CURLOPT_SOCKOPTFUNCTION(3) Library Functions Manual CURLOPT_SOCKOPTFUNCTION(3)
NAME
CURLOPT_SOCKOPTFUNCTION - callback for setting socket options
SYNOPSIS
#include <curl/curl.h>
typedef enum {
CURLSOCKTYPE_IPCXN, /* socket created for a specific IP connection */
CURLSOCKTYPE_ACCEPT, /* socket created by accept() call */
CURLSOCKTYPE_LAST /* never use */
} curlsocktype;
#define CURL_SOCKOPT_OK 0
#define CURL_SOCKOPT_ERROR 1 /* causes libcurl to abort and return
CURLE_ABORTED_BY_CALLBACK */
#define CURL_SOCKOPT_ALREADY_CONNECTED 2
int sockopt_callback(void *clientp,
curl_socket_t curlfd,
curlsocktype purpose);
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SOCKOPTFUNCTION, sockopt_callback);
DESCRIPTION
Pass a pointer to your callback function, which should match the proto-
type shown above.
When set, this callback function gets called by libcurl when the socket
has been created, but before the connect call to allow applications to
change specific socket options. The callback's purpose argument identi-
fies the exact purpose for this particular socket:
CURLSOCKTYPE_IPCXN for actively created connections or since 7.28.0
CURLSOCKTYPE_ACCEPT for FTP when the connection was setup with PORT/EPSV
(in earlier versions these sockets were not passed to this callback).
Future versions of libcurl may support more purposes. libcurl passes the
newly created socket descriptor to the callback in the curlfd parameter
so additional setsockopt() calls can be done at the user's discretion.
The clientp pointer contains whatever user-defined value set using the
CURLOPT_SOCKOPTDATA(3) function.
Return CURL_SOCKOPT_OK from the callback on success. Return CURL_SOCK-
OPT_ERROR from the callback function to signal an unrecoverable error to
the library and it closes the socket and returns CURLE_COULDNT_CONNECT.
Alternatively, the callback function can return CURL_SOCKOPT_AL-
READY_CONNECTED, to tell libcurl that the socket is already connected
and then libcurl does no attempt to connect. This allows an application
to pass in an already connected socket with CURLOPT_OPENSOCKETFUNC-
TION(3) and then have this function make libcurl not attempt to connect
(again).
DEFAULT
NULL
PROTOCOLS
This functionality affects all supported protocols
EXAMPLE
/* make libcurl use the already established socket 'sockfd' */
static curl_socket_t opensocket(void *clientp,
curlsocktype purpose,
struct curl_sockaddr *address)
{
curl_socket_t sockfd;
sockfd = *(curl_socket_t *)clientp;
/* the actual externally set socket is passed in via the OPENSOCKETDATA
option */
return sockfd;
}
static int sockopt_callback(void *clientp, curl_socket_t curlfd,
curlsocktype purpose)
{
/* This return code was added in libcurl 7.21.5 */
return CURL_SOCKOPT_ALREADY_CONNECTED;
}
int main(void)
{
CURL *curl = curl_easy_init();
if(curl) {
CURLcode res;
int sockfd; /* our custom file descriptor */
/* libcurl thinks that you connect to the host
* and port that you specify in the URL option. */
curl_easy_setopt(curl, CURLOPT_URL, "http://99.99.99.99:9999");
/* call this function to get a socket */
curl_easy_setopt(curl, CURLOPT_OPENSOCKETFUNCTION, opensocket);
curl_easy_setopt(curl, CURLOPT_OPENSOCKETDATA, &sockfd);
/* call this function to set options for the socket */
curl_easy_setopt(curl, CURLOPT_SOCKOPTFUNCTION, sockopt_callback);
res = curl_easy_perform(curl);
curl_easy_cleanup(curl);
}
}
AVAILABILITY
Added in curl 7.16.0
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
CURLOPT_OPENSOCKETFUNCTION(3), CURLOPT_SEEKFUNCTION(3), CURLOPT_SOCKOPT-
DATA(3)
libcurl 2026-07-12 CURLOPT_SOCKOPTFUNCTION(3)
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