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1. Introduction

This tutorial is for developers who want to learn how they can add HTTP serving capabilities to their applications with the GNU libmicrohttpd library, abbreviated MHD. The reader will learn how to implement basic HTTP functions from simple executable sample programs that implement various features.

The text is supposed to be a supplement to the API reference manual of GNU libmicrohttpd and for that reason does not explain many of the parameters. Therefore, the reader should always consult the manual to find the exact meaning of the functions used in the tutorial. Furthermore, the reader is encouraged to study the relevant RFCs, which document the HTTP standard.

GNU libmicrohttpd is assumed to be already installed. This tutorial is written for version 0.9.16. At the time being, this tutorial has only been tested on GNU/Linux machines even though efforts were made not to rely on anything that would prevent the samples from being built on similar systems.


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1.1 History

This tutorial was originally written by Sebastian Gerhardt for MHD 0.4.0. It was slighly polished and updated to MHD 0.9.0 by Christian Grothoff.


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2. Hello browser example

The most basic task for a HTTP server is to deliver a static text message to any client connecting to it. Given that this is also easy to implement, it is an excellent problem to start with.

For now, the particular URI the client asks for shall have no effect on the message that will be returned. In addition, the server shall end the connection after the message has been sent so that the client will know there is nothing more to expect.

The C program hellobrowser.c, which is to be found in the examples section, does just that. If you are very eager, you can compile and start it right away but it is advisable to type the lines in by yourself as they will be discussed and explained in detail.

After the necessary includes and the definition of the port which our server should listen on

#include <sys/types.h>
#include <sys/select.h>
#include <sys/socket.h>
#include <microhttpd.h>

#define PORT 8888

the desired behaviour of our server when HTTP request arrive has to be implemented. We already have agreed that it should not care about the particular details of the request, such as who is requesting what. The server will respond merely with the same small HTML page to every request.

The function we are going to write now will be called by GNU libmicrohttpd every time an appropriate request comes in. While the name of this callback function is arbitrary, its parameter list has to follow a certain layout. So please, ignore the lot of parameters for now, they will be explained at the point they are needed. We have to use only one of them, struct MHD_Connection *connection, for the minimalistic functionality we want to archive at the moment.

This parameter is set by the libmicrohttpd daemon and holds the necessary information to relate the call with a certain connection. Keep in mind that a server might have to satisfy hundreds of concurrent connections and we have to make sure that the correct data is sent to the destined client. Therefore, this variable is a means to refer to a particular connection if we ask the daemon to sent the reply.

Talking about the reply, it is defined as a string right after the function header

int answer_to_connection (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 **con_cls)
{
  const char *page  = "<html><body>Hello, browser!</body></html>";

HTTP is a rather strict protocol and the client would certainly consider it "inappropriate" if we just sent the answer string "as is". Instead, it has to be wrapped with additional information stored in so-called headers and footers. Most of the work in this area is done by the library for us—we just have to ask. Our reply string packed in the necessary layers will be called a "response". To obtain such a response we hand our data (the reply–string) and its size over to the MHD_create_response_from_buffer function. The last two parameters basically tell MHD that we do not want it to dispose the message data for us when it has been sent and there also needs no internal copy to be done because the constant string won’t change anyway.

  struct MHD_Response *response;
  int ret;

  response = MHD_create_response_from_buffer (strlen (page),
                                            (void*) page, MHD_RESPMEM_PERSISTENT);

Now that the the response has been laced up, it is ready for delivery and can be queued for sending. This is done by passing it to another GNU libmicrohttpd function. As all our work was done in the scope of one function, the recipient is without doubt the one associated with the local variable connection and consequently this variable is given to the queue function. Every HTTP response is accompanied by a status code, here "OK", so that the client knows this response is the intended result of his request and not due to some error or malfunction.

Finally, the packet is destroyed and the return value from the queue returned, already being set at this point to either MHD_YES or MHD_NO in case of success or failure.

  ret = MHD_queue_response (connection, MHD_HTTP_OK, response);
  MHD_destroy_response (response);

  return ret;
}

With the primary task of our server implemented, we can start the actual server daemon which will listen on PORT for connections. This is done in the main function.

int main ()
{
  struct MHD_Daemon *daemon;

  daemon = MHD_start_daemon (MHD_USE_SELECT_INTERNALLY, PORT, NULL, NULL, 
                             &answer_to_connection, NULL, MHD_OPTION_END);
  if (NULL == daemon) return 1;

The first parameter is one of three possible modes of operation. Here we want the daemon to run in a separate thread and to manage all incoming connections in the same thread. This means that while producing the response for one connection, the other connections will be put on hold. In this example, where the reply is already known and therefore the request is served quickly, this poses no problem.

We will allow all clients to connect regardless of their name or location, therefore we do not check them on connection and set the forth and fifth parameter to NULL.

Parameter six is the address of the function we want to be called whenever a new connection has been established. Our answer_to_connection knows best what the client wants and needs no additional information (which could be passed via the next parameter) so the next parameter is NULL. Likewise, we do not need to pass extra options to the daemon so we just write the MHD_OPTION_END as the last parameter.

As the server daemon runs in the background in its own thread, the execution flow in our main function will contine right after the call. Because of this, we must delay the execution flow in the main thread or else the program will terminate prematurely. We let it pause in a processing-time friendly manner by waiting for the enter key to be pressed. In the end, we stop the daemon so it can do its cleanup tasks.

  getchar (); 

  MHD_stop_daemon (daemon);
  return 0;
}

The first example is now complete.

Compile it with

cc hellobrowser.c -o hellobrowser -I$PATH_TO_LIBMHD_INCLUDES 
  -L$PATH_TO_LIBMHD_LIBS -lmicrohttpd

with the two paths set accordingly and run it.

Now open your favorite Internet browser and go to the address http://localhost:8888/, provided that 8888 is the port you chose. If everything works as expected, the browser will present the message of the static HTML page it got from our minimal server.

Remarks

To keep this first example as small as possible, some drastic shortcuts were taken and are to be discussed now.

Firstly, there is no distinction made between the kinds of requests a client could send. We implied that the client sends a GET request, that means, that he actually asked for some data. Even when it is not intended to accept POST requests, a good server should at least recognize that this request does not constitute a legal request and answer with an error code. This can be easily implemented by checking if the parameter method equals the string "GET" and returning a MHD_NO if not so.

Secondly, the above practice of queuing a response upon the first call of the callback function brings with it some limitations. This is because the content of the message body will not be received if a response is queued in the first iteration. Furthermore, the connection will be closed right after the response has been transferred then. This is typically not what you want as it disables HTTP pipelining. The correct approach is to simply not queue a message on the first callback unless there is an error. The void** argument to the callback provides a location for storing information about the history of the connection; for the first call, the pointer will point to NULL. A simplistic way to differenciate the first call from others is to check if the pointer is NULL and set it to a non-NULL value during the first call.

Both of these issues you will find addressed in the official minimal_example.c residing in the src/examples directory of the MHD package. The source code of this program should look very familiar to you by now and easy to understand.

For our example, the must_copy and must_free parameter at the response construction function could be set to MHD_NO. In the usual case, responses cannot be sent immediately after being queued. For example, there might be other data on the system that needs to be sent with a higher priority. Nevertheless, the queue function will return successfully—raising the problem that the data we have pointed to may be invalid by the time it is about being sent. This is not an issue here because we can expect the page string, which is a constant string literal here, to be static. That means it will be present and unchanged for as long as the program runs. For dynamic data, one could choose to either have MHD free the memory page points to itself when it is not longer needed or, alternatively, have the library to make and manage its own copy of it.

Exercises


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3. Exploring requests

This chapter will deal with the information which the client sends to the server at every request. We are going to examine the most useful fields of such an request and print them out in a readable manner. This could be useful for logging facilities.

The starting point is the hellobrowser program with the former response removed.

This time, we just want to collect information in the callback function, thus we will just return MHD_NO after we have probed the request. This way, the connection is closed without much ado by the server.

static int 
answer_to_connection (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 **con_cls)
{
  ...  
  return MHD_NO;
}

The ellipsis marks the position where the following instructions shall be inserted.

We begin with the most obvious information available to the server, the request line. You should already have noted that a request consists of a command (or "HTTP method") and a URI (e.g. a filename). It also contains a string for the version of the protocol which can be found in version. To call it a "new request" is justified because we return only MHD_NO, thus ensuring the function will not be called again for this connection.

printf ("New %s request for %s using version %s\n", method, url, version);

The rest of the information is a bit more hidden. Nevertheless, there is lot of it sent from common Internet browsers. It is stored in "key-value" pairs and we want to list what we find in the header. As there is no mandatory set of keys a client has to send, each key-value pair is printed out one by one until there are no more left. We do this by writing a separate function which will be called for each pair just like the above function is called for each HTTP request. It can then print out the content of this pair.

int print_out_key (void *cls, enum MHD_ValueKind kind, 
                   const char *key, const char *value)
{
  printf ("%s: %s\n", key, value);
  return MHD_YES;
}

To start the iteration process that calls our new function for every key, the line

MHD_get_connection_values (connection, MHD_HEADER_KIND, &print_out_key, NULL);

needs to be inserted in the connection callback function too. The second parameter tells the function that we are only interested in keys from the general HTTP header of the request. Our iterating function print_out_key does not rely on any additional information to fulfill its duties so the last parameter can be NULL.

All in all, this constitutes the complete logging.c program for this chapter which can be found in the examples section.

Connecting with any modern Internet browser should yield a handful of keys. You should try to interpret them with the aid of RFC 2616. Especially worth mentioning is the "Host" key which is often used to serve several different websites hosted under one single IP address but reachable by different domain names (this is called virtual hosting).

Conclusion

The introduced capabilities to itemize the content of a simple GET request—especially the URI—should already allow the server to satisfy clients’ requests for small specific resources (e.g. files) or even induce alteration of server state. However, the latter is not recommended as the GET method (including its header data) is by convention considered a "safe" operation, which should not change the server’s state in a significant way. By convention, GET operations can thus be performed by crawlers and other automatic software. Naturally actions like searching for a passed string are fine.

Of course, no transmission can occur while the return value is still set to MHD_NO in the callback function.

Exercises


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4. Response headers

Now that we are able to inspect the incoming request in great detail, this chapter discusses the means to enrich the outgoing responses likewise.

As you have learned in the Hello, Browser chapter, some obligatory header fields are added and set automatically for simple responses by the library itself but if more advanced features are desired, additional fields have to be created. One of the possible fields is the content type field and an example will be developed around it. This will lead to an application capable of correctly serving different types of files.

