CURLOPT_OP...ETFUNCTION(3) Library Functions Manual CURLOPT_OP...ETFUNCTION(3)
NAME
CURLOPT_OPENSOCKETFUNCTION - callback for opening socket
SYNOPSIS
#include <curl/curl.h>
typedef enum {
CURLSOCKTYPE_IPCXN, /* socket created for a specific IP connection */
} curlsocktype;
struct curl_sockaddr {
int family;
int socktype;
int protocol;
unsigned int addrlen;
struct sockaddr addr;
};
curl_socket_t opensocket_callback(void *clientp,
curlsocktype purpose,
struct curl_sockaddr *address);
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_OPENSOCKETFUNCTION, opensocket_callback);
DESCRIPTION
Pass a pointer to your callback function, which should match the proto-
type shown above.
This callback function gets called by libcurl instead of the socket(2)
call. The callback's purpose argument identifies the exact purpose for
this particular socket. CURLSOCKTYPE_IPCXN is for IP based connections
and is the only purpose currently used in libcurl. Future versions of
libcurl may support more purposes.
The clientp pointer contains whatever user-defined value set using the
CURLOPT_OPENSOCKETDATA(3) function.
The callback gets the resolved peer address as the address argument and
is allowed to modify the address or refuse to connect completely. The
callback function should return the newly created socket or
CURL_SOCKET_BAD in case no connection could be established or another
error was detected. Any additional setsockopt(2) calls can of course be
done on the socket at the user's discretion.
If CURL_SOCKET_BAD is returned by the callback then libcurl treats it as
a failed connection and tries to open a socket to connect to a different
IP address associated with the transfer. If there are no more addresses
to try then libcurl fails the transfer with error code
CURLE_COULDNT_CONNECT.
You can get the IP address that curl is opening the socket for by cast-
ing address->addr to sockaddr_in if address->family is AF_INET, or to
sockaddr_in6 if address->family is AF_INET6. For an example of how that
data can be compared against refer to docs/examples/block_ip.c.
If you want to pass in a socket with an already established connection,
pass the socket back with this callback and then use CURLOPT_SOCKOPT-
FUNCTION(3) to signal that it already is connected.
DEFAULT
The equivalent of this:
return socket(addr->family, addr->socktype, addr->protocol);
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;
extern int sockfd; /* the already connected one */
/* 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 7.17.1
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_CLOSESOCKETFUNCTION(3), CURLOPT_OPENSOCKETFUNCTION(3), CUR-
LOPT_SOCKOPTFUNCTION(3)
libcurl 2026-07-12 CURLOPT_OP...ETFUNCTION(3)
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