curl_multi_perform(3)           libcurl Manual           curl_multi_perform(3)

NAME
       curl_multi_perform - reads/writes available data from each easy handle

SYNOPSIS
       #include <curl/curl.h>

       CURLMcode  curl_multi_perform(CURLM  *multi_handle,  int  *running_han-
       dles);

DESCRIPTION
       This function handles transfers on all  the  added  handles  that  need
       attention in an non-blocking fashion.

       When  an  application  has  found  out  there's  data available for the
       multi_handle or a timeout has elapsed, the application should call this
       function  to  read/write  whatever  there is to read or write right now
       etc.  curl_multi_perform(3) returns as soon  as  the  reads/writes  are
       done.  This  function  does not require that there actually is any data
       available for reading or that data can be written,  it  can  be  called
       just  in  case. It will write the number of handles that still transfer
       data in the second argument's integer-pointer.

       If the amount of running_handles is changed from the previous call  (or
       is  less than the amount of easy handles you've added to the multi han-
       dle), you know that there is one or more transfers less "running".  You
       can  then  call  curl_multi_info_read(3)  to get information about each
       individual completed transfer, and that returned info includes CURLcode
       and  more.  If  an  added  handle  fails  very quickly, it may never be
       counted as a running_handle.

       When running_handles is set to zero(0) on the return of this function,
       there is no longer any transfers in progress.

EXAMPLE
       #ifdef _WIN32
       #define SHORT_SLEEP Sleep(100)
       #else
       #define SHORT_SLEEP usleep(100000)
       #endif

       fd_set fdread;
       fd_set fdwrite;
       fd_set fdexcep;
       int maxfd = -1;

       long curl_timeo;

       curl_multi_timeout(multi_handle, &curl_timeo);
       if(curl_timeo < 0)
         curl_timeo = 1000;

       timeout.tv_sec = curl_timeo / 1000;
       timeout.tv_usec = (curl_timeo % 1000) * 1000;

       FD_ZERO(&fdread);
       FD_ZERO(&fdwrite);
       FD_ZERO(&fdexcep);

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

       if(maxfd == -1) {
         SHORT_SLEEP;
         rc = 0;
       }
       else
         rc = select(maxfd+1, &fdread, &fdwrite, &fdexcep, &timeout);

       switch(rc) {
       case -1:
         /* select error */
         break;
       case 0:
       default:
         /* timeout or readable/writable sockets */
         curl_multi_perform(multi_handle, &still_running);
         break;
       }

       /* if there are still transfers, loop! */

RETURN VALUE
       CURLMcode type, general libcurl multi interface error code.

       Before  version  7.20.0:  If you receive CURLM_CALL_MULTI_PERFORM, this
       basically means  that  you  should  call  curl_multi_perform(3)  again,
       before  you  select()  on more actions. You don't have to do it immedi-
       ately, but the return code means that libcurl may have more data avail-
       able  to return or that there may be more data to send off before it is
       "satisfied".   Do   note   that   curl_multi_perform(3)   will   return
       CURLM_CALL_MULTI_PERFORM  only when it wants to be called again immedi-
       ately. When things are fine and there is  nothing  immediate  it  wants
       done,  it'll return CURLM_OK and you need to wait for "action" and then
       call this function again.

       This function only returns errors etc regarding the whole multi  stack.
       Problems  still  might  have occurred on individual transfers even when
       this function returns CURLM_OK. Use curl_multi_info_read(3)  to  figure
       out how individual transfers did.

TYPICAL USAGE
       Most  applications  will  use curl_multi_fdset(3) to get the multi_han-
       dle's file descriptors, and curl_multi_timeout(3)  to  get  a  suitable
       timeout  period,  then  it'll  wait  for action on the file descriptors
       using select(3). As soon as one  or  more  file  descriptor  is  ready,
       curl_multi_perform(3) gets called.

SEE ALSO
       curl_multi_cleanup(3),      curl_multi_init(3),     curl_multi_wait(3),
       curl_multi_fdset(3), curl_multi_info_read(3), libcurl-errors(3)

libcurl 7.54.0                 February 03, 2016         curl_multi_perform(3)