GNU libmicrohttpd: a library for creating an embedded HTTP server

GNU libmicrohttpd

About

GNU libmicrohttpd is a small C library that is supposed to make it
easy to run an HTTP server as part of another application. GNU
libmicrohttpd is free software and part of the GNU project. Key features that distinguish
libmicrohttpd from other projects are:

libmicrohttpd was started because the author needed an easy way to add
a concurrent HTTP server to other projects. Existing alternatives
were either non-free, not reentrant, standalone, of terrible code
quality or a combination thereof. Do not use libmicrohttpd if you are
looking for a standalone HTTP server, there are many other projects
out there that provide that kind of functionality already. However,
if you want to be able to serve simple WWW pages from within your C or
C++ application, check it out.

libmicrohttpd is licensed under the GNU LGPL. If you
disable HTTPS/SSL support, you can also choose the second license,
the eCos License.
If you have questions about licensing, please contact the
maintainer.

Download

You can find the current release including binaries for Windows
here and on
GNU ftp mirrors.
The latest version of the source code can be browsed from
here or obtained using

$ svn checkout https://gnunet.org/svn/libmicrohttpd/

libmicrohttpd can be used without any dependencies; however,
for SSL/TLS support we require
libgcrypt and
libgnutls.
Furthermore, the testcases use libcurl. Some extended
testcases also use zzuf and socat (to simulate
clients that violate the HTTP protocols). You can compile and use
libmicrohttpd without installing libgcrypt, libgnutls, libcurl, zzuf or
socat.

If you want to be notified about updates, subscribe to libmicrohttpd on
freshmeat A public mailinglist for libmicrohttpd is hosted at http://lists.gnu.org/mailman/listinfo/libmicrohttpd.

Using libmicrohttpd

In addition to the brief documentation on this webpage, we
have various other forms of documentation available:

microhttpd.h
This include file documents most of the API in detail.
Manual
The libmicrohttpd manual is available as one document in
pdf and html
formats.
Tutorial
The libmicrohttpd tutorial is available as one document in
pdf and html
formats.

The rest of this section gives a general overview.

Before including the microhttpd.h header, you may need to
include the headers of your operating system that define the size_t,
fd_set, socklen_t and struct sockaddr data
types and define MHD_PLATFORM_H. Otherwise,
the microhttpd.h header will attempt to include the
appropriate headers automatically, which may fail for more exotic
platforms.

Here is a minimal example for GNU/Linux (included in the distribution):

#include <microhttpd.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#define PAGE "<html><head><title>libmicrohttpd demo</title>"\
             "</head><body>libmicrohttpd demo</body></html>"
static int ahc_echo(void * cls,
		    struct MHD_Connection * connection,
		    const char * url,
		    const char * method,
                    const char * version,
		    const char * upload_data,
		    size_t * upload_data_size,
                    void ** ptr) {
  static int dummy;
  const char * page = cls;
  struct MHD_Response * response;
  int ret;
  if (0 != strcmp(method, "GET"))
    return MHD_NO; /* unexpected method */
  if (&dummy != *ptr)
    {
      /* The first time only the headers are valid,
         do not respond in the first round... */
      *ptr = &dummy;
      return MHD_YES;
    }
  if (0 != *upload_data_size)
    return MHD_NO; /* upload data in a GET!? */
  *ptr = NULL; /* clear context pointer */
  response = MHD_create_response_from_data(strlen(page),
					   (void*) page,
					   MHD_NO,
					   MHD_NO);
  ret = MHD_queue_response(connection,
			   MHD_HTTP_OK,
			   response);
  MHD_destroy_response(response);
  return ret;
}
int main(int argc,
	 char ** argv) {
  struct MHD_Daemon * d;
  if (argc != 2) {
    printf("%s PORT\n",
	   argv[0]);
    return 1;
  }
  d = MHD_start_daemon(MHD_USE_THREAD_PER_CONNECTION,
		       atoi(argv[1]),
		       NULL,
		       NULL,
		       &ahc_echo,
		       PAGE,
		       MHD_OPTION_END);
  if (d == NULL)
    return 1;
  (void) getc ();
  MHD_stop_daemon(d);
  return 0;
}

Threading models

The example above uses the simplest threading model,
MHD_USE_THREAD_PER_CONNECTION. In this model, MHD starts one
thread to listen on the port for new connections and then spawns a new
thread to handle each connection. This model is great if the HTTP
server has hardly any state that is shared between connections (no
synchronization issues!) and may need to perform blocking operations
(such as extensive IO or running of code) to handle an individual
connection.

The second threading model, MHD_USE_SELECT_INTERNALLY, uses
only a single thread to handle listening on the port and processing of
requests. This model is preferable if spawning a thread for each
connection would be costly. If the HTTP server is able to quickly
produce responses without much computational overhead for each
connection, this model can be a great choice. Note that MHD will
still start a single thread for itself -- this way, the main program
can continue with its operations after calling MHD_daemon_start.
Naturally, if the HTTP server needs to interact with
shared state in the main application, synchronization will be
required. If such synchronization in code providing a response
results in blocking, all HTTP server operations on all connections
will stall. This mode is a bad choice if response data (for responses
generated using the MHD_create_response_from_callback
function) cannot always be provided instantly. The reason is that the
code generating responses should not block (since that would block all
other connections) and on the other hand, if response data is not
available immediately, MHD will start to busy wait on it. Use the
first model if you want to block on providing response data in the
callback, or the last model if you want to use a more event-driven
model with one big select loop.

