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Embedding LibHTTP

LibHTTP is primarily designed so applications can easily add HTTP and HTTPS server as well as WebSocket functionality. For example, an application server could use LibHTTP to enable a web service interface for automation or remote control.

It can deliver static files and offers built-in server side Lua, JavaScript and CGI support. Some instructions how to build the stand-alone server can be found in Building.md.

Files

There is just a small set of files to compile in to the application, but if a library is desired, see Building.md

Regarding the INL file extension

The INL file extension represents code that is statically included inline in a source file. Slightly different from C++ where it means "inline" code which is technically not the same as static code. LibHTTP overloads this extension for the sake of clarity as opposed to having .c extensions on files that should not be directly compiled.

HTTP Server Source Files

These files constitute the LibHTTP library. They do not contain a main function, but all functions required to run a HTTP server.

  • HTTP server API
    • include/libhttp.h
  • C implementation
    • src/libhttp.c
    • src/md5.inl (MD5 calculation)
    • src/handle_form.inl (HTML form handling functions)

Quick Start

By default, the server will automatically serve up files like a normal HTTP server. An embedded server is most likely going to overload this functionality.

C

  • Include the C interface libhttp.h.
  • Use httplib_start() to start the server.
    • Use options to select the port and document root among other things.
    • Use callbacks to add your own hooks.
  • Use httplib_set_request_handler() to easily add your own request handlers.
  • Use httplib_stop() to stop the server.

JavaScript Support

LibHTTP can be built with server side JavaScript support by including the Duktape library.

LibHTTP internals

LibHTTP is multithreaded web server. httplib_start() function allocates web server context (struct httplib_context), which holds all information about web server instance:

  • configuration options. Note that LibHTTP makes internal copies of passed options.
  • SSL context, if any
  • user-defined callbacks
  • opened listening sockets
  • a queue for accepted sockets
  • mutexes and condition variables for inter-thread synchronization

When httplib_start() returns, all initialization is guaranteed to be complete (e.g. listening ports are opened, SSL is initialized, etc). httplib_start() starts some threads: a master thread, that accepts new connections, and several worker threads, that process accepted connections. The number of worker threads is configurable via num_threads configuration option. That number puts a limit on number of simultaneous requests that can be handled by LibHTTP. If you embed LibHTTP into a program that uses SSL outside LibHTTP as well, you may need to initialize SSL before calling httplib_start(), and set the pre- processor define SSL_ALREADY_INITIALIZED. This is not required if SSL is used only within LibHTTP.

When master thread accepts new a connection, a new accepted socket (described by struct socket) it placed into the accepted sockets queue, which has size of MGSQLEN (default 20). Any idle worker thread can grab accepted sockets from that queue. If all worker threads are busy, master thread can accept and queue up to 20 more TCP connections, filling up the queue. In the attempt to queue even more accepted connection, the master thread blocks until there is space in the queue. When the master thread is blocked on a full queue, the operating system can also queue incoming connection. The number is limited by the listen() call parameter, which is SOMAXCONN and depends on the platform.

Worker threads are running in an infinite loop, which in a simplified form looks something like this:

static void *worker_thread() {
  while (consume_socket()) {
    process_new_connection();
  }
}

Function consume_socket() gets a new accepted socket from the LibHTTP socket queue, atomically removing it from the queue. If the queue is empty, consume_socket() blocks and waits until a new socket is placed in the queue by the master thread.

process_new_connection() actually processes the connection, i.e. reads the request, parses it, and performs appropriate action depending on the parsed request.

Master thread uses poll() and accept() to accept new connections on listening sockets. poll() is used to avoid FD_SETSIZE limitation of select(). Since there are only a few listening sockets, there is no reason to use hi-performance alternatives like epoll() or kqueue(). Worker threads use blocking IO on accepted sockets for reading and writing data. All accepted sockets have SO_RCVTIMEO and SO_SNDTIMEO socket options set (controlled by the request_timeout_ms LibHTTP option, 30 seconds default) which specifies a read/write timeout on client connections.