When we responded with HTML page packed in the static string previously, the client had no choice but guessing about how to handle the response, because the server had not told him. What if we had sent a picture or a sound file? Would the message have been understood or merely been displayed as an endless stream of random characters in the browser? This is what the mime content types are for. The header of the response is extended by certain information about how the data is to be interpreted.

To introduce the concept, a picture of the format PNG will be sent to the client and labeled accordingly with image/png. Once again, we can base the new example on the hellobrowser program.

#define FILENAME "picture.png"
#define MIMETYPE "image/png"

static int 
answer_to_connection (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 **con_cls)
{
  unsigned char *buffer = NULL;
  struct MHD_Response *response;

We want the program to open the file for reading and determine its size:

  int fd;
  int ret;
  struct stat sbuf;

  if (0 != strcmp (method, "GET"))
    return MHD_NO;
  if ( (-1 == (fd = open (FILENAME, O_RDONLY))) ||
       (0 != fstat (fd, &sbuf)) )
    {
     /* error accessing file */
      /* ... (see below) */
    }
 /* ... (see below) */

When dealing with files, there is a lot that could go wrong on the server side and if so, the client should be informed with MHD_HTTP_INTERNAL_SERVER_ERROR.

      /* error accessing file */
     if (fd != -1) close (fd);
      const char *errorstr =
        "<html><body>An internal server error has occured!\
                              </body></html>";
      response =
	MHD_create_response_from_buffer (strlen (errorstr), 
				         (void *) errorstr, 
				         MHD_RESPMEM_PERSISTENT);
      if (response)
        {
          ret =
            MHD_queue_response (connection, MHD_HTTP_INTERNAL_SERVER_ERROR,
                                response);
          MHD_destroy_response (response);

          return MHD_YES;
        }
      else
        return MHD_NO;
  if (!ret) 
    {
      const char *errorstr = "<html><body>An internal server error has occured!\
                              </body></html>";

      if (buffer) free(buffer);
    
      response = MHD_create_response_from_buffer (strlen(errorstr), (void*) errorstr,
                                                  MHD_RESPMEM_PERSISTENT);

      if (response)
        {     
          ret = MHD_queue_response (connection, 
	      			    MHD_HTTP_INTERNAL_SERVER_ERROR, 
				    response);
          MHD_destroy_response (response);

          return MHD_YES;    
        } 
      else return MHD_NO;
    }

Note that we nevertheless have to create a response object even for sending a simple error code. Otherwise, the connection would just be closed without comment, leaving the client curious about what has happened.

But in the case of success a response will be constructed directly from the file descriptor:

     /* error accessing file */
     /* ... (see above) */
    }

  response =
    MHD_create_response_from_fd_at_offset (sbuf.st_size, fd, 0);
  MHD_add_response_header (response, "Content-Type", MIMETYPE);
  ret = MHD_queue_response (connection, MHD_HTTP_OK, response);
  MHD_destroy_response (response);

Note that the response object will take care of closing the file desciptor for us.

Up to this point, there was little new. The actual novelty is that we enhance the header with the meta data about the content. Aware of the field’s name we want to add, it is as easy as that:

MHD_add_response_header(response, "Content-Type", MIMETYPE);

We do not have to append a colon expected by the protocol behind the first field—GNU libhttpdmicro will take care of this.

The function finishes with the well-known lines

  ret = MHD_queue_response (connection, MHD_HTTP_OK, response);
  MHD_destroy_response (response);
  return ret;
}

The complete program responseheaders.c is in the examples section as usual. Find a PNG file you like and save it to the directory the example is run from under the name picture.png. You should find the image displayed on your browser if everything worked well.

Remarks

The include file of the MHD library comes with the header types mentioned in RFC 2616 already defined as macros. Thus, we could have written MHD_HTTP_HEADER_CONTENT_TYPE instead of "Content-Type" as well. However, one is not limited to these standard headers and could add custom response headers without violating the protocol. Whether, and how, the client would react to these custom header is up to the receiver. Likewise, the client is allowed to send custom request headers to the server as well, opening up yet more possibilities how client and server could communicate with each other.

The method of creating the response from a file on disk only works for static content. Serving dynamically created responses will be a topic of a future chapter.

Exercises


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5. Supporting basic authentication

With the small exception of IP address based access control, requests from all connecting clients where served equally until now. This chapter discusses a first method of client’s authentication and its limits.

A very simple approach feasible with the means already discussed would be to expect the password in the URI string before granting access to the secured areas. The password could be separated from the actual resource identifier by a certain character, thus the request line might look like

GET /picture.png?mypassword

In the rare situation where the client is customized enough and the connection occurs through secured lines (e.g., a embedded device directly attached to another via wire) and where the ability to embedd a password in the URI or to pass on a URI with a password are desired, this can be a reasonable choice.

But when it is assumed that the user connecting does so with an ordinary Internet browser, this implementation brings some problems about. For example, the URI including the password stays in the address field or at least in the history of the browser for anybody near enough to see. It will also be inconvenient to add the password manually to any new URI when the browser does not know how to compose this automatically.

At least the convenience issue can be addressed by employing the simplest built-in password facilities of HTTP compliant browsers, hence we want to start there. It will however turn out to have still severe weaknesses in terms of security which need consideration.

Before we will start implementing Basic Authentication as described in RFC 2617, we should finally abandon the bad practice of responding every request the first time our callback is called for a given connection. This is becoming more important now because the client and the server will have to talk in a more bi-directional way than before to

But how can we tell whether the callback has been called before for the particular connection? Initially, the pointer this parameter references is set by MHD in the callback. But it will also be "remembered" on the next call (for the same connection). Thus, we will generate no response until the parameter is non-null—implying the callback was called before at least once. We do not need to share information between different calls of the callback, so we can set the parameter to any adress that is assured to be not null. The pointer to the connection structure will be pointing to a legal address, so we take this.

The first time answer_to_connection is called, we will not even look at the headers.

static int 
answer_to_connection (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 **con_cls)
{
  if (0 != strcmp(method, "GET")) return MHD_NO;
  if (NULL == *con_cls) {*con_cls = connection; return MHD_YES;}

  ... 
  /* else respond accordingly */
  ...
}

Note how we lop off the connection on the first condition (no "GET" request), but return asking for more on the other one with MHD_YES. With this minor change, we can proceed to implement the actual authentication process.

Request for authentication

Let us assume we had only files not intended to be handed out without the correct username/password, so every "GET" request will be challenged. RFC 2617 describes how the server shall ask for authentication by adding a WWW-Authenticate response header with the name of the realm protected. MHD can generate and queue such a failure response for you using the MHD_queue_basic_auth_fail_response API. The only thing you need to do is construct a response with the error page to be shown to the user if he aborts basic authentication. But first, you should check if the proper credentials were already supplied using the MHD_basic_auth_get_username_password call.

Your code would then look like this:

static int
answer_to_connection (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 **con_cls)
{
  char *user;
  char *pass;
  int fail;
  struct MHD_Response *response;

  if (0 != strcmp (method, MHD_HTTP_METHOD_GET))
    return MHD_NO;
  if (NULL == *con_cls)
    {
      *con_cls = connection;
      return MHD_YES;
    }
  pass = NULL;
  user = MHD_basic_auth_get_username_password (connection, &pass);
  fail = ( (user == NULL) ||
	   (0 != strcmp (user, "root")) ||
	   (0 != strcmp (pass, "pa$$w0rd") ) );  
  if (user != NULL) free (user);
  if (pass != NULL) free (pass);
  if (fail)
    {
      const char *page = "<html><body>Go away.</body></html>";
      response =
	MHD_create_response_from_buffer (strlen (page), (void *) page, 
				       MHD_RESPMEM_PERSISTENT);
      ret = MHD_queue_basic_auth_fail_response (connection,
						"my realm",
						response);
    }
  else
    {
      const char *page = "<html><body>A secret.</body></html>";
      response =
	MHD_create_response_from_buffer (strlen (page), (void *) page, 
				       MHD_RESPMEM_PERSISTENT);
      ret = MHD_queue_response (connection, MHD_HTTP_OK, response);
    }
  MHD_destroy_response (response);
  return ret;
}

See the examples directory for the complete example file.

Remarks

For a proper server, the conditional statements leading to a return of MHD_NO should yield a response with a more precise status code instead of silently closing the connection. For example, failures of memory allocation are best reported as internal server error and unexpected authentication methods as 400 bad request.

Exercises


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6. Processing POST data

The previous chapters already have demonstrated a variety of possibilities to send information to the HTTP server, but it is not recommended that the GET method is used to alter the way the server operates. To induce changes on the server, the POST method is preferred over and is much more powerful than GET and will be introduced in this chapter.

We are going to write an application that asks for the visitor’s name and, after the user has posted it, composes an individual response text. Even though it was not mandatory to use the POST method here, as there is no permanent change caused by the POST, it is an illustrative example on how to share data between different functions for the same connection. Furthermore, the reader should be able to extend it easily.

GET request

When the first GET request arrives, the server shall respond with a HTML page containing an edit field for the name.

const char* askpage = "<html><body>\
                       What's your name, Sir?<br>\
                       <form action=\"/namepost\" method=\"post\">\
                       <input name=\"name\" type=\"text\"\
                       <input type=\"submit\" value=\" Send \"></form>\
                       </body></html>";

The action entry is the URI to be called by the browser when posting, and the name will be used later to be sure it is the editbox’s content that has been posted.

We also prepare the answer page, where the name is to be filled in later, and an error page as the response for anything but proper GET and POST requests:

const char* greatingpage="<html><body><h1>Welcome, %s!</center></h1></body></html>";

const char* errorpage="<html><body>This doesn't seem to be right.</body></html>";

Whenever we need to send a page, we use an extra function int send_page(struct MHD_Connection *connection, const char* page) for this, which does not contain anything new and whose implementation is therefore not discussed further in the tutorial.

POST request

Posted data can be of arbitrary and considerable size; for example, if a user uploads a big image to the server. Similar to the case of the header fields, there may also be different streams of posted data, such as one containing the text of an editbox and another the state of a button. Likewise, we will have to register an iterator function that is going to be called maybe several times not only if there are different POSTs but also if one POST has only been received partly yet and needs processing before another chunk can be received.

