevent *bev;
      SSL *ssl;

      ssl = create_ssl(ssl_ctx);
      bev = bufferevent_openssl_socket_new(
          evbase, -1, ssl, BUFFEREVENT_SSL_CONNECTING,
          BEV_OPT_DEFER_CALLBACKS | BEV_OPT_CLOSE_ON_FREE);
      bufferevent_enable(bev, EV_READ | EV_WRITE);
      bufferevent_setcb(bev, readcb, writecb, eventcb, session_data);
      rv = bufferevent_socket_connect_hostname(bev, session_data->dnsbase,
                                               AF_UNSPEC, host, port);

      if (rv != 0) {
        errx(1, "Could not connect to the remote host %s", host);
      }
      session_data->bev = bev;
    }

`initiate_connection()` creates a bufferevent for the connection and sets up three callbacks: `readcb`, `writecb`, and `eventcb`.

The `eventcb()` is invoked by the libevent event loop when an event (e.g. connection has been established, timeout, etc.) occurs on the underlying network socket:

    static void eventcb(struct bufferevent *bev, short events, void *ptr) {
      http2_session_data *session_data = (http2_session_data *)ptr;
      if (events & BEV_EVENT_CONNECTED) {
        int fd = bufferevent_getfd(bev);
        int val = 1;
        const unsigned char *alpn = NULL;
        unsigned int alpnlen = 0;
        SSL *ssl;

        fprintf(stderr, "Connected\n");

        ssl = bufferevent_openssl_get_ssl(session_data->bev);

        SSL_get0_alpn_selected(ssl, &alpn, &alpnlen);

        if (alpn == NULL || alpnlen != 2 || memcmp("h2", alpn, 2) != 0) {
          fprintf(stderr, "h2 is not negotiated\n");
          delete_http2_session_data(session_data);
          return;
        }

        setsockopt(fd, IPPROTO_TCP, TCP_NODELAY, (char *)&val, sizeof(val));
        initialize_nghttp2_session(session_data);
        send_client_connection_header(session_data);
        submit_request(session_data);
        if (session_send(session_data) != 0) {
          delete_http2_session_data(session_data);
        }
        return;
      }
      if (events & BEV_EVENT_EOF) {
        warnx("Disconnected from the remote host");
      } else if (events & BEV_EVENT_ERROR) {
        warnx("Network error");
      } else if (events & BEV_EVENT_TIMEOUT) {
        warnx("Timeout");
      }
      delete_http2_session_data(session_data);
    }

Here we validate that HTTP/2 is negotiated, and if not, drop connection.

For `BEV_EVENT_EOF`, `BEV_EVENT_ERROR`, and `BEV_EVENT_TIMEOUT` events, we just simply tear down the connection.

The `BEV_EVENT_CONNECTED` event is invoked when the SSL/TLS handshake has completed successfully. After this we’re ready to begin communicating via HTTP/2.

The `initialize_nghttp2_session()` function initializes the nghttp2 session object and several callbacks:

    static void initialize_nghttp2_session(http2_session_data *session_data) {
      nghttp2_session_callbacks *callbacks;

      nghttp2_session_callbacks_new(&callbacks);

      nghttp2_session_callbacks_set_send_callback2(callbacks, send_callback);

      nghttp2_session_callbacks_set_on_frame_recv_callback(callbacks,
                                                           on_frame_recv_callback);

      nghttp2_session_callbacks_set_on_data_chunk_recv_callback(
          callbacks, on_data_chunk_recv_callback);

      nghttp2_session_callbacks_set_on_stream_close_callback(
          callbacks, on_stream_close_callback);

      nghttp2_session_callbacks_set_on_header_callback(callbacks,
                                                       on_header_callback);

      nghttp2_session_callbacks_set_on_begin_headers_callback(
          callbacks, on_begin_headers_callback);

      nghttp2_session_client_new(&session_data->session, callbacks, session_data);

      nghttp2_session_callbacks_del(callbacks);
    }

Since we are creating a client, we use `nghttp2_session_client_new()` to initialize the nghttp2 session object. The callbacks setup are explained later.

The `delete_http2_session_data()` function destroys `session_data` and frees its bufferevent, so the underlying connection is closed. It also calls `nghttp2_session_del()` to delete the nghttp2 session object.

