CURLOPT_CLOSESOCKETFUNCTION(3) Library Functions Manual
NAME
CURLOPT_CLOSESOCKETFUNCTION - callback to socket close replacement
SYNOPSIS
#include <curl/curl.h>
int closesocket_callback(void *clientp, curl_socket_t item);
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_CLOSESOCKETFUNCTION,
closesocket_callback);
DESCRIPTION
Pass a pointer to your callback function, which should match the
prototype shown above.
This callback function gets called by libcurl instead of the close(3)
or closesocket(3) call when sockets are closed (not for any other file
descriptors). This is pretty much the reverse to the
CURLOPT_OPENSOCKETFUNCTION(3) option. Return 0 to signal success and 1
if there was an error.
The clientp pointer is set with CURLOPT_CLOSESOCKETDATA(3). item is the
socket libcurl wants to be closed.
Note that when using multi/share handles, your callback may get invoked
even after the easy handle has been cleaned up. The callback and data
is inherited by a new connection and that connection may live longer
than the transfer itself in the multi/share handle's connection cache.
NOTES ON CONNECTION REUSE
The close socket callback is invoked when libcurl closes a socket it
owns. When using the multi interface, the callback and
CURLOPT_CLOSESOCKETDATA(3) are copied from the first easy handle that
creates the socket used for a connection; changing this option on a
subsequent easy handle that reuses the same connection has no effect
for that connection. The callback is stored with the connection
because the connection and its associated socket may outlive the easy
handle that created it, so that libcurl can still invoke it when the
socket is closed even after that handle has been cleaned up.
DEFAULT
Use the standard socket close function.
PROTOCOLS
This functionality affects all supported protocols
EXAMPLE
struct priv {
void *custom;
};
static int closesocket(void *clientp, curl_socket_t item)
{
struct priv *my = clientp;
printf("our ptr: %p\n", my->custom);
printf("libcurl wants to close %d now\n", (int)item);
return 0;
}
int main(void)
{
struct priv myown;
CURL *curl = curl_easy_init();
if(curl) {
CURLcode result;
/* call this function to close sockets */
curl_easy_setopt(curl, CURLOPT_CLOSESOCKETFUNCTION, closesocket);
curl_easy_setopt(curl, CURLOPT_CLOSESOCKETDATA, &myown);
result = curl_easy_perform(curl);
curl_easy_cleanup(curl);
}
}
AVAILABILITY
Added in curl 7.21.7
RETURN VALUE
curl_easy_setopt(3) returns a CURLcode indicating success or error.
CURLE_OK (0) means everything was OK, non-zero means an error occurred,
see libcurl-errors(3).
SEE ALSO
CURLMOPT_SOCKETFUNCTION(3), CURLOPT_CLOSESOCKETDATA(3),
CURLOPT_OPENSOCKETFUNCTION(3)
libcurl 2026-09-03 CURLOPT_CLOSESOCKETFUNCTION(3)
curl 8.22.0 - Generated Fri Sep 4 15:59:20 CDT 2026