Such an iterator function is called by a postprocessor, which must be created upon arriving of the post request. We want the iterator function to read the first post data which is tagged name and to create an individual greeting string based on the template and the name. But in order to pass this string to other functions and still be able to differentiate different connections, we must first define a structure to share the information, holding the most import entries.

struct connection_info_struct
{
  int connectiontype;
  char *answerstring;
  struct MHD_PostProcessor *postprocessor; 
};

With these information available to the iterator function, it is able to fulfill its task. Once it has composed the greeting string, it returns MHD_NO to inform the post processor that it does not need to be called again. Note that this function does not handle processing of data for the same key. If we were to expect that the name will be posted in several chunks, we had to expand the namestring dynamically as additional parts of it with the same key came in. But in this example, the name is assumed to fit entirely inside one single packet.

static 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)
{
  struct connection_info_struct *con_info = coninfo_cls;

  if (0 == strcmp (key, "name"))
    {
      if ((size > 0) && (size <= MAXNAMESIZE))
        {
          char *answerstring;
          answerstring = malloc (MAXANSWERSIZE);
          if (!answerstring) return MHD_NO;
      
          snprintf (answerstring, MAXANSWERSIZE, greatingpage, data);
          con_info->answerstring = answerstring;      
        } 
      else con_info->answerstring = NULL;

      return MHD_NO;
    }

  return MHD_YES;
}

Once a connection has been established, it can be terminated for many reasons. As these reasons include unexpected events, we have to register another function that cleans up any resources that might have been allocated for that connection by us, namely the post processor and the greetings string. This cleanup function must take into account that it will also be called for finished requests other than POST requests.

void request_completed (void *cls, struct MHD_Connection *connection, 
     		        void **con_cls,
                        enum MHD_RequestTerminationCode toe)
{
  struct connection_info_struct *con_info = *con_cls;

  if (NULL == con_info) return;
  if (con_info->connectiontype == POST)
    {
      MHD_destroy_post_processor (con_info->postprocessor);        
      if (con_info->answerstring) free (con_info->answerstring);
    }
  
  free (con_info);
  *con_cls = NULL;   
}

GNU libmicrohttpd is informed that it shall call the above function when the daemon is started in the main function.

...
daemon = MHD_start_daemon (MHD_USE_SELECT_INTERNALLY, PORT, NULL, NULL,
                           &answer_to_connection, NULL, 
			   MHD_OPTION_NOTIFY_COMPLETED, &request_completed, NULL,
			   MHD_OPTION_END);
...

Request handling

With all other functions prepared, we can now discuss the actual request handling.

On the first iteration for a new request, we start by allocating a new instance of a struct connection_info_struct structure, which will store all necessary information for later iterations and other functions.

static int 
answer_to_connection (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 **con_cls)
{
  if(NULL == *con_cls) 
    {
      struct connection_info_struct *con_info;

      con_info = malloc (sizeof (struct connection_info_struct));
      if (NULL == con_info) return MHD_NO;
      con_info->answerstring = NULL;

If the new request is a POST, the postprocessor must be created now. In addition, the type of the request is stored for convenience.

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

          if (NULL == con_info->postprocessor) 
            {
              free (con_info); 
              return MHD_NO;
            }
          con_info->connectiontype = POST;
        } 
      else con_info->connectiontype = GET;

The address of our structure will both serve as the indicator for successive iterations and to remember the particular details about the connection.

      *con_cls = (void*) con_info; 
      return MHD_YES;
    }

The rest of the function will not be executed on the first iteration. A GET request is easily satisfied by sending the question form.

  if (0 == strcmp (method, "GET")) 
    {
      return send_page (connection, askpage);     
    } 

In case of POST, we invoke the post processor for as long as data keeps incoming, setting *upload_data_size to zero in order to indicate that we have processed—or at least have considered—all of it.

  if (0 == strcmp (method, "POST")) 
    {
      struct connection_info_struct *con_info = *con_cls;

      if (*upload_data_size != 0) 
        {
          MHD_post_process (con_info->postprocessor, upload_data,	
	                    *upload_data_size);
          *upload_data_size = 0;
          
          return MHD_YES;
        } 
      else if (NULL != con_info->answerstring) 
        return send_page (connection, con_info->answerstring);
    } 

Finally, if they are neither GET nor POST requests, the error page is returned.

  return send_page(connection, errorpage); 
}

These were the important parts of the program simplepost.c.


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7. Improved processing of POST data

The previous chapter introduced a way to upload data to the server, but the developed example program has some shortcomings, such as not being able to handle larger chunks of data. In this chapter, we are going to discuss a more advanced server program that allows clients to upload a file in order to have it stored on the server’s filesystem. The server shall also watch and limit the number of clients concurrently uploading, responding with a proper busy message if necessary.

Prepared answers

We choose to operate the server with the SELECT_INTERNALLY method. This makes it easier to synchronize the global states at the cost of possible delays for other connections if the processing of a request is too slow. One of these variables that needs to be shared for all connections is the total number of clients that are uploading.

#define MAXCLIENTS      2
static unsigned int    nr_of_uploading_clients = 0;

If there are too many clients uploading, we want the server to respond to all requests with a busy message.

const char* busypage = 
  "<html><body>This server is busy, please try again later.</body></html>";

Otherwise, the server will send a form that informs the user of the current number of uploading clients, and ask her to pick a file on her local filesystem which is to be uploaded.

const char* askpage = "<html><body>\n\
                       Upload a file, please!<br>\n\
                       There are %u clients uploading at the moment.<br>\n\
                       <form action=\"/filepost\" method=\"post\" \
                         enctype=\"multipart/form-data\">\n\
                       <input name=\"file\" type=\"file\">\n\
                       <input type=\"submit\" value=\" Send \"></form>\n\
                       </body></html>";

If the upload has succeeded, the server will respond with a message saying so.

const char* completepage = "<html><body>The upload has been completed.</body></html>";

We want the server to report internal errors, such as memory shortage or file access problems, adequately.

const char* servererrorpage 
  = "<html><body>An internal server error has occured.</body></html>";
const char* fileexistspage
  = "<html><body>This file already exists.</body></html>";

It would be tolerable to send all these responses undifferentiated with a 200 HTTP_OK status code but in order to improve the HTTP conformance of our server a bit, we extend the send_page function so that it accepts individual status codes.

static int 
send_page (struct MHD_Connection *connection, 
	   const char* page, int status_code)
{
  int ret;
  struct MHD_Response *response;
  
  response = MHD_create_response_from_buffer (strlen (page), (void*) page, 
  	     				      MHD_RESPMEM_MUST_COPY);
  if (!response) return MHD_NO;
 
  ret = MHD_queue_response (connection, status_code, response);
  MHD_destroy_response (response);

  return ret;
}

Note how we ask MHD to make its own copy of the message data. The reason behind this will become clear later.

Connection cycle

The decision whether the server is busy or not is made right at the beginning of the connection. To do that at this stage is especially important for POST requests because if no response is queued at this point, and MHD_YES returned, MHD will not sent any queued messages until a postprocessor has been created and the post iterator is called at least once.

static int 
answer_to_connection (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 **con_cls)
{
  if (NULL == *con_cls) 
    {
      struct connection_info_struct *con_info;

      if (nr_of_uploading_clients >= MAXCLIENTS) 
        return send_page(connection, busypage, MHD_HTTP_SERVICE_UNAVAILABLE);

If the server is not busy, the connection_info structure is initialized as usual, with the addition of a filepointer for each connection.

      con_info = malloc (sizeof (struct connection_info_struct));
      if (NULL == con_info) return MHD_NO;
      con_info->fp = 0;

      if (0 == strcmp (method, "POST")) 
        {  
          ...
        } 
      else con_info->connectiontype = GET;

      *con_cls = (void*) con_info;
 
      return MHD_YES;
    }

For POST requests, the postprocessor is created and we register a new uploading client. From this point on, there are many possible places for errors to occur that make it necessary to interrupt the uploading process. We need a means of having the proper response message ready at all times. Therefore, the connection_info structure is extended to hold the most current response message so that whenever a response is sent, the client will get the most informative message. Here, the structure is initialized to "no error".

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

          if (NULL == con_info->postprocessor) 
            {
              free (con_info); 
              return MHD_NO;
            }

          nr_of_uploading_clients++;
          
          con_info->connectiontype = POST;
          con_info->answercode = MHD_HTTP_OK;
          con_info->answerstring = completepage;
        } 
      else con_info->connectiontype = GET;

If the connection handler is called for the second time, GET requests will be answered with the form. We can keep the buffer under function scope, because we asked MHD to make its own copy of it for as long as it is needed.

  if (0 == strcmp (method, "GET")) 
    {
      int ret;
      char buffer[1024];
        
      sprintf (buffer, askpage, nr_of_uploading_clients);
      return send_page (connection, buffer, MHD_HTTP_OK);     
    } 

The rest of the answer_to_connection function is very similar to the simplepost.c example, except the more flexible content of the responses. The POST data is processed until there is none left and the execution falls through to return an error page if the connection constituted no expected request method.

  if (0 == strcmp (method, "POST")) 
    {
      struct connection_info_struct *con_info = *con_cls;
       
      if (0 != *upload_data_size) 
        { 
          MHD_post_process (con_info->postprocessor,
	                    upload_data, *upload_data_size);
          *upload_data_size = 0;
          
          return MHD_YES;
        } 
      else 
        return send_page (connection, con_info->answerstring, 
	       		  con_info->answercode);
    } 

  return send_page(connection, errorpage, MHD_HTTP_BAD_REQUEST);
}

Storing to data

Unlike the simplepost.c example, here it is to be expected that post iterator will be called several times now. This means that for any given connection (there might be several concurrent of them) the posted data has to be written to the correct file. That is why we store a file handle in every connection_info, so that the it is preserved between successive iterations.

static 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)
{
  struct connection_info_struct *con_info = coninfo_cls;

Because the following actions depend heavily on correct file processing, which might be error prone, we default to reporting internal errors in case anything will go wrong.

con_info->answerstring = servererrorpage;
con_info->answercode = MHD_HTTP_INTERNAL_SERVER_ERROR;

In the "askpage" form, we told the client to label its post data with the "file" key. Anything else would be an error.

  if (0 != strcmp (key, "file")) return MHD_NO;

If the iterator is called for the first time, no file will have been opened yet. The filename string contains the name of the file (without any paths) the user selected on his system. We want to take this as the name the file will be stored on the server and make sure no file of that name exists (or is being uploaded) before we create one (note that the code below technically contains a race between the two "fopen" calls, but we will overlook this for portability sake).

  if (!con_info->fp)
    {
      if (NULL != (fp = fopen (filename, "rb")) )
        {
          fclose (fp);
          con_info->answerstring = fileexistspage;
          con_info->answercode = MHD_HTTP_FORBIDDEN;
          return MHD_NO;
        }
      
      con_info->fp = fopen (filename, "ab");
      if (!con_info->fp) return MHD_NO;    
    }

Occasionally, the iterator function will be called even when there are 0 new bytes to process. The server only needs to write data to the file if there is some.

if (size > 0) 
    {  
      if (!fwrite (data, size, sizeof(char), con_info->fp))
        return MHD_NO;
    }

If this point has been reached, everything worked well for this iteration and the response can be set to success again. If the upload has finished, this iterator function will not be called again.

  con_info->answerstring = completepage;
  con_info->answercode = MHD_HTTP_OK;

  return MHD_YES;
}

The new client was registered when the postprocessor was created. Likewise, we unregister the client on destroying the postprocessor when the request is completed.

void request_completed (void *cls, struct MHD_Connection *connection, 
     		        void **con_cls,
                        enum MHD_RequestTerminationCode toe)
{
  struct connection_info_struct *con_info = *con_cls;

  if (NULL == con_info) return;

  if (con_info->connectiontype == POST)
    {
      if (NULL != con_info->postprocessor) 
        {
          MHD_destroy_post_processor (con_info->postprocessor); 
          nr_of_uploading_clients--;
        }

      if (con_info->fp) fclose (con_info->fp); 
    }

  free (con_info);
  *con_cls = NULL;      
}

This is essentially the whole example largepost.c.

