# Tutorial: HTTP/2 server

In this tutorial, we are going to write a single-threaded, event-based HTTP/2 web server, which supports HTTPS only. It can handle concurrent multiple requests, but only the GET method is supported. The complete source code, libevent-server.c, is attached at the end of this page. The source also resides in the examples directory in the archive or repository.

This simple server takes 3 arguments: The port number to listen on, the path to your SSL/TLS private key file, and the path to your certificate file. The synopsis is:

    $ libevent-server PORT /path/to/server.key /path/to/server.crt

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

The server 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 callback. The ALPN callback is used by the server to select application protocols offered by client. In ALPN, client sends the list of supported application protocols, and server selects one of them. We provide the callback for it:

    static int alpn_select_proto_cb(SSL *ssl _U_, const unsigned char **out,
                                    unsigned char *outlen, const unsigned char *in,
                                    unsigned int inlen, void *arg _U_) {
      int rv;

      rv = nghttp2_select_alpn(out, outlen, in, inlen);

      if (rv != 1) {
        return SSL_TLSEXT_ERR_NOACK;
      }

      return SSL_TLSEXT_ERR_OK;
    }

    static SSL_CTX *create_ssl_ctx(const char *key_file, const char *cert_file) {
      SSL_CTX *ssl_ctx;
      EC_KEY *ecdh;

      ssl_ctx = SSL_CTX_new(SSLv23_server_method());

      ...

      SSL_CTX_set_alpn_select_cb(ssl_ctx, alpn_select_proto_cb, NULL);

      return ssl_ctx;
    }

In `alpn_select_proto_cb()`, we use `nghttp2_select_alpn()` to select application protocol. The `nghttp2_select_alpn()` returns 1 only if it selected h2 (ALPN identifier for HTTP/2), and out parameters were assigned accordingly.

Next, let's take a look at the main structures used by the example application:

We use the `app_context` structure to store application-wide data:

    struct app_context {
      SSL_CTX *ssl_ctx;
      struct event_base *evbase;
    };

We use the `http2_session_data` structure to store session-level (which corresponds to one HTTP/2 connection) data:

    typedef struct http2_session_data {
      struct http2_stream_data root;
      struct bufferevent *bev;
      app_context *app_ctx;
      nghttp2_session *session;
      char *client_addr;
    } http2_session_data;

We use the `http2_stream_data` structure to store stream-level data:

    typedef struct http2_stream_data {
      struct http2_stream_data *prev, *next;
      char *request_path;
      int32_t stream_id;
      int fd;
    } http2_stream_data;

A single HTTP/2 session can have multiple streams. To manage them, we use a doubly linked list: The first element of this list is pointed to by the `root->next` in `http2_session_data`. Initially, `root->next` is `NULL`.

libevent's bufferevent structure is used to perform network I/O, with the pointer to the bufferevent stored in the `http2_session_data` structure. Note that the bufferevent object is kept in `http2_session_data` and not in `http2_stream_data`. This is because `http2_stream_data` is just a logical stream multiplexed over the single connection managed by the bufferevent in `http2_session_data`.

We first create a listener object to accept incoming connections. libevent's `struct evconnlistener` is used for this purpose:

    static void start_listen(struct event_base *evbase, const char *service,
                             app_context *app_ctx) {
      int rv;
      struct addrinfo hints;
      struct addrinfo *res, *rp;

      memset(&hints, 0, sizeof(hints));
      hints.ai_family = AF_UNSPEC;
      hints.ai_socktype = SOCK_STREAM;
      hints.ai_flags = AI_PASSIVE;
    #ifdef AI_ADDRCONFIG
      hints.ai_flags |= AI_ADDRCONFIG;
    #endif /* AI_ADDRCONFIG */

      rv = getaddrinfo(NULL, service, &hints, &res);
      if (rv != 0) {
        errx(1, NULL);
      }
      for (rp = res; rp; rp = rp->ai_next) {
        struct evconnlistener *listener;
        listener = evconnlistener_new_bind(
            evbase, acceptcb, app_ctx, LEV_OPT_CLOSE_ON_FREE | LEV_OPT_REUSEABLE,
            16, rp->ai_addr, (int)rp->ai_addrlen);
        if (listener) {
          freeaddrinfo(res);

          return;
        }
      }
      errx(1, "Could not start listener");
    }

We specify the `acceptcb` callback, which is called when a new connection is accepted:

    static void acceptcb(struct evconnlistener *listener _U_, int fd,
                         struct sockaddr *addr, int addrlen, void *arg) {
      app_context *app_ctx = (app_context *)arg;
      http2_session_data *session_data;

      session_data = create_http2_session_data(app_ctx, fd, addr, addrlen);

      bufferevent_setcb(session_data->bev, readcb, writecb, eventcb, session_data);
    }

Here we create the `http2_session_data` object. The connection's bufferevent is initialized at the same time. We specify three callbacks for the bufferevent: `readcb`, `writecb`, and `eventcb`.

