CURLMOPT_SOCKETFUNCTION(3) Library Functions Manual CURLMOPT_SOCKETFUNCTION(3)
NAME
CURLMOPT_SOCKETFUNCTION - callback informed about what to wait for
SYNOPSIS
#include <curl/curl.h>
int socket_callback(CURL *easy, /* easy handle */
curl_socket_t s, /* socket */
int what, /* describes the socket */
void *clientp, /* private callback pointer */
void *socketp); /* private socket pointer */
CURLMcode curl_multi_setopt(CURLM *handle, CURLMOPT_SOCKETFUNCTION, socket_callback);
DESCRIPTION
Pass a pointer to your callback function, which should match the proto-
type shown above.
When the curl_multi_socket_action(3) function is called, it uses this
callback to inform the application about updates in the socket (file de-
scriptor) status by doing none, one, or multiple calls to the
socket_callback. The callback function gets status updates with changes
since the previous time the callback was called. If the given callback
pointer is set to NULL, no callback is called.
libcurl then expects the application to monitor the sockets for the spe-
cific activities and tell libcurl again when something happens on one of
them. Tell libcurl by calling curl_multi_socket_action(3).
This callback may get invoked at any time when interacting with libcurl.
This may even happen after all transfers are done and is likely to hap-
pen during a call to curl_multi_cleanup(3) when cached connections are
shut down.
CALLBACK ARGUMENTS
easy identifies the specific transfer for which this update is related.
Since this callback manages a whole multi handle, an application should
not make assumptions about which particular handle that is passed here.
It might even be an internal easy handle that the application did not
add itself.
s is the specific socket this function invocation concerns. If the what
argument is not CURL_POLL_REMOVE then it holds information about what
activity on this socket the application is supposed to monitor. Subse-
quent calls to this callback might update the what bits for a socket
that is already monitored.
The socket 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.
clientp is set with CURLMOPT_SOCKETDATA(3).
socketp is set with curl_multi_assign(3) or NULL.
The what parameter informs the callback on the status of the given
socket. It can hold one of these values:
CURL_POLL_IN
Wait for incoming data. For the socket to become readable.
CURL_POLL_OUT
Wait for outgoing data. For the socket to become writable.
CURL_POLL_INOUT
Wait for incoming and outgoing data. For the socket to become
readable or writable.
CURL_POLL_REMOVE
The specified socket/file descriptor is no longer used by libcurl
for any active transfer. It might soon be added again.
DEFAULT
NULL (no callback)
PROTOCOLS
This functionality affects all supported protocols
EXAMPLE
struct priv {
void *ours;
};
static int sock_cb(CURL *e, curl_socket_t s, int what, void *cbp, void *sockp)
{
struct priv *p = sockp;
printf("our ptr: %p\n", p->ours);
if(what == CURL_POLL_REMOVE) {
/* remove the socket from our collection */
}
if(what & CURL_POLL_IN) {
/* wait for read on this socket */
}
if(what & CURL_POLL_OUT) {
/* wait for write on this socket */
}
return 0;
}
int main(void)
{
struct priv setup;
CURLM *multi = curl_multi_init();
/* ... use socket callback and custom pointer */
curl_multi_setopt(multi, CURLMOPT_SOCKETFUNCTION, sock_cb);
curl_multi_setopt(multi, CURLMOPT_SOCKETDATA, &setup);
}
AVAILABILITY
Added in curl 7.15.4
RETURN VALUE
Returns CURLM_OK.
SEE ALSO
CURLMOPT_SOCKETDATA(3), CURLMOPT_TIMERFUNCTION(3), curl_multi_socket_ac-
tion(3)
libcurl 2026-07-12 CURLMOPT_SOCKETFUNCTION(3)
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