**************************************************************************
* _ _ ____ _
* Project ___| | | | _ \| |
* / __| | | | |_) | |
* | (__| |_| | _ <| |___
* \___|\___/|_| \_\_____|
*
* Copyright (C) 1998 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.
*
* This software is licensed as described in the file COPYING, which
* you should have received as part of this distribution. The terms
* are also available at https://curl.haxx.se/docs/copyright.html.
*
* You may opt to use, copy, modify, merge, publish, distribute and/or sell
* copies of the Software, and permit persons to whom the Software is
* furnished to do so, under the terms of the COPYING file.
*
* This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
* KIND, either express or implied.
*
**************************************************************************

CURLOPT_SSL_CTX_DATA - custom pointer passed to ssl_ctx callback #include <curl/curl.h> CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSL_CTX_DATA, void *pointer); Data pointer to pass to the ssl context callback set by the option CURLOPT_SSL_CTX_FUNCTION(3), this is the pointer you'll get as third parameter. NULL All TLS based protocols: HTTPS, FTPS, IMAPS, POP3S, SMTPS etc.

/* 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 = (char *)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);
 if(cert == 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 */
 X509_free(cert);
 BIO_free(bio);

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

int main(void)
{
 CURL * ch;
 CURLcode rv;
 char *mypem = /* example CA cert PEM - shortened */
 "-----BEGIN CERTIFICATE-----\\n"
 "MIIHPTCCBSWgAwIBAgIBADANBgkqhkiG9w0BAQQFADB5MRAwDgYDVQQKEwdSb290\\n"
 "IENBMR4wHAYDVQQLExVodHRwOi8vd3d3LmNhY2VydC5vcmcxIjAgBgNVBAMTGUNB\\n"
 "IENlcnQgU2lnbmluZyBBdXRob3JpdHkxITAfBgkqhkiG9w0BCQEWEnN1cHBvcnRA\\n"
 "Y2FjZXJ0Lm9yZzAeFw0wMzAzMzAxMjI5NDlaFw0zMzAzMjkxMjI5NDlaMHkxEDAO\\n"
 "GCSNe9FINSkYQKyTYOGWhlC0elnYjyELn8+CkcY7v2vcB5G5l1YjqrZslMZIBjzk\\n"
 "zk6q5PYvCdxTby78dOs6Y5nCpqyJvKeyRKANihDjbPIky/qbn3BHLt4Ui9SyIAmW\\n"
 "omTxJBzcoTWcFbLUvFUufQb1nA5V9FrWk9p2rSVzTMVD\\n"
 "-----END CERTIFICATE-----\\n";

 rv=curl_global_init(CURL_GLOBAL_ALL);
 ch=curl_easy_init();
 rv=curl_easy_setopt(ch, CURLOPT_SSLCERTTYPE, "PEM");
 rv=curl_easy_setopt(ch, CURLOPT_SSL_VERIFYPEER, 1L);
 rv=curl_easy_setopt(ch, CURLOPT_URL, "https://www.example.com/");

 /* Retrieve page using cacerts' certificate -> will succeed
 * load the certificate by installing a function doing the necessary
 * "modifications" to the SSL CONTEXT just before link init
 */
 rv=curl_easy_setopt(ch, CURLOPT_SSL_CTX_FUNCTION, *sslctx_function);
 rv=curl_easy_setopt(ch, CURLOPT_SSL_CTX_DATA, mypem);
 rv=curl_easy_perform(ch);
 if(rv==CURLE_OK)
 printf("*** transfer succeeded ***\\n");
 else
 printf("*** transfer failed ***\\n");

 curl_easy_cleanup(ch);
 curl_global_cleanup();
 return rv;
}

Added in 7.11.0 for OpenSSL, in 7.42.0 for wolfSSL and in 7.54.0 for mbedTLS. Other SSL backends are not supported. CURLE_OK if supported; or an error such as: CURLE_NOT_BUILT_IN - Not supported by the SSL backend CURLE_UNKNOWN_OPTION CURLOPT_SSL_CTX_FUNCTION "(3), " CURLOPT_SSLVERSION "(3), "