docs/example/localport.c: show off CURLOPT_LOCALPORT

Closes #12230

docs/examples/Makefile.inc

@@ -78,6 +78,7 @@ check_PROGRAMS = \
   imap-tls \
   interface \
   ipv6 \
+  localport \
   maxconnects  \
   multi-app \
   multi-debugcallback \

docs/examples/localport.c

@@ -0,0 +1,53 @@
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  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_LOCALPORT to control local port number
+ * </DESC>
+ */
+#include <stdio.h>
+#include <curl/curl.h>
+
+int main(void)
+{
+  CURL *curl;
+  CURLcode res = CURLE_OK;
+
+  curl = curl_easy_init();
+  if(curl) {
+    /* Try to use a local port number between 20000-20009 */
+    curl_easy_setopt(curl, CURLOPT_LOCALPORT, 20000L);
+    /* 10 means number of attempts, which starts with the number set in
+       CURLOPT_LOCALPORT. The lowe value set, the smaller the change it will
+       work. */
+    curl_easy_setopt(curl, CURLOPT_LOCALPORTRANGE, 10L);
+    curl_easy_setopt(curl, CURLOPT_URL, "https://curl.se/");
+
+    res = curl_easy_perform(curl);
+
+    /* always cleanup */
+    curl_easy_cleanup(curl);
+  }
+
+  return (int)res;
+}