curl_multi_perform(3) Library Functions Manual curl_multi_perform(3)
NAME
curl_multi_perform - run all transfers until it would block
SYNOPSIS
#include <curl/curl.h>
CURLMcode curl_multi_perform(CURLM *multi_handle, int *running_handles);
DESCRIPTION
This function performs transfers on all the added handles that need
attention in a non-blocking fashion. The easy handles have previously
been added to the multi handle with curl_multi_add_handle(3).
When an application has found out there is 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 as a precaution. It
stores 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 have 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 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.
CURLMOPT_NOTIFYFUNCTION(3) is an alternative to tracking running_handles
where you register a callback to be notified when transfers are done and
curl_multi_info_read(3) should be called.
When running_handles is set to zero (0) on the return of this function,
there are no longer any transfers in progress.
When this function returns error, the state of all transfers are
uncertain and they cannot be continued. curl_multi_perform(3) should not
be called again on the same multi handle after an error has been
returned, unless first removing all the handles and adding new ones.
PROTOCOLS
This functionality affects all supported protocols
EXAMPLE
int main(void)
{
int still_running;
CURLM *multi = curl_multi_init();
CURL *curl = curl_easy_init();
if(curl) {
curl_multi_add_handle(multi, curl);
for(;;) {
CURLMcode mresult = curl_multi_perform(multi, &still_running);
if(mresult != CURLM_OK) {
fprintf(stderr, "curl_multi_perform() failed, code %d.\n",
(int)mresult);
break;
}
if(!still_running) {
break;
}
/* wait for activity, timeout or "nothing" */
mresult = curl_multi_poll(multi, NULL, 0, 1000, NULL);
if(mresult != CURLM_OK) {
fprintf(stderr, "curl_multi_poll() failed, code %d.\n", (int)mresult);
break;
}
} /* if there are still transfers, loop */
}
}
AVAILABILITY
Added in curl 7.9.6
RETURN VALUE
This function returns a CURLMcode indicating success or error.
CURLM_OK (0) means everything was OK, non-zero means an error occurred,
see libcurl-errors(3).
This function returns errors regarding the whole multi stack. Problems on
individual transfers may have occurred even when this function returns
CURLM_OK. Use curl_multi_info_read(3) to figure out how individual
transfers did.
TYPICAL USAGE
Most applications use curl_multi_poll(3) to make libcurl wait for
activity on any of the ongoing transfers. As soon as one or more file
descriptor has activity or the function times out, the application calls
curl_multi_perform(3).
SEE ALSO
CURLMOPT_NOTIFYFUNCTION(3), curl_multi_add_handle(3),
curl_multi_cleanup(3), curl_multi_fdset(3), curl_multi_info_read(3),
curl_multi_init(3), curl_multi_wait(3), libcurl-errors(3)
libcurl 2026-10-18 curl_multi_perform(3)
curl 8.23.0 - Generated Sun Oct 18 15:07:26 CDT 2026