A HTTP/2 connection begins by sending the client connection preface, which is a 24 byte magic byte string (`NGHTTP2_CLIENT_MAGIC`), followed by a SETTINGS frame. The 24 byte magic string is sent automatically by nghttp2. We send the SETTINGS frame in `send_client_connection_header()`:

    static void send_client_connection_header(http2_session_data *session_data) {
      nghttp2_settings_entry iv[1] = {
          {NGHTTP2_SETTINGS_MAX_CONCURRENT_STREAMS, 100}};
      int rv;

      /* client 24 bytes magic string will be sent by nghttp2 library */
      rv = nghttp2_submit_settings(session_data->session, NGHTTP2_FLAG_NONE, iv,
                                   ARRLEN(iv));
      if (rv != 0) {
        errx(1, "Could not submit SETTINGS: %s", nghttp2_strerror(rv));
      }
    }

Here we specify SETTINGS_MAX_CONCURRENT_STREAMS as 100. This is not needed for this tiny example program, it just demonstrates use of the SETTINGS frame. To queue the SETTINGS frame for transmission, we call `nghttp2_submit_settings()`. Note that `nghttp2_submit_settings()` only queues the frame for transmission, and doesn’t actually send it. All `nghttp2_submit_*()` family functions have this property. To actually send the frame, `nghttp2_session_send()` has to be called, which is described (and called) later.

After the transmission of the client connection header, we enqueue the HTTP request in the `submit_request()` function:

    static void submit_request(http2_session_data *session_data) {
      int32_t stream_id;
      http2_stream_data *stream_data = session_data->stream_data;
      const char *uri = stream_data->uri;
      const urlparse_url *u = stream_data->u;
      nghttp2_nv hdrs[] = {
          MAKE_NV2(":method", "GET"),
          MAKE_NV(":scheme", &uri[u->field_data[URLPARSE_SCHEMA].off],
                  u->field_data[URLPARSE_SCHEMA].len),
          MAKE_NV(":authority", stream_data->authority, stream_data->authoritylen),
          MAKE_NV(":path", stream_data->path, stream_data->pathlen)};
      fprintf(stderr, "Request headers:\n");
      print_headers(stderr, hdrs, ARRLEN(hdrs));
      stream_id = nghttp2_submit_request2(session_data->session, NULL, hdrs,
                                          ARRLEN(hdrs), NULL, stream_data);
      if (stream_id < 0) {
        errx(1, "Could not submit HTTP request: %s", nghttp2_strerror(stream_id));
      }

      stream_data->stream_id = stream_id;
    }

We build the HTTP request header fields in `hdrs`, which is an array of `nghttp2_nv`. There are four header fields to be sent: `:method`, `:scheme`, `:authority`, and `:path`. To queue the HTTP request, we call `nghttp2_submit_request2()`. The `stream_data` is passed via the stream_user_data parameter, which is helpfully later passed back to callback functions.

`nghttp2_submit_request2()` returns the newly assigned stream ID for the request.

The next bufferevent callback is `readcb()`, which is invoked when data is available to read from the bufferevent input buffer:

    static void readcb(struct bufferevent *bev, void *ptr) {
      http2_session_data *session_data = (http2_session_data *)ptr;
      nghttp2_ssize readlen;
      struct evbuffer *input = bufferevent_get_input(bev);
      size_t datalen = evbuffer_get_length(input);
      unsigned char *data = evbuffer_pullup(input, -1);

      readlen = nghttp2_session_mem_recv2(session_data->session, data, datalen);
      if (readlen < 0) {
        warnx("Fatal error: %s", nghttp2_strerror((int)readlen));
        delete_http2_session_data(session_data);
        return;
      }
      if (evbuffer_drain(input, (size_t)readlen) != 0) {
        warnx("Fatal error: evbuffer_drain failed");
        delete_http2_session_data(session_data);
        return;
      }
      if (session_send(session_data) != 0) {
        delete_http2_session_data(session_data);
        return;
      }
    }

In this function we feed all unprocessed, received data to the nghttp2 session object using the `nghttp2_session_mem_recv2()` function. `nghttp2_session_mem_recv2()` processes the received data and may invoke nghttp2 callbacks and queue frames for transmission. Since there may be pending frames for transmission, we call immediately `session_send()` to send them. `session_send()` is defined as follows:

    static int session_send(http2_session_data *session_data) {
      int rv;

      rv = nghttp2_session_send(session_data->session);
      if (rv != 0) {
        warnx("Fatal error: %s", nghttp2_strerror(rv));
        return -1;
      }
      return 0;
    }

The `nghttp2_session_send()` function serializes pending frames into wire format and calls the `send_callback()` function to send them. `send_callback()` has type `nghttp2_send_callback2` and is defined as:

    static nghttp2_ssize send_callback(nghttp2_session *session _U_,
                                       const uint8_t *data, size_t length,
                                       int flags _U_, void *user_data) {
      http2_session_data *session_data = (http2_session_data *)user_data;
      struct bufferevent *bev = session_data->bev;
      bufferevent_write(bev, data, length);
      return (nghttp2_ssize)length;
    }

