examples: add TCP echo server & client from documentation

examples/sock_tcp_echo/Makefile

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+# name of your application
+APPLICATION = sock_tcp_echo
+
+# If no BOARD is found in the environment, use this default:
+BOARD ?= native
+
+# default to using GNRC
+LWIP ?= 0
+
+# This has to be the absolute path to the RIOT base directory:
+RIOTBASE ?= $(CURDIR)/../..
+
+USEMODULE += sock_tcp
+USEMODULE += netdev_default
+
+USEMODULE += shell
+USEMODULE += shell_cmds_default
+USEMODULE += ps
+USEMODULE += netutils
+
+ifeq (1, $(LWIP))
+  USEMODULE += ipv6_addr
+  USEMODULE += lwip_ipv6_autoconfig
+  USEMODULE += lwip_netdev
+else
+  USEMODULE += auto_init_gnrc_netif
+  USEMODULE += gnrc_ipv6_default
+  # we want to be able to open two sockets
+  CFLAGS += -DCONFIG_GNRC_TCP_RCV_BUFFERS=2
+endif
+
+# Comment this out to disable code in RIOT that does safety checking
+# which is not needed in a production environment but helps in the
+# development process:
+DEVELHELP ?= 1
+
+# As there is a .config we want to explicitly disable Kconfig by setting
+# the variable to empty
+SHOULD_RUN_KCONFIG ?=
+
+include $(RIOTBASE)/Makefile.include

examples/sock_tcp_echo/Makefile.ci

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+BOARD_INSUFFICIENT_MEMORY := \
+    arduino-duemilanove \
+    arduino-leonardo \
+    arduino-mega2560 \
+    arduino-nano \
+    arduino-uno \
+    atmega328p \
+    atmega328p-xplained-mini \
+    atmega8 \
+    atxmega-a3bu-xplained \
+    bluepill-stm32f030c8 \
+    derfmega128 \
+    i-nucleo-lrwan1 \
+    im880b \
+    m1284p \
+    microduino-corerf \
+    msb-430 \
+    msb-430h \
+    nucleo-c031c6 \
+    nucleo-f030r8 \
+    nucleo-f031k6 \
+    nucleo-f042k6 \
+    nucleo-f303k8 \
+    nucleo-f334r8 \
+    nucleo-l011k4 \
+    nucleo-l031k6 \
+    nucleo-l053r8 \
+    olimex-msp430-h1611 \
+    olimex-msp430-h2618 \
+    samd10-xmini \
+    slstk3400a \
+    stk3200 \
+    stm32f030f4-demo \
+    stm32f0discovery \
+    stm32g0316-disco \
+    stm32l0538-disco \
+    telosb \
+    waspmote-pro \
+    weact-g030f6 \
+    z1 \
+    zigduino \
+    #

examples/sock_tcp_echo/README.md

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+TCP Echo Server / Client
+========================
+
+This is a simple TCP echo server / client that uses the SOCK API.
+It can make use of both the GNRC and the LWIP network stack.
+The default is GNRC, to chose LWIP set `LWIP=1` when compiling the example.
+
+## Echo Server
+
+The echo server will echo back any data that it receives.
+To start the echo server on port 12345 run
+
+    listen 12345
+
+## Echo Client
+
+The echo client will connect to a server, send some data and wait for the reply.
+To send data with the echo client to a server running on 2001:db8::1 run
+
+    send 2001:db8::1 12345 some data