Remarks

Now that the clients are able to create files on the server, security aspects are becoming even more important than before. Aside from proper client authentication, the server should always make sure explicitly that no files will be created outside of a dedicated upload directory. In particular, filenames must be checked to not contain strings like "../".


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8. Session management

This chapter discusses how one should manage sessions, that is, share state between multiple HTTP requests from the same user. We use a simple example where the user submits multiple forms and the server is supposed to accumulate state from all of these forms. Naturally, as this is a network protocol, our session mechanism must support having many users with many concurrent sessions at the same time.

In order to track users, we use a simple session cookie. A session cookie expires when the user closes the browser. Changing from session cookies to persistent cookies only requires adding an expiration time to the cookie. The server creates a fresh session cookie whenever a request without a cookie is received, or if the supplied session cookie is not known to the server.

Looking up the cookie

Since MHD parses the HTTP cookie header for us, looking up an existing cookie is straightforward:

FIXME.

Here, FIXME is the name we chose for our session cookie.

Setting the cookie header

MHD requires the user to provide the full cookie format string in order to set cookies. In order to generate a unique cookie, our example creates a random 64-character text string to be used as the value of the cookie:

FIXME.

Given this cookie value, we can then set the cookie header in our HTTP response as follows:

FIXME.

Remark: Session expiration

It is of course possible that clients stop their interaction with the server at any time. In order to avoid using too much storage, the server must thus discard inactive sessions at some point. Our example implements this by discarding inactive sessions after a certain amount of time. Alternatively, the implementation may limit the total number of active sessions. Which bounds are used for idle sessions or the total number of sessions obviously depends largely on the type of the application and available server resources.

Example code

A sample application implementing a website with multiple forms (which are dynamically created using values from previous POST requests from the same session) is available as the example sessions.c.

Note that the example uses a simple, $O(n)$ linked list traversal to look up sessions and to expire old sessions. Using a hash table and a heap would be more appropriate if a large number of concurrent sessions is expected.

Remarks

Naturally, it is quite conceivable to store session data in a database instead of in memory. Still, having mechanisms to expire data associated with long-time idle sessions (where the business process has still not finished) is likely a good idea.


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9. Adding a layer of security

We left the basic authentication chapter with the unsatisfactory conclusion that any traffic, including the credentials, could be intercepted by anyone between the browser client and the server. Protecting the data while it is sent over unsecured lines will be the goal of this chapter.

Since version 0.4, the MHD library includes support for encrypting the traffic by employing SSL/TSL. If GNU libmicrohttpd has been configured to support these, encryption and decryption can be applied transparently on the data being sent, with only minimal changes to the actual source code of the example.

Preparation

First, a private key for the server will be generated. With this key, the server will later be able to authenticate itself to the client—preventing anyone else from stealing the password by faking its identity. The OpenSSL suite, which is available on many operating systems, can generate such a key. For the scope of this tutorial, we will be content with a 1024 bit key:

> openssl genrsa -out server.key 1024

In addition to the key, a certificate describing the server in human readable tokens is also needed. This certificate will be attested with our aforementioned key. In this way, we obtain a self-signed certificate, valid for one year.

> openssl req -days 365 -out server.pem -new -x509 -key server.key

To avoid unnecessary error messages in the browser, the certificate needs to have a name that matches the URI, for example, "localhost" or the domain. If you plan to have a publicly reachable server, you will need to ask a trusted third party, called Certificate Authority, or CA, to attest the certificate for you. This way, any visitor can make sure the server’s identity is real.

Whether the server’s certificate is signed by us or a third party, once it has been accepted by the client, both sides will be communicating over encrypted channels. From this point on, it is the client’s turn to authenticate itself. But this has already been implemented in the basic authentication scheme.

Changing the source code

We merely have to extend the server program so that it loads the two files into memory,

int
main ()
{
  struct MHD_Daemon *daemon;
  char *key_pem;
  char *cert_pem;

  key_pem = load_file (SERVERKEYFILE);
  cert_pem = load_file (SERVERCERTFILE);

  if ((key_pem == NULL) || (cert_pem == NULL))
  {
    printf ("The key/certificate files could not be read.\n");
    return 1;
  }

and then we point the MHD daemon to it upon initalization.

  daemon = MHD_start_daemon (MHD_USE_SELECT_INTERNALLY | MHD_USE_SSL, 
  	   		     PORT, NULL, NULL,
                             &answer_to_connection, NULL, 
                             MHD_OPTION_HTTPS_MEM_KEY, key_pem,
                             MHD_OPTION_HTTPS_MEM_CERT, cert_pem,
                             MHD_OPTION_END);

  if (NULL == daemon)
    {
      printf ("%s\n", cert_pem);
  
      free (key_pem);
      free (cert_pem);
  
      return 1;
    }

The rest consists of little new besides some additional memory cleanups.

  getchar ();

  MHD_stop_daemon (daemon);
  free (key_pem);
  free (cert_pem);
  
  return 0;
}

The rather unexciting file loader can be found in the complete example tlsauthentication.c.

Remarks

Client authentication

You can also use MHD to authenticate the client via SSL/TLS certificates (as an alternative to using the password-based Basic or Digest authentication). To do this, you will need to link your application against gnutls. Next, when you start the MHD daemon, you must specify the root CA that you’re willing to trust:

  daemon = MHD_start_daemon (MHD_USE_SELECT_INTERNALLY | MHD_USE_SSL, 
  	   		     PORT, NULL, NULL,
                             &answer_to_connection, NULL, 
                             MHD_OPTION_HTTPS_MEM_KEY, key_pem,
                             MHD_OPTION_HTTPS_MEM_CERT, cert_pem,
			     MHD_OPTION_HTTPS_MEM_TRUST, root_ca_pem,
                             MHD_OPTION_END);

With this, you can then obtain client certificates for each session. In order to obtain the identity of the client, you first need to obtain the raw GnuTLS session handle from MHD using MHD_get_connection_info.

#include <gnutls/gnutls.h>
#include <gnutls/x509.h>

gnutls_session_t tls_session;
union MHD_ConnectionInfo *ci;

ci = MHD_get_connection_info (connection, 
                              MHD_CONNECTION_INFO_GNUTLS_SESSION);
tls_session = ci->tls_session;

You can then extract the client certificate:

/**
 * Get the client's certificate
 *
 * @param tls_session the TLS session
 * @return NULL if no valid client certificate could be found, a pointer
 *  	to the certificate if found 
 */
static gnutls_x509_crt_t
get_client_certificate (gnutls_session_t tls_session) 
{
  unsigned int listsize;
  const gnutls_datum_t * pcert;
  gnutls_certificate_status_t client_cert_status;
  gnutls_x509_crt_t client_cert;

  if (tls_session == NULL) 
    return NULL;
  if (gnutls_certificate_verify_peers2(tls_session,
				       &client_cert_status)) 
    return NULL;
  pcert = gnutls_certificate_get_peers(tls_session, 
				       &listsize);
  if ( (pcert == NULL) || 
       (listsize == 0)) 
    {
      fprintf (stderr,
	       "Failed to retrieve client certificate chain\n");
      return NULL;
    }    
  if (gnutls_x509_crt_init(&client_cert)) 
    {
      fprintf (stderr,
	       "Failed to initialize client certificate\n");
      return NULL;
    }
  /* Note that by passing values between 0 and listsize here, you
     can get access to the CA's certs */
  if (gnutls_x509_crt_import(client_cert, 
			     &pcert[0],
			     GNUTLS_X509_FMT_DER)) 
    {
      fprintf (stderr,
	       "Failed to import client certificate\n");
      gnutls_x509_crt_deinit(client_cert);
      return NULL;
    }  
  return client_cert;
}

Using the client certificate, you can then get the client’s distinguished name and alternative names:

/**
 * Get the distinguished name from the client's certificate
 *
 * @param client_cert the client certificate
 * @return NULL if no dn or certificate could be found, a pointer
 * 			to the dn if found
 */
char *
cert_auth_get_dn(gnutls_x509_crt_c client_cert) 
{
  char* buf;
  size_t lbuf;  

  lbuf = 0;
  gnutls_x509_crt_get_dn(client_cert, NULL, &lbuf);
  buf = malloc(lbuf);
  if (buf == NULL) 
    {
      fprintf (stderr,
	       "Failed to allocate memory for certificate dn\n");
      return NULL;
    }
  gnutls_x509_crt_get_dn(client_cert, buf, &lbuf);
  return buf;
}


/**
 * Get the alternative name of specified type from the client's certificate
 *
 * @param client_cert the client certificate
 * @param nametype The requested name type
 * @param index The position of the alternative name if multiple names are
 * 			matching the requested type, 0 for the first matching name
 * @return NULL if no matching alternative name could be found, a pointer
 * 			to the alternative name if found
 */
char *
MHD_cert_auth_get_alt_name(gnutls_x509_crt_t client_cert,
			   int nametype, 
			   unsigned int index) 
{
  char* buf;
  size_t lbuf;
  unsigned int seq;
  unsigned int subseq;
  unsigned int type;
  int result;

  subseq = 0;
  for (seq=0;;seq++) 
    {
      lbuf = 0;
      result = gnutls_x509_crt_get_subject_alt_name2(client_cert, seq, NULL, &lbuf,
						     &type, NULL);
      if (result == GNUTLS_E_REQUESTED_DATA_NOT_AVAILABLE)
	return NULL;
      if (nametype != (int) type)
	continue;
      if (subseq == index) 
	break;
      subseq++;
    }
  buf = malloc(lbuf);
  if (buf == NULL) 
    {
      fprintf (stderr,
	       "Failed to allocate memory for certificate alt name\n");
      return NULL;
    }
  result = gnutls_x509_crt_get_subject_alt_name2(client_cert, 
						 seq,
						 buf,
						 &lbuf, 
						 NULL, NULL);
  if (result != nametype)
    {
      fprintf (stderr,
	       "Unexpected return value from gnutls: %d\n",
	       result);
      free (buf);
      return NULL;
    }
  return buf;
}

Finally, you should release the memory associated with the client certificate:

gnutls_x509_crt_deinit (client_cert);

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A. Bibliography

API reference


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B. GNU Free Documentation License

Version 1.3, 3 November 2008

 
Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
http://fsf.org/

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
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    You may also lend copies, under the same conditions stated above, and you may publicly display copies.