Since we use bufferevent to abstract network I/O, we just write the data to the bufferevent object. Note that `nghttp2_session_send()` continues to write all frames queued so far. If we were writing the data to the non-blocking socket directly using the `write()` system call, we’d soon receive an `EAGAIN` or `EWOULDBLOCK` error, since sockets have a limited send buffer. If that happens, it’s possible to return `NGHTTP2_ERR_WOULDBLOCK` to signal the nghttp2 library to stop sending further data. When writing to a bufferevent, you should regulate the amount of data written, to avoid possible huge memory consumption. In this example client however we don’t implement a limit. To see how to regulate the amount of buffered data, see the `send_callback()` in the server tutorial.

The third bufferevent callback is `writecb()`, which is invoked when all data written in the bufferevent output buffer has been sent:

    static void writecb(struct bufferevent *bev _U_, void *ptr) {
      http2_session_data *session_data = (http2_session_data *)ptr;
      if (nghttp2_session_want_read(session_data->session) == 0 &&
          nghttp2_session_want_write(session_data->session) == 0 &&
          evbuffer_get_length(bufferevent_get_output(session_data->bev)) == 0) {
        delete_http2_session_data(session_data);
      }
    }

As described earlier, we just write off all data in `send_callback()`, so there is no data to write in this function. All we have to do is check if the connection should be dropped or not. The nghttp2 session object keeps track of reception and transmission of GOAWAY frames and other error conditions. Using this information, the nghttp2 session object can state whether the connection should be dropped or not. More specifically, when both `nghttp2_session_want_read()` and `nghttp2_session_want_write()` return 0, the connection is no-longer required and can be closed. Since we’re using bufferevent and its deferred callback option, the bufferevent output buffer may still contain pending data when the `writecb()` is called. To handle this situation, we also check whether the output buffer is empty or not. If all of these conditions are met, then we drop the connection.

Now let’s look at the remaining nghttp2 callbacks setup in the `initialize_nghttp2_setup()` function.

A server responds to the request by first sending a HEADERS frame. The HEADERS frame consists of response header name/value pairs, and the `on_header_callback()` is called for each name/value pair:

    static int on_header_callback(nghttp2_session *session _U_,
                                  const nghttp2_frame *frame, const uint8_t *name,
                                  size_t namelen, const uint8_t *value,
                                  size_t valuelen, uint8_t flags _U_,
                                  void *user_data) {
      http2_session_data *session_data = (http2_session_data *)user_data;
      switch (frame->hd.type) {
      case NGHTTP2_HEADERS:
        if (frame->headers.cat == NGHTTP2_HCAT_RESPONSE &&
            session_data->stream_data->stream_id == frame->hd.stream_id) {
          /* Print response headers for the initiated request. */
          print_header(stderr, name, namelen, value, valuelen);
          break;
        }
      }
      return 0;
    }

In this tutorial, we just print the name/value pairs on stderr.

After the HEADERS frame has been fully received (and thus all response header name/value pairs have been received), the `on_frame_recv_callback()` function is called:

    static int on_frame_recv_callback(nghttp2_session *session _U_,
                                      const nghttp2_frame *frame, void *user_data) {
      http2_session_data *session_data = (http2_session_data *)user_data;
      switch (frame->hd.type) {
      case NGHTTP2_HEADERS:
        if (frame->headers.cat == NGHTTP2_HCAT_RESPONSE &&
            session_data->stream_data->stream_id == frame->hd.stream_id) {
          fprintf(stderr, "All headers received\n");
        }
        break;
      }
      return 0;
    }

`on_frame_recv_callback()` is called for other frame types too.

In this tutorial, we are just interested in the HTTP response HEADERS frame. We check the frame type and its category (it should be `NGHTTP2_HCAT_RESPONSE` for HTTP response HEADERS). We also check its stream ID.

Next, zero or more DATA frames can be received. The `on_data_chunk_recv_callback()` function is invoked when a chunk of data is received from the remote peer:

    static int on_data_chunk_recv_callback(nghttp2_session *session _U_,
                                           uint8_t flags _U_, int32_t stream_id,
                                           const uint8_t *data, size_t len,
                                           void *user_data) {
      http2_session_data *session_data = (http2_session_data *)user_data;
      if (session_data->stream_data->stream_id == stream_id) {
        fwrite(data, len, 1, stdout);
      }
      return 0;
    }

In our case, a chunk of data is HTTP response body. After checking the stream ID, we just write the received data to stdout. Note the output in the terminal may be corrupted if the response body contains some binary data.

