Embedding.md

Embedding CivetWeb

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

However, it can also be used as a stand-alone executable. 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. CivetWeb 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 CivetWeb library. They do not contain a main function, but all functions required to run a HTTP server.

• HTTP server API
  • include/civetweb.h
• C implementation
  • src/civetweb.c
  • src/md5.inl (MD5 calculation)
  • src/sha1.inl (SHA calculation)
  • src/handle_form.inl (HTML form handling functions)
  • src/timer.inl (optional timer support)
• Optional: C++ wrapper
  • include/CivetServer.h (C++ interface)
  • src/CivetServer.cpp (C++ wrapper implementation)
• Optional: Third party components
  • src/third_party/* (third party components, mainly used for the standalone server)
  • src/mod_*.inl (modules to access third party components from civetweb)

Note: The C++ wrapper uses the official C interface (civetweb.h) and does not add new features to the server. Several features available in the C interface are missing in the C++ interface. While all features should be accessible using the C interface, this is not a design goal of the C++ interface.

Additional Source Files for Executables

These files can be used to build a server executable. They contain a main function starting the HTTP server.

• Stand-alone C Server
  • src/main.c
• Reference embedded C Server
  • examples/embedded_c/embedded_c.c
• Reference embedded C++ Server
  • examples/embedded_cpp/embedded_cpp.cpp

Note: The "embedded" example is actively maintained, updated, extended and tested. Other examples in the examples/ folder might be outdated and remain there for reference.

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 civetweb.h.
• Use mg_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 mg_set_request_handler() to easily add your own request handlers.
• Use mg_stop() to stop the server.

C++

• Note that CivetWeb is Clean C, and C++ interface CivetServer.h is only a wrapper layer around the C interface. Not all CivetWeb features available in C are also available in C++.
• Create CivetHandlers for each URI.
• Register the handlers with CivetServer::addHandler()
• CivetServer starts on contruction and stops on destruction.
• Use contructor options to select the port and document root among other things.
• Use constructor callbacks to add your own hooks.

Alternative quick start: Have a look at the examples embedded_c and embedded_cpp

Lua Support

Lua is a server side include functionality. Files ending in .lua will be processed with Lua.

Add the following CFLAGS

• -DLUA_COMPAT_ALL
• -DUSE_LUA
• -DUSE_LUA_SQLITE3
• -DUSE_LUA_FILE_SYSTEM

Add the following sources

• src/mod_lua.inl
• src/third_party/lua-5.2.4/src
  • lapi.c
  • lauxlib.c
  • lbaselib.c
  • lbitlib.c
  • lcode.c
  • lcorolib.c
  • lctype.c
  • ldblib.c
  • ldebug.c
  • ldo.c
  • ldump.c
  • lfunc.c
  • lgc.c
  • linit.c
  • liolib.c
  • llex.c
  • lmathlib.c
  • lmem.c
  • loadlib.c
  • lobject.c
  • lopcodes.c
  • loslib.c
  • lparser.c
  • lstate.c
  • lstring.c
  • lstrlib.c
  • ltable.c
  • ltablib.c
  • ltm.c
  • lundump.c
  • lvm.c
  • lzio.c
• src/third_party/sqlite3.c
• src/third_party/sqlite3.h
• src/third_party/lsqlite3.c
• src/third_party/lfs.c
• src/third_party/lfs.h

This build is valid for Lua version Lua 5.2. It is also possible to build with Lua 5.1 (including LuaJIT) or Lua 5.3.

JavaScript Support

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

CivetWeb internals

CivetWeb is multithreaded web server. mg_start() function allocates web server context (struct mg_context), which holds all information about web server instance:

• configuration options. Note that CivetWeb 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 mg_start() returns, all initialization is guaranteed to be complete (e.g. listening ports are opened, SSL is initialized, etc). mg_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 CivetWeb. If you embed CivetWeb into a program that uses SSL outside CivetWeb as well, you may need to initialize SSL before calling mg_start(), and set the pre- processor define SSL_ALREADY_INITIALIZED. This is not required if SSL is used only within CivetWeb.

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 CivetWeb 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 CivetWeb option, 30 seconds default) which specifies a read/write timeout on client connections.

A minimal example

Initializing a HTTP server

{
    /* Server context handle */
    struct mg_context *ctx;

    /* Initialize the library */
    mg_init_library(0);

    /* Start the server */
    ctx = mg_start(NULL, 0, NULL);

    /* Add some handler */
    mg_set_request_handler(ctx, "/hello", handler, "Hello world");

    ... Run the application ...
    
    /* Stop the server */
    mg_stop(ctx);

    /* Un-initialize the library */
    mg_exit_library();
}

A simple callback

static int
handler(struct mg_connection *conn, void *ignored)
{
	const char *msg = "Hello world";
	unsigned long len = (unsigned long)strlen(msg);

	mg_printf(conn,
	          "HTTP/1.1 200 OK\r\n"
	          "Content-Length: %lu\r\n"
	          "Content-Type: text/plain\r\n"
	          "Connection: close\r\n\r\n",
	          len);

	mg_write(conn, msg, len);

	return 200;
}