examples: add multi-poll.c

Show how curl_multi_poll() makes it even easier to use the multi
interface.

Closes #4596

docs/examples/Makefile.inc

@@ -36,7 +36,7 @@ check_PROGRAMS = 10-at-a-time anyauthput cookie_interface debug fileupload \
   ftpuploadresume sslbackend postit2-formadd multi-formadd                 \
   shared-connection-cache sftpuploadresume http2-pushinmemory parseurl     \
   urlapi imap-authzid pop3-authzid smtp-authzid http3 altsvc               \
-  http3-present
+  http3-present multi-poll
 
 # 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/multi-poll.c

@@ -0,0 +1,76 @@
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  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.
+ *
+ ***************************************************************************/
+/* <DESC>
+ * single download with the multi interface's curl_multi_poll
+ * </DESC>
+ */
+
+#include <stdio.h>
+#include <string.h>
+
+/* somewhat unix-specific */
+#include <sys/time.h>
+#include <unistd.h>
+
+/* curl stuff */
+#include <curl/curl.h>
+
+int main(void)
+{
+  CURL *http_handle;
+  CURLM *multi_handle;
+  int still_running = 1; /* keep number of running handles */
+
+  curl_global_init(CURL_GLOBAL_DEFAULT);
+
+  http_handle = curl_easy_init();
+
+  curl_easy_setopt(http_handle, CURLOPT_URL, "https://www.example.com/");
+
+  multi_handle = curl_multi_init();
+
+  curl_multi_add_handle(multi_handle, http_handle);
+
+  while(still_running) {
+    CURLMcode mc; /* curl_multi_poll() return code */
+    int numfds;
+
+    /* we start some action by calling perform right away */
+    mc = curl_multi_perform(multi_handle, &still_running);
+
+    if(still_running)
+      /* wait for activity, timeout or "nothing" */
+      mc = curl_multi_poll(multi_handle, NULL, 0, 1000, &numfds);
+
+    if(mc != CURLM_OK) {
+      fprintf(stderr, "curl_multi_wait() failed, code %d.\n", mc);
+      break;
+    }
+  }
+
+  curl_multi_remove_handle(multi_handle, http_handle);
+  curl_easy_cleanup(http_handle);
+  curl_multi_cleanup(multi_handle);
+  curl_global_cleanup();
+
+  return 0;
+}