The `on_stream_close_callback()` function is invoked when the stream is about to close:

    static int on_stream_close_callback(nghttp2_session *session, int32_t stream_id,
                                        nghttp2_error_code error_code,
                                        void *user_data) {
      http2_session_data *session_data = (http2_session_data *)user_data;
      int rv;

      if (session_data->stream_data->stream_id == stream_id) {
        fprintf(stderr, "Stream %d closed with error_code=%d\n", stream_id,
                error_code);
        rv = nghttp2_session_terminate_session(session, NGHTTP2_NO_ERROR);
        if (rv != 0) {
          return NGHTTP2_ERR_CALLBACK_FAILURE;
        }
      }
      return 0;
    }

If the stream ID matches the one we initiated, it means that its stream is going to be closed. Since we have finished receiving resource we wanted (or the stream was reset by RST_STREAM from the remote peer), we call `nghttp2_session_terminate_session()` to commence closure of the HTTP/2 session gracefully. If you have some data associated for the stream to be closed, you may delete it here.

## libevent-client.c

    /*
     * nghttp2 - HTTP/2 C Library
     *
     * Copyright (c) 2013 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.
     */
    #ifdef __sgi
    #  include <string.h>
    #  define errx(exitcode, format, args...)                                      \
        {                                                                          \
          warnx(format, ##args);                                                   \
          exit(exitcode);                                                          \
        }
    #  define warnx(format, args...) fprintf(stderr, format "\n", ##args)
    char *strndup(const char *s, size_t size);
    #endif /* defined(__sgi) */

    #ifdef HAVE_CONFIG_H
    #  include <config.h>
    #endif /* defined(HAVE_CONFIG_H) */

    #include <sys/types.h>
    #ifdef HAVE_UNISTD_H
    #  include <unistd.h>
    #endif /* defined(HAVE_UNISTD_H) */
    #ifdef HAVE_SYS_SOCKET_H
    #  include <sys/socket.h>
    #endif /* defined(HAVE_SYS_SOCKET_H) */
    #ifdef HAVE_NETINET_IN_H
    #  include <netinet/in.h>
    #endif /* defined(HAVE_NETINET_IN_H) */
    #include <netinet/tcp.h>
    #ifndef __sgi
    #  include <err.h>
    #endif /* !defined(__sgi) */
    #include <signal.h>
    #include <string.h>

    #include <openssl/ssl.h>
    #include <openssl/err.h>
    #include <openssl/conf.h>

    #include <event.h>
    #include <event2/event.h>
    #include <event2/bufferevent_ssl.h>
    #include <event2/dns.h>

    #define NGHTTP2_NO_SSIZE_T
    #include <nghttp2/nghttp2.h>

    #include "urlparse.h"

    #define ARRLEN(x) (sizeof(x) / sizeof(x[0]))

    typedef struct {
      /* The NULL-terminated URI string to retrieve. */
      const char *uri;
      /* Parsed result of the |uri| */
      urlparse_url *u;
      /* The authority portion of the |uri|, not NULL-terminated */
      char *authority;
      /* The path portion of the |uri|, including query, not
         NULL-terminated */
      char *path;
      /* The length of the |authority| */
      size_t authoritylen;
      /* The length of the |path| */
      size_t pathlen;
      /* The stream ID of this stream */
      int32_t stream_id;
    } http2_stream_data;

    typedef struct {
      nghttp2_session *session;
      struct evdns_base *dnsbase;
      struct bufferevent *bev;
      http2_stream_data *stream_data;
    } http2_session_data;

    static http2_stream_data *create_http2_stream_data(const char *uri,
                                                       urlparse_url *u) {
      /* MAX 5 digits (max 65535) + 1 ':' + 1 NULL (because of snprintf) */
      size_t extra = 7;
      http2_stream_data *stream_data = malloc(sizeof(http2_stream_data));

      stream_data->uri = uri;
      stream_data->u = u;
      stream_data->stream_id = -1;

      stream_data->authoritylen = u->field_data[URLPARSE_HOST].len;
      stream_data->authority = malloc(stream_data->authoritylen + extra);
      memcpy(stream_data->authority, &uri[u->field_data[URLPARSE_HOST].off],
             u->field_data[URLPARSE_HOST].len);
      if (u->field_set & (1 << URLPARSE_PORT)) {
        stream_data->authoritylen += (size_t)snprintf(
          stream_data->authority + u->field_data[URLPARSE_HOST].len, extra, ":%u",
          u->port);
      }

