CURLMOPT_NOTIFYFUNCTION(3) Library Functions Manual
NAME
CURLMOPT_NOTIFYFUNCTION - callback receiving notifications
SYNOPSIS
#include <curl/curl.h>
void notify_callback(CURLM *multi, /* multi handle */
unsigned int notification, /* notification type */
CURL *easy, /* easy handle */
void *notifyp); /* private notify pointer */
CURLMcode curl_multi_setopt(CURLM *handle, CURLMOPT_NOTIFYFUNCTION, notify_callback);
DESCRIPTION
Pass a pointer to your callback function, which should match the
prototype shown above.
When the multi handle processes transfers, changes can be observed by
receiving notifications about them. This can eliminate the need to
constantly interrogate the multi handle to observe such changes to act
on them.
Notifications are collected and dispatched to the application's
callback function at an appropriate time.
The notify callback is different from other callbacks in that it can
use more libcurl API functions. Apart from curl_multi_perform(3),
curl_multi_socket(3), curl_multi_socket_action(3),
curl_multi_socket_all(3) and curl_multi_cleanup(3) it may call all
other methods on the multi and easy handles. This includes adding and
removing easy handles to/from the multi handle.
This callback may get invoked at any time when interacting with
libcurl. This may even happen after all transfers are done and may
also happen during a call to curl_multi_cleanup(3) when cached
connections are shut down.
CALLBACK ARGUMENTS
multi identifies the multi handle that triggered the notification.
notification is the type of notification, e.g. what happened. The
following types are available right now. In the future, new ones might
be added.
CURLMNOTIFY_INFO_READ
When enabled via curl_multi_notify_enable(3), this informs the
application that there are new messages to be processed via
curl_multi_info_read(3).
This notification happens whenever a message is added to an
empty message stack in the multi handle and not for subsequent
additions. The notification callback is then expected to read
all available message, emptying the stack, so a subsequent
addition triggers the notification again.
The easy handle passed is an internal handle.
CURLMNOTIFY_EASY_DONE
When enabled via curl_multi_notify_enable(3), this notification
is triggered when an easy handle has finished. This happens both
for successful and failed transfers.
The easy handle passed is the transfer that is done. This may be
an internal handle when DoH or other features are used.
easy identifies the transfer involved. This may be one of the
application's own easy handle or an internal handle.
notifyp is set with CURLMOPT_NOTIFYDATA(3).
DEFAULT
NULL (no callback)
PROTOCOLS
This functionality affects all supported protocols
EXAMPLE
struct priv {
void *ours;
};
static void notify_cb(CURLM *multi, unsigned int notification,
CURL *easy, void *notifyp)
{
struct priv *p = notifyp;
printf("my ptr: %p\n", p->ours);
/* ... */
}
int main(void)
{
struct priv setup;
CURLM *multi = curl_multi_init();
/* ... use socket callback and custom pointer */
curl_multi_setopt(multi, CURLMOPT_NOTIFYFUNCTION, notify_cb);
curl_multi_setopt(multi, CURLMOPT_NOTIFYDATA, &setup);
curl_multi_notify_enable(multi, CURLMNOTIFY_INFO_READ);
}
AVAILABILITY
Added in curl 8.17.0
RETURN VALUE
Returns CURLM_OK.
SEE ALSO
CURLMOPT_NOTIFYDATA(3), curl_multi_notify_disable(3),
curl_multi_notify_enable(3), curl_multi_socket_action(3)
libcurl 2026-03-23 CURLMOPT_NOTIFYFUNCTION(3)
curl 8.19.0 - Generated Tue Mar 31 18:37:12 CDT 2026