#include <curl/curl.h>
CURLcode ssl_ctx_callback(CURL *curl, void *ssl_ctx, void *clientp);
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSL_CTX_FUNCTION,

Pass a pointer to your callback function, which should match the prototype shown above.

This callback function gets called by libcurl just before the initialization of an SSL connection after having processed all other SSL related options to give a last chance to an application to modify the behavior of the SSL initialization. The ssl_ctx parameter is a pointer to the SSL library's SSL_CTX for OpenSSL or wolfSSL, a pointer to mbedtls_ssl_config for mbedTLS or a pointer to br_ssl_client_context for BearSSL. If an error is returned from the callback no attempt to establish a connection is made and the perform operation returns the callback's error code. Set the clientp argument passed in to this callback with the CURLOPT_SSL_CTX_DATA(3) option.

This function gets called for all new connections made to a server, during the SSL negotiation. While ssl_ctx points to a newly initialized object each time, the pointer may still be the same as in a prior call.

To use this callback, a non-trivial amount of knowledge of your SSL library is necessary. For example, you can use this function to call library-specific callbacks to add additional validation code for certificates, and even to change the actual URI of an HTTPS request.

For OpenSSL, asynchronous certificate verification via SSL_set_retry_verify is supported. (Added in 8.3.0)

The CURLOPT_SSL_CTX_FUNCTION(3) callback allows the application to reach in and modify SSL details in the connection without libcurl itself knowing anything about it, which then subsequently can lead to libcurl unknowingly reusing SSL connections with different properties. To remedy this you may set CURLOPT_FORBID_REUSE(3) from the callback function.

If you are using DNS-over-HTTPS (DoH) via CURLOPT_DOH_URL(3) then this callback is also called for those transfers and the curl handle is set to an internal handle. This behavior is subject to change. We recommend setting CURLOPT_PRIVATE(3) on your curl handle so you can identify it correctly in the context callback. If you have a reason to modify DoH SSL context please let us know on the curl-library mailing list because we are considering removing this capability.

libcurl does not guarantee the lifetime of the passed in object once this callback function has returned. Your application must not assume that it can keep using the SSL context or data derived from it once this function is completed.


All TLS based protocols: HTTPS, FTPS, IMAPS, POP3S, SMTPS etc.

This option works only with the following TLS backends: BearSSL, OpenSSL, mbedTLS and wolfSSL

/* OpenSSL specific */
#include <openssl/ssl.h>
#include <curl/curl.h>
#include <stdio.h>
static CURLcode sslctx_function(CURL *curl, void *sslctx, void *parm)
  X509_STORE *store;
  X509 *cert = NULL;
  BIO *bio;
  char *mypem = parm;
  /* 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);
    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 */
  /* all set to go */
  return CURLE_OK;
int main(void)
  CURL *ch;
  CURLcode rv;
  char *mypem = /* example CA cert PEM - shortened */
    "-----BEGIN CERTIFICATE-----\n"
    "-----END CERTIFICATE-----\n";
  ch = curl_easy_init();
  curl_easy_setopt(ch, CURLOPT_SSLCERTTYPE, "PEM");
  curl_easy_setopt(ch, CURLOPT_SSL_VERIFYPEER, 1L);
  curl_easy_setopt(ch, CURLOPT_URL, "");
  curl_easy_setopt(ch, CURLOPT_SSL_CTX_FUNCTION, *sslctx_function);
  curl_easy_setopt(ch, CURLOPT_SSL_CTX_DATA, mypem);
  rv = curl_easy_perform(ch);
    printf("*** transfer succeeded ***\n");
    printf("*** transfer failed ***\n");
  return rv;

OpenSSL (added in 7.11.0), wolfSSL (added in 7.42.0), mbedTLS (added in 7.54.0) or BearSSL (added in 7.83.0)

CURLE_OK if supported; or an error such as:

CURLE_NOT_BUILT_IN - Not supported by the SSL backend


2024-05-22 libcurl