      /* If we don't have path in URI, we use "/" as path. */
      stream_data->pathlen = 1;
      if (u->field_set & (1 << URLPARSE_PATH)) {
        stream_data->pathlen = u->field_data[URLPARSE_PATH].len;
      }
      if (u->field_set & (1 << URLPARSE_QUERY)) {
        /* +1 for '?' character */
        stream_data->pathlen += (size_t)(u->field_data[URLPARSE_QUERY].len + 1);
      }

      stream_data->path = malloc(stream_data->pathlen);
      if (u->field_set & (1 << URLPARSE_PATH)) {
        memcpy(stream_data->path, &uri[u->field_data[URLPARSE_PATH].off],
               u->field_data[URLPARSE_PATH].len);
      } else {
        stream_data->path[0] = '/';
      }
      if (u->field_set & (1 << URLPARSE_QUERY)) {
        stream_data
          ->path[stream_data->pathlen - u->field_data[URLPARSE_QUERY].len - 1] =
          '?';
        memcpy(stream_data->path + stream_data->pathlen -
                 u->field_data[URLPARSE_QUERY].len,
               &uri[u->field_data[URLPARSE_QUERY].off],
               u->field_data[URLPARSE_QUERY].len);
      }

      return stream_data;
    }

    static void delete_http2_stream_data(http2_stream_data *stream_data) {
      free(stream_data->path);
      free(stream_data->authority);
      free(stream_data);
    }

    /* Initializes |session_data| */
    static http2_session_data *
    create_http2_session_data(struct event_base *evbase) {
      http2_session_data *session_data = malloc(sizeof(http2_session_data));

      memset(session_data, 0, sizeof(http2_session_data));
      session_data->dnsbase = evdns_base_new(evbase, 1);
      return session_data;
    }

    static void delete_http2_session_data(http2_session_data *session_data) {
      SSL *ssl = bufferevent_openssl_get_ssl(session_data->bev);

      if (ssl) {
        SSL_shutdown(ssl);
      }
      bufferevent_free(session_data->bev);
      session_data->bev = NULL;
      evdns_base_free(session_data->dnsbase, 1);
      session_data->dnsbase = NULL;
      nghttp2_session_del(session_data->session);
      session_data->session = NULL;
      if (session_data->stream_data) {
        delete_http2_stream_data(session_data->stream_data);
        session_data->stream_data = NULL;
      }
      free(session_data);
    }

    static void print_header(FILE *f, const uint8_t *name, size_t namelen,
                             const uint8_t *value, size_t valuelen) {
      fwrite(name, 1, namelen, f);
      fprintf(f, ": ");
      fwrite(value, 1, valuelen, f);
      fprintf(f, "\n");
    }

    /* Print HTTP headers to |f|. Please note that this function does not
       take into account that header name and value are sequence of
       octets, therefore they may contain non-printable characters. */
    static void print_headers(FILE *f, nghttp2_nv *nva, size_t nvlen) {
      size_t i;
      for (i = 0; i < nvlen; ++i) {
        print_header(f, nva[i].name, nva[i].namelen, nva[i].value, nva[i].valuelen);
      }
      fprintf(f, "\n");
    }

    /* nghttp2_send_callback2. Here we transmit the |data|, |length|
       bytes, to the network. Because we are using libevent bufferevent,
       we just write those bytes into bufferevent buffer. */
    static nghttp2_ssize send_callback(nghttp2_session *session,
                                       const uint8_t *data, size_t length,
                                       int flags, void *user_data) {
      http2_session_data *session_data = (http2_session_data *)user_data;
      struct bufferevent *bev = session_data->bev;
      (void)session;
      (void)flags;

      bufferevent_write(bev, data, length);
      return (nghttp2_ssize)length;
    }

    /* nghttp2_on_header_callback: Called when nghttp2 library emits
       single header name/value pair. */
    static int 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) {
      http2_session_data *session_data = (http2_session_data *)user_data;
      (void)session;
      (void)flags;

      switch (frame->hd.type) {
      case NGHTTP2_HEADERS:
        if (frame->headers.cat == NGHTTP2_HCAT_RESPONSE &&
            session_data->stream_data->stream_id == frame->hd.stream_id) {
          /* Print response headers for the initiated request. */
          print_header(stderr, name, namelen, value, valuelen);
          break;
        }
      }
      return 0;
    }

