esp32之 http request

使用esp32进行 http request 是一项非常重要的应用
源码路径：esp-idf/examples/protocols/http_request
源码使用：

/* HTTP GET Example using plain POSIX sockets

   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 
#include "freertos/FreeRTOS.h"
#include "freertos/task.h"
#include "freertos/event_groups.h"
#include "esp_system.h"
#include "esp_wifi.h"
#include "esp_event_loop.h"
#include "esp_log.h"
#include "nvs_flash.h"

#include "lwip/err.h"
#include "lwip/sockets.h"
#include "lwip/sys.h"
#include "lwip/netdb.h"
#include "lwip/dns.h"

/* The examples use simple WiFi configuration that you can set via
   'make menuconfig'.

   If you'd rather not, just change the below entries to strings with
   the config you want - ie #define EXAMPLE_WIFI_SSID "mywifissid"
*/
#define EXAMPLE_WIFI_SSID CONFIG_WIFI_SSID
#define EXAMPLE_WIFI_PASS CONFIG_WIFI_PASSWORD

/* FreeRTOS event group to signal when we are connected & ready to make a request */
static EventGroupHandle_t wifi_event_group;

/* The event group allows multiple bits for each event,
   but we only care about one event - are we connected
   to the AP with an IP? */
const int CONNECTED_BIT = BIT0;

/* Constants that aren't configurable in menuconfig */
#define WEB_SERVER "example.com"
#define WEB_PORT 80
#define WEB_URL "http://example.com/"

static const char *TAG = "example";

static const char *REQUEST = "GET " WEB_URL " HTTP/1.0\r\n"
    "Host: "WEB_SERVER"\r\n"
    "User-Agent: esp-idf/1.0 esp32\r\n"
    "\r\n";

static esp_err_t event_handler(void *ctx, system_event_t *event)
{
    switch(event->event_id) {
    case SYSTEM_EVENT_STA_START:
        esp_wifi_connect();
        break;
    case SYSTEM_EVENT_STA_GOT_IP:
        xEventGroupSetBits(wifi_event_group, CONNECTED_BIT);
        break;
    case SYSTEM_EVENT_STA_DISCONNECTED:
        /* This is a workaround as ESP32 WiFi libs don't currently
           auto-reassociate. */
        esp_wifi_connect();
        xEventGroupClearBits(wifi_event_group, CONNECTED_BIT);
        break;
    default:
        break;
    }
    return ESP_OK;
}

static void initialise_wifi(void)
{
    tcpip_adapter_init();
    wifi_event_group = xEventGroupCreate();
    ESP_ERROR_CHECK( esp_event_loop_init(event_handler, NULL) );
    wifi_init_config_t cfg = WIFI_INIT_CONFIG_DEFAULT();
    ESP_ERROR_CHECK( esp_wifi_init(&cfg) );
    ESP_ERROR_CHECK( esp_wifi_set_storage(WIFI_STORAGE_RAM) );
    wifi_config_t wifi_config = {
        .sta = {
            .ssid = EXAMPLE_WIFI_SSID,
            .password = EXAMPLE_WIFI_PASS,
        },
    };
    ESP_LOGI(TAG, "Setting WiFi configuration SSID %s...", wifi_config.sta.ssid);
    ESP_ERROR_CHECK( esp_wifi_set_mode(WIFI_MODE_STA) );
    ESP_ERROR_CHECK( esp_wifi_set_config(ESP_IF_WIFI_STA, &wifi_config) );
    ESP_ERROR_CHECK( esp_wifi_start() );
}

static void http_get_task(void *pvParameters)
{
    const struct addrinfo hints = {
        .ai_family = AF_INET,		// IPV4
        .ai_socktype = SOCK_STREAM,	// TCP协议
    };
    struct addrinfo *res;
    struct in_addr *addr;
    int s, r;
    char recv_buf[64];

    while(1) {
        /* Wait for the callback to set the CONNECTED_BIT in the
           event group.
        */
        xEventGroupWaitBits(wifi_event_group, CONNECTED_BIT,
                            false, true, portMAX_DELAY);
        ESP_LOGI(TAG, "Connected to AP");

        int err = getaddrinfo(WEB_SERVER, "80", &hints, &res);

        if(err != 0 || res == NULL) {
            ESP_LOGE(TAG, "DNS lookup failed err=%d res=%p", err, res);
            vTaskDelay(1000 / portTICK_PERIOD_MS);
            continue;
        }

        /* Code to print the resolved IP.

