NAME

       curl_multi_fdset - extract file descriptor information from a multi handle

SYNOPSIS

       #include <curl/curl.h>

       CURLMcode curl_multi_fdset(CURLM *multi_handle,
                                  fd_set *read_fd_set,
                                  fd_set *write_fd_set,
                                  fd_set *exc_fd_set,
                                  int *max_fd);

DESCRIPTION

       This  function  extracts  file  descriptor information from a given multi_handle.  libcurl
       returns its fd_set sets. The application can use these to select()  on,  but  be  sure  to
       FD_ZERO  them  before  calling  this  function  as  curl_multi_fdset(3)  only adds its own
       descriptors, it does not zero or otherwise remove any  others.  The  curl_multi_perform(3)
       function should be called as soon as one of them is ready to be read from or written to.

       The  read_fd_set  argument  should  point  to  an  object  of  type fd_set that on returns
       specifies the file descriptors to be checked for being ready to read.

       The write_fd_set argument should point  to  an  object  of  type  fd_set  that  on  return
       specifies the file descriptors to be checked for being ready to write.

       The  exc_fd_set argument should point to an object of type fd_set that on return specifies
       the file descriptors to be checked for error conditions.

       If no file descriptors are set by libcurl, max_fd contain -1 when this  function  returns.
       Otherwise  it  contains the highest descriptor number libcurl set. When libcurl returns -1
       in max_fd, it is because libcurl currently does something that is not  possible  for  your
       application  to monitor with a socket and unfortunately you can then not know exactly when
       the current action is completed using select(). You then need to wait a while  before  you
       proceed    and   call   curl_multi_perform(3)   anyway.   How   long   to   wait?   Unless
       curl_multi_timeout(3) gives you a lower number, we suggest 100 milliseconds or so, but you
       may want to test it out in your own particular conditions to find a suitable value.

       When  doing  select(), you should use curl_multi_timeout(3) to figure out how long to wait
       for action. Call curl_multi_perform(3) even if no activity has been seen  on  the  fd_sets
       after  the  timeout expires as otherwise internal retries and timeouts may not work as you
       would think and want.

       If one of the sockets used by libcurl happens to be larger than what  can  be  set  in  an
       fd_set,  which  on POSIX systems means that the file descriptor is larger than FD_SETSIZE,
       then libcurl tries to not set it. Setting a too large file descriptor in an fd_set implies
       an  out  of  bounds  write which can cause crashes, or worse. The effect of NOT storing it
       might possibly save you from the crash, but makes your program NOT  wait  for  sockets  it
       should wait for...

PROTOCOLS

       This functionality affects all supported protocols

EXAMPLE

       int main(void)
       {
         fd_set fdread;
         fd_set fdwrite;
         fd_set fdexcep;
         int maxfd;
         int rc;
         CURLMcode mc;
         struct timeval timeout = {1, 0};

         CURLM *multi = curl_multi_init();

         do {

           /* call curl_multi_perform() */

           /* get file descriptors from the transfers */
           mc = curl_multi_fdset(multi, &fdread, &fdwrite, &fdexcep, &maxfd);

           if(mc != CURLM_OK) {
             fprintf(stderr, "curl_multi_fdset() failed, code %d.\n", mc);
             break;
           }

           /* wait for activity on one of the sockets */
           rc = select(maxfd + 1, &fdread, &fdwrite, &fdexcep, &timeout);

         } while(!mc);
       }

AVAILABILITY

       Added in curl 7.9.6

RETURN VALUE

       CURLMcode type, general libcurl multi interface error code. See libcurl-errors(3)

SEE ALSO

       curl_multi_cleanup(3),  curl_multi_init(3),  curl_multi_perform(3), curl_multi_timeout(3),
       curl_multi_wait(3), curl_multi_waitfds(3), select(2)