The `eventcb()` callback 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 _U_, short events, void *ptr) {
      http2_session_data *session_data = (http2_session_data *)ptr;
      if (events & BEV_EVENT_CONNECTED) {
        const unsigned char *alpn = NULL;
        unsigned int alpnlen = 0;
        SSL *ssl;

        fprintf(stderr, "%s connected\n", session_data->client_addr);

        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, "%s h2 is not negotiated\n", session_data->client_addr);
          delete_http2_session_data(session_data);
          return;
        }

        initialize_nghttp2_session(session_data);

        if (send_server_connection_header(session_data) != 0 ||
            session_send(session_data) != 0) {
          delete_http2_session_data(session_data);
          return;
        }

        return;
      }
      if (events & BEV_EVENT_EOF) {
        fprintf(stderr, "%s EOF\n", session_data->client_addr);
      } else if (events & BEV_EVENT_ERROR) {
        fprintf(stderr, "%s network error\n", session_data->client_addr);
      } else if (events & BEV_EVENT_TIMEOUT) {
        fprintf(stderr, "%s timeout\n", session_data->client_addr);
      }
      delete_http2_session_data(session_data);
    }

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

For the `BEV_EVENT_EOF`, `BEV_EVENT_ERROR`, and `BEV_EVENT_TIMEOUT` events, we just simply tear down the connection. The `delete_http2_session_data()` function destroys the `http2_session_data` object and its associated bufferevent member. As a result, the underlying connection is closed.

The `BEV_EVENT_CONNECTED` event is invoked when SSL/TLS handshake has completed successfully. After this we are 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_callback(callbacks, send_callback);

      nghttp2_session_callbacks_set_on_frame_recv_callback(callbacks,
                                                           on_frame_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_server_new(&session_data->session, callbacks, session_data);

      nghttp2_session_callbacks_del(callbacks);
    }

Since we are creating a server, we use `nghttp2_session_server_new()` to initialize the nghttp2 session object. We also setup 5 callbacks for the nghttp2 session, these are explained later.

The server now begins by sending the server connection preface, which always consists of a SETTINGS frame. `send_server_connection_header()` configures and submits it:

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

      rv = nghttp2_submit_settings(session_data->session, NGHTTP2_FLAG_NONE, iv,
                                   ARRLEN(iv));
      if (rv != 0) {
        warnx("Fatal error: %s", nghttp2_strerror(rv));
        return -1;
      }
      return 0;
    }

In the example SETTINGS frame we've set SETTINGS_MAX_CONCURRENT_STREAMS to 100. `nghttp2_submit_settings()` is used to queue the frame for transmission, but note it only queues the frame for transmission, and doesn't actually send it. All functions in the `nghttp2_submit_*()` family have this property. To actually send the frame, `nghttp2_session_send()` should be used, as described later.

Since bufferevent may buffer more than the first 24 bytes from the client, we have to process them here since libevent won't invoke callback functions for this pending data. To process the received data, we call the `session_recv()` function:

    static int session_recv(http2_session_data *session_data) {
      ssize_t readlen;
      struct evbuffer *input = bufferevent_get_input(session_data->bev);
      size_t datalen = evbuffer_get_length(input);
      unsigned char *data = evbuffer_pullup(input, -1);

      readlen = nghttp2_session_mem_recv(session_data->session, data, datalen);
      if (readlen < 0) {
        warnx("Fatal error: %s", nghttp2_strerror((int)readlen));
        return -1;
      }
      if (evbuffer_drain(input, (size_t)readlen) != 0) {
        warnx("Fatal error: evbuffer_drain failed");
        return -1;
      }
      if (session_send(session_data) != 0) {
        return -1;
      }
      return 0;
    }

In this function, we feed all unprocessed but already received data to the nghttp2 session object using the `nghttp2_session_mem_recv()` function. The `nghttp2_session_mem_recv()` function processes the data and may both invoke the previously setup callbacks and also queue outgoing frames. To send any pending outgoing frames, we immediately call `session_send()`.

The `session_send()` function 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 the frame into wire format and calls the `send_callback()`, which is of type `nghttp2_send_callback`. The `send_callback()` is defined as follows:

    static ssize_t 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;
      /* Avoid excessive buffering in server side. */
      if (evbuffer_get_length(bufferevent_get_output(session_data->bev)) >=
          OUTPUT_WOULDBLOCK_THRESHOLD) {
        return NGHTTP2_ERR_WOULDBLOCK;
      }
      bufferevent_write(bev, data, length);
      return (ssize_t)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 a non-blocking socket directly using the `write()` system call in the `send_callback()`, 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. But here, when writing to the bufferevent, we have to regulate the amount data to buffered ourselves to avoid using huge amounts of memory. To achieve this, we check the size of the output buffer and if it reaches more than or equal to `OUTPUT_WOULDBLOCK_THRESHOLD` bytes, we stop writing data and return `NGHTTP2_ERR_WOULDBLOCK`.

