CURLOPT_SOCKOPTFUNCTION(3)

💖 NAME 💖

okay so welcome to CURLOPT_SOCKOPTFUNCTION(3) from the Library Functions Manual — it’s the callback for setting socket options, and yes bestie, it’s lowkey iconic.

✨ 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, and it should match the prototype shown above. Like, exactly bestie — int sockopt_callback(void *clientp, curl_socket_t curlfd, curlsocktype purpose) energy only. 💅

When set, this callback function gets called by libcurl when the socket has been created, but before the connect call, so applications can change specific socket options. The callback’s purpose argument identifies the exact purpose for this particular socket:

• CURLSOCKTYPE_IPCXN — for actively created connections.
• CURLSOCKTYPE_ACCEPT — since 7.28.0, 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_SOCKOPT_ERROR from the callback function to signal an unrecoverable error to the library, and it closes the socket and returns CURLE_COULDNT_CONNECT for CURLSOCKTYPE_IPCXN and CURLE_ABORTED_BY_CALLBACK for CURLSOCKTYPE_ACCEPT. 😭

The callback function may return CURL_SOCKOPT_ALREADY_CONNECTED for CURLSOCKTYPE_IPCXN, 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_OPENSOCKETFUNCTION(3) and then have this function make libcurl not attempt to connect (again). Slay — already connected means already connected. ✨

🎀 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)
{
  return CURL_SOCKOPT_ALREADY_CONNECTED;
}

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

    result = 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_SOCKOPTDATA(3)

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🫶 xoxo, libcurl 💖 — page date 2026-10-12, source/version: libcurl. stay socket-y, bestie! 🎀