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_handles);

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 handle), 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.  You could use curl_multi_info_read(3) to track actual status of the added
       handles in that case.

       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 immediately, but the return code means that libcurl may have more data
       available  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  immediately. 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_handle'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)