[libmicrohttpd] Re: libmicrohttpd - question on tutorial

[Christian Grothoff]

Hi!

I just tried this:

$ echo test > picture.png
$ gcc -o srv -g -I /usr/local/include/ \
    -L/usr/local/lib/ -lmicrohttpd responseheaders.c     
$ ./srv &
$ telnet localhost 8888
Trying ::1...
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
GET /dontcare HTTP/1.1
Host: itsme

HTTP/1.1 200 OK
Content-Length: 5
Content-Type: image/png
Date: Thu, 17 Feb 2011 14:47:53 GMT

test
Connection closed by foreign host.


So this worked on my system (which is Debian, but should make no difference) 
using the (except #includes) unmodified responseheaders.c against MHD 0.9.7.  

So this is rather confusing, especially given that you did use the same code 
(!?).  Does your image file have some access permissions that might cause 
problems? Try running the server with 'strace -f' and see what the system 
calls do -- there SHOULD be one to 'sendfile' for the actual transmission 
(shortly after your 'open' call to picture.png).  That might help.  Here is 
what mine looks like:

After telnet accept, select waits for 'GET' (I post the entire request at once 
with middle-mouse click):
[pid 19650] select(6, [4 5], [], [], {1, 0}) = 1 (in [5], left {0, 556551}) 
[pid 19650] recvfrom(5, "GET / HTTP/1.1\r\nHost: me\r\n\r\n", 2048, 
MSG_DONTWAIT|MSG_NOSIGNAL, NULL, NULL) = 28

Here is the open call:
[pid 19650] open("picture.png", O_RDONLY) = 6
[pid 19650] fstat(6, {st_mode=S_IFREG|0644, st_size=5, ...}) = 0
[pid 19650] shutdown(5, 0 /* receive */) = 0

Part of MHD header response creation involves getting system time:
[pid 19650] open("/etc/localtime", O_RDONLY) = 7
[pid 19650] fstat(7, {st_mode=S_IFREG|0644, st_size=2309, ...}) = 0
[pid 19650] fstat(7, {st_mode=S_IFREG|0644, st_size=2309, ...}) = 0
[pid 19650] mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, 
-1, 0) = 0x7fe5da3b7000
[pid 19650] read(7, 
"TZif2\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\10\0\0\0\10\0\0\0\0"..., 4096) = 
2309
[pid 19650] lseek(7, -1467, SEEK_CUR)   = 842
[pid 19650] read(7, 
"TZif2\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\t\0\0\0\t\0\0\0\0"..., 4096) = 1467
[pid 19650] close(7)                    = 0
[pid 19650] munmap(0x7fe5da3b7000, 4096) = 0

Then MHD sends the header (cork on):
[pid 19650] setsockopt(5, SOL_TCP, TCP_CORK, [1], 4) = 0
[pid 19650] select(6, [4], [5], [], {1, 0}) = 1 (out [5], left {0, 999998})
[pid 19650] sendto(5, "HTTP/1.1 200 OK\r\nContent-Length:"..., 100, 
MSG_DONTWAIT|MSG_NOSIGNAL, NULL, 0) = 100

Then MHD waits again to send the body (cork off):
[pid 19650] select(6, [4], [5], [], {1, 0}) = 1 (out [5], left {0, 999998})
[pid 19650] sendfile(5, 6, [0], 5)      = 5
[pid 19650] setsockopt(5, SOL_TCP, TCP_CORK, [0], 4) = 0

Finally, we're done and close the file and socket:
[pid 19650] close(6)                    = 0
[pid 19650] shutdown(5, 2 /* send and receive */) = 0
[pid 19650] close(5)                    = 0

Back to waiting for more:
[pid 19650] select(5, [4], [], [], {1, 0}) = 0 (Timeout)


Any changes from this sequence (return values, etc.) might help explain what's 
going on...

Happy hacking,

Christian

On Thursday 17 February 2011 15:26:05 Neil D'Souza wrote:
> Hello Christian,
> 
> First of all Thank you for the excellently written tutorial. I myself am
> working on a language for scripting questionnaires for the Market Research
> industry. You can see the project here
> http://sourceforge.net/projects/xtcc - the compiler is called qscript (for
> Questionnaire Script).
> 
> It currently generates code for simple data entry in an ncurses interface.
> Installing the compiler requires the presence of gcc, ncurses and compiling
> qscript from source - which can be big barriers for adoption/trying. I am
> trying to embed the generated code inside a web-server and demo what the
> interface looks like and the programming language looks like - hence my
> interest in libmicrohttpd. I am going through the tutorial so that I can
> understand libmicrohttpd usage.
> 
> I have reached Chapter 4 where we send the png image. below is a slightly
> modified function just for me to be sure that things were working. However
> the image did not appear in the browser. So I telnetted to port 8888. As
> you can see the png file is not sent - otherwise there should have been a
> bytestream of the image data. It seems that after getting enqueued the
> file is not transmitted. Just wanted to check if I am missing something. I
> tried with Konqueror and Firefox before resorting to telnet.
> 
> Also wanted to add that you need to include
> #include <sys/stat.h>
> #include <fcntl.h>
>  to responseheaders.c to compile with gcc version 4.4.1 . I am using ubuntu
> 9.10 in case it is of any use and microhttpd version is 0.9.7 and was
> compiled by me from source and installed in /usr/local .
> 
> Many thanks for your help in advance.
> 
> Kind Regards,
> Neil
> 
> 
> /media/sda3/home/nxd/Prog/C/microhttpd/tutorial>telnet localhost 8888
> Trying 127.0.0.1...
> Connected to localhost.
> Escape character is '^]'.
> GET /dontcare HTTP/1.1
> Host: itsme
> 
> HTTP/1.1 200 OK
> Content-Length: 1340341
> Content-Type: image/png
> Date: Thu, 17 Feb 2011 13:54:52 GMT
> 
> Connection closed by foreign host.
> 
> 
> 
> // ==================
> 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;
>   printf ("New %s request for %s using version %s\n", method, url,
> version);
> 
> 
>   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);
>   if (ret == MHD_YES)
>       printf ("queued response\n");
>   else
>       printf ("could not queue response\n");
>   MHD_destroy_response (response);
> 
>   return ret;
> }
> 
> // ===============