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

    static void readcb(struct bufferevent *bev _U_, void *ptr) {
      http2_session_data *session_data = (http2_session_data *)ptr;
      if (session_recv(session_data) != 0) {
        delete_http2_session_data(session_data);
        return;
      }
    }

In this function, we just call `session_recv()` to process incoming data.

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

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

First we check whether we should drop the connection or not. The nghttp2 session object keeps track of reception and transmission of GOAWAY frames and other error conditions as well. Using this information, the nghttp2 session object can state whether the connection should be dropped or not. More specifically, if both `nghttp2_session_want_read()` and `nghttp2_session_want_write()` return 0, the connection is no-longer required and can be closed. Since we are using bufferevent and its deferred callback option, the bufferevent output buffer may still contain pending data when the `writecb()` is called. To handle this, we check whether the output buffer is empty or not. If all of these conditions are met, we drop connection.

Otherwise, we call `session_send()` to process the pending output data. Remember that in `send_callback()`, we must not write all data to bufferevent to avoid excessive buffering. We continue processing pending data when the output buffer becomes empty.

We have already described the nghttp2 callback `send_callback()`. Let's learn about the remaining nghttp2 callbacks setup in `initialize_nghttp2_setup()` function.

The `on_begin_headers_callback()` function is invoked when the reception of a header block in HEADERS or PUSH_PROMISE frame is started:

    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;
      http2_stream_data *stream_data;

      if (frame->hd.type != NGHTTP2_HEADERS ||
          frame->headers.cat != NGHTTP2_HCAT_REQUEST) {
        return 0;
      }
      stream_data = create_http2_stream_data(session_data, frame->hd.stream_id);
      nghttp2_session_set_stream_user_data(session, frame->hd.stream_id,
                                           stream_data);
      return 0;
    }

We are only interested in the HEADERS frame in this function. Since the HEADERS frame has several roles in the HTTP/2 protocol, we check that it is a request HEADERS, which opens new stream. If the frame is a request HEADERS, we create a `http2_stream_data` object to store the stream related data. We associate the created `http2_stream_data` object with the stream in the nghttp2 session object using `nghttp2_set_stream_user_data()`. The `http2_stream_data` object can later be easily retrieved from the stream, without searching through the doubly linked list.

In this example server, we want to serve files relative to the current working directory in which the program was invoked. Each header name/value pair is emitted via `on_header_callback` function, which is called after `on_begin_headers_callback()`:

    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 _U_,
                                  void *user_data _U_) {
      http2_stream_data *stream_data;
      const char PATH[] = ":path";
      switch (frame->hd.type) {
      case NGHTTP2_HEADERS:
        if (frame->headers.cat != NGHTTP2_HCAT_REQUEST) {
          break;
        }
        stream_data =
            nghttp2_session_get_stream_user_data(session, frame->hd.stream_id);
        if (!stream_data || stream_data->request_path) {
          break;
        }
        if (namelen == sizeof(PATH) - 1 && memcmp(PATH, name, namelen) == 0) {
          size_t j;
          for (j = 0; j < valuelen && value[j] != '?'; ++j)
            ;
          stream_data->request_path = percent_decode(value, j);
        }
        break;
      }
      return 0;
    }

We search for the `:path` header field among the request headers and store the requested path in the `http2_stream_data` object. In this example program, we ignore the `:method` header field and always treat the request as a GET request.

The `on_frame_recv_callback()` function is invoked when a frame is fully received:

    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;
      http2_stream_data *stream_data;
      switch (frame->hd.type) {
      case NGHTTP2_DATA:
      case NGHTTP2_HEADERS:
        /* Check that the client request has finished */
        if (frame->hd.flags & NGHTTP2_FLAG_END_STREAM) {
          stream_data =
              nghttp2_session_get_stream_user_data(session, frame->hd.stream_id);
          /* For DATA and HEADERS frame, this callback may be called after
             on_stream_close_callback. Check that stream still alive. */
          if (!stream_data) {
            return 0;
          }
          return on_request_recv(session, session_data, stream_data);
        }
        break;
      default:
        break;
      }
      return 0;
    }

First we retrieve the `http2_stream_data` object associated with the stream in `on_begin_headers_callback()` using `nghttp2_session_get_stream_user_data()`. If the requested path cannot be served for some reason (e.g. file is not found), we send a 404 response using `error_reply()`. Otherwise, we open the requested file and send its content. We send the header field `:status` as a single response header.

