Allow VFS file descriptors in select()

examples/peripherals/uart_select/Makefile

@@ -0,0 +1,8 @@
+#
+# This is a project Makefile. It is assumed the directory this Makefile resides in is a
+# project subdirectory.
+#
+
+PROJECT_NAME := uart_select
+
+include $(IDF_PATH)/make/project.mk

examples/peripherals/uart_select/README.md

@@ -0,0 +1,13 @@
+# UART Select Example
+
+The UART select example is for demonstrating the use of `select()` for
+synchronous I/O multiplexing on the UART interface. The example waits for a
+character from UART using `select()` until a blocking read without delay or a
+successful non-blocking read is possible.
+
+Please note that the same result can be achieved by using `uart_read_bytes()`
+but the use of `select()` allows to use it together with other virtual
+file system (VFS) drivers, e.g. LWIP sockets. For a more comprehensive example
+please refer to `system/select`.
+
+See the README.md file in the upper level 'examples' directory for more information about examples.

examples/peripherals/uart_select/main/component.mk

@@ -0,0 +1,3 @@
+#
+# Main Makefile. This is basically the same as a component makefile.
+#

examples/peripherals/uart_select/main/uart_select_example_main.c

@@ -0,0 +1,90 @@
+/* UART Select Example
+
+   This example code is in the Public Domain (or CC0 licensed, at your option.)
+
+   Unless required by applicable law or agreed to in writing, this
+   software is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+   CONDITIONS OF ANY KIND, either express or implied.
+*/
+#include <stdio.h>
+#include <sys/fcntl.h>
+#include <sys/errno.h>
+#include <sys/unistd.h>
+#include <sys/select.h>
+#include "freertos/FreeRTOS.h"
+#include "freertos/task.h"
+#include "esp_log.h"
+#include "esp_vfs.h"
+#include "esp_vfs_dev.h"
+#include "driver/uart.h"
+
+static const char* TAG = "uart_select_example";
+
+static void uart_select_task()
+{
+    uart_config_t uart_config = {
+        .baud_rate = 115200,
+        .data_bits = UART_DATA_8_BITS,
+        .parity    = UART_PARITY_DISABLE,
+        .stop_bits = UART_STOP_BITS_1,
+        .flow_ctrl = UART_HW_FLOWCTRL_DISABLE
+    };
+    uart_param_config(UART_NUM_0, &uart_config);
+    uart_driver_install(UART_NUM_0, 2*1024, 0, 0, NULL, 0);
+
+    while (1) {
+        int fd;
+
+        if ((fd = open("/dev/uart/0", O_RDWR)) == -1) {
+            ESP_LOGE(TAG, "Cannot open UART");
+            vTaskDelay(5000 / portTICK_PERIOD_MS);
+            continue;
+        }
+
+        // We have a driver now installed so set up the read/write functions to use driver also.
+        esp_vfs_dev_uart_use_driver(0);
+
+        while (1) {
+            int s;
+            fd_set rfds;
+            struct timeval tv = {
+                .tv_sec = 5,
+                .tv_usec = 0,
+            };
+
+            FD_ZERO(&rfds);
+            FD_SET(fd, &rfds);
+
+            s = select(fd + 1, &rfds, NULL, NULL, &tv);
+
+            if (s < 0) {
+                ESP_LOGE(TAG, "Select failed: errno %d", errno);
+                break;
+            } else if (s == 0) {
+                ESP_LOGI(TAG, "Timeout has been reached and nothing has been received");
+            } else {
+                if (FD_ISSET(fd, &rfds)) {
+                    char buf;
+                    if (read(fd, &buf, 1) > 0) {
+                            ESP_LOGI(TAG, "Received: %c", buf);
+                    } else {
+                        ESP_LOGE(TAG, "UART read error");
+                        break;
+                    }
+                } else {
+                    ESP_LOGE(TAG, "No FD has been set in select()");
+                    break;
+                }
+            }
+        }
+
+        close(fd);
+    }
+
+    vTaskDelete(NULL);
+}
+
+void app_main()
+{
+    xTaskCreate(uart_select_task, "uart_select_task", 4*1024, NULL, 5, NULL);
+}