CURLOPT_OP...ETFUNCTION(3)

🌸 NAME 🌸

CURLOPT_OPENSOCKETFUNCTION — callback for opening socket

okay so this page is CURLOPT_OP...ETFUNCTION(3) from the Library Functions Manual, and it's literally the whole vibe for making libcurl call your callback when it needs to open a socket. bestie, we're talking custom socket energy. 🫶

✨ 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 💖

okay so you're passing a pointer to your callback function, and it needs to match the prototype up there exactly. no improvising the signature, bestie. 💅

This callback gets called by libcurl instead of the socket(2) call. The purpose argument tells you 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. iykyk. ✨

The clientp pointer contains whatever user-defined value you set using CURLOPT_OPENSOCKETDATA(3). so it's your lil custom-data bag. 🎀

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. love that for you. 🦋

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. no cap, that's the fail path. 😭

You can get the IP address that curl is opening the socket for by casting 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. cute lil reference file. 🌈

If you want to pass in a socket with an already established connection, pass the socket back with this callback and then use CURLOPT_SOCKOPTFUNCTION(3) to signal that it already is connected. that's the whole trick, periodt. 👑

🎀 DEFAULT 🎀

The equivalent of this:

return socket(addr->family, addr->socktype, addr->protocol);

that's the default vibe. lowkey simple. 🍓

🌈 PROTOCOLS 🌈

This functionality affects all supported protocols. literally all of them. ⭐

🧚 EXAMPLE 🧚

okay bestie, here's the full example from the page, and yes it's a whole little program:

/* 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)
{
  return CURL_SOCKOPT_ALREADY_CONNECTED;
}

int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    CURLcode result;
    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);

    result = curl_easy_perform(curl);

    curl_easy_cleanup(curl);
  }
}

⭐ AVAILABILITY ⭐

Added in curl 7.17.1. iconic since then. 💜

💀 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). so like, zero is the slay result. 💖

🦋 SEE ALSO 🦋

• CURLOPT_CLOSESOCKETFUNCTION(3) — bestie cross-reference
• CURLOPT_OPENSOCKETFUNCTION(3) — yes, this one
• CURLOPT_SOCKOPTFUNCTION(3) — the sockopt callback reference

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mwah 💋 — this page is from libcurl, dated 2026-10-12, and its manual title is CURLOPT_OP...ETFUNCTION(3). periodt. 🎀