Sending the file content is performed by the `send_response()` function:

    static int send_response(nghttp2_session *session, int32_t stream_id,
                             nghttp2_nv *nva, size_t nvlen, int fd) {
      int rv;
      nghttp2_data_provider data_prd;
      data_prd.source.fd = fd;
      data_prd.read_callback = file_read_callback;

      rv = nghttp2_submit_response(session, stream_id, nva, nvlen, &data_prd);
      if (rv != 0) {
        warnx("Fatal error: %s", nghttp2_strerror(rv));
        return -1;
      }
      return 0;
    }

nghttp2 uses the `nghttp2_data_provider` structure to send the entity body to the remote peer. The `source` member of this structure is a union, which can be either a void pointer or an int (which is intended to be used as file descriptor). In this example server, we use it as a file descriptor. We also set the `file_read_callback()` callback function to read the contents of the file:

    static ssize_t file_read_callback(nghttp2_session *session _U_,
                                      int32_t stream_id _U_, uint8_t *buf,
                                      size_t length, uint32_t *data_flags,
                                      nghttp2_data_source *source,
                                      void *user_data _U_) {
      int fd = source->fd;
      ssize_t r;
      while ((r = read(fd, buf, length)) == -1 && errno == EINTR)
        ;
      if (r == -1) {
        return NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE;
      }
      if (r == 0) {
        *data_flags |= NGHTTP2_DATA_FLAG_EOF;
      }
      return r;
    }

If an error occurs while reading the file, we return `NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE`. This tells the library to send RST_STREAM to the stream. When all data has been read, the `NGHTTP2_DATA_FLAG_EOF` flag is set to signal nghttp2 that we have finished reading the file.

The `nghttp2_submit_response()` function is used to send the response to the remote peer.

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,
                                        uint32_t error_code _U_, void *user_data) {
      http2_session_data *session_data = (http2_session_data *)user_data;
      http2_stream_data *stream_data;

      stream_data = nghttp2_session_get_stream_user_data(session, stream_id);
      if (!stream_data) {
        return 0;
      }
      remove_stream(session_data, stream_data);
      delete_http2_stream_data(stream_data);
      return 0;
    }

Lastly, we destroy the `http2_stream_data` object in this function, since the stream is about to close and we no longer need the object.

## libevent-server.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
    #  define errx(exitcode, format, args...)                                      \
        {                                                                          \
          warnx(format, ##args);                                                   \
          exit(exitcode);                                                          \
        }
    #  define warn(format, args...) warnx(format ": %s", ##args, strerror(errno))
    #  define warnx(format, args...) fprintf(stderr, format "\n", ##args)
    #endif

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

    #include <sys/types.h>
    #ifdef HAVE_SYS_SOCKET_H
    #  include <sys/socket.h>
    #endif /* HAVE_SYS_SOCKET_H */
    #ifdef HAVE_NETDB_H
    #  include <netdb.h>
    #endif /* HAVE_NETDB_H */
    #include <signal.h>
    #ifdef HAVE_UNISTD_H
    #  include <unistd.h>
    #endif /* HAVE_UNISTD_H */
    #include <sys/stat.h>
    #ifdef HAVE_FCNTL_H
    #  include <fcntl.h>
    #endif /* HAVE_FCNTL_H */
    #include <ctype.h>
    #ifdef HAVE_NETINET_IN_H
    #  include <netinet/in.h>
    #endif /* HAVE_NETINET_IN_H */
    #include <netinet/tcp.h>
    #ifndef __sgi
    #  include <err.h>
    #endif
    #include <string.h>
    #include <errno.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/listener.h>

    #include <nghttp2/nghttp2.h>

    #define OUTPUT_WOULDBLOCK_THRESHOLD (1 << 16)

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

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

    struct app_context;
    typedef struct app_context app_context;

    typedef struct http2_stream_data {
      struct http2_stream_data *prev, *next;
      char *request_path;
      int32_t stream_id;
      int fd;
    } http2_stream_data;

    typedef struct http2_session_data {
      struct http2_stream_data root;
      struct bufferevent *bev;
      app_context *app_ctx;
      nghttp2_session *session;
      char *client_addr;
    } http2_session_data;

    struct app_context {
      SSL_CTX *ssl_ctx;
      struct event_base *evbase;
    };

