docs/examples: demonstrate how to select SSL backends

The newly-introduced curl_global_sslset() function deserves to be
show-cased.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

docs/INSTALL.md

@@ -222,6 +222,9 @@ subdirectory run following command from mentioned subdirectory:
 In order to build sample program simplessl.c an SSL enabled libcurl is
 required, as well as the OpenSSL libeay32.lib and ssleay32.lib libraries.
 
+In order to build sample program `sslbackend.c`, an SSL enabled libcurl
+is required.
+
 ## Disabling Specific Protocols in Windows builds
 
 The configure utility, unfortunately, is not available for the Windows

docs/examples/.gitignore

@@ -70,6 +70,7 @@ smtp-multi
 smtp-ssl
 smtp-tls
 smtp-vrfy
+sslbackend
 url2file
 usercertinmem
 xmlstream

docs/examples/Makefile.inc

@@ -33,7 +33,7 @@ check_PROGRAMS = 10-at-a-time anyauthput cookie_interface debug fileupload \
   imap-search imap-create imap-delete imap-copy imap-noop imap-ssl         \
   imap-tls imap-multi url2file sftpget ftpsget postinmemory http2-download \
   http2-upload http2-serverpush getredirect ftpuploadfrommem               \
-  ftpuploadresume
+  ftpuploadresume sslbackend
 
 # These examples require external dependencies that may not be commonly
 # available on POSIX systems, so don't bother attempting to compile them here.

docs/examples/sslbackend.c

@@ -0,0 +1,75 @@
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * Copyright (C) 1998 - 2017, 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.
+ *
+ ***************************************************************************/
+/* <DESC>
+ * Shows HTTPS usage with client certs and optional ssl engine use.
+ * </DESC>
+ */
+#include <assert.h>
+#include <ctype.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include <curl/curl.h>
+
+/*
+ * An SSL-enabled libcurl is required for this sample to work (at least one
+ * SSL backend has to be configured).
+ *
+ *  **** This example only works with libcurl 7.56.0 and later! ****
+*/
+
+int main(int argc, char **argv)
+{
+  const char *name = argc > 1 ? argv[1] : "openssl";
+  CURLsslset result;
+
+  if(!strcmp("list", name)) {
+    const curl_ssl_backend **list;
+    int i;
+
+    result = curl_global_sslset(-1, NULL, &list);
+    assert(result == CURLSSLSET_UNKNOWN_BACKEND);
+
+    for(i = 0; list[i]; i++)
+      printf("SSL backend #%d: '%s' (ID: %d)\n",
+             i, list[i]->name, list[i]->id);
+
+    return 0;
+  } else if(isdigit(*name)) {
+    curl_sslbackend id = (curl_sslbackend)atoi(name);
+
+    result = curl_global_sslset(id, NULL, NULL);
+  } else
+    result = curl_global_sslset(-1, name, NULL);
+
+  if(result == CURLSSLSET_UNKNOWN_BACKEND) {
+    fprintf(stderr, "Unknown SSL backend id: %s\n", name);
+    return 1;
+  }
+
+  assert(result == CURLSSLSET_OK);
+
+  printf("Version with SSL backend '%s':\n\n\t%s\n", name, curl_version());
+
+  return 0;
+}