  4. COPYING IN QUANTITY

    If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document’s license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.

    If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.

    If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard network protocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.

    It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.

  5. MODIFICATIONS

    You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:

    1. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.
    2. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has fewer than five), unless they release you from this requirement.
    3. State on the Title page the name of the publisher of the Modified Version, as the publisher.
    4. Preserve all the copyright notices of the Document.
    5. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.
    6. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below.
    7. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document’s license notice.
    8. Include an unaltered copy of this License.
    9. Preserve the section Entitled “History”, Preserve its Title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section Entitled “History” in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.
    10. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the “History” section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.
    11. For any section Entitled “Acknowledgements” or “Dedications”, Preserve the Title of the section, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein.
    12. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.
    13. Delete any section Entitled “Endorsements”. Such a section may not be included in the Modified Version.
    14. Do not retitle any existing section to be Entitled “Endorsements” or to conflict in title with any Invariant Section.
    15. Preserve any Warranty Disclaimers.

    If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version’s license notice. These titles must be distinct from any other section titles.

    You may add a section Entitled “Endorsements”, provided it contains nothing but endorsements of your Modified Version by various parties—for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.

    You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.

    The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.

  6. COMBINING DOCUMENTS

    You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.

    The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.

    In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”

  7. COLLECTIONS OF DOCUMENTS

    You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

    You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.

  8. AGGREGATION WITH INDEPENDENT WORKS

    A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation’s users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.

    If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document’s Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.

  9. TRANSLATION

    Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.

    If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.

  10. TERMINATION

    You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License.

    However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.

    Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.

    Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it.

  11. FUTURE REVISIONS OF THIS LICENSE

    The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.

    Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a proxy can decide which future versions of this License can be used, that proxy’s public statement of acceptance of a version permanently authorizes you to choose that version for the Document.

  12. RELICENSING

    “Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in the site means any set of copyrightable works thus published on the MMC site.

    “CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization.

    “Incorporate” means to publish or republish a Document, in whole or in part, as part of another Document.

    An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.

    The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.

ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

 
  Copyright (C)  year  your name.
  Permission is granted to copy, distribute and/or modify this document
  under the terms of the GNU Free Documentation License, Version 1.3
  or any later version published by the Free Software Foundation;
  with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
  Texts.  A copy of the license is included in the section entitled ``GNU
  Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with…Texts.” line with this:

 
    with the Invariant Sections being list their titles, with
    the Front-Cover Texts being list, and with the Back-Cover Texts
    being list.

If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.


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C. Example programs


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C.1 hellobrowser.c

 
#include <sys/types.h>
#include <sys/select.h>
#include <sys/socket.h>
#include <microhttpd.h>

#define PORT 8888

static int
answer_to_connection (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 **con_cls)
{
  const char *page = "<html><body>Hello, browser!</body></html>";
  struct MHD_Response *response;
  int ret;
  
  response =
    MHD_create_response_from_buffer (strlen (page), (void *) page, 
				     MHD_RESPMEM_PERSISTENT);
  ret = MHD_queue_response (connection, MHD_HTTP_OK, response);
  MHD_destroy_response (response);

  return ret;
}

int
main ()
{
  struct MHD_Daemon *daemon;

  daemon = MHD_start_daemon (MHD_USE_SELECT_INTERNALLY, PORT, NULL, NULL,
                             &answer_to_connection, NULL, MHD_OPTION_END);
  if (NULL == daemon)
    return 1;

  getchar ();

  MHD_stop_daemon (daemon);
  return 0;
}

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C.2 logging.c

 
#include <sys/types.h>
#include <sys/select.h>
#include <sys/socket.h>
#include <microhttpd.h>

#define PORT 8888


static int
print_out_key (void *cls, enum MHD_ValueKind kind, const char *key,
               const char *value)
{
  printf ("%s: %s\n", key, value);
  return MHD_YES;
}

static int
answer_to_connection (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 **con_cls)
{
  printf ("New %s request for %s using version %s\n", method, url, version);

  MHD_get_connection_values (connection, MHD_HEADER_KIND, print_out_key,
                             NULL);

  return MHD_NO;
}

int
main ()
{
  struct MHD_Daemon *daemon;

  daemon = MHD_start_daemon (MHD_USE_SELECT_INTERNALLY, PORT, NULL, NULL,
                             &answer_to_connection, NULL, MHD_OPTION_END);
  if (NULL == daemon)
    return 1;

  getchar ();

  MHD_stop_daemon (daemon);
  return 0;
}

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C.3 responseheaders.c

 
#include <sys/types.h>
#include <sys/select.h>
#include <sys/socket.h>
#include <microhttpd.h>
#include <time.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>

#define PORT 8888
#define FILENAME "picture.png"
#define MIMETYPE "image/png"

static int
answer_to_connection (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 **con_cls)
{
  struct MHD_Response *response;
  int fd;
  int ret;
  struct stat sbuf;

  if (0 != strcmp (method, "GET"))
    return MHD_NO;

  if ( (-1 == (fd = open (FILENAME, O_RDONLY))) ||
       (0 != fstat (fd, &sbuf)) )
    {
      /* error accessing file */
      if (fd != -1) close (fd);
      const char *errorstr =
        "<html><body>An internal server error has occured!\
                              </body></html>";
      response =
	MHD_create_response_from_buffer (strlen (errorstr), 
					 (void *) errorstr, 
					 MHD_RESPMEM_PERSISTENT);
      if (response)
        {
          ret =
            MHD_queue_response (connection, MHD_HTTP_INTERNAL_SERVER_ERROR,
                                response);
          MHD_destroy_response (response);

          return MHD_YES;
        }
      else
        return MHD_NO;
    }
  response =
    MHD_create_response_from_fd_at_offset (sbuf.st_size, fd, 0);
  MHD_add_response_header (response, "Content-Type", MIMETYPE);
  ret = MHD_queue_response (connection, MHD_HTTP_OK, response);
  MHD_destroy_response (response);

  return ret;
}


int
main ()
{
  struct MHD_Daemon *daemon;

  daemon = MHD_start_daemon (MHD_USE_SELECT_INTERNALLY, PORT, NULL, NULL,
                             &answer_to_connection, NULL, MHD_OPTION_END);
  if (NULL == daemon)
    return 1;

  getchar ();

  MHD_stop_daemon (daemon);

  return 0;
}

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C.4 basicauthentication.c

 
#include <sys/types.h>
#include <sys/select.h>
#include <sys/socket.h>
#include <microhttpd.h>
#include <time.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>

#define PORT 8888


static int
answer_to_connection (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 **con_cls)
{
  char *user;
  char *pass;
  int fail;
  int ret;
  struct MHD_Response *response;

  if (0 != strcmp (method, "GET"))
    return MHD_NO;
  if (NULL == *con_cls)
    {
      *con_cls = connection;
      return MHD_YES;
    }
  pass = NULL;
  user = MHD_basic_auth_get_username_password (connection, &pass);
  fail = ( (user == NULL) ||
	   (0 != strcmp (user, "root")) ||
	   (0 != strcmp (pass, "pa$$w0rd") ) );  
  if (user != NULL) free (user);
  if (pass != NULL) free (pass);
  if (fail)
    {
      const char *page = "<html><body>Go away.</body></html>";
      response =
	MHD_create_response_from_buffer (strlen (page), (void *) page,
					 MHD_RESPMEM_PERSISTENT);
      ret = MHD_queue_basic_auth_fail_response (connection,
						"my realm",
						response);
    }
  else
    {
      const char *page = "<html><body>A secret.</body></html>";
      response =
	MHD_create_response_from_buffer (strlen (page), (void *) page,
					 MHD_RESPMEM_PERSISTENT);
      ret = MHD_queue_response (connection, MHD_HTTP_OK, response);
    }
  MHD_destroy_response (response);
  return ret;
}


int
main ()
{
  struct MHD_Daemon *daemon;

  daemon = MHD_start_daemon (MHD_USE_SELECT_INTERNALLY, PORT, NULL, NULL,
                             &answer_to_connection, NULL, MHD_OPTION_END);
  if (NULL == daemon)
    return 1;

  getchar ();

  MHD_stop_daemon (daemon);
  return 0;
}

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C.5 simplepost.c

 
#include <sys/types.h>
#include <sys/select.h>
#include <sys/socket.h>
#include <microhttpd.h>

#define PORT            8888
#define POSTBUFFERSIZE  512
#define MAXNAMESIZE     20
#define MAXANSWERSIZE   512

#define GET             0
#define POST            1

struct connection_info_struct
{
  int connectiontype;
  char *answerstring;
  struct MHD_PostProcessor *postprocessor;
};

const char *askpage = "<html><body>\
                       What's your name, Sir?<br>\
                       <form action=\"/namepost\" method=\"post\">\
                       <input name=\"name\" type=\"text\"\
                       <input type=\"submit\" value=\" Send \"></form>\
                       </body></html>";

const char *greetingpage =
  "<html><body><h1>Welcome, %s!</center></h1></body></html>";

const char *errorpage =
  "<html><body>This doesn't seem to be right.</body></html>";


static int
send_page (struct MHD_Connection *connection, const char *page)
{
  int ret;
  struct MHD_Response *response;


  response =
    MHD_create_response_from_buffer (strlen (page), (void *) page,
				     MHD_RESPMEM_PERSISTENT);
  if (!response)
    return MHD_NO;

  ret = MHD_queue_response (connection, MHD_HTTP_OK, response);
  MHD_destroy_response (response);

  return ret;
}


static 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)
{
  struct connection_info_struct *con_info = coninfo_cls;

  if (0 == strcmp (key, "name"))
    {
      if ((size > 0) && (size <= MAXNAMESIZE))
        {
          char *answerstring;
          answerstring = malloc (MAXANSWERSIZE);
          if (!answerstring)
            return MHD_NO;

          snprintf (answerstring, MAXANSWERSIZE, greetingpage, data);
          con_info->answerstring = answerstring;
        }
      else
        con_info->answerstring = NULL;

      return MHD_NO;
    }

  return MHD_YES;
}

static void
request_completed (void *cls, struct MHD_Connection *connection,
                   void **con_cls, enum MHD_RequestTerminationCode toe)
{
  struct connection_info_struct *con_info = *con_cls;

  if (NULL == con_info)
    return;

  if (con_info->connectiontype == POST)
    {
      MHD_destroy_post_processor (con_info->postprocessor);
      if (con_info->answerstring)
        free (con_info->answerstring);
    }

  free (con_info);
  *con_cls = NULL;
}


static int
answer_to_connection (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 **con_cls)
{
  if (NULL == *con_cls)
    {
      struct connection_info_struct *con_info;

      con_info = malloc (sizeof (struct connection_info_struct));
      if (NULL == con_info)
        return MHD_NO;
      con_info->answerstring = NULL;

