docs/examples/interface.c: show CURLOPT_INTERFACE use

Although super simple.

Closes #12229

docs/examples/Makefile.inc

@@ -76,6 +76,7 @@ check_PROGRAMS = \
   imap-ssl \
   imap-store \
   imap-tls \
+  interface \
   ipv6 \
   maxconnects  \
   multi-app \

docs/examples/interface.c

@@ -0,0 +1,52 @@
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * Copyright (C) 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.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.
+ *
+ * SPDX-License-Identifier: curl
+ *
+ ***************************************************************************/
+/* <DESC>
+ * Use CURLOPT_INTERFACE to bind the outgoing socket to an interface
+ * </DESC>
+ */
+#include <stdio.h>
+#include <curl/curl.h>
+
+int main(void)
+{
+  CURL *curl;
+  CURLcode res = CURLE_OK;
+
+  curl = curl_easy_init();
+  if(curl) {
+    /* The interface needs to be a local existing interface over which you can
+       connect to the host in the URL. It can also specify an IP address, but
+       that address needs to be assigned one of the local network
+       interfaces. */
+    curl_easy_setopt(curl, CURLOPT_INTERFACE, "enp3s0");
+    curl_easy_setopt(curl, CURLOPT_URL, "https://curl.se/");
+
+    res = curl_easy_perform(curl);
+
+    /* always cleanup */
+    curl_easy_cleanup(curl);
+  }
+
+  return (int)res;
+}