focal (3) curl_multi_perform.3.gz

Provided by: libcurl4-doc_7.68.0-1ubuntu2.25_all bug

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)