GNURLOPT_SOCKOPTFUNCTION(3)

💅✨ GNURLOPT_SOCKOPTFUNCTION(3) ✨💅

okay so bestie, welcome to the curl_easy_setopt options manual page for GNURLOPT_SOCKOPTFUNCTION(3) — yes, that whole mouthful. it's giving callback-socket-glow-up energy, and we're gonna get through it together. 🫶

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🎀 NAME 🎀

CURLOPT_SOCKOPTFUNCTION — set the callback for setting socket options. literally the lil function that lets you tweak socket options before the connect era begins. 💅

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✨ SYNOPSIS ✨

bestie, here's the exact shape of it — no cap:

#include <gnurl/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);

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🌸 DESCRIPTION 🌸

okay so you pass a pointer to your callback function, and it should match the prototype shown above. no cap, that's the whole assignment. 💖

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 weren't 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. go off, bestie. ✨

the clientp pointer contains whatever user-defined value you set using CURLOPT_SOCKOPTDATA(3). 🫶

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 will close the socket and return CURLE_COULDNT_CONNECT. that one is not a vibe. 💀

alternatively, the callback function can return CURL_SOCKOPT_ALREADY_CONNECTED, to tell libcurl that the socket is already connected, and then libcurl will not attempt to connect it. 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). 🔥

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🦋 DEFAULT 🦋

by default, this callback is NULL and unused. iconic nothing-burger behavior. 💅

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🌈 PROTOCOLS 🌈

All — every protocol, bestie. periodt. ✨

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🍓 EXAMPLE 🍓

here's the example exactly as the page gives it — make libcurl use the already established socket 'sockfd':

/* 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;
}

curl = curl_easy_init();
if(curl) {
  /* libcurl will internally think 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);

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👑 AVAILABILITY 👑

added in 7.16.0. the CURL_SOCKOPT_ALREADY_CONNECTED return code was added in 7.21.5. so yes, that's the timeline, bestie. 💜

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⭐ RETURN VALUE ⭐

returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not. simple vibe check. ✅

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🎀 SEE ALSO 🎀

• CURLOPT_SOCKOPTDATA(3)
• CURLOPT_OPENSOCKETFUNCTION(3)

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mwah 💋 — this lil page is from libcurl 7.37.0, dated 16 Jun 2014, and it's the GNURLOPT_SOCKOPTFUNCTION(3) manual page. byeeee 🫶✨