           Note: inet_ntoa is non-reentrant, look at ipaddr_ntoa_r for "real" code */
        addr = &((struct sockaddr_in *)res->ai_addr)->sin_addr;
        ESP_LOGI(TAG, "DNS lookup succeeded. IP=%s", inet_ntoa(*addr));

        s = socket(res->ai_family, res->ai_socktype, 0);
        if(s < 0) {
            ESP_LOGE(TAG, "... Failed to allocate socket.");
            freeaddrinfo(res);
            vTaskDelay(1000 / portTICK_PERIOD_MS);
            continue;
        }
        ESP_LOGI(TAG, "... allocated socket");

        if(connect(s, res->ai_addr, res->ai_addrlen) != 0) {
            ESP_LOGE(TAG, "... socket connect failed errno=%d", errno);
            close(s);
            freeaddrinfo(res);
            vTaskDelay(4000 / portTICK_PERIOD_MS);
            continue;
        }

        ESP_LOGI(TAG, "... connected");
        freeaddrinfo(res);

		printf("REQUEST: %s\n", REQUEST);
        if (write(s, REQUEST, strlen(REQUEST)) < 0) {
            ESP_LOGE(TAG, "... socket send failed");
            close(s);
            vTaskDelay(4000 / portTICK_PERIOD_MS);
            continue;
        }
        ESP_LOGI(TAG, "... socket send success");

        struct timeval receiving_timeout;
        receiving_timeout.tv_sec = 5;
        receiving_timeout.tv_usec = 0;
        if (setsockopt(s, SOL_SOCKET, SO_RCVTIMEO, &receiving_timeout,
                sizeof(receiving_timeout)) < 0) {
            ESP_LOGE(TAG, "... failed to set socket receiving timeout");
            close(s);
            vTaskDelay(4000 / portTICK_PERIOD_MS);
            continue;
        }
        ESP_LOGI(TAG, "... set socket receiving timeout success");

        /* Read HTTP response */
        do {
            bzero(recv_buf, sizeof(recv_buf));
            r = read(s, recv_buf, sizeof(recv_buf)-1);	// 考虑结束符\0
            for(int i = 0; i < r; i++) {
                putchar(recv_buf[i]);
            }
        } while(r > 0);

        ESP_LOGI(TAG, "... done reading from socket. Last read return=%d errno=%d\r\n", r, errno);
        close(s);
        for(int countdown = 10; countdown >= 0; countdown--) {
            ESP_LOGI(TAG, "%d... ", countdown);
            vTaskDelay(1000 / portTICK_PERIOD_MS);
        }
        ESP_LOGI(TAG, "Starting again!");
    }
}

void app_main()
{
    ESP_ERROR_CHECK( nvs_flash_init() );
    initialise_wifi();
    xTaskCreate(&http_get_task, "http_get_task", 4096, NULL, 5, NULL);
}

运行结果如下：

REQUEST: GET http://example.com/ HTTP/1.0
Host: example.com
User-Agent: esp-idf/1.0 esp32


I (3420157) example: ... socket send success
I (3420157) example: ... set socket receiving timeout success
HTTP/1.0 200 OK
Accept-Ranges: bytes
Cache-Control: max-age=604800
Content-Type: text/html; charset=UTF-8
Date: Sun, 28 Oct 2018 01:55:09 GMT
Etag: "1541025663"
Expires: Sun, 04 Nov 2018 01:55:09 GMT
Last-Modified: Fri, 09 Aug 2013 23:54:35 GMT
Server: ECS (oxr/83B7)
Vary: Accept-Encoding
X-Cache: HIT
Content-Length: 1270
Connection: close




    

    
    
    
        




    
Example Domain
    
This domain is established to be used for illustrative examples in documents. You may use this
    domain in examples without prior coordination or asking for permission.
    
More information...

可以看到 HTTP GET 请求成功，返回页面代码。
总结esp32上使用 http request 的方法：

1.使用 initialise_wifi，初始化WIFI，连接互联网
2.使用 getaddrinfo 获取HTTP服务器地址信息，并填充 struct addrinfo 结构体
3.使用 socket 创建套接字，HTTP 协议实质是 TCP 协议
4.使用 connect 建立 TCP 连接
5.使用 write 进行 TCP send，等价于发送 GET 请求
6.使用 read 进行 TCP recv，等价于接收 RESPONSE 响应
7.使用 close 释放套接字

所以使用 HTTP 请求协议是非常简单的，因为其实质是 TCP 协议的调用，只是稍微封装了一下 TCP 协议而已，而大部分接口还是一致的，需要注意的是 HTTP 协议请求格式问题，毕竟它是 HTTP 协议，得遵循一定的请求规则，例如本例中的请求：

static const char *REQUEST = "GET " WEB_URL " HTTP/1.0\r\n"
    "Host: "WEB_SERVER"\r\n"
    "User-Agent: esp-idf/1.0 esp32\r\n"
    "\r\n";

上面这字符串就是遵循了 HTTP 协议要求使用 TCP 传送而已

编程注意事项：字符串中使用宏的方法 " WEB_URL "