    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;
      (void)ssl;
      (void)arg;

      rv = nghttp2_select_alpn(out, outlen, in, inlen);

      if (rv != 1) {
        return SSL_TLSEXT_ERR_NOACK;
      }

      return SSL_TLSEXT_ERR_OK;
    }

    /* Create SSL_CTX. */
    static SSL_CTX *create_ssl_ctx(const char *key_file, const char *cert_file) {
      SSL_CTX *ssl_ctx;

      ssl_ctx = SSL_CTX_new(TLS_server_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);
    #if OPENSSL_VERSION_NUMBER >= 0x30000000L
      if (SSL_CTX_set1_curves_list(ssl_ctx, "P-256") != 1) {
        errx(1, "SSL_CTX_set1_curves_list failed: %s",
             ERR_error_string(ERR_get_error(), NULL));
      }
    #else  /* !(OPENSSL_VERSION_NUMBER >= 0x30000000L) */
      {
        EC_KEY *ecdh;
        ecdh = EC_KEY_new_by_curve_name(NID_X9_62_prime256v1);
        if (!ecdh) {
          errx(1, "EC_KEY_new_by_curv_name failed: %s",
               ERR_error_string(ERR_get_error(), NULL));
        }
        SSL_CTX_set_tmp_ecdh(ssl_ctx, ecdh);
        EC_KEY_free(ecdh);
      }
    #endif /* !(OPENSSL_VERSION_NUMBER >= 0x30000000L) */

      if (SSL_CTX_use_PrivateKey_file(ssl_ctx, key_file, SSL_FILETYPE_PEM) != 1) {
        errx(1, "Could not read private key file %s", key_file);
      }
      if (SSL_CTX_use_certificate_chain_file(ssl_ctx, cert_file) != 1) {
        errx(1, "Could not read certificate file %s", cert_file);
      }

      SSL_CTX_set_alpn_select_cb(ssl_ctx, alpn_select_proto_cb, NULL);

      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 add_stream(http2_session_data *session_data,
                           http2_stream_data *stream_data) {
      stream_data->next = session_data->root.next;
      session_data->root.next = stream_data;
      stream_data->prev = &session_data->root;
      if (stream_data->next) {
        stream_data->next->prev = stream_data;
      }
    }

    static void remove_stream(http2_session_data *session_data,
                              http2_stream_data *stream_data) {
      (void)session_data;

      stream_data->prev->next = stream_data->next;
      if (stream_data->next) {
        stream_data->next->prev = stream_data->prev;
      }
    }

    static http2_stream_data *
    create_http2_stream_data(http2_session_data *session_data, int32_t stream_id) {
      http2_stream_data *stream_data;
      stream_data = malloc(sizeof(http2_stream_data));
      memset(stream_data, 0, sizeof(http2_stream_data));
      stream_data->stream_id = stream_id;
      stream_data->fd = -1;

      add_stream(session_data, stream_data);
      return stream_data;
    }

    static void delete_http2_stream_data(http2_stream_data *stream_data) {
      if (stream_data->fd != -1) {
        close(stream_data->fd);
      }
      free(stream_data->request_path);
      free(stream_data);
    }

    static http2_session_data *create_http2_session_data(app_context *app_ctx,
                                                         int fd,
                                                         struct sockaddr *addr,
                                                         int addrlen) {
      int rv;
      http2_session_data *session_data;
      SSL *ssl;
      char host[NI_MAXHOST];
      int val = 1;

      ssl = create_ssl(app_ctx->ssl_ctx);
      session_data = malloc(sizeof(http2_session_data));
      memset(session_data, 0, sizeof(http2_session_data));
      session_data->app_ctx = app_ctx;
      setsockopt(fd, IPPROTO_TCP, TCP_NODELAY, (char *)&val, sizeof(val));
      session_data->bev = bufferevent_openssl_socket_new(
          app_ctx->evbase, fd, ssl, BUFFEREVENT_SSL_ACCEPTING,
          BEV_OPT_CLOSE_ON_FREE | BEV_OPT_DEFER_CALLBACKS);
      bufferevent_enable(session_data->bev, EV_READ | EV_WRITE);
      rv = getnameinfo(addr, (socklen_t)addrlen, host, sizeof(host), NULL, 0,
                       NI_NUMERICHOST);
      if (rv != 0) {
        session_data->client_addr = strdup("(unknown)");
      } else {
        session_data->client_addr = strdup(host);
      }

      return session_data;
    }

    static void delete_http2_session_data(http2_session_data *session_data) {
      http2_stream_data *stream_data;
      SSL *ssl = bufferevent_openssl_get_ssl(session_data->bev);
      fprintf(stderr, "%s disconnected\n", session_data->client_addr);
      if (ssl) {
        SSL_shutdown(ssl);
      }
      bufferevent_free(session_data->bev);
      nghttp2_session_del(session_data->session);
      for (stream_data = session_data->root.next; stream_data;) {
        http2_stream_data *next = stream_data->next;
        delete_http2_stream_data(stream_data);
        stream_data = next;
      }
      free(session_data->client_addr);
      free(session_data);
    }