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

          if (NULL == con_info->postprocessor)
            {
              free (con_info);
              return MHD_NO;
            }

          con_info->connectiontype = POST;
        }
      else
        con_info->connectiontype = GET;

      *con_cls = (void *) con_info;

      return MHD_YES;
    }

  if (0 == strcmp (method, "GET"))
    {
      return send_page (connection, askpage);
    }

  if (0 == strcmp (method, "POST"))
    {
      struct connection_info_struct *con_info = *con_cls;

      if (*upload_data_size != 0)
        {
          MHD_post_process (con_info->postprocessor, upload_data,
                            *upload_data_size);
          *upload_data_size = 0;

          return MHD_YES;
        }
      else if (NULL != con_info->answerstring)
        return send_page (connection, con_info->answerstring);
    }

  return send_page (connection, errorpage);
}

int
main ()
{
  struct MHD_Daemon *daemon;

  daemon = MHD_start_daemon (MHD_USE_SELECT_INTERNALLY, PORT, NULL, NULL,
                             &answer_to_connection, NULL,
                             MHD_OPTION_NOTIFY_COMPLETED, request_completed,
                             NULL, MHD_OPTION_END);
  if (NULL == daemon)
    return 1;

  getchar ();

  MHD_stop_daemon (daemon);

  return 0;
}

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C.6 largepost.c

 
#include <sys/types.h>
#include <sys/select.h>
#include <sys/socket.h>
#include <microhttpd.h>

#define PORT            8888
#define POSTBUFFERSIZE  512
#define MAXCLIENTS      2

#define GET             0
#define POST            1

static unsigned int nr_of_uploading_clients = 0;

struct connection_info_struct
{
  int connectiontype;
  struct MHD_PostProcessor *postprocessor;
  FILE *fp;
  const char *answerstring;
  int answercode;
};

const char *askpage = "<html><body>\n\
                       Upload a file, please!<br>\n\
                       There are %u clients uploading at the moment.<br>\n\
                       <form action=\"/filepost\" method=\"post\" enctype=\"multipart/form-data\">\n\
                       <input name=\"file\" type=\"file\">\n\
                       <input type=\"submit\" value=\" Send \"></form>\n\
                       </body></html>";

const char *busypage =
  "<html><body>This server is busy, please try again later.</body></html>";

const char *completepage =
  "<html><body>The upload has been completed.</body></html>";

const char *errorpage =
  "<html><body>This doesn't seem to be right.</body></html>";
const char *servererrorpage =
  "<html><body>An internal server error has occured.</body></html>";
const char *fileexistspage =
  "<html><body>This file already exists.</body></html>";


static int
send_page (struct MHD_Connection *connection, const char *page,
           int status_code)
{
  int ret;
  struct MHD_Response *response;

  response =
    MHD_create_response_from_buffer (strlen (page), (void *) page,
				     MHD_RESPMEM_PERSISTENT);
  if (!response)
    return MHD_NO;

  ret = MHD_queue_response (connection, status_code, response);
  MHD_destroy_response (response);

  return ret;
}


static 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)
{
  struct connection_info_struct *con_info = coninfo_cls;
  FILE *fp;

  con_info->answerstring = servererrorpage;
  con_info->answercode = MHD_HTTP_INTERNAL_SERVER_ERROR;

  if (0 != strcmp (key, "file"))
    return MHD_NO;

  if (!con_info->fp)
    {
      if (NULL != (fp = fopen (filename, "rb")))
        {
          fclose (fp);
          con_info->answerstring = fileexistspage;
          con_info->answercode = MHD_HTTP_FORBIDDEN;
          return MHD_NO;
        }

      con_info->fp = fopen (filename, "ab");
      if (!con_info->fp)
        return MHD_NO;
    }

  if (size > 0)
    {
      if (!fwrite (data, size, sizeof (char), con_info->fp))
        return MHD_NO;
    }

  con_info->answerstring = completepage;
  con_info->answercode = MHD_HTTP_OK;

  return MHD_YES;
}

static void
request_completed (void *cls, struct MHD_Connection *connection,
                   void **con_cls, enum MHD_RequestTerminationCode toe)
{
  struct connection_info_struct *con_info = *con_cls;

  if (NULL == con_info)
    return;

  if (con_info->connectiontype == POST)
    {
      if (NULL != con_info->postprocessor)
        {
          MHD_destroy_post_processor (con_info->postprocessor);
          nr_of_uploading_clients--;
        }

      if (con_info->fp)
        fclose (con_info->fp);
    }

  free (con_info);
  *con_cls = NULL;
}


static int
answer_to_connection (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 **con_cls)
{
  if (NULL == *con_cls)
    {
      struct connection_info_struct *con_info;

      if (nr_of_uploading_clients >= MAXCLIENTS)
        return send_page (connection, busypage, MHD_HTTP_SERVICE_UNAVAILABLE);

      con_info = malloc (sizeof (struct connection_info_struct));
      if (NULL == con_info)
        return MHD_NO;

      con_info->fp = NULL;

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

          if (NULL == con_info->postprocessor)
            {
              free (con_info);
              return MHD_NO;
            }

          nr_of_uploading_clients++;

          con_info->connectiontype = POST;
          con_info->answercode = MHD_HTTP_OK;
          con_info->answerstring = completepage;
        }
      else
        con_info->connectiontype = GET;

      *con_cls = (void *) con_info;

      return MHD_YES;
    }

  if (0 == strcmp (method, "GET"))
    {
      int ret;
      char buffer[1024];

      sprintf (buffer, askpage, nr_of_uploading_clients);
      return send_page (connection, buffer, MHD_HTTP_OK);
    }

  if (0 == strcmp (method, "POST"))
    {
      struct connection_info_struct *con_info = *con_cls;

      if (0 != *upload_data_size)
        {
          MHD_post_process (con_info->postprocessor, upload_data,
                            *upload_data_size);
          *upload_data_size = 0;

          return MHD_YES;
        }
      else
        return send_page (connection, con_info->answerstring,
                          con_info->answercode);
    }

  return send_page (connection, errorpage, MHD_HTTP_BAD_REQUEST);
}

int
main ()
{
  struct MHD_Daemon *daemon;


  daemon = MHD_start_daemon (MHD_USE_SELECT_INTERNALLY, PORT, NULL, NULL,
                             &answer_to_connection, NULL,
                             MHD_OPTION_NOTIFY_COMPLETED, request_completed,
                             NULL, MHD_OPTION_END);
  if (NULL == daemon)
    return 1;

  getchar ();

  MHD_stop_daemon (daemon);

  return 0;
}

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C.7 sessions.c

 
/*
     This file is part of libmicrohttpd
     (C) 2011 Christian Grothoff (and other contributing authors)

     This library is free software; you can redistribute it and/or
     modify it under the terms of the GNU Lesser General Public
     License as published by the Free Software Foundation; either
     version 2.1 of the License, or (at your option) any later version.

     This library is distributed in the hope that it will be useful,
     but WITHOUT ANY WARRANTY; without even the implied warranty of
     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
     Lesser General Public License for more details.

     You should have received a copy of the GNU Lesser General Public
     License along with this library; if not, write to the Free Software
     Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
*/
/**
 * @file post_example.c
 * @brief example for processing POST requests using libmicrohttpd
 * @author Christian Grothoff
 */

/* needed for asprintf */
#define _GNU_SOURCE


#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <errno.h>
#include <time.h>
#include <microhttpd.h>

/**
 * Invalid method page.
 */
#define METHOD_ERROR "<html><head><title>Illegal request</title></head><body>Go away.</body></html>"

/**
 * Invalid URL page.
 */
#define NOT_FOUND_ERROR "<html><head><title>Not found</title></head><body>Go away.</body></html>"

/**
 * Front page. (/)
 */
#define MAIN_PAGE "<html><head><title>Welcome</title></head><body><form action=\"/2\" method=\"post\">What is your name? <input type=\"text\" name=\"v1\" value=\"%s\" /><input type=\"submit\" value=\"Next\" /></body></html>"

/**
 * Second page. (/2)
 */
#define SECOND_PAGE "<html><head><title>Tell me more</title></head><body><a href=\"/\">previous</a> <form action=\"/S\" method=\"post\">%s, what is your job? <input type=\"text\" name=\"v2\" value=\"%s\" /><input type=\"submit\" value=\"Next\" /></body></html>"

/**
 * Second page (/S)
 */
#define SUBMIT_PAGE "<html><head><title>Ready to submit?</title></head><body><form action=\"/F\" method=\"post\"><a href=\"/2\">previous </a> <input type=\"hidden\" name=\"DONE\" value=\"yes\" /><input type=\"submit\" value=\"Submit\" /></body></html>"

/**
 * Last page.
 */
#define LAST_PAGE "<html><head><title>Thank you</title></head><body>Thank you.</body></html>"

/**
 * Name of our cookie.
 */
#define COOKIE_NAME "session"


/**
 * State we keep for each user/session/browser.
 */
struct Session
{
  /**
   * We keep all sessions in a linked list.
   */
  struct Session *next;

  /**
   * Unique ID for this session. 
   */
  char sid[33];

  /**
   * Reference counter giving the number of connections
   * currently using this session.
   */
  unsigned int rc;

  /**
   * Time when this session was last active.
   */
  time_t start;

  /**
   * String submitted via form.
   */
  char value_1[64];

  /**
   * Another value submitted via form.
   */
  char value_2[64];

};


/**
 * Data kept per request.
 */
struct Request
{

  /**
   * Associated session.
   */
  struct Session *session;

  /**
   * Post processor handling form data (IF this is
   * a POST request).
   */
  struct MHD_PostProcessor *pp;

  /**
   * URL to serve in response to this POST (if this request 
   * was a 'POST')
   */
  const char *post_url;

};


/**
 * Linked list of all active sessions.  Yes, O(n) but a
 * hash table would be overkill for a simple example...
 */
static struct Session *sessions;




