civetweb/docs/APIReference.md

CivetWeb API Reference

CivetWeb is often used as HTTP and HTTPS library inside a larger application. An API is available to integrate the CivetWeb functionality in a larger codebase. This document describes the API. Basic usage examples of the API can be found in Embedding.md.

Functions

mg_check_feature( feature )

Parameters

• unsigned feature - a value indicating the feature to be checked

Returns

• unsigned - a value indicating if a feature is available

Description

The function mg_check_feature() can be called from an application program to check of specific features have been compiled in the civetweb version which the application has been linked to. The feature to check is provided as an unsigned integer parameter. If the function is available in the currently linked library version, a value > 0 is returned. Otherwise the function mg_check_feature() returns the value 0.

The following parameter values can be used:

• 1 - Serve files (NO_FILES not set during compilation). If this feature is available, the webserver is able to serve files directly from a directory tree.
• 2 - Support HTTPS (NO_SSL not set during compiletion). If this feature is available, the webserver van use encryption in the client-server connection. SSLv2, SSLv3, TLSv1.0, TLSv1.1 and TLSv1.2 are supported, but which protocols are used effectively is dependent on the options used when the server is started.
• 4 - support CGI (NO_CGI not set during compilation). If this feature is available, external CGI scripts can be called by the webserver.
• 8 - support IPv6 (USE_IPV6 set during compilation). The CivetWeb library is capable of communicating over both IPv4 and IPv6, but IPv6 support is only available if it has been enabled at compile time.
• 16 - support WebSocket (USE_WEBSOCKET set during compilation). WebSockets support is available in the CivetWeb library if the proper options has been used during cimpile time.
• 32 - support Lua scripts and Lua server pages (USE_LUA set during compilation). CivetWeb supports server side scripting through the Lua language, if that has been enabled at compile time. Lua is an efficient scripting language which is less resource heavy than for example PHP.
• 64 - support server side JavaScript (USE_DUKTAPE set during compilation). Server side JavaScript can be used for dynamic page generation if the proper options have been set at compile time. Please note that client side JavaScript execution is always available if it has been enabled in the connecting browser.
• 128 - support caching (NO_CACHING not set during compilation). The webserver will support caching, if it has not been disabled while compiling the library.

Parameter values other than the values mentioned above will give undefined results.

mg_start( callbacks, user_data, options )

Parameters

• const struct mg_callbacks *callbacks - a structure with optional callback functions to process requests from the web server.
• void *user_data - a pointer to optional user data
• char **options - a list of options used to initialize the web server

Returns

• struct mg_context * - a pointer to a context structure when successful, or NULL in case of failure

Description

The function mg_start() is the only function needed to call to initialize the webserver. After the function returns and a pointer to a contect structure is provided, it is guaranteed that the server has started and is listening on the designated ports. In case of failure a NULL pointer is returned. The behaviour of the web server is controlled by a list of callback functions and a list of options. The callback functions can do application specific processing of events which are encountered by the webserver. If a specific callback function is not provided, the webserver uses their default callback routines. The options list controls how the webserver should be started and contains settings for for example the ports to listen on, the maximum number of threads created to handle requests in parallel and if settings for SSL encryption.

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/handle_form.inl (HTML form handling functions)
• 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. Some features available in the C interface might be missing in 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.

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.