Theo Borm's example, as was posted here:

http://curl.haxx.se/mail/lib-2005-08/0163.html

docs/examples/Makefile.am

@@ -11,7 +11,7 @@ EXTRA_DIST = README curlgtk.c sepheaders.c simple.c postit2.c		\
  multi-post.c fopen.c simplepost.c makefile.dj curlx.c https.c		\
  multi-debugcallback.c fileupload.c getinfo.c ftp3rdparty.c debug.c	\
  anyauthput.c htmltitle.cc htmltidy.c opensslthreadlock.c		\
- cookie_interface.c
+ cookie_interface.c cacertinmem.c
 
 all:
 	@echo "done"

docs/examples/cacertinmem.c

@@ -0,0 +1,138 @@
+/*****************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * $Id$
+ *
+ * Example using a "in core" PEM certificate to retrieve a https page.
+ * Written by Theo Borm
+ */
+
+/* on a netBSD system with OPENSSL& LIBCURL installed from
+ * pkgsrc (using default paths) this program can be compiled using:
+ * gcc -I/usr/pkg/include -L/usr/pkg/lib -lcurl -Wl,-R/usr/pkg/lib -lssl
+ * -lcrypto -lz -o curlcacerttest curlcacerttest.c
+ * on other operating systems you may want to change paths to headers
+ * and libraries
+*/
+#include <openssl/ssl.h>
+#include <curl/curl.h>
+#include <stdio.h>
+
+size_t writefunction( void *ptr, size_t size, size_t nmemb, void *stream)
+{
+  fwrite(ptr,size,nmemb,stream);
+  return(nmemb*size);
+}
+
+static CURLcode sslctx_function(CURL * curl, void * sslctx, void * parm)
+{
+  X509_STORE * store;
+  X509 * cert=NULL;
+  BIO * bio;
+  char * mypem = /* www.cacert.org */
+    "-----BEGIN CERTIFICATE-----\n"\
+    "MIIHPTCCBSWgAwIBAgIBADANBgkqhkiG9w0BAQQFADB5MRAwDgYDVQQKEwdSb290\n"\
+    "IENBMR4wHAYDVQQLExVodHRwOi8vd3d3LmNhY2VydC5vcmcxIjAgBgNVBAMTGUNB\n"\
+    "IENlcnQgU2lnbmluZyBBdXRob3JpdHkxITAfBgkqhkiG9w0BCQEWEnN1cHBvcnRA\n"\
+    "Y2FjZXJ0Lm9yZzAeFw0wMzAzMzAxMjI5NDlaFw0zMzAzMjkxMjI5NDlaMHkxEDAO\n"\
+    "BgNVBAoTB1Jvb3QgQ0ExHjAcBgNVBAsTFWh0dHA6Ly93d3cuY2FjZXJ0Lm9yZzEi\n"\
+    "MCAGA1UEAxMZQ0EgQ2VydCBTaWduaW5nIEF1dGhvcml0eTEhMB8GCSqGSIb3DQEJ\n"\
+    "ARYSc3VwcG9ydEBjYWNlcnQub3JnMIICIjANBgkqhkiG9w0BAQEFAAOCAg8AMIIC\n"\
+    "CgKCAgEAziLA4kZ97DYoB1CW8qAzQIxL8TtmPzHlawI229Z89vGIj053NgVBlfkJ\n"\
+    "8BLPRoZzYLdufujAWGSuzbCtRRcMY/pnCujW0r8+55jE8Ez64AO7NV1sId6eINm6\n"\
+    "zWYyN3L69wj1x81YyY7nDl7qPv4coRQKFWyGhFtkZip6qUtTefWIonvuLwphK42y\n"\
+    "fk1WpRPs6tqSnqxEQR5YYGUFZvjARL3LlPdCfgv3ZWiYUQXw8wWRBB0bF4LsyFe7\n"\
+    "w2t6iPGwcswlWyCR7BYCEo8y6RcYSNDHBS4CMEK4JZwFaz+qOqfrU0j36NK2B5jc\n"\
+    "G8Y0f3/JHIJ6BVgrCFvzOKKrF11myZjXnhCLotLddJr3cQxyYN/Nb5gznZY0dj4k\n"\
+    "epKwDpUeb+agRThHqtdB7Uq3EvbXG4OKDy7YCbZZ16oE/9KTfWgu3YtLq1i6L43q\n"\
+    "laegw1SJpfvbi1EinbLDvhG+LJGGi5Z4rSDTii8aP8bQUWWHIbEZAWV/RRyH9XzQ\n"\
+    "QUxPKZgh/TMfdQwEUfoZd9vUFBzugcMd9Zi3aQaRIt0AUMyBMawSB3s42mhb5ivU\n"\
+    "fslfrejrckzzAeVLIL+aplfKkQABi6F1ITe1Yw1nPkZPcCBnzsXWWdsC4PDSy826\n"\
+    "YreQQejdIOQpvGQpQsgi3Hia/0PsmBsJUUtaWsJx8cTLc6nloQsCAwEAAaOCAc4w\n"\
+    "ggHKMB0GA1UdDgQWBBQWtTIb1Mfz4OaO873SsDrusjkY0TCBowYDVR0jBIGbMIGY\n"\