    /* nghttp2_on_begin_headers_callback: Called when nghttp2 library gets
       started to receive header block. */
    static int on_begin_headers_callback(nghttp2_session *session,
                                         const nghttp2_frame *frame,
                                         void *user_data) {
      http2_session_data *session_data = (http2_session_data *)user_data;
      (void)session;

      switch (frame->hd.type) {
      case NGHTTP2_HEADERS:
        if (frame->headers.cat == NGHTTP2_HCAT_RESPONSE &&
            session_data->stream_data->stream_id == frame->hd.stream_id) {
          fprintf(stderr, "Response headers for stream ID=%d:\n",
                  frame->hd.stream_id);
        }
        break;
      }
      return 0;
    }

    /* nghttp2_on_frame_recv_callback: Called when nghttp2 library
       received a complete frame from the remote peer. */
    static int on_frame_recv_callback(nghttp2_session *session,
                                      const nghttp2_frame *frame, void *user_data) {
      http2_session_data *session_data = (http2_session_data *)user_data;
      (void)session;

      switch (frame->hd.type) {
      case NGHTTP2_HEADERS:
        if (frame->headers.cat == NGHTTP2_HCAT_RESPONSE &&
            session_data->stream_data->stream_id == frame->hd.stream_id) {
          fprintf(stderr, "All headers received\n");
        }
        break;
      }
      return 0;
    }

    /* nghttp2_on_data_chunk_recv_callback: Called when DATA frame is
       received from the remote peer. In this implementation, if the frame
       is meant to the stream we initiated, print the received data in
       stdout, so that the user can redirect its output to the file
       easily. */
    static int 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) {
      http2_session_data *session_data = (http2_session_data *)user_data;
      (void)session;
      (void)flags;

      if (session_data->stream_data->stream_id == stream_id) {
        fwrite(data, 1, len, stdout);
      }
      return 0;
    }

    /* nghttp2_on_stream_close_callback: Called when a stream is about to
       closed. This example program only deals with 1 HTTP request (1
       stream), if it is closed, we send GOAWAY and tear down the
       session */
    static int on_stream_close_callback(nghttp2_session *session, int32_t stream_id,
                                        uint32_t error_code, void *user_data) {
      http2_session_data *session_data = (http2_session_data *)user_data;
      int rv;

      if (session_data->stream_data->stream_id == stream_id) {
        fprintf(stderr, "Stream %d closed with error_code=%u\n", stream_id,
                error_code);
        rv = nghttp2_session_terminate_session(session, NGHTTP2_NO_ERROR);
        if (rv != 0) {
          return NGHTTP2_ERR_CALLBACK_FAILURE;
        }
      }
      return 0;
    }

    /* Create SSL_CTX. */
    static SSL_CTX *create_ssl_ctx(void) {
      SSL_CTX *ssl_ctx;
      ssl_ctx = SSL_CTX_new(TLS_client_method());
      if (!ssl_ctx) {
        errx(1, "Could not create SSL/TLS context: %s",
             ERR_error_string(ERR_get_error(), NULL));
      }
      SSL_CTX_set_options(ssl_ctx, SSL_OP_ALL | SSL_OP_NO_SSLv2 | SSL_OP_NO_SSLv3 |
                                     SSL_OP_NO_COMPRESSION |
                                     SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION);

      SSL_CTX_set_alpn_protos(ssl_ctx, (const unsigned char *)"\x02h2", 3);

      return ssl_ctx;
    }

    /* Create SSL object */
    static SSL *create_ssl(SSL_CTX *ssl_ctx) {
      SSL *ssl;
      ssl = SSL_new(ssl_ctx);
      if (!ssl) {
        errx(1, "Could not create SSL/TLS session object: %s",
             ERR_error_string(ERR_get_error(), NULL));
      }
      return ssl;
    }

    static void initialize_nghttp2_session(http2_session_data *session_data) {
      nghttp2_session_callbacks *callbacks;

      nghttp2_session_callbacks_new(&callbacks);

      nghttp2_session_callbacks_set_send_callback2(callbacks, send_callback);

      nghttp2_session_callbacks_set_on_frame_recv_callback(callbacks,
                                                           on_frame_recv_callback);

      nghttp2_session_callbacks_set_on_data_chunk_recv_callback(
        callbacks, on_data_chunk_recv_callback);

      nghttp2_session_callbacks_set_on_stream_close_callback(
        callbacks, on_stream_close_callback);

      nghttp2_session_callbacks_set_on_header_callback(callbacks,
                                                       on_header_callback);

      nghttp2_session_callbacks_set_on_begin_headers_callback(
        callbacks, on_begin_headers_callback);

      nghttp2_session_client_new(&session_data->session, callbacks, session_data);

      nghttp2_session_callbacks_del(callbacks);
    }

    static void send_client_connection_header(http2_session_data *session_data) {
      nghttp2_settings_entry iv[1] = {
        {NGHTTP2_SETTINGS_MAX_CONCURRENT_STREAMS, 100}};
      int rv;

