curl / libcurl / API / curl_multi_setopt / CURLMOPT_SOCKETFUNCTION



CURLMOPT_SOCKETFUNCTION - callback informed about what to wait for


#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);


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

When the curl_multi_socket_action function is called, it uses this callback to inform the application about updates in the socket (file descriptor) 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 specific activities and tell libcurl again when something happens on one of them. Tell libcurl by calling curl_multi_socket_action.

Callback arguments

easy identifies the specific transfer for which this update is related.

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. Subsequent 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.

socketp is set with curl_multi_assign or NULL.

The what parameter informs the callback on the status of the given socket. It can hold one of these values:


Wait for incoming data. For the socket to become readable.


Wait for outgoing data. For the socket to become writable.


Wait for incoming and outgoing data. For the socket to become readable or writable.


The specified socket/file descriptor is no longer used by libcurl for any active transfer. It might soon be added again.


NULL (no callback)


This functionality affects all supported protocols


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);


Added in curl 7.15.4

Return value

Returns CURLM_OK.

See also

CURLMOPT_SOCKETDATA(3), CURLMOPT_TIMERFUNCTION(3), curl_multi_socket_action(3)

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