CURLMOPT_TIMERFUNCTION - callback to receive timeout values

#include <curl/curl.h>
int timer_callback(CURLM *multi,    /* multi handle */
                   long timeout_ms, /* timeout in number of ms */
                   void *clientp);    /* private callback pointer */
CURLMcode curl_multi_setopt(CURLM *handle, CURLMOPT_TIMERFUNCTION, timer_callback);

Pass a pointer to your callback function, which should match the prototype shown above.

Certain features, such as timeouts and retries, require you to call libcurl even when there is no activity on the file descriptors.

Your callback function timer_callback should install a non-repeating timer with an expire time of timeout_ms milliseconds. When that timer fires, call either curl_multi_socket_action(3) or curl_multi_perform(3), depending on which interface you use.

A timeout_ms value of -1 passed to this callback means you should delete the timer. All other values are valid expire times in number of milliseconds.

The timer_callback is called when the timeout expire time is changed.

The clientp pointer is set with CURLMOPT_TIMERDATA(3).

The timer callback should return 0 on success, and -1 on error. If this callback returns error, all transfers currently in progress in this multi handle are aborted and made to fail.

This callback can be used instead of, or in addition to, curl_multi_timeout(3).

WARNING: do not call libcurl directly from within the callback itself when the timeout_ms value is zero, since it risks triggering an unpleasant recursive behavior that immediately calls another call to the callback with a zero timeout...



struct priv {
  void *custom;
static int timerfunc(CURLM *multi, long timeout_ms, void *clientp)
 struct priv *mydata = clientp;
 printf("our ptr: %p\n", mydata->custom);
 if(timeout_ms) {
   /* this is the new single timeout to wait for */
 else {
   /* delete the timeout, nothing to wait for now */
int main(void)
  struct priv mydata;
  CURLM *multi = curl_multi_init();
  curl_multi_setopt(multi, CURLMOPT_TIMERFUNCTION, timerfunc);
  curl_multi_setopt(multi, CURLMOPT_TIMERDATA, &mydata);

Added in 7.16.0

Returns CURLM_OK if the option is supported, and CURLM_UNKNOWN_OPTION if not.


2024-05-22 libcurl