1 Now that we are able to inspect the incoming request in great detail, 2 this chapter discusses the means to enrich the outgoing responses likewise. 3 4 As you have learned in the @emph{Hello, Browser} chapter, some obligatory 5 header fields are added and set automatically for simple responses by the library 6 itself but if more advanced features are desired, additional fields have to be created. 7 One of the possible fields is the content type field and an example will be developed around it. 8 This will lead to an application capable of correctly serving different types of files. 9 10 11 When we responded with HTML page packed in the static string previously, the client had no choice 12 but guessing about how to handle the response, because the server had not told him. 13 What if we had sent a picture or a sound file? Would the message have been understood 14 or merely been displayed as an endless stream of random characters in the browser? 15 This is what the mime content types are for. The header of the response is extended 16 by certain information about how the data is to be interpreted. 17 18 To introduce the concept, a picture of the format @emph{PNG} will be sent to the client 19 and labeled accordingly with @code{image/png}. 20 Once again, we can base the new example on the @code{hellobrowser} program. 21 22 @verbatim 23 #define FILENAME "picture.png" 24 #define MIMETYPE "image/png" 25 26 static int 27 answer_to_connection (void *cls, struct MHD_Connection *connection, 28 const char *url, 29 const char *method, const char *version, 30 const char *upload_data, 31 size_t *upload_data_size, void **con_cls) 32 { 33 unsigned char *buffer = NULL; 34 struct MHD_Response *response; 35 @end verbatim 36 @noindent 37 38 We want the program to open the file for reading and determine its size: 39 @verbatim 40 int fd; 41 int ret; 42 struct stat sbuf; 43 44 if (0 != strcmp (method, "GET")) 45 return MHD_NO; 46 if ( (-1 == (fd = open (FILENAME, O_RDONLY))) || 47 (0 != fstat (fd, &sbuf)) ) 48 { 49 /* error accessing file */ 50 /* ... (see below) */ 51 } 52 /* ... (see below) */ 53 @end verbatim 54 @noindent 55 56 When dealing with files, there is a lot that could go wrong on the 57 server side and if so, the client should be informed with @code{MHD_HTTP_INTERNAL_SERVER_ERROR}. 58 59 @verbatim 60 /* error accessing file */ 61 if (fd != -1) close (fd); 62 const char *errorstr = 63 "<html><body>An internal server error has occured!\ 64 </body></html>"; 65 response = 66 MHD_create_response_from_buffer (strlen (errorstr), 67 (void *) errorstr, 68 MHD_RESPMEM_PERSISTENT); 69 if (response) 70 { 71 ret = 72 MHD_queue_response (connection, MHD_HTTP_INTERNAL_SERVER_ERROR, 73 response); 74 MHD_destroy_response (response); 75 76 return MHD_YES; 77 } 78 else 79 return MHD_NO; 80 if (!ret) 81 { 82 const char *errorstr = "<html><body>An internal server error has occured!\ 83 </body></html>"; 84 85 if (buffer) free(buffer); 86 87 response = MHD_create_response_from_buffer (strlen(errorstr), (void*) errorstr, 88 MHD_RESPMEM_PERSISTENT); 89 90 if (response) 91 { 92 ret = MHD_queue_response (connection, 93 MHD_HTTP_INTERNAL_SERVER_ERROR, 94 response); 95 MHD_destroy_response (response); 96 97 return MHD_YES; 98 } 99 else return MHD_NO; 100 } 101 @end verbatim 102 @noindent 103 104 Note that we nevertheless have to create a response object even for sending a simple error code. 105 Otherwise, the connection would just be closed without comment, leaving the client curious about 106 what has happened. 107 108 But in the case of success a response will be constructed directly from the file descriptor: 109 110 @verbatim 111 /* error accessing file */ 112 /* ... (see above) */ 113 } 114 115 response = 116 MHD_create_response_from_fd_at_offset (sbuf.st_size, fd, 0); 117 MHD_add_response_header (response, "Content-Type", MIMETYPE); 118 ret = MHD_queue_response (connection, MHD_HTTP_OK, response); 119 MHD_destroy_response (response); 120 @end verbatim 121 @noindent 122 123 Note that the response object will take care of closing the file desciptor for us. 124 125 Up to this point, there was little new. The actual novelty is that we enhance the header with the 126 meta data about the content. Aware of the field's name we want to add, it is as easy as that: 127 @verbatim 128 MHD_add_response_header(response, "Content-Type", MIMETYPE); 129 @end verbatim 130 @noindent 131 We do not have to append a colon expected by the protocol behind the first 132 field---@emph{GNU libhttpdmicro} will take care of this. 133 134 The function finishes with the well-known lines 135 @verbatim 136 ret = MHD_queue_response (connection, MHD_HTTP_OK, response); 137 MHD_destroy_response (response); 138 return ret; 139 } 140 @end verbatim 141 @noindent 142 143 The complete program @code{responseheaders.c} is in the @code{examples} section as usual. 144 Find a @emph{PNG} file you like and save it to the directory the example is run from under the name 145 @code{picture.png}. You should find the image displayed on your browser if everything worked well. 146 147 @heading Remarks 148 The include file of the @emph{MHD} library comes with the header types mentioned in @emph{RFC 2616} 149 already defined as macros. Thus, we could have written @code{MHD_HTTP_HEADER_CONTENT_TYPE} instead 150 of @code{"Content-Type"} as well. However, one is not limited to these standard headers and could 151 add custom response headers without violating the protocol. Whether, and how, the client would react 152 to these custom header is up to the receiver. Likewise, the client is allowed to send custom request 153 headers to the server as well, opening up yet more possibilities how client and server could 154 communicate with each other. 155 156 The method of creating the response from a file on disk only works for static content. 157 Serving dynamically created responses will be a topic of a future chapter. 158 159 @heading Exercises 160 @itemize @bullet 161 162 @item 163 Remember that the original program was written under a few assumptions---a static response 164 using a local file being one of them. In order to simulate a very large or hard to reach file that cannot be provided 165 instantly, postpone the queuing in the callback with the @code{sleep} function for 30 seconds 166 @emph{if} the file @code{/big.png} is requested (but deliver the same as above). A request for 167 @code{/picture.png} should provide just the same but without any artificial delays. 168 169 Now start two instances of your browser (or even use two machines) and see how the second client 170 is put on hold while the first waits for his request on the slow file to be fulfilled. 171 172 Finally, change the sourcecode to use @code{MHD_USE_THREAD_PER_CONNECTION} when the daemon is 173 started and try again. 174 175 176 @item 177 Did you succeed in implementing the clock exercise yet? This time, let the server save the 178 program's start time @code{t} and implement a response simulating a countdown that reaches 0 at 179 @code{t+60}. Returning a message saying on which point the countdown is, the response should 180 ultimately be to reply "Done" if the program has been running long enough, 181 182 An unofficial, but widely understood, response header line is @code{Refresh: DELAY; url=URL} with 183 the uppercase words substituted to tell the client it should request the given resource after 184 the given delay again. Improve your program in that the browser (any modern browser should work) 185 automatically reconnects and asks for the status again every 5 seconds or so. The URL would have 186 to be composed so that it begins with "http://", followed by the @emph{URI} the server is reachable 187 from the client's point of view. 188 189 Maybe you want also to visualize the countdown as a status bar by creating a 190 @code{<table>} consisting of one row and @code{n} columns whose fields contain small images of either 191 a red or a green light. 192 193 @end itemize 194