The third model combines a thread pool with
the MHD_USE_SELECT_INTERNALLY mode, which can benefit
implementations that require scalability. As said before, by default
this mode only uses a single thread. When combined with the thread pool option, it
is possible to handle multiple connections with multiple threads. The
number of threads is specified using the
MHD_OPTION_THREAD_POOL_SIZE; any value greater than one for
this option will activate the use of the thread pool. In contrast to
the MHD_USE_THREAD_PER_CONNECTION mode (where each thread
handles one and only one connection), threads in the pool can handle a
large number of concurrent connections. Using
MHD_USE_SELECT_INTERNALLY in combination with a thread pool
is typically the most scalable (but also hardest to debug) mode of
operation for MHD.

The fourth threading model (used when no specific flag is given), uses
no threads. Instead, the main application must (periodically) request
file descriptor sets from MHD, perform a select call and then call
MHD_run. MHD_run will then process HTTP requests as
usual and return. MHD_run is guaranteed to not block;
however, access handlers and response processing callbacks that it
invokes may block. This mode is useful if a single-threaded
implementation is desired and in particular if the main application
already uses a select loop for its processing. If the application is
not ready to provide a response, it can just return zero for the
number of bytes read and use its file descriptors in the external
select loop to wake up and continue once the data is ready -- MHD will
unlist the socket from the write set if the application failed to
provide response data (this only happens in this mode).

The testcases provided include examples for using each of the
threading modes.

Responses

MHD provides various functions to create struct MHD_Response
objects. A response consists of a set of HTTP headers and a (possibly
empty) body. The three main ways to create a response are either by
specifying a given (fixed-size) body
(MHD_create_response_from_data), by providing a function of
type MHD_ContentReaderCallback which provides portions of the
response as needed or by providing an open file descriptor
(MHD_create_response_from_fd). The first response
construction is great for small and in particular static webpages that
fit into memory. The second response type should be used for response
objects where the size is initially not known or where the response
maybe too large to fit into memory. Finally, using a file descriptor
can be used on Linux systems to use the highly efficient
sendfile call for the file transfer.

A response is used by calling MHD_queue_response which sends
the response back to the client on the specified connection. Once
created, a response object can be used any number of times.
Internally, each response uses a reference counter. The response is
freed once the reference counter reaches zero. The HTTP server should
call MHD_destroy_response when a response object is no longer
needed, that is, the server will not call MHD_queue_response
again using this response object. Note that this does not mean that
the response will be immediately destroyed -- destruction may be
delayed until sending of the response is complete on all connections
that have the response in the queue.

Queueing responses

Clients should never create a "100 CONTINUE" response. MHD
handles "100 CONTINUE" internally and only allows clients to
queue a single response per connection. Furthermore, clients must not
queue a response before the request has been fully received (except
in the case of rejecting PUT or POST operations in HTTP 1.1). If a
client attempts to queue multiple responses or attempts to queue a
response early, MHD_queue_response will fail (and return
MHD_NO).

The callback function for the respective URL will be called at least
twice. The first call happens after the server has received the
headers. The client should use the last void** argument to
store internal context for the session. The first call to the
callback function is mostly for this type of initialization and for
internal access checks. At least, the callback function should
"remember" that the first call with just the headers has
happened. Queueing a response during the first call (for a given
connection) should only be used for errors -- if the client queues a
response during this first call, a 100 CONTINUE response will
be suppressed, the request body will not be read and the connection
will be closed after sending the response. After the first call, the
callback function will be called with upload data. Until
*upload_data_size is zero, the callback may not queue a
response, any such attempt will fail. The callback function should
update *upload_data_size to indicate how many bytes were
processed. Depending on available buffer space, incremental
processing of the upload maybe required. Once all of the upload data
has been processed, MHD will call the callback a second time with
*upload_data_size being zero. At this point, the callback
should queue a "normal" response. If queueing a response is
not possible, the callback may either block or simply not queue a
response depending on the threading model that is used. If the
callback does not queue a response at this point, MHD will either
(eventually) timeout the connection or keep calling it.

Parsing of POST requests

MHD includes a set of three functions for parsing and processing data
received in POST requests. The functions allow incremental parsing
and processing of POST data. Only a tiny fraction of the overall POST
data needs to fit into memory. As a result, applications using MHD
can support POST requests of arbitrary size. POST data is processed
by providing MHD with a callback function that is called on portions
of the received values. The POST parser itself is invoked repeatedly
whenever more input bytes become available. MHD supports both uri-
and multipart/form-encoded POST data.

Memory Management

The application can determine the size of buffers that MHD should use
for handling of HTTP requests and parsing of POST data. This way, MHD
users can trade-off processing time and memory utilization.
Applications can limit the overall number of connections MHD will
accept, as well as the total amount of memory used per connection.
MHD will gracefully handle all out-of-memory situations (by closing
the connection and cleaning up any remaining state).

Bugtrack

libmicrohttpd uses Mantis for bugtracking. Visit https://gnunet.org/bugs/ to
report bugs. You need to sign up for a reporter account. Please make
sure you report bugs under libmicrohttpd and not
under any of the other projects.

For questions and discussions please use the
GNU libmicrohttpd mailinglist.
Messages to the list from non-subscribers are subject to manual moderation
and are hence likely to be delayed significantly.

Applications using libmicrohttpd

If you write an application that uses libmicrohttpd, please
let us know so that we can add you to the list!

Alternatives

If you are aware of a competing library that might be a better fit for
some developers, please let us know so that we can add it to the list!

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