---
c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
SPDX-License-Identifier: curl
Title: CURLMOPT_SOCKETFUNCTION
Section: 3
Source: libcurl
See-also:
  - CURLMOPT_SOCKETDATA (3)
  - CURLMOPT_TIMERFUNCTION (3)
  - curl_multi_socket_action (3)
Protocol:
  - All
Added-in: 7.15.4
---

# NAME

CURLMOPT_SOCKETFUNCTION - callback informed about what to wait for

# SYNOPSIS

~~~c
#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 prototype
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
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(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
happen *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. 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(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%

# EXAMPLE

~~~c
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%

# RETURN VALUE

Returns CURLM_OK.
