1The previous chapters already have demonstrated a variety of possibilities to send information
2to the HTTP server, but it is not recommended that the @emph{GET} method is used to alter the way
3the server operates. To induce changes on the server, the @emph{POST} method is preferred over
4and is much more powerful than @emph{GET} and will be introduced in this chapter.
5
6We are going to write an application that asks for the visitor's name and, after the user has posted it,
7composes an individual response text. Even though it was not mandatory to use the @emph{POST} method here,
8as there is no permanent change caused by the POST, it is an illustrative example on how to share data
9between different functions for the same connection. Furthermore, the reader should be able to extend
10it easily.
11
12@heading GET request
13When the first @emph{GET} request arrives, the server shall respond with a HTML page containing an
14edit field for the name.
15
16@verbatim
17const char* askpage = "<html><body>\
18                       What's your name, Sir?<br>\
19                       <form action=\"/namepost\" method=\"post\">\
20                       <input name=\"name\" type=\"text\"\
21                       <input type=\"submit\" value=\" Send \"></form>\
22                       </body></html>";
23@end verbatim
24@noindent
25
26The @code{action} entry is the @emph{URI} to be called by the browser when posting, and the
27@code{name} will be used later to be sure it is the editbox's content that has been posted.
28
29We also prepare the answer page, where the name is to be filled in later, and an error page
30as the response for anything but proper @emph{GET} and @emph{POST} requests:
31
32@verbatim
33const char* greatingpage="<html><body><h1>Welcome, %s!</center></h1></body></html>";
34
35const char* errorpage="<html><body>This doesn't seem to be right.</body></html>";
36@end verbatim
37@noindent
38
39Whenever we need to send a page, we use an extra function
40@code{int send_page(struct MHD_Connection *connection, const char* page)}
41for this, which does not contain anything new and whose implementation is therefore
42not discussed further in the tutorial.
43
44
45@heading POST request
46Posted data can be of arbitrary and considerable size; for example, if a user uploads a big
47image to the server. Similar to the case of the header fields, there may also be different streams
48of posted data, such as one containing the text of an editbox and another the state of a button.
49Likewise, we will have to register an iterator function that is going to be called maybe several times
50not only if there are different POSTs but also if one POST has only been received partly yet and
51needs processing before another chunk can be received.
52
53Such an iterator function is called by a @emph{postprocessor}, which must be created upon arriving
54of the post request.  We want the iterator function to read the first post data which is tagged
55@code{name} and to create an individual greeting string based on the template and the name.
56But in order to pass this string to other functions and still be able to differentiate different
57connections, we must first define a structure to share the information, holding the most import entries.
58
59@verbatim
60struct connection_info_struct
61{
62  int connectiontype;
63  char *answerstring;
64  struct MHD_PostProcessor *postprocessor;
65};
66@end verbatim
67@noindent
68
69With these information available to the iterator function, it is able to fulfill its task.
70Once it has composed the greeting string, it returns @code{MHD_NO} to inform the post processor
71that it does not need to be called again. Note that this function does not handle processing
72of data for the same @code{key}. If we were to expect that the name will be posted in several
73chunks, we had to expand the namestring dynamically as additional parts of it with the same @code{key}
74came in. But in this example, the name is assumed to fit entirely inside one single packet.
75
76@verbatim
77static int
78iterate_post (void *coninfo_cls, enum MHD_ValueKind kind, const char *key,
79              const char *filename, const char *content_type,
80              const char *transfer_encoding, const char *data,
81	      uint64_t off, size_t size)
82{
83  struct connection_info_struct *con_info = coninfo_cls;
84
85  if (0 == strcmp (key, "name"))
86    {
87      if ((size > 0) && (size <= MAXNAMESIZE))
88        {
89          char *answerstring;
90          answerstring = malloc (MAXANSWERSIZE);
91          if (!answerstring) return MHD_NO;
92
93          snprintf (answerstring, MAXANSWERSIZE, greatingpage, data);
94          con_info->answerstring = answerstring;
95        }
96      else con_info->answerstring = NULL;
97
98      return MHD_NO;
99    }
100
101  return MHD_YES;
102}
103@end verbatim
104@noindent
105
106Once a connection has been established, it can be terminated for many reasons. As these
107reasons include unexpected events, we have to register another function that cleans up any resources
108that might have been allocated for that connection by us, namely the post processor and the greetings
109string. This cleanup function must take into account that it will also be called for finished
110requests other than @emph{POST} requests.
