samples: posix: gettimeofday: Demo gettimeofday() returning correct time

Achieved using CONFIG_NET_CONFIG_CLOCK_SNTP_INIT option, i.e. querying
time over network using SNTP.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>

samples/index.rst

@@ -22,6 +22,7 @@ Samples and Demos
    display/*
    shields/*
    portability/*
+   posix/*
    gui/*
    video/*
 

samples/posix/gettimeofday/CMakeLists.txt

@@ -0,0 +1,9 @@
+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.13.1)
+
+include($ENV{ZEPHYR_BASE}/cmake/app/boilerplate.cmake NO_POLICY_SCOPE)
+project(gettimeofday)
+
+#target_include_directories(app PRIVATE $ENV{ZEPHYR_BASE}/include/posix)
+target_sources(app PRIVATE src/main.c)

samples/posix/gettimeofday/Makefile.posix

@@ -0,0 +1,4 @@
+# This makefile builds the sample for a POSIX system, like Linux
+
+gettimeofday: src/main.c
+	$(CC) $^ -o $@

samples/posix/gettimeofday/README.rst

@@ -0,0 +1,33 @@
+.. _posix-gettimeofday-sample:
+
+POSIX gettimeofday() with clock initialization over SNTP
+########################################################
+
+Overview
+********
+
+This sample application demonstrates using the POSIX gettimeofday()
+function to display the absolute wall clock time every second. At
+system startup, the current time is queried using the SNTP networking
+protocol, enabled by setting the :option:`CONFIG_NET_CONFIG_CLOCK_SNTP_INIT`
+and :option:`CONFIG_NET_CONFIG_SNTP_INIT_SERVER` options.
+
+Building and Running
+********************
+
+This project outputs to the console.  It can be built and executed
+on QEMU as follows:
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/posix/gettimeofday
+   :host-os: unix
+   :board: qemu_x86
+   :goals: run
+   :compact:
+
+For comparison, a version for native POSIX operating systems (e.g. Linux)
+can be built using:
+
+.. code-block:: console
+
+   make -f Makefile.posix

samples/posix/gettimeofday/prj.conf

@@ -0,0 +1,30 @@
+# General config
+CONFIG_NEWLIB_LIBC=y
+CONFIG_POSIX_API=y
+CONFIG_SNTP=y
+CONFIG_NET_CONFIG_CLOCK_SNTP_INIT=y
+CONFIG_NET_CONFIG_SNTP_INIT_SERVER="time.nist.gov"
+
+# Networking config
+CONFIG_NETWORKING=y
+CONFIG_NET_IPV4=y
+CONFIG_NET_IPV6=y
+CONFIG_NET_TCP=y
+CONFIG_NET_SOCKETS=y
+
+CONFIG_DNS_RESOLVER=y
+CONFIG_DNS_SERVER_IP_ADDRESSES=y
+CONFIG_DNS_SERVER1="192.0.2.2"
+
+# Network driver config
+CONFIG_TEST_RANDOM_GENERATOR=y
+
+# Network address config
+CONFIG_NET_CONFIG_SETTINGS=y
+CONFIG_NET_CONFIG_NEED_IPV4=y
+CONFIG_NET_CONFIG_MY_IPV4_ADDR="192.0.2.1"
+CONFIG_NET_CONFIG_PEER_IPV4_ADDR="192.0.2.2"
+CONFIG_NET_CONFIG_MY_IPV4_GW="192.0.2.2"
+
+# Network debug config
+CONFIG_NET_LOG=y

samples/posix/gettimeofday/src/main.c

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2019 Linaro Limited
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <errno.h>
+#include <sys/time.h>
+#include <unistd.h>
+
+int main(void)
+{
+	struct timeval tv;
+
+	while (1) {
+		int res = gettimeofday(&tv, NULL);
+
+		if (res < 0) {
+			printf("Error in gettimeofday(): %d\n", errno);
+			return 1;
+		}
+
+		printf("gettimeofday(): HI(tv_sec)=%d, LO(tv_sec)=%d, "
+		       "tv_usec=%d\n", (unsigned int)(tv.tv_sec >> 32),
+		       (unsigned int)tv.tv_sec, (unsigned int)tv.tv_usec);
+		sleep(1);
+	}
+}

samples/posix/posix.rst

@@ -0,0 +1,10 @@
+.. _posix-samples:
+
+POSIX Subsystem Samples
+#######################
+
+.. toctree::
+   :maxdepth: 1
+   :glob:
+
+   **/*