Recommending – libmicrohttpd library

The GNU libmicrohttpd is a small, yet very powerful, C library that allows you to run HTTP 1.1 compliant server as part of another application.

The library provides two threading modes – MHD_USE_SELECT_INTERNALLY that works like SWILL, i.e. requests are processed one by one, and MHD_USE_THREAD_PER_CONNECTION that uses one thread per request, allowing for parallel processing.

The libmicrohttpd library allows you to

  • accept or reject connection via a callback function,
  • incrementally process POST data,
  • send a reply even if the size of reply data is not known ahead,
  • support SSL autentification (requires libgcrypt),

and much more…

The life of a request is as follows (most of the steps may be omitted if not required):

  • Accept or reject the connection,
  • Authenticate the user,
  • Initialize any data that you need for the request processing,
  • Process POST data,
  • Create a response to the request,
  • Send the response to the client,
  • Do the clean up.

There is much, much more to say about libmicrohttpd, don’t forget to check out Manual and Tutorial.

I’ll finish off with a simple template of server, that uses one thread per connection, allows refusal of client per IP (IP filtering), and processes incoming POST data.
It might look fussy at first, but the structure is very clean and neat. Cheers to Christian Grothoff!

// std lib includes
#include <stdio.h>
#include <string.h>
#include <sstream>

// HTTP server includes
#include "platform.h"
#include "microhttpd.h"

//! Buffer chunk size for \a POST data loading - not too big, not too small.
#define POSTBUFFERSIZE  2048

//! HTTP method \a GET.
#define GET             0
//! HTTP method \a POST.
#define POST            1

//! Structure holding all connection related data.
struct connection_info_struct
    int connection_type;                  //!< \a GET or \a POST.
    MHD_PostProcessor* post_processor;    //!< Processor for chunks of \a POST data.
    std::string data;                     //!< Loaded \a POST data.

        connection_type = GET;
        post_processor = NULL;

        if (NULL != post_processor)
        post_processor = NULL;

int send_page(MHD_Connection* connection, const char* page, int status_code)
    MHD_Response* response;
    response = MHD_create_response_from_data(strlen(page),
        (void*) page, MHD_NO, MHD_YES);
    if (!response)
        return MHD_NO;

    int ret = MHD_queue_response(connection, status_code, response);
    return ret;

int on_client_connect(void* cls, const sockaddr* addr, socklen_t addrlen)
    //! \todo Here you can do any statistics, IP filtering, etc.
    char buf[INET6_ADDRSTRLEN];
    printf("Incoming IP: %s\n", inet_ntop(addr->sa_family,
        addr->sa_data + 2, buf, INET6_ADDRSTRLEN));
    //! \todo Return MHD_NO if you want to refuse the connection.
    return MHD_YES;

int iterate_post(void *coninfo_cls, enum MHD_ValueKind kind, const char *key,
                 const char *filename, const char *content_type,
                 const char *transfer_encoding, const char *data, uint64_t off,
                 size_t size)
    connection_info_struct* con_info = (connection_info_struct*) coninfo_cls;

    /*! \todo: This is where we want to do the processing of POST data.
     * Here we simply append it to \a data member for further use. */
    if (size > 0)
        con_info->data += std::string(data, size);

    return MHD_YES;

int answer_to_connection(void* cls, MHD_Connection* connection,
                         const char* url, const char* method,
                         const char* version, const char* upload_data,
                         size_t* upload_data_size, void** con_cls)
    if (NULL == *con_cls)    // Incoming!... prepare all.
        connection_info_struct* con_info = new connection_info_struct;
        if (NULL == con_info)
            return MHD_NO;

        if (0 == strcmp(method, "POST"))
            con_info->post_processor =
                MHD_create_post_processor(connection, POSTBUFFERSIZE,
                iterate_post, (void*) con_info);

            if (NULL == con_info->post_processor)
                delete con_info;
                return MHD_NO;
            con_info->connection_type = POST;
            con_info->connection_type = GET;

        *con_cls = (void*) con_info;

        return MHD_YES;

    if (0 == strcmp(method, "GET"))
        //! \todo Send an appropriate reply to GET request, based on \a url.
        return send_page(connection, "Some reply...", MHD_HTTP_OK);

    if (0 == strcmp(method, "POST"))
        //! \todo Process a POST request.
        connection_info_struct* con_info = (connection_info_struct*) *con_cls;

        if (0 != *upload_data_size)        // chunk of POST data?
            // we're still receiving the POST data, process them.
                upload_data, *upload_data_size);
            *upload_data_size = 0;    // 0 means "successfully processed"
            return MHD_YES;
        else    // awaiting reply
            //! \todo Send an appropriate reply to POST request, based on \a url.
            return send_page(connection, "Some reply...", MHD_HTTP_OK);
    return MHD_YES;

void request_completed(void* cls, MHD_Connection* connection,
                       void** con_cls, MHD_RequestTerminationCode toe)
    //! \todo Here we do the clean up on connection close.
    connection_info_struct* con_info = (connection_info_struct*) *con_cls;
    if (NULL != con_info)
        delete con_info;
    con_info = NULL;
    *con_cls = NULL;

int main(int argc, char** argv)
    int port = 1234;
    MHD_Daemon* daemon = MHD_start_daemon(
        on_client_connect, NULL,
        &answer_to_connection, NULL,
        MHD_OPTION_NOTIFY_COMPLETED, request_completed,
    if (NULL == daemon)
        return 1;

    //! \todo Here we just sleep... you can play chess here, or whatever you want.
    while (stop_condition)
        usleep(100000);    // sleep 0.1 sec

    return 0;

Happy coding! 🙂

7 responses to “Recommending – libmicrohttpd library

  1. Interesting, I see that you include “platform.h” as does the tutorial and the examples. My “make install” of libmicrohttpd-0.4.2.tar.gz does not seem to add that file to those installed. For me only the microhttpd.h gets installed.

    The “Using libmicrohttpd” on the home page does not include platform.h. This comment in microhttpd.h also implies that it is not used any longer:

    * NEW: Before including “microhttpd.h” you should add the necessary
    * includes to define the “uint64_t”, “size_t”, “fd_set”, “socklen_t”
    * and “struct sockaddr” data types (which headers are needed may
    * depend on your platform; for possible suggestions consult
    * “platform.h” in the MHD distribution).

    I’m not sure what is proper here.

    – Bruce

  2. I think that the platform.h file is generated by `configure`, but can’t verify it right now.
    The file can be found in the source tree after the build – that’s how I got it, and since I very rarely use `make install`, I can’t tell you whether the file was copied for me or not.

    If the code works fine for you without the header file, go ahead, I guess it’s not required; if you get some errors like missing symbols etc. just copy it from the source tree wherever you need to.

    Sorry I couldn’t help more.

  3. This is probably moot by now, but I have gathered from the documentation that platform.h is just a convenience header which includes all of libmicrohttpd’s dependencies. You needn’t use it, and it’s not officially part of the library, but it does help out.

    I’m not using it myself; I prefer to include all the headers which microhttpd.h depends on manually.

  4. I have one questiong concerning libmicrohttpd cleanup. I am passing a string allocated on heap to function MHD_create_response_from_data, so I have to free it somewhere. But where? If I free it after MHD_destroy_response, some functions are still working with it (MHD_connection_handle_idle?). If I free it in function registered via MHD_OPTION_NOTIFY_COMPLETED, still some functions are using it. Any ideas?

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