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 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...

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 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), select(2)

libcurl 8.5.0                                   December 04, 2023                            curl_multi_fdset(3)