+    "gBQWtTIb1Mfz4OaO873SsDrusjkY0aF9pHsweTEQMA4GA1UEChMHUm9vdCBDQTEe\n"\
+    "MBwGA1UECxMVaHR0cDovL3d3dy5jYWNlcnQub3JnMSIwIAYDVQQDExlDQSBDZXJ0\n"\
+    "IFNpZ25pbmcgQXV0aG9yaXR5MSEwHwYJKoZIhvcNAQkBFhJzdXBwb3J0QGNhY2Vy\n"\
+    "dC5vcmeCAQAwDwYDVR0TAQH/BAUwAwEB/zAyBgNVHR8EKzApMCegJaAjhiFodHRw\n"\
+    "czovL3d3dy5jYWNlcnQub3JnL3Jldm9rZS5jcmwwMAYJYIZIAYb4QgEEBCMWIWh0\n"\
+    "dHBzOi8vd3d3LmNhY2VydC5vcmcvcmV2b2tlLmNybDA0BglghkgBhvhCAQgEJxYl\n"\
+    "aHR0cDovL3d3dy5jYWNlcnQub3JnL2luZGV4LnBocD9pZD0xMDBWBglghkgBhvhC\n"\
+    "AQ0ESRZHVG8gZ2V0IHlvdXIgb3duIGNlcnRpZmljYXRlIGZvciBGUkVFIGhlYWQg\n"\
+    "b3ZlciB0byBodHRwOi8vd3d3LmNhY2VydC5vcmcwDQYJKoZIhvcNAQEEBQADggIB\n"\
+    "ACjH7pyCArpcgBLKNQodgW+JapnM8mgPf6fhjViVPr3yBsOQWqy1YPaZQwGjiHCc\n"\
+    "nWKdpIevZ1gNMDY75q1I08t0AoZxPuIrA2jxNGJARjtT6ij0rPtmlVOKTV39O9lg\n"\
+    "18p5aTuxZZKmxoGCXJzN600BiqXfEVWqFcofN8CCmHBh22p8lqOOLlQ+TyGpkO/c\n"\
+    "gr/c6EWtTZBzCDyUZbAEmXZ/4rzCahWqlwQ3JNgelE5tDlG+1sSPypZt90Pf6DBl\n"\
+    "Jzt7u0NDY8RD97LsaMzhGY4i+5jhe1o+ATc7iwiwovOVThrLm82asduycPAtStvY\n"\
+    "sONvRUgzEv/+PDIqVPfE94rwiCPCR/5kenHA0R6mY7AHfqQv0wGP3J8rtsYIqQ+T\n"\
+    "SCX8Ev2fQtzzxD72V7DX3WnRBnc0CkvSyqD/HMaMyRa+xMwyN2hzXwj7UfdJUzYF\n"\
+    "CpUCTPJ5GhD22Dp1nPMd8aINcGeGG7MW9S/lpOt5hvk9C8JzC6WZrG/8Z7jlLwum\n"\
+    "GCSNe9FINSkYQKyTYOGWhlC0elnYjyELn8+CkcY7v2vcB5G5l1YjqrZslMZIBjzk\n"\
+    "zk6q5PYvCdxTby78dOs6Y5nCpqyJvKeyRKANihDjbPIky/qbn3BHLt4Ui9SyIAmW\n"\
+    "omTxJBzcoTWcFbLUvFUufQb1nA5V9FrWk9p2rSVzTMVD\n"\
+    "-----END CERTIFICATE-----\n";
+  /* 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");
+
+  /* all set to go */
+  return CURLE_OK ;
+}
+
+int main(void)
+{
+  CURL * ch;
+  CURLcode rv;
+
+  rv=curl_global_init(CURL_GLOBAL_ALL);
+  ch=curl_easy_init();
+  rv=curl_easy_setopt(ch,CURLOPT_VERBOSE, 0);
+  rv=curl_easy_setopt(ch,CURLOPT_HEADER, 0);
+  rv=curl_easy_setopt(ch,CURLOPT_NOPROGRESS, 1);
+  rv=curl_easy_setopt(ch,CURLOPT_NOSIGNAL, 1);
+  rv=curl_easy_setopt(ch,CURLOPT_WRITEFUNCTION, *writefunction);
+  rv=curl_easy_setopt(ch,CURLOPT_WRITEDATA, stdout);
+  rv=curl_easy_setopt(ch,CURLOPT_HEADERFUNCTION, *writefunction);
+  rv=curl_easy_setopt(ch,CURLOPT_WRITEHEADER, stderr);
+  rv=curl_easy_setopt(ch,CURLOPT_SSLCERTTYPE,"PEM");
+  rv=curl_easy_setopt(ch,CURLOPT_SSL_VERIFYPEER,1);
+  rv=curl_easy_setopt(ch, CURLOPT_URL, "https://www.cacert.org/");
+
+  /* first try: retrieve page without cacerts' certificate -> will fail
+   */
+  rv=curl_easy_perform(ch);
+  if (rv==CURLE_OK)
+    printf("*** transfer succeeded ***\n");
+  else
+    printf("*** transfer failed ***\n");
+
+  /* second try: retrieve page using cacerts' certificate -> will succeed
+   * load the certificate by installing a function doing the nescessary
+   * "modifications" to the SSL CONTEXT just before link init
+   */
+  rv=curl_easy_setopt(ch,CURLOPT_SSL_CTX_FUNCTION, *sslctx_function);
+  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();
+}