---
c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
SPDX-License-Identifier: curl
Title: curl_multi_assign
Section: 3
Source: libcurl
See-also:
  - curl_multi_setopt (3)
  - curl_multi_socket_action (3)
  - CURLMOPT_SOCKETFUNCTION (3)
Protocol:
  - All
Added-in: 7.15.5
---

# NAME

curl_multi_assign - set data to associate with an internal socket

# SYNOPSIS

~~~c
#include <curl/curl.h>

CURLMcode curl_multi_assign(CURLM *multi_handle, curl_socket_t sockfd,
                            void *sockptr);
~~~

# DESCRIPTION

This function creates an association in the multi handle between the given
socket and a private pointer of the application. This is designed for
curl_multi_socket_action(3) uses.

When set, the *sockptr* pointer is passed to all future socket callbacks for
the specific *sockfd* socket, until the socket stops being monitored
(CURL_POLL_REMOVE is sent to the CURLMOPT_SOCKETFUNCTION(3) callback).

If the given *sockfd* is not already in use by libcurl, this function returns
an error.

libcurl only keeps one single pointer associated with a socket, so calling
this function several times for the same socket makes the last set pointer get
used.

It is acceptable to call this function from your multi callback functions.

# %PROTOCOLS%

# EXAMPLE

~~~c
int main(void)
{
  CURLM *multi = curl_multi_init();
  int private_data = 123;
  curl_socket_t fd = 0; /* file descriptor to associate our data with */

  /* make our struct pointer associated with socket fd */
  CURLMcode mresult = curl_multi_assign(multi, fd, &private_data);
  if(mresult)
    printf("error: %s\n", curl_multi_strerror(mresult));
}
~~~

# TYPICAL USAGE

In a typical application you allocate a struct or at least use some kind of
semi-dynamic data for each socket that we must wait for action on when using
the curl_multi_socket_action(3) approach.

When our socket-callback gets called by libcurl and we get to know about yet
another socket to wait for, we can use curl_multi_assign(3) to point out the
particular data so that when we get updates about this same socket again, we
do not have to find the struct associated with this socket by ourselves.

# %AVAILABILITY%

# RETURN VALUE

This function returns a CURLMcode indicating success or error.

CURLM_OK (0) means everything was OK, non-zero means an error occurred, see
libcurl-errors(3).
