NAME

       curl_multi_fdset - extracts 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.

       If  the  read_fd_set argument is not a null pointer, it points to an object of type fd_set
       that on returns specifies the file descriptors to be checked for being ready to read.

       If the write_fd_set argument is not a null pointer, it points to an object of type  fd_set
       that on return specifies the file descriptors to be checked for being ready to write.

       If  the  exc_fd_set  argument is not a null pointer, it points to an object of type fd_set
       that on return specifies the file descriptors to be checked for error conditions pending.

       If no file descriptors are set by libcurl, max_fd  will  contain  -1  when  this  function
       returns. Otherwise it will contain 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 will try 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 will possibly save you from the crash, but will make your program NOT wait  for
       sockets it should wait for...

EXAMPLE

        /* get file descriptors from the transfers */
        mc = curl_multi_fdset(multi_handle, &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);

AVAILABILITY

       Added in 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_wait(3),  curl_multi_timeout(3),
       curl_multi_perform(3), select(2)