    /* 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;
    }

    /* Read the data in the bufferevent and feed them into nghttp2 library
       function. Invocation of nghttp2_session_mem_recv() may make
       additional pending frames, so call session_send() at the end of the
       function. */
    static int session_recv(http2_session_data *session_data) {
      ssize_t readlen;
      struct evbuffer *input = bufferevent_get_input(session_data->bev);
      size_t datalen = evbuffer_get_length(input);
      unsigned char *data = evbuffer_pullup(input, -1);

      readlen = nghttp2_session_mem_recv(session_data->session, data, datalen);
      if (readlen < 0) {
        warnx("Fatal error: %s", nghttp2_strerror((int)readlen));
        return -1;
      }
      if (evbuffer_drain(input, (size_t)readlen) != 0) {
        warnx("Fatal error: evbuffer_drain failed");
        return -1;
      }
      if (session_send(session_data) != 0) {
        return -1;
      }
      return 0;
    }

    static ssize_t 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;

      /* Avoid excessive buffering in server side. */
      if (evbuffer_get_length(bufferevent_get_output(session_data->bev)) >=
          OUTPUT_WOULDBLOCK_THRESHOLD) {
        return NGHTTP2_ERR_WOULDBLOCK;
      }
      bufferevent_write(bev, data, length);
      return (ssize_t)length;
    }

    /* Returns nonzero if the string |s| ends with the substring |sub| */
    static int ends_with(const char *s, const char *sub) {
      size_t slen = strlen(s);
      size_t sublen = strlen(sub);
      if (slen < sublen) {
        return 0;
      }
      return memcmp(s + slen - sublen, sub, sublen) == 0;
    }

    /* Returns int value of hex string character |c| */
    static uint8_t hex_to_uint(uint8_t c) {
      if ('0' <= c && c <= '9') {
        return (uint8_t)(c - '0');
      }
      if ('A' <= c && c <= 'F') {
        return (uint8_t)(c - 'A' + 10);
      }
      if ('a' <= c && c <= 'f') {
        return (uint8_t)(c - 'a' + 10);
      }
      return 0;
    }

    /* Decodes percent-encoded byte string |value| with length |valuelen|
       and returns the decoded byte string in allocated buffer. The return
       value is NULL terminated. The caller must free the returned
       string. */
    static char *percent_decode(const uint8_t *value, size_t valuelen) {
      char *res;

      res = malloc(valuelen + 1);
      if (valuelen > 3) {
        size_t i, j;
        for (i = 0, j = 0; i < valuelen - 2;) {
          if (value[i] != '%' || !isxdigit(value[i + 1]) ||
              !isxdigit(value[i + 2])) {
            res[j++] = (char)value[i++];
            continue;
          }
          res[j++] =
              (char)((hex_to_uint(value[i + 1]) << 4) + hex_to_uint(value[i + 2]));
          i += 3;
        }
        memcpy(&res[j], &value[i], 2);
        res[j + 2] = '\0';
      } else {
        memcpy(res, value, valuelen);
        res[valuelen] = '\0';
      }
      return res;
    }

    static ssize_t file_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) {
      int fd = source->fd;
      ssize_t r;
      (void)session;
      (void)stream_id;
      (void)user_data;

      while ((r = read(fd, buf, length)) == -1 && errno == EINTR)
        ;
      if (r == -1) {
        return NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE;
      }
      if (r == 0) {
        *data_flags |= NGHTTP2_DATA_FLAG_EOF;
      }
      return r;
    }

    static int send_response(nghttp2_session *session, int32_t stream_id,
                             nghttp2_nv *nva, size_t nvlen, int fd) {
      int rv;
      nghttp2_data_provider data_prd;
      data_prd.source.fd = fd;
      data_prd.read_callback = file_read_callback;

      rv = nghttp2_submit_response(session, stream_id, nva, nvlen, &data_prd);
      if (rv != 0) {
        warnx("Fatal error: %s", nghttp2_strerror(rv));
        return -1;
      }
      return 0;
    }

    static const char ERROR_HTML[] = "<html><head><title>404</title></head>"
                                     "<body><h1>404 Not Found</h1></body></html>";

    static int error_reply(nghttp2_session *session,
                           http2_stream_data *stream_data) {
      int rv;
      ssize_t writelen;
      int pipefd[2];
      nghttp2_nv hdrs[] = {MAKE_NV(":status", "404")};

      rv = pipe(pipefd);
      if (rv != 0) {
        warn("Could not create pipe");
        rv = nghttp2_submit_rst_stream(session, NGHTTP2_FLAG_NONE,
                                       stream_data->stream_id,
                                       NGHTTP2_INTERNAL_ERROR);
        if (rv != 0) {
          warnx("Fatal error: %s", nghttp2_strerror(rv));
          return -1;
        }
        return 0;
      }

      writelen = write(pipefd[1], ERROR_HTML, sizeof(ERROR_HTML) - 1);
      close(pipefd[1]);

      if (writelen != sizeof(ERROR_HTML) - 1) {
        close(pipefd[0]);
        return -1;
      }

      stream_data->fd = pipefd[0];

      if (send_response(session, stream_data->stream_id, hdrs, ARRLEN(hdrs),
                        pipefd[0]) != 0) {
        close(pipefd[0]);
        return -1;
      }
      return 0;
    }

