1 The previous chapters already have demonstrated a variety of possibilities to send information 2 to the HTTP server, but it is not recommended that the @emph{GET} method is used to alter the way 3 the server operates. To induce changes on the server, the @emph{POST} method is preferred over 4 and is much more powerful than @emph{GET} and will be introduced in this chapter. 5 6 We are going to write an application that asks for the visitor's name and, after the user has posted it, 7 composes an individual response text. Even though it was not mandatory to use the @emph{POST} method here, 8 as there is no permanent change caused by the POST, it is an illustrative example on how to share data 9 between different functions for the same connection. Furthermore, the reader should be able to extend 10 it easily. 11 12 @heading GET request 13 When the first @emph{GET} request arrives, the server shall respond with a HTML page containing an 14 edit field for the name. 15 16 @verbatim 17 const 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 26 The @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 29 We also prepare the answer page, where the name is to be filled in later, and an error page 30 as the response for anything but proper @emph{GET} and @emph{POST} requests: 31 32 @verbatim 33 const char* greatingpage="<html><body><h1>Welcome, %s!</center></h1></body></html>"; 34 35 const char* errorpage="<html><body>This doesn't seem to be right.</body></html>"; 36 @end verbatim 37 @noindent 38 39 Whenever we need to send a page, we use an extra function 40 @code{int send_page(struct MHD_Connection *connection, const char* page)} 41 for this, which does not contain anything new and whose implementation is therefore 42 not discussed further in the tutorial. 43 44 45 @heading POST request 46 Posted data can be of arbitrary and considerable size; for example, if a user uploads a big 47 image to the server. Similar to the case of the header fields, there may also be different streams 48 of posted data, such as one containing the text of an editbox and another the state of a button. 49 Likewise, we will have to register an iterator function that is going to be called maybe several times 50 not only if there are different POSTs but also if one POST has only been received partly yet and 51 needs processing before another chunk can be received. 52 53 Such an iterator function is called by a @emph{postprocessor}, which must be created upon arriving 54 of 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. 56 But in order to pass this string to other functions and still be able to differentiate different 57 connections, we must first define a structure to share the information, holding the most import entries. 58 59 @verbatim 60 struct connection_info_struct 61 { 62 int connectiontype; 63 char *answerstring; 64 struct MHD_PostProcessor *postprocessor; 65 }; 66 @end verbatim 67 @noindent 68 69 With these information available to the iterator function, it is able to fulfill its task. 70 Once it has composed the greeting string, it returns @code{MHD_NO} to inform the post processor 71 that it does not need to be called again. Note that this function does not handle processing 72 of data for the same @code{key}. If we were to expect that the name will be posted in several 73 chunks, we had to expand the namestring dynamically as additional parts of it with the same @code{key} 74 came in. But in this example, the name is assumed to fit entirely inside one single packet. 75 76 @verbatim 77 static int 78 iterate_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 105 106 Once a connection has been established, it can be terminated for many reasons. As these 107 reasons include unexpected events, we have to register another function that cleans up any resources 108 that might have been allocated for that connection by us, namely the post processor and the greetings 109 string. This cleanup function must take into account that it will also be called for finished 110 requests other than @emph{POST} requests. 111 112 @verbatim 113 void 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 133 in the main function. 134 135 @verbatim 136 ... 137 daemon = 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 146 With all other functions prepared, we can now discuss the actual request handling. 147 148 On 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 150 iterations and other functions. 151 152 @verbatim 153 static int 154 answer_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 169 170 If the new request is a @emph{POST}, the postprocessor must be created now. In addition, the type 171 of 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 189 190 The address of our structure will both serve as the indicator for successive iterations and to remember 191 the particular details about the connection. 192 @verbatim 193 *con_cls = (void*) con_info; 194 return MHD_YES; 195 } 196 @end verbatim 197 @noindent 198 199 The rest of the function will not be executed on the first iteration. A @emph{GET} request is easily 200 satisfied 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 208 209 In 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 211 considered---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 230 231 Finally, 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 237 238 These were the important parts of the program @code{simplepost.c}. 239