      /* client 24 bytes magic string will be sent by nghttp2 library */
      rv = nghttp2_submit_settings(session_data->session, NGHTTP2_FLAG_NONE, iv,
                                   ARRLEN(iv));
      if (rv != 0) {
        errx(1, "Could not submit SETTINGS: %s", nghttp2_strerror(rv));
      }
    }

    #define MAKE_NV(NAME, VALUE, VALUELEN)                                         \
      {                                                                            \
        (uint8_t *)NAME, (uint8_t *)VALUE,     sizeof(NAME) - 1,                   \
        VALUELEN,        NGHTTP2_NV_FLAG_NONE,                                     \
      }

    #define MAKE_NV2(NAME, VALUE)                                                  \
      {                                                                            \
        (uint8_t *)NAME,   (uint8_t *)VALUE,     sizeof(NAME) - 1,                 \
        sizeof(VALUE) - 1, NGHTTP2_NV_FLAG_NONE,                                   \
      }

    /* Send HTTP request to the remote peer */
    static void submit_request(http2_session_data *session_data) {
      int32_t stream_id;
      http2_stream_data *stream_data = session_data->stream_data;
      const char *uri = stream_data->uri;
      const urlparse_url *u = stream_data->u;
      nghttp2_nv hdrs[] = {
        MAKE_NV2(":method", "GET"),
        MAKE_NV(":scheme", &uri[u->field_data[URLPARSE_SCHEMA].off],
                u->field_data[URLPARSE_SCHEMA].len),
        MAKE_NV(":authority", stream_data->authority, stream_data->authoritylen),
        MAKE_NV(":path", stream_data->path, stream_data->pathlen)};
      fprintf(stderr, "Request headers:\n");
      print_headers(stderr, hdrs, ARRLEN(hdrs));
      stream_id = nghttp2_submit_request2(session_data->session, NULL, hdrs,
                                          ARRLEN(hdrs), NULL, stream_data);
      if (stream_id < 0) {
        errx(1, "Could not submit HTTP request: %s", nghttp2_strerror(stream_id));
      }

      stream_data->stream_id = stream_id;
    }

    /* Serialize the frame and send (or buffer) the data to
       bufferevent. */
    static int session_send(http2_session_data *session_data) {
      int rv;

      rv = nghttp2_session_send(session_data->session);
      if (rv != 0) {
        warnx("Fatal error: %s", nghttp2_strerror(rv));
        return -1;
      }
      return 0;
    }

    /* readcb for bufferevent. Here we get the data from the input buffer
       of bufferevent and feed them to nghttp2 library. This may invoke
       nghttp2 callbacks. It may also queues the frame in nghttp2 session
       context. To send them, we call session_send() in the end. */
    static void readcb(struct bufferevent *bev, void *ptr) {
      http2_session_data *session_data = (http2_session_data *)ptr;
      nghttp2_ssize readlen;
      struct evbuffer *input = bufferevent_get_input(bev);
      size_t datalen = evbuffer_get_length(input);
      unsigned char *data = evbuffer_pullup(input, -1);

      readlen = nghttp2_session_mem_recv2(session_data->session, data, datalen);
      if (readlen < 0) {
        warnx("Fatal error: %s", nghttp2_strerror((int)readlen));
        delete_http2_session_data(session_data);
        return;
      }
      if (evbuffer_drain(input, (size_t)readlen) != 0) {
        warnx("Fatal error: evbuffer_drain failed");
        delete_http2_session_data(session_data);
        return;
      }
      if (session_send(session_data) != 0) {
        delete_http2_session_data(session_data);
        return;
      }
    }

    /* writecb for bufferevent. To greaceful shutdown after sending or
       receiving GOAWAY, we check the some conditions on the nghttp2
       library and output buffer of bufferevent. If it indicates we have
       no business to this session, tear down the connection. */
    static void writecb(struct bufferevent *bev, void *ptr) {
      http2_session_data *session_data = (http2_session_data *)ptr;
      (void)bev;

      if (nghttp2_session_want_read(session_data->session) == 0 &&
          nghttp2_session_want_write(session_data->session) == 0 &&
          evbuffer_get_length(bufferevent_get_output(session_data->bev)) == 0) {
        delete_http2_session_data(session_data);
      }
    }