examples/sock_tcp_echo/main.c

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+/*
+ * Copyright (C) 2016 Freie Universität Berlin
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @{
+ *
+ * @file
+ * @author  Martine Lenders <[email protected]>
+ * @author  Benjamin Valentin <[email protected]>
+ */
+
+#include <stdio.h>
+
+#include "net/sock/tcp.h"
+#include "net/ipv6/addr.h"
+#include "net/utils.h"
+#include "shell.h"
+
+#define SOCK_QUEUE_LEN  1
+
+static char _echo_server_stack[THREAD_STACKSIZE_DEFAULT];
+
+static int _cmd_tcp_send(int argc, char **argv)
+{
+    int res;
+    sock_tcp_t sock;
+    sock_tcp_ep_t remote;
+    netif_t *netif;
+    char buf[128];
+
+    if (argc < 4) {
+        printf("usage: %s <addr> <port> <data>\n", argv[0]);
+        return -1;
+    }
+
+    if (netutils_get_ipv6((ipv6_addr_t *)&remote.addr,
+                          &netif, argv[1]) < 0) {
+        printf("can't resolve %s\n", argv[1]);
+        return -1;
+    }
+
+    remote.family = AF_INET6;
+    remote.netif  = netif ? netif_get_id(netif) : 0;
+    remote.port   = atoi(argv[2]);
+
+    if (remote.port == 0) {
+        printf("Invalid port: %s\n", argv[2]);
+        return -1;
+    }
+
+    if ((res = sock_tcp_connect(&sock, &remote, 0, 0)) < 0) {
+        printf("Error connecting sock: %s\n", strerror(-res));
+        return -1;
+    }
+
+    if ((res = sock_tcp_write(&sock, argv[3], strlen(argv[3]))) < 0) {
+        printf("Errored on write: %s\n", strerror(-res));
+        goto error;
+    }
+
+    if ((res = sock_tcp_read(&sock, &buf, sizeof(buf), SOCK_NO_TIMEOUT)) <= 0) {
+        printf("Disconnected: %s\n", strerror(-res));
+        goto error;
+    }
+
+    buf[res] = 0;
+    printf("Read: \"%s\"\n", buf);
+
+error:
+    sock_tcp_disconnect(&sock);
+
+    return 0;
+}
+SHELL_COMMAND(send, "send data over TCP", _cmd_tcp_send);
+
+static void *_run_echo_server(void *ctx)
+{
+    sock_tcp_t sock_queue[SOCK_QUEUE_LEN];
+    char buf[128];
+    uint16_t port = (uintptr_t)ctx;
+
+    sock_tcp_ep_t local = SOCK_IPV6_EP_ANY;
+    sock_tcp_queue_t queue;
+    local.port = port;
+    if (sock_tcp_listen(&queue, &local, sock_queue, SOCK_QUEUE_LEN, 0) < 0) {
+        puts("Error creating listening queue");
+        return NULL;
+    }
+    printf("Listening on port %u\n", port);
+    while (1) {
+        sock_tcp_t *sock;
+        if (sock_tcp_accept(&queue, &sock, SOCK_NO_TIMEOUT) < 0) {
+            puts("Error accepting new sock");
+            break;
+        }
+
+        int res = 0;
+        puts("Reading data");
+        while (res >= 0) {
+            res = sock_tcp_read(sock, &buf, sizeof(buf), SOCK_NO_TIMEOUT);
+            if (res <= 0) {
+                printf("Disconnected: %s\n", strerror(-res));
+                break;
+            }
+            buf[res] = 0;
+            printf("Read: \"%s\"\n", buf);
+            if ((res = sock_tcp_write(sock, &buf, res)) < 0) {
+                printf("Errored on write: %s\n", strerror(-res));
+            }
+        }
+        sock_tcp_disconnect(sock);
+    }
+    sock_tcp_stop_listen(&queue);
+    return NULL;
+}
+
+static int _cmd_tcp_listen(int argc, char **argv)
+{
+    static kernel_pid_t pid;
+    uint16_t port;
+
+    if (argc < 2) {
+        printf("usage: %s <port>\n", argv[0]);
+        return -1;
+    }
+
+    if (pid) {
+        puts("server already running");
+        return -1;
+    }
+
+    port = atoi(argv[1]);
+
+    if (port == 0) {
+        printf("invalid port: %s\n", argv[1]);
+        return -1;
+    }
+
+    pid = thread_create(_echo_server_stack, sizeof(_echo_server_stack),
+                        THREAD_PRIORITY_MAIN - 1, 0,
+                        _run_echo_server, (void*)(uintptr_t)port, "echo_server");
+    return 0;
+}
+SHELL_COMMAND(listen, "start echo server", _cmd_tcp_listen);
+
+int main(void)
+{
+    puts("RIOT TCP client example application");
+
+    /* start shell */
+    puts("All up, running the shell now");
+    char line_buf[SHELL_DEFAULT_BUFSIZE];
+    shell_run(NULL, line_buf, SHELL_DEFAULT_BUFSIZE);
+
+    /* should be never reached */
+    return 0;
+}
+/** @} */