oracular (3) curl_multi_fdset.3.gz

Provided by: libcurl4-doc_8.9.1-2ubuntu2.2_all bug

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)