/**
 * Return the session handle for this connection, or 
 * create one if this is a new user.
 */
static struct Session *
get_session (struct MHD_Connection *connection)
{
  struct Session *ret;
  const char *cookie;

  cookie = MHD_lookup_connection_value (connection,
					MHD_COOKIE_KIND,
					COOKIE_NAME);
  if (cookie != NULL)
    {
      /* find existing session */
      ret = sessions;
      while (NULL != ret)
	{
	  if (0 == strcmp (cookie, ret->sid))
	    break;
	  ret = ret->next;
	}
      if (NULL != ret)
	{
	  ret->rc++;
	  return ret;
	}
    }
  /* create fresh session */
  ret = calloc (1, sizeof (struct Session));
  if (NULL == ret)
    {						
      fprintf (stderr, "calloc error: %s\n", strerror (errno));
      return NULL; 
    }
  /* not a super-secure way to generate a random session ID,
     but should do for a simple example... */
  snprintf (ret->sid,
	    sizeof (ret->sid),
	    "%X%X%X%X",
	    (unsigned int) random (),
	    (unsigned int) random (),
	    (unsigned int) random (),
	    (unsigned int) random ());
  ret->rc++;  
  ret->start = time (NULL);
  ret->next = sessions;
  sessions = ret;
  return ret;
}


/**
 * Type of handler that generates a reply.
 *
 * @param cls content for the page (handler-specific)
 * @param mime mime type to use
 * @param session session information
 * @param connection connection to process
 * @param MHD_YES on success, MHD_NO on failure
 */
typedef int (*PageHandler)(const void *cls,
			   const char *mime,
			   struct Session *session,
			   struct MHD_Connection *connection);


/**
 * Entry we generate for each page served.
 */ 
struct Page
{
  /**
   * Acceptable URL for this page.
   */
  const char *url;

  /**
   * Mime type to set for the page.
   */
  const char *mime;

  /**
   * Handler to call to generate response.
   */
  PageHandler handler;

  /**
   * Extra argument to handler.
   */ 
  const void *handler_cls;
};


/**
 * Add header to response to set a session cookie.
 *
 * @param session session to use
 * @param response response to modify
 */ 
static void
add_session_cookie (struct Session *session,
		    struct MHD_Response *response)
{
  char cstr[256];
  snprintf (cstr,
	    sizeof (cstr),
	    "%s=%s",
	    COOKIE_NAME,
	    session->sid);
  if (MHD_NO == 
      MHD_add_response_header (response,
			       MHD_HTTP_HEADER_SET_COOKIE,
			       cstr))
    {
      fprintf (stderr, 
	       "Failed to set session cookie header!\n");
    }
}


/**
 * Handler that returns a simple static HTTP page that
 * is passed in via 'cls'.
 *
 * @param cls a 'const char *' with the HTML webpage to return
 * @param mime mime type to use
 * @param session session handle 
 * @param connection connection to use
 */
static int
serve_simple_form (const void *cls,
		   const char *mime,
		   struct Session *session,
		   struct MHD_Connection *connection)
{
  int ret;
  const char *form = cls;
  struct MHD_Response *response;

  /* return static form */
  response = MHD_create_response_from_buffer (strlen (form),
					      (void *) form,
					      MHD_RESPMEM_PERSISTENT);
  add_session_cookie (session, response);
  MHD_add_response_header (response,
			   MHD_HTTP_HEADER_CONTENT_ENCODING,
			   mime);
  ret = MHD_queue_response (connection, 
			    MHD_HTTP_OK, 
			    response);
  MHD_destroy_response (response);
  return ret;
}


/**
 * Handler that adds the 'v1' value to the given HTML code.
 *
 * @param cls a 'const char *' with the HTML webpage to return
 * @param mime mime type to use
 * @param session session handle 
 * @param connection connection to use
 */
static int
fill_v1_form (const void *cls,
	      const char *mime,
	      struct Session *session,
	      struct MHD_Connection *connection)
{
  int ret;
  const char *form = cls;
  char *reply;
  struct MHD_Response *response;

  if (-1 == asprintf (&reply,
		      form,
		      session->value_1))
    {
      /* oops */
      return MHD_NO;
    }
  /* return static form */
  response = MHD_create_response_from_buffer (strlen (reply),
					      (void *) reply,
					      MHD_RESPMEM_MUST_FREE);
  add_session_cookie (session, response);
  MHD_add_response_header (response,
			   MHD_HTTP_HEADER_CONTENT_ENCODING,
			   mime);
  ret = MHD_queue_response (connection, 
			    MHD_HTTP_OK, 
			    response);
  MHD_destroy_response (response);
  return ret;
}


/**
 * Handler that adds the 'v1' and 'v2' values to the given HTML code.
 *
 * @param cls a 'const char *' with the HTML webpage to return
 * @param mime mime type to use
 * @param session session handle 
 * @param connection connection to use
 */
static int
fill_v1_v2_form (const void *cls,
		 const char *mime,
		 struct Session *session,
		 struct MHD_Connection *connection)
{
  int ret;
  const char *form = cls;
  char *reply;
  struct MHD_Response *response;

  if (-1 == asprintf (&reply,
		      form,
		      session->value_1,
		      session->value_2))
    {
      /* oops */
      return MHD_NO;
    }
  /* return static form */
  response = MHD_create_response_from_buffer (strlen (reply),
					      (void *) reply,
					      MHD_RESPMEM_MUST_FREE);
  add_session_cookie (session, response);
  MHD_add_response_header (response,
			   MHD_HTTP_HEADER_CONTENT_ENCODING,
			   mime);
  ret = MHD_queue_response (connection, 
			    MHD_HTTP_OK, 
			    response);
  MHD_destroy_response (response);
  return ret;
}


/**
 * Handler used to generate a 404 reply.
 *
 * @param cls a 'const char *' with the HTML webpage to return
 * @param mime mime type to use
 * @param session session handle 
 * @param connection connection to use
 */
static int
not_found_page (const void *cls,
		const char *mime,
		struct Session *session,
		struct MHD_Connection *connection)
{
  int ret;
  struct MHD_Response *response;

  /* unsupported HTTP method */
  response = MHD_create_response_from_buffer (strlen (NOT_FOUND_ERROR),
					      (void *) NOT_FOUND_ERROR,
					      MHD_RESPMEM_PERSISTENT);
  ret = MHD_queue_response (connection, 
			    MHD_HTTP_NOT_FOUND, 
			    response);
  MHD_add_response_header (response,
			   MHD_HTTP_HEADER_CONTENT_ENCODING,
			   mime);
  MHD_destroy_response (response);
  return ret;
}


/**
 * List of all pages served by this HTTP server.
 */
static struct Page pages[] = 
  {
    { "/", "text/html",  &fill_v1_form, MAIN_PAGE },
    { "/2", "text/html", &fill_v1_v2_form, SECOND_PAGE },
    { "/S", "text/html", &serve_simple_form, SUBMIT_PAGE },
    { "/F", "text/html", &serve_simple_form, LAST_PAGE },
    { NULL, NULL, &not_found_page, NULL } /* 404 */
  };



/**
 * Iterator over key-value pairs where the value
 * maybe made available in increments and/or may
 * not be zero-terminated.  Used for processing
 * POST data.
 *
 * @param cls user-specified closure
 * @param kind type of the value
 * @param key 0-terminated key for the value
 * @param filename name of the uploaded file, NULL if not known
 * @param content_type mime-type of the data, NULL if not known
 * @param transfer_encoding encoding of the data, NULL if not known
 * @param data pointer to size bytes of data at the
 *              specified offset
 * @param off offset of data in the overall value
 * @param size number of bytes in data available
 * @return MHD_YES to continue iterating,
 *         MHD_NO to abort the iteration
 */
static int
post_iterator (void *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)
{
  struct Request *request = cls;
  struct Session *session = request->session;

  if (0 == strcmp ("DONE", key))
    {
      fprintf (stdout,
	       "Session `%s' submitted `%s', `%s'\n",
	       session->sid,
	       session->value_1,
	       session->value_2);
      return MHD_YES;
    }
  if (0 == strcmp ("v1", key))
    {
      if (size + off > sizeof(session->value_1))
	size = sizeof (session->value_1) - off;
      memcpy (&session->value_1[off],
	      data,
	      size);
      if (size + off < sizeof (session->value_1))
	session->value_1[size+off] = '\0';
      return MHD_YES;
    }
  if (0 == strcmp ("v2", key))
    {
      if (size + off > sizeof(session->value_2))
	size = sizeof (session->value_2) - off;
      memcpy (&session->value_2[off],
	      data,
	      size);
      if (size + off < sizeof (session->value_2))
	session->value_2[size+off] = '\0';
      return MHD_YES;
    }
  fprintf (stderr, "Unsupported form value `%s'\n", key);
  return MHD_YES;
}


/**
 * Main MHD callback for handling requests.
 *
 *
 * @param cls argument given together with the function
 *        pointer when the handler was registered with MHD
 * @param url the requested url
 * @param method the HTTP method used ("GET", "PUT", etc.)
 * @param version the HTTP version string (i.e. "HTTP/1.1")
 * @param upload_data the data being uploaded (excluding HEADERS,
 *        for a POST that fits into memory and that is encoded
 *        with a supported encoding, the POST data will NOT be
 *        given in upload_data and is instead available as
 *        part of MHD_get_connection_values; very large POST
 *        data *will* be made available incrementally in
 *        upload_data)
 * @param upload_data_size set initially to the size of the
 *        upload_data provided; the method must update this
 *        value to the number of bytes NOT processed;
 * @param con_cls pointer that the callback can set to some
 *        address and that will be preserved by MHD for future
 *        calls for this request; since the access handler may
 *        be called many times (i.e., for a PUT/POST operation
 *        with plenty of upload data) this allows the application
 *        to easily associate some request-specific state.
 *        If necessary, this state can be cleaned up in the
 *        global "MHD_RequestCompleted" callback (which
 *        can be set with the MHD_OPTION_NOTIFY_COMPLETED).
 *        Initially, <tt>*con_cls</tt> will be NULL.
 * @return MHS_YES if the connection was handled successfully,
 *         MHS_NO if the socket must be closed due to a serios
 *         error while handling the request
 */
static int
create_response (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)
{
  struct MHD_Response *response;
  struct Request *request;
  struct Session *session;
  int ret;
  unsigned int i;

