---
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,
                    curl_socket_t socket,
                    int what,
                    void *clientp,
                    void *socketp);

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.

libcurl may use a number of internal file descriptors for name resolving,
Happy Eyeballs racing, internal communication and more, in addition to the
main sockets used for network transfers. All of those file descriptors might
get passed to this callback as "sockets".

# 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.

**socket** 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.

When a socket is given a CURL_POLL_REMOVE value, it might be because libcurl
is going to close it, but it might also mean that it does not need any more
monitoring for the moment. An application cannot assume either. The same
socket might appear soon in a call asking for monitoring again.

A socket that is removed like this loses its assigned pointer as set with
curl_multi_assign(3).

# 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 = cbp;
  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.