    /* eventcb for bufferevent. For the purpose of simplicity and
       readability of the example program, we omitted the certificate and
       peer verification. After SSL/TLS handshake is over, initialize
       nghttp2 library session, and send client connection header. Then
       send HTTP request. */
    static void eventcb(struct bufferevent *bev, short events, void *ptr) {
      http2_session_data *session_data = (http2_session_data *)ptr;
      if (events & BEV_EVENT_CONNECTED) {
        int fd = bufferevent_getfd(bev);
        int val = 1;
        const unsigned char *alpn = NULL;
        unsigned int alpnlen = 0;
        SSL *ssl;

        fprintf(stderr, "Connected\n");

        ssl = bufferevent_openssl_get_ssl(session_data->bev);

        if (alpn == NULL) {
          SSL_get0_alpn_selected(ssl, &alpn, &alpnlen);
        }

        if (alpn == NULL || alpnlen != 2 || memcmp("h2", alpn, 2) != 0) {
          fprintf(stderr, "h2 is not negotiated\n");
          delete_http2_session_data(session_data);
          return;
        }

        setsockopt(fd, IPPROTO_TCP, TCP_NODELAY, (char *)&val, sizeof(val));
        initialize_nghttp2_session(session_data);
        send_client_connection_header(session_data);
        submit_request(session_data);
        if (session_send(session_data) != 0) {
          delete_http2_session_data(session_data);
        }
        return;
      }
      if (events & BEV_EVENT_EOF) {
        warnx("Disconnected from the remote host");
      } else if (events & BEV_EVENT_ERROR) {
        warnx("Network error");
      } else if (events & BEV_EVENT_TIMEOUT) {
        warnx("Timeout");
      }
      delete_http2_session_data(session_data);
    }

    /* Start connecting to the remote peer |host:port| */
    static void initiate_connection(struct event_base *evbase, SSL_CTX *ssl_ctx,
                                    const char *host, uint16_t port,
                                    http2_session_data *session_data) {
      int rv;
      struct bufferevent *bev;
      SSL *ssl;

      ssl = create_ssl(ssl_ctx);
      bev = bufferevent_openssl_socket_new(
        evbase, -1, ssl, BUFFEREVENT_SSL_CONNECTING,
        BEV_OPT_DEFER_CALLBACKS | BEV_OPT_CLOSE_ON_FREE);
      bufferevent_enable(bev, EV_READ | EV_WRITE);
      bufferevent_setcb(bev, readcb, writecb, eventcb, session_data);
      rv = bufferevent_socket_connect_hostname(bev, session_data->dnsbase,
                                               AF_UNSPEC, host, port);

      if (rv != 0) {
        errx(1, "Could not connect to the remote host %s", host);
      }
      session_data->bev = bev;
    }

    /* Get resource denoted by the |uri|. The debug and error messages are
       printed in stderr, while the response body is printed in stdout. */
    static void run(const char *uri) {
      urlparse_url u;
      char *host;
      uint16_t port;
      int rv;
      SSL_CTX *ssl_ctx;
      struct event_base *evbase;
      http2_session_data *session_data;

      /* Parse the |uri| and stores its components in |u| */
      rv = urlparse_parse_url(uri, strlen(uri), 0, &u);
      if (rv != 0) {
        errx(1, "Could not parse URI %s", uri);
      }
      host = strndup(&uri[u.field_data[URLPARSE_HOST].off],
                     u.field_data[URLPARSE_HOST].len);
      if (!(u.field_set & (1 << URLPARSE_PORT))) {
        port = 443;
      } else {
        port = u.port;
      }

      ssl_ctx = create_ssl_ctx();

      evbase = event_base_new();

      session_data = create_http2_session_data(evbase);
      session_data->stream_data = create_http2_stream_data(uri, &u);

      initiate_connection(evbase, ssl_ctx, host, port, session_data);
      free(host);
      host = NULL;

      event_base_loop(evbase, 0);

      event_base_free(evbase);
      SSL_CTX_free(ssl_ctx);
    }

    int main(int argc, char **argv) {
      struct sigaction act;

      if (argc < 2) {
        fprintf(stderr, "Usage: libevent-client HTTPS_URI\n");
        exit(EXIT_FAILURE);
      }

      memset(&act, 0, sizeof(struct sigaction));
      act.sa_handler = SIG_IGN;
      sigaction(SIGPIPE, &act, NULL);

      run(argv[1]);
      return 0;
    }