  request = *ptr;
  if (NULL == request)
    {
      request = calloc (1, sizeof (struct Request));
      if (NULL == request)
	{
	  fprintf (stderr, "calloc error: %s\n", strerror (errno));
	  return MHD_NO;
	}
      *ptr = request;
      if (0 == strcmp (method, MHD_HTTP_METHOD_POST))
	{
	  request->pp = MHD_create_post_processor (connection, 1024,
						   &post_iterator, request);
	  if (NULL == request->pp)
	    {
	      fprintf (stderr, "Failed to setup post processor for `%s'\n",
		       url);
	      return MHD_NO; /* internal error */
	    }
	}
      return MHD_YES;
    }
  if (NULL == request->session)
    {
      request->session = get_session (connection);
      if (NULL == request->session)
	{
	  fprintf (stderr, "Failed to setup session for `%s'\n",
		   url);
	  return MHD_NO; /* internal error */
	}
    }
  session = request->session;
  session->start = time (NULL);
  if (0 == strcmp (method, MHD_HTTP_METHOD_POST))
    {      
      /* evaluate POST data */
      MHD_post_process (request->pp,
			upload_data,
			*upload_data_size);
      if (0 != *upload_data_size)
	{
	  *upload_data_size = 0;
	  return MHD_YES;
	}
      /* done with POST data, serve response */
      MHD_destroy_post_processor (request->pp);
      request->pp = NULL;
      method = MHD_HTTP_METHOD_GET; /* fake 'GET' */
      if (NULL != request->post_url)
	url = request->post_url;
    }

  if ( (0 == strcmp (method, MHD_HTTP_METHOD_GET)) ||
       (0 == strcmp (method, MHD_HTTP_METHOD_HEAD)) )
    {
      /* find out which page to serve */
      i=0;
      while ( (pages[i].url != NULL) &&
	      (0 != strcmp (pages[i].url, url)) )
	i++;
      ret = pages[i].handler (pages[i].handler_cls, 
			      pages[i].mime,
			      session, connection);
      if (ret != MHD_YES)
	fprintf (stderr, "Failed to create page for `%s'\n",
		 url);
      return ret;
    }
  /* unsupported HTTP method */
  response = MHD_create_response_from_buffer (strlen (METHOD_ERROR),
					      (void *) METHOD_ERROR,
					      MHD_RESPMEM_PERSISTENT);
  ret = MHD_queue_response (connection, 
			    MHD_HTTP_METHOD_NOT_ACCEPTABLE, 
			    response);
  MHD_destroy_response (response);
  return ret;
}


/**
 * Callback called upon completion of a request.
 * Decrements session reference counter.
 *
 * @param cls not used
 * @param connection connection that completed
 * @param con_cls session handle
 * @param toe status code
 */
static void
request_completed_callback (void *cls,
			    struct MHD_Connection *connection,
			    void **con_cls,
			    enum MHD_RequestTerminationCode toe)
{
  struct Request *request = *con_cls;

  if (NULL == request)
    return;
  if (NULL != request->session)
    request->session->rc--;
  if (NULL != request->pp)
    MHD_destroy_post_processor (request->pp);
  free (request);
}


/**
 * Clean up handles of sessions that have been idle for
 * too long.
 */
static void
expire_sessions ()
{
  struct Session *pos;
  struct Session *prev;
  struct Session *next;
  time_t now;

  now = time (NULL);
  prev = NULL;
  pos = sessions;
  while (NULL != pos)
    {
      next = pos->next;
      if (now - pos->start > 60 * 60)
	{
	  /* expire sessions after 1h */
	  if (NULL == prev)
	    sessions = pos->next;
	  else
	    prev->next = next;
	  free (pos);
	}
      else
        prev = pos;
      pos = next;
    }      
}


/**
 * Call with the port number as the only argument.
 * Never terminates (other than by signals, such as CTRL-C).
 */
int
main (int argc, char *const *argv)
{
  struct MHD_Daemon *d;
  struct timeval tv;
  struct timeval *tvp;
  fd_set rs;
  fd_set ws;
  fd_set es;
  int max;
  unsigned MHD_LONG_LONG mhd_timeout;

  if (argc != 2)
    {
      printf ("%s PORT\n", argv[0]);
      return 1;
    }
  /* initialize PRNG */
  srandom ((unsigned int) time (NULL));
  d = MHD_start_daemon (MHD_USE_DEBUG,
                        atoi (argv[1]),
                        NULL, NULL, 
			&create_response, NULL, 
			MHD_OPTION_CONNECTION_TIMEOUT, (unsigned int) 15,
			MHD_OPTION_NOTIFY_COMPLETED, &request_completed_callback, NULL,
			MHD_OPTION_END);
  if (NULL == d)
    return 1;
  while (1)
    {
      expire_sessions ();
      max = 0;
      FD_ZERO (&rs);
      FD_ZERO (&ws);
      FD_ZERO (&es);
      if (MHD_YES != MHD_get_fdset (d, &rs, &ws, &es, &max))
	break; /* fatal internal error */
      if (MHD_get_timeout (d, &mhd_timeout) == MHD_YES)	
	{
	  tv.tv_sec = mhd_timeout / 1000;
	  tv.tv_usec = (mhd_timeout - (tv.tv_sec * 1000)) * 1000;
	  tvp = &tv;	  
	}
      else
	tvp = NULL;
      select (max + 1, &rs, &ws, &es, tvp);
      MHD_run (d);
    }
  MHD_stop_daemon (d);
  return 0;
}


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C.8 tlsauthentication.c

 
#include <sys/types.h>
#include <sys/select.h>
#include <sys/socket.h>
#include <microhttpd.h>

#define PORT 8888

#define REALM     "\"Maintenance\""
#define USER      "a legitimate user"
#define PASSWORD  "and his password"

#define SERVERKEYFILE "server.key"
#define SERVERCERTFILE "server.pem"


static char *
string_to_base64 (const char *message)
{
  const char *lookup =
    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
  unsigned long l;
  int i;
  char *tmp;
  size_t length = strlen (message);

  tmp = malloc (length * 2);
  if (NULL == tmp)
    return tmp;

  tmp[0] = 0;

  for (i = 0; i < length; i += 3)
    {
      l = (((unsigned long) message[i]) << 16)
        | (((i + 1) < length) ? (((unsigned long) message[i + 1]) << 8) : 0)
        | (((i + 2) < length) ? ((unsigned long) message[i + 2]) : 0);


      strncat (tmp, &lookup[(l >> 18) & 0x3F], 1);
      strncat (tmp, &lookup[(l >> 12) & 0x3F], 1);

      if (i + 1 < length)
        strncat (tmp, &lookup[(l >> 6) & 0x3F], 1);
      if (i + 2 < length)
        strncat (tmp, &lookup[l & 0x3F], 1);
    }

  if (length % 3)
    strncat (tmp, "===", 3 - length % 3);

  return tmp;
}


static long
get_file_size (const char *filename)
{
  FILE *fp;

  fp = fopen (filename, "rb");
  if (fp)
    {
      long size;

      if ((0 != fseek (fp, 0, SEEK_END)) || (-1 == (size = ftell (fp))))
        size = 0;

      fclose (fp);

      return size;
    }
  else
    return 0;
}

static char *
load_file (const char *filename)
{
  FILE *fp;
  char *buffer;
  long size;

  size = get_file_size (filename);
  if (size == 0)
    return NULL;

  fp = fopen (filename, "rb");
  if (!fp)
    return NULL;

  buffer = malloc (size);
  if (!buffer)
    {
      fclose (fp);
      return NULL;
    }

  if (size != fread (buffer, 1, size, fp))
    {
      free (buffer);
      buffer = NULL;
    }

  fclose (fp);
  return buffer;
}

static int
ask_for_authentication (struct MHD_Connection *connection, const char *realm)
{
  int ret;
  struct MHD_Response *response;
  char *headervalue;
  const char *strbase = "Basic realm=";

  response = MHD_create_response_from_buffer (0, NULL, 
					      MHD_RESPMEM_PERSISTENT);
  if (!response)
    return MHD_NO;

  headervalue = malloc (strlen (strbase) + strlen (realm) + 1);
  if (!headervalue)
    return MHD_NO;

  strcpy (headervalue, strbase);
  strcat (headervalue, realm);

  ret = MHD_add_response_header (response, "WWW-Authenticate", headervalue);
  free (headervalue);
  if (!ret)
    {
      MHD_destroy_response (response);
      return MHD_NO;
    }

  ret = MHD_queue_response (connection, MHD_HTTP_UNAUTHORIZED, response);

  MHD_destroy_response (response);

  return ret;
}

static int
is_authenticated (struct MHD_Connection *connection,
                  const char *username, const char *password)
{
  const char *headervalue;
  char *expected_b64, *expected;
  const char *strbase = "Basic ";
  int authenticated;

  headervalue =
    MHD_lookup_connection_value (connection, MHD_HEADER_KIND,
                                 "Authorization");
  if (NULL == headervalue)
    return 0;
  if (0 != strncmp (headervalue, strbase, strlen (strbase)))
    return 0;

  expected = malloc (strlen (username) + 1 + strlen (password) + 1);
  if (NULL == expected)
    return 0;

  strcpy (expected, username);
  strcat (expected, ":");
  strcat (expected, password);

  expected_b64 = string_to_base64 (expected);
  free (expected);
  if (NULL == expected_b64)
    return 0;

  authenticated =
    (strcmp (headervalue + strlen (strbase), expected_b64) == 0);

  free (expected_b64);

  return authenticated;
}


static int
secret_page (struct MHD_Connection *connection)
{
  int ret;
  struct MHD_Response *response;
  const char *page = "<html><body>A secret.</body></html>";

  response =
    MHD_create_response_from_buffer (strlen (page), (void *) page, 
				     MHD_RESPMEM_PERSISTENT);
  if (!response)
    return MHD_NO;

  ret = MHD_queue_response (connection, MHD_HTTP_OK, response);
  MHD_destroy_response (response);

  return ret;
}


static int
answer_to_connection (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 **con_cls)
{
  if (0 != strcmp (method, "GET"))
    return MHD_NO;
  if (NULL == *con_cls)
    {
      *con_cls = connection;
      return MHD_YES;
    }

  if (!is_authenticated (connection, USER, PASSWORD))
    return ask_for_authentication (connection, REALM);

  return secret_page (connection);
}


int
main ()
{
  struct MHD_Daemon *daemon;
  char *key_pem;
  char *cert_pem;

  key_pem = load_file (SERVERKEYFILE);
  cert_pem = load_file (SERVERCERTFILE);

  if ((key_pem == NULL) || (cert_pem == NULL))
    {
      printf ("The key/certificate files could not be read.\n");
      return 1;
    }

  daemon =
    MHD_start_daemon (MHD_USE_SELECT_INTERNALLY | MHD_USE_SSL, PORT, NULL,
                      NULL, &answer_to_connection, NULL,
                      MHD_OPTION_HTTPS_MEM_KEY, key_pem,
                      MHD_OPTION_HTTPS_MEM_CERT, cert_pem, MHD_OPTION_END);
  if (NULL == daemon)
    {
      printf ("%s\n", cert_pem);

      free (key_pem);
      free (cert_pem);

      return 1;
    }

  getchar ();

  MHD_stop_daemon (daemon);
  free (key_pem);
  free (cert_pem);

  return 0;
}

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