    /* 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_stream_data *stream_data;
      const char PATH[] = ":path";
      (void)flags;
      (void)user_data;

      switch (frame->hd.type) {
      case NGHTTP2_HEADERS:
        if (frame->headers.cat != NGHTTP2_HCAT_REQUEST) {
          break;
        }
        stream_data =
            nghttp2_session_get_stream_user_data(session, frame->hd.stream_id);
        if (!stream_data || stream_data->request_path) {
          break;
        }
        if (namelen == sizeof(PATH) - 1 && memcmp(PATH, name, namelen) == 0) {
          size_t j;
          for (j = 0; j < valuelen && value[j] != '?'; ++j)
            ;
          stream_data->request_path = percent_decode(value, j);
        }
        break;
      }
      return 0;
    }

    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;
      http2_stream_data *stream_data;

      if (frame->hd.type != NGHTTP2_HEADERS ||
          frame->headers.cat != NGHTTP2_HCAT_REQUEST) {
        return 0;
      }
      stream_data = create_http2_stream_data(session_data, frame->hd.stream_id);
      nghttp2_session_set_stream_user_data(session, frame->hd.stream_id,
                                           stream_data);
      return 0;
    }

    /* Minimum check for directory traversal. Returns nonzero if it is
       safe. */
    static int check_path(const char *path) {
      /* We don't like '\' in url. */
      return path[0] && path[0] == '/' && strchr(path, '\\') == NULL &&
             strstr(path, "/../") == NULL && strstr(path, "/./") == NULL &&
             !ends_with(path, "/..") && !ends_with(path, "/.");
    }

    static int on_request_recv(nghttp2_session *session,
                               http2_session_data *session_data,
                               http2_stream_data *stream_data) {
      int fd;
      nghttp2_nv hdrs[] = {MAKE_NV(":status", "200")};
      char *rel_path;

      if (!stream_data->request_path) {
        if (error_reply(session, stream_data) != 0) {
          return NGHTTP2_ERR_CALLBACK_FAILURE;
        }
        return 0;
      }
      fprintf(stderr, "%s GET %s\n", session_data->client_addr,
              stream_data->request_path);
      if (!check_path(stream_data->request_path)) {
        if (error_reply(session, stream_data) != 0) {
          return NGHTTP2_ERR_CALLBACK_FAILURE;
        }
        return 0;
      }
      for (rel_path = stream_data->request_path; *rel_path == '/'; ++rel_path)
        ;
      fd = open(rel_path, O_RDONLY);
      if (fd == -1) {
        if (error_reply(session, stream_data) != 0) {
          return NGHTTP2_ERR_CALLBACK_FAILURE;
        }
        return 0;
      }
      stream_data->fd = fd;

      if (send_response(session, stream_data->stream_id, hdrs, ARRLEN(hdrs), fd) !=
          0) {
        close(fd);
        return NGHTTP2_ERR_CALLBACK_FAILURE;
      }
      return 0;
    }

    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;
      http2_stream_data *stream_data;
      switch (frame->hd.type) {
      case NGHTTP2_DATA:
      case NGHTTP2_HEADERS:
        /* Check that the client request has finished */
        if (frame->hd.flags & NGHTTP2_FLAG_END_STREAM) {
          stream_data =
              nghttp2_session_get_stream_user_data(session, frame->hd.stream_id);
          /* For DATA and HEADERS frame, this callback may be called after
             on_stream_close_callback. Check that stream still alive. */
          if (!stream_data) {
            return 0;
          }
          return on_request_recv(session, session_data, stream_data);
        }
        break;
      default:
        break;
      }
      return 0;
    }

    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;
      http2_stream_data *stream_data;
      (void)error_code;

      stream_data = nghttp2_session_get_stream_user_data(session, stream_id);
      if (!stream_data) {
        return 0;
      }
      remove_stream(session_data, stream_data);
      delete_http2_stream_data(stream_data);
      return 0;
    }

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

      nghttp2_session_callbacks_new(&callbacks);

      nghttp2_session_callbacks_set_send_callback(callbacks, send_callback);

      nghttp2_session_callbacks_set_on_frame_recv_callback(callbacks,
                                                           on_frame_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_server_new(&session_data->session, callbacks, session_data);

      nghttp2_session_callbacks_del(callbacks);
    }

    /* Send HTTP/2 client connection header, which includes 24 bytes
       magic octets and SETTINGS frame */
    static int send_server_connection_header(http2_session_data *session_data) {
      nghttp2_settings_entry iv[1] = {
          {NGHTTP2_SETTINGS_MAX_CONCURRENT_STREAMS, 100}};
      int rv;

      rv = nghttp2_submit_settings(session_data->session, NGHTTP2_FLAG_NONE, iv,
                                   ARRLEN(iv));
      if (rv != 0) {
        warnx("Fatal error: %s", nghttp2_strerror(rv));
        return -1;
      }
      return 0;
    }