111
112@verbatim
113void request_completed (void *cls, struct MHD_Connection *connection,
114     		        void **con_cls,
115                        enum MHD_RequestTerminationCode toe)
116{
117  struct connection_info_struct *con_info = *con_cls;
118
119  if (NULL == con_info) return;
120  if (con_info->connectiontype == POST)
121    {
122      MHD_destroy_post_processor (con_info->postprocessor);
123      if (con_info->answerstring) free (con_info->answerstring);
124    }
125
126  free (con_info);
127  *con_cls = NULL;
128}
129@end verbatim
130@noindent
131
132@emph{GNU libmicrohttpd} is informed that it shall call the above function when the daemon is started
133in the main function.
134
135@verbatim
136...
137daemon = MHD_start_daemon (MHD_USE_SELECT_INTERNALLY, PORT, NULL, NULL,
138                           &answer_to_connection, NULL,
139			   MHD_OPTION_NOTIFY_COMPLETED, &request_completed, NULL,
140			   MHD_OPTION_END);
141...
142@end verbatim
143@noindent
144
145@heading Request handling
146With all other functions prepared, we can now discuss the actual request handling.
147
148On the first iteration for a new request, we start by allocating a new instance of a
149@code{struct connection_info_struct} structure, which will store all necessary information for later
150iterations and other functions.
151
152@verbatim
153static int
154answer_to_connection (void *cls, struct MHD_Connection *connection,
155		      const char *url,
156                      const char *method, const char *version,
157		      const char *upload_data,
158                      size_t *upload_data_size, void **con_cls)
159{
160  if(NULL == *con_cls)
161    {
162      struct connection_info_struct *con_info;
163
164      con_info = malloc (sizeof (struct connection_info_struct));
165      if (NULL == con_info) return MHD_NO;
166      con_info->answerstring = NULL;
167@end verbatim
168@noindent
169
170If the new request is a @emph{POST}, the postprocessor must be created now. In addition, the type
171of the request is stored for convenience.
172@verbatim
173      if (0 == strcmp (method, "POST"))
174        {
175          con_info->postprocessor
176	    = MHD_create_post_processor (connection, POSTBUFFERSIZE,
177                                         iterate_post, (void*) con_info);
178
179          if (NULL == con_info->postprocessor)
180            {
181              free (con_info);
182              return MHD_NO;
183            }
184          con_info->connectiontype = POST;
185        }
186      else con_info->connectiontype = GET;
187@end verbatim
188@noindent
189
190The address of our structure will both serve as the indicator for successive iterations and to remember
191the particular details about the connection.
192@verbatim
193      *con_cls = (void*) con_info;
194      return MHD_YES;
195    }
196@end verbatim
197@noindent
198
199The rest of the function will not be executed on the first iteration. A @emph{GET} request is easily
200satisfied by sending the question form.
201@verbatim
202  if (0 == strcmp (method, "GET"))
203    {
204      return send_page (connection, askpage);
205    }
206@end verbatim
207@noindent
208
209In case of @emph{POST}, we invoke the post processor for as long as data keeps incoming, setting
210@code{*upload_data_size} to zero in order to indicate that we have processed---or at least have
211considered---all of it.
212@verbatim
213  if (0 == strcmp (method, "POST"))
214    {
215      struct connection_info_struct *con_info = *con_cls;
216
217      if (*upload_data_size != 0)
218        {
219          MHD_post_process (con_info->postprocessor, upload_data,
220	                    *upload_data_size);
221          *upload_data_size = 0;
222
223          return MHD_YES;
224        }
225      else if (NULL != con_info->answerstring)
226        return send_page (connection, con_info->answerstring);
227    }
228@end verbatim
229@noindent
230
231Finally, if they are neither @emph{GET} nor @emph{POST} requests, the error page is returned.
232@verbatim
233  return send_page(connection, errorpage);
234}
235@end verbatim
236@noindent
237
238These were the important parts of the program @code{simplepost.c}.
239