curl_multi_assign(3) Library Functions Manual curl_multi_assign(3)
NAME
curl_multi_assign - set data to associate with an internal socket
SYNOPSIS
#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.
If the given sockfd is not already in use by libcurl, this function re-
turns an error.
libcurl only keeps one single pointer associated with a socket, so call-
ing this function several times for the same socket makes the last set
pointer get used.
The idea here being that this association (socket to private pointer) is
something that just about every application that uses this API needs and
then libcurl can just as well do it since it already has the necessary
functionality.
It is acceptable to call this function from your multi callback func-
tions.
PROTOCOLS
This functionality affects all supported protocols
EXAMPLE
int main(void)
{
CURLM *multi = curl_multi_init();
int private = 123;
curl_socket_t fd = 0; /* file descriptor to associate our data with */
/* make our struct pointer associated with socket fd */
CURLMcode mc = curl_multi_assign(multi, fd, &private);
if(mc)
printf("error: %s\n", curl_multi_strerror(mc));
}
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
Added in curl 7.15.5
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).
SEE ALSO
curl_multi_setopt(3), curl_multi_socket_action(3)
libcurl 2026-07-12 curl_multi_assign(3)
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