    /* readcb for bufferevent after client connection header was
       checked. */
    static void readcb(struct bufferevent *bev, void *ptr) {
      http2_session_data *session_data = (http2_session_data *)ptr;
      (void)bev;

      if (session_recv(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. If the
       connection is not going to shutdown, we call session_send() to
       process pending data in the output buffer. This is necessary
       because we have a threshold on the buffer size to avoid too much
       buffering. See send_callback(). */
    static void writecb(struct bufferevent *bev, void *ptr) {
      http2_session_data *session_data = (http2_session_data *)ptr;
      if (evbuffer_get_length(bufferevent_get_output(bev)) > 0) {
        return;
      }
      if (nghttp2_session_want_read(session_data->session) == 0 &&
          nghttp2_session_want_write(session_data->session) == 0) {
        delete_http2_session_data(session_data);
        return;
      }
      if (session_send(session_data) != 0) {
        delete_http2_session_data(session_data);
        return;
      }
    }

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

        fprintf(stderr, "%s connected\n", session_data->client_addr);

        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, "%s h2 is not negotiated\n", session_data->client_addr);
          delete_http2_session_data(session_data);
          return;
        }

        initialize_nghttp2_session(session_data);

        if (send_server_connection_header(session_data) != 0 ||
            session_send(session_data) != 0) {
          delete_http2_session_data(session_data);
          return;
        }

        return;
      }
      if (events & BEV_EVENT_EOF) {
        fprintf(stderr, "%s EOF\n", session_data->client_addr);
      } else if (events & BEV_EVENT_ERROR) {
        fprintf(stderr, "%s network error\n", session_data->client_addr);
      } else if (events & BEV_EVENT_TIMEOUT) {
        fprintf(stderr, "%s timeout\n", session_data->client_addr);
      }
      delete_http2_session_data(session_data);
    }

    /* callback for evconnlistener */
    static void acceptcb(struct evconnlistener *listener, int fd,
                         struct sockaddr *addr, int addrlen, void *arg) {
      app_context *app_ctx = (app_context *)arg;
      http2_session_data *session_data;
      (void)listener;

      session_data = create_http2_session_data(app_ctx, fd, addr, addrlen);

      bufferevent_setcb(session_data->bev, readcb, writecb, eventcb, session_data);
    }

    static void start_listen(struct event_base *evbase, const char *service,
                             app_context *app_ctx) {
      int rv;
      struct addrinfo hints;
      struct addrinfo *res, *rp;

      memset(&hints, 0, sizeof(hints));
      hints.ai_family = AF_UNSPEC;
      hints.ai_socktype = SOCK_STREAM;
      hints.ai_flags = AI_PASSIVE;
    #ifdef AI_ADDRCONFIG
      hints.ai_flags |= AI_ADDRCONFIG;
    #endif /* AI_ADDRCONFIG */

      rv = getaddrinfo(NULL, service, &hints, &res);
      if (rv != 0) {
        errx(1, "Could not resolve server address");
      }
      for (rp = res; rp; rp = rp->ai_next) {
        struct evconnlistener *listener;
        listener = evconnlistener_new_bind(
            evbase, acceptcb, app_ctx, LEV_OPT_CLOSE_ON_FREE | LEV_OPT_REUSEABLE,
            16, rp->ai_addr, (int)rp->ai_addrlen);
        if (listener) {
          freeaddrinfo(res);

          return;
        }
      }
      errx(1, "Could not start listener");
    }

    static void initialize_app_context(app_context *app_ctx, SSL_CTX *ssl_ctx,
                                       struct event_base *evbase) {
      memset(app_ctx, 0, sizeof(app_context));
      app_ctx->ssl_ctx = ssl_ctx;
      app_ctx->evbase = evbase;
    }

    static void run(const char *service, const char *key_file,
                    const char *cert_file) {
      SSL_CTX *ssl_ctx;
      app_context app_ctx;
      struct event_base *evbase;

      ssl_ctx = create_ssl_ctx(key_file, cert_file);
      evbase = event_base_new();
      initialize_app_context(&app_ctx, ssl_ctx, evbase);
      start_listen(evbase, service, &app_ctx);

      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 < 4) {
        fprintf(stderr, "Usage: libevent-server PORT KEY_FILE CERT_FILE\n");
        exit(EXIT_FAILURE);
      }

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

      run(argv[1], argv[2], argv[3]);
      return 0;
    }