curl_multi_perform - reads/writes available data from each easy handle

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.

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 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 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 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 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.

When this function returns error, the state of all transfers are uncertain and they cannot be continued. curl_multi_perform 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);
    do {
      CURLMcode mc = curl_multi_perform(multi, &still_running);
 
      if(!mc && still_running)
        /* wait for activity, timeout or "nothing" */
        mc = curl_multi_poll(multi, NULL, 0, 1000, NULL);
 
      if(mc) {
        fprintf(stderr, "curl_multi_poll() failed, code %d.\n", (int)mc);
        break;
      }
 
    /* if there are still transfers, loop */
    } while(still_running);
  }
}

Availability

Added in curl 7.9.6

Return value

CURLMcode type, general libcurl multi interface error code.

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 to figure out how individual transfers did.

Typical usage

Most applications use curl_multi_poll 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.