CURLOPT_SSL_CTX_DATA(3)

💅 CURLOPT_SSL_CTX_DATA 💅

okay bestie, welcome to the Library Functions Manual entry for CURLOPT_SSL_CTX_DATA(3) — yes that's the whole name, yes it's a mouthful, no we're not abbreviating it, she earned her title ✨

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🌸 NAME 🌸

CURLOPT_SSL_CTX_DATA — the pointer you hand off to your SSL context callback, lowkey the little sidekick of the whole operation 🫶

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✨ SYNOPSIS ✨

#include <curl/curl.h>

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSL_CTX_DATA, void *pointer);

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🎀 DESCRIPTION 🎀

so here's the tea 💅 — this is the data pointer you pass along to the SSL context callback that you set with the option CURLOPT_SSL_CTX_FUNCTION(3). it's literally the pointer you get as the third parameter in that callback. no cap, that's the whole gig. main character energy for one specific job 🔥

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🦋 DEFAULT 🦋

NULL

(empty vibes until you say otherwise, ngl 🙈)

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🌈 PROTOCOLS 🌈

this functionality affects all TLS based protocols, like: HTTPS, FTPS, IMAPS, POP3S, SMTPS etc. — the whole S-suffixed squad 👑

heads up bestie: this option works only with the following TLS backends: OpenSSL, mbedTLS and wolfSSL 🧚

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⭐ EXAMPLE ⭐

/* OpenSSL specific */

#include <openssl/ssl.h>
#include <curl/curl.h>
#include <stdio.h>

static CURLcode sslctx_function(CURL *curl, void *sslctx, void *pointer)
{
  X509_STORE *store;
  X509 *cert = NULL;
  BIO *bio;
  char *mypem = pointer;
  /* get a BIO */
  bio = BIO_new_mem_buf(mypem, -1);
  /* use it to read the PEM formatted certificate from memory into an
   * X509 structure that SSL can use
   */
  PEM_read_bio_X509(bio, &cert, 0, NULL);
  if(!cert)
    printf("PEM_read_bio_X509 failed...\n");

  /* get a pointer to the X509 certificate store (which may be empty) */
  store = SSL_CTX_get_cert_store((SSL_CTX *)sslctx);

  /* add our certificate to this store */
  if(X509_STORE_add_cert(store, cert) == 0)
    printf("error adding certificate\n");

  /* decrease reference counts */
  X509_free(cert);
  BIO_free(bio);

  /* all set to go */
  return CURLE_OK;
}

int main(void)
{
  CURL *curl;
  CURLcode result;
  /* CA cert in PEM format, replace the XXXs */
  char *mypem =
    "-----BEGIN CERTIFICATE-----\n"
    "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX\n"
    "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX\n"
    "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX\n"
    "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX\n"
    "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX\n"
    "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX\n"
    "-----END CERTIFICATE-----\n";

  curl_global_init(CURL_GLOBAL_ALL);
  curl = curl_easy_init();

  curl_easy_setopt(curl, CURLOPT_SSLCERTTYPE, "PEM");
  curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, 1L);
  curl_easy_setopt(curl, CURLOPT_URL, "https://www.example.com/");

  curl_easy_setopt(curl, CURLOPT_SSL_CTX_FUNCTION, *sslctx_function);
  curl_easy_setopt(curl, CURLOPT_SSL_CTX_DATA, mypem);
  result = curl_easy_perform(curl);
  if(!result)
    printf("*** transfer succeeded ***\n");
  else
    printf("*** transfer failed ***\n");

  curl_easy_cleanup(curl);
  curl_global_cleanup();
  return (int)result;
}

notice how mypem gets yeeted into CURLOPT_SSL_CTX_DATA and then just... shows up as pointer in the callback? that's the pipeline, bestie — iconic behaviour honestly 🌟

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👑 HISTORY 👑

added in 7.11.0 for OpenSSL, in 7.42.0 for wolfSSL, in 7.54.0 for mbedTLS — a whole glow-up timeline fr fr 💜

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💖 AVAILABILITY 💖

added in curl 7.10.6 🎀

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🔥 RETURN VALUE 🔥

CURLE_OK if it's supported — slay. otherwise you get an error, such as:

• CURLE_NOT_BUILT_IN — not supported by the SSL backend 😭
• CURLE_UNKNOWN_OPTION

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🍓 SEE ALSO 🍓

• CURLOPT_SSLVERSION(3)
• CURLOPT_SSL_CTX_FUNCTION(3)

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and that's the whole page, bestie 🫶 written by the libcurl girlies, dated 2026-10-12, and yeah — libcurl, 2026-10-12, CURLOPT_SSL_CTX_DATA(3), that's the footer, love you bye 💅✨