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)

libcurl 7.81.0                                  October 31, 2021                             curl_multi_fdset(3)