add example code for a telnet echo server (still WIP)

examples/telnet_echo/Makefile

@@ -0,0 +1,4 @@
+# Makefile for access_point example
+PROGRAM=telnet_echo
+
+include ../../common.mk

examples/telnet_echo/telnet_echo.c

@@ -0,0 +1,167 @@
+/**
+ * Very basic example showing usage of netconn data receive echo over telnet.
+ * the connection.
+ *
+ * This example code is in the public domain.
+ */
+#include <string.h>
+
+#include <espressif/esp_common.h>
+#include <esp/uart.h>
+#include <FreeRTOS.h>
+#include <task.h>
+#include <queue.h>
+
+#include <ssid_config.h>
+
+#include <lwip/api.h>
+
+#define TELNET_PORT 23
+#define ECHO_PREFIX "\r\nYou entered: "
+#define ECHO_POSTFIX "\r\n"
+#define INPUT_ERROR "\r\nERROR: input line too long - try again.\r\n"
+#define BYE "\r\nBYE\r\n"
+
+static void telnetTask(void *pvParameters);
+
+static void heartbeatTask(void *pvParameters){
+    while(1){
+        vTaskDelay(1000 / portTICK_PERIOD_MS);
+        printf("Uptime %d seconds\r\n", xTaskGetTickCount() * portTICK_PERIOD_MS / 1000);
+    }
+}
+
+void user_init(void)
+{
+    uart_set_baud(0, 115200);
+    printf("SDK version:%s\n", sdk_system_get_sdk_version());
+
+    struct sdk_station_config config = {
+        .ssid = WIFI_SSID,
+        .password = WIFI_PASS,
+    };
+
+    sdk_wifi_set_opmode(STATION_MODE);
+    sdk_wifi_station_set_config(&config);
+    sdk_wifi_station_connect();
+
+    xTaskCreate(telnetTask, "telnetTask", 512, NULL, 2, NULL);
+    xTaskCreate(heartbeatTask, "heardbeatTask", 512, NULL, 2, NULL);
+}
+
+/* Telnet task listens on port 23, returns some status information and then closes
+ the connection if you connect to it.
+ */
+static void telnetTask(void *pvParameters)
+{
+    struct netconn *nc = netconn_new(NETCONN_TCP);
+    if (!nc)
+    {
+        printf("Status monitor: Failed to allocate socket.\r\n");
+        return;
+    }
+    netconn_bind(nc, IP_ANY_TYPE, TELNET_PORT);
+    netconn_listen(nc);
+
+    while (1)
+    {
+        struct netconn *client = NULL;
+#ifdef DEBUG
+        printf("waiting for new connection\n");
+#endif
+        err_t err = netconn_accept(nc, &client);
+
+        if (err != ERR_OK)
+        {
+            if (client){
+                netconn_delete(client);
+            }
+            continue;
+        }
+
+        ip_addr_t client_addr;
+        uint16_t port_ignore;
+        netconn_peer(client, &client_addr, &port_ignore);
+        // TODO check return codes
+        char buf[80];
+        snprintf(buf, sizeof(buf), "Uptime %d seconds\r\n", xTaskGetTickCount() * portTICK_PERIOD_MS / 1000);
+        netconn_write(client, buf, strlen(buf), NETCONN_COPY);
+        snprintf(buf, sizeof(buf), "Free heap %d bytes\r\n", (int) xPortGetFreeHeapSize());
+        netconn_write(client, buf, strlen(buf), NETCONN_COPY);
+        char abuf[40];
+        snprintf(buf, sizeof(buf), "Your address is %s\r\n\r\n", ipaddr_ntoa_r(&client_addr, abuf, sizeof(abuf)));
+        netconn_write(client, buf, strlen(buf), NETCONN_COPY);
+
+        char input_line[160] = {'\0'};
+        size_t buffer_pos = 0;
+        while(err == ERR_OK)
+        {
+            struct netbuf *nb;
+            if ((err = netconn_recv(client, &nb)) == ERR_OK) {
+                char *data_in = NULL;
+                uint16_t data_len = 0;
+                err = netbuf_data(nb, (void*)&data_in, &data_len);
+
+                if(err != ERR_OK){
+                    if(nb != NULL){
+                        netbuf_delete(nb);
+                    }
+                    break;
+                }
+#ifdef DEBUG
+                printf("input[%d]: %s\n", data_len, data_in);
+#endif
+                // check for termination request
+                if((data_len == 1 && data_in[0] == 0x04) || (data_len == 2 && data_in[0] == 0xFF && data_in[1] == 0xEC)){
+#ifdef DEBUG
+                    printf("exit requested\n");
+#endif
+                    err = netconn_write(client, BYE, strlen(BYE), NETCONN_NOFLAG);
+                    if(nb != NULL){
+                        netbuf_delete(nb);
+                    }
+                    break;
+                }
+
+                // check if there's space left in the buffer
+                if(buffer_pos + data_len < sizeof(input_line)) {
+                    // copy data into input buffer
+                    if(data_len > 0){
+                        memcpy(&input_line[buffer_pos], (char*)data_in, strlen(data_in));
+                        buffer_pos += data_len;
+                        // check for line ending
+                        if((data_len >= 2) &&
+                            (((data_in[data_len-2] == 0x0D) && (data_in[data_len-1] == 0x00))
+                            ||
+                            ((data_in[data_len-2] == '\r') && (data_in[data_len-1] == '\n')))){ // \r\n
+#ifdef DEBUG
+                            printf("got line ending\n");
+#endif
+                            netconn_write(client, ECHO_PREFIX, strlen(ECHO_PREFIX), NETCONN_NOFLAG);
+                            netconn_write(client, input_line, strlen(input_line), NETCONN_COPY);
+                            netconn_write(client, ECHO_POSTFIX, strlen(ECHO_POSTFIX), NETCONN_NOFLAG);
+
+                            // set buffer to zero, otherwise we would have to add the null byte by hand
+                            memset(input_line, 0, sizeof(input_line));
+                            buffer_pos = 0;
+                        }
+                    }
+                } else {
+                    netconn_write(client, INPUT_ERROR, strlen(INPUT_ERROR), NETCONN_NOFLAG);
+                    memset(input_line, 0, sizeof(input_line));
+                    buffer_pos = 0;
+                }
+            }
+            if(nb != NULL){
+                netbuf_delete(nb);
+            }
+        }
+        memset(input_line, 0, sizeof(input_line));
+        buffer_pos = 0;
+        netconn_close(client);
+        netconn_delete(client);
+#ifdef DEBUG
+        printf("connection close\n");
+#endif
+    }
+}