使用esp32進行 http request 是一項非常重要的應用 原始碼路徑：esp-idf/examples/protocols/http_request 原始碼使用：

/* 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 <string.h>
#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協議
    };
    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);	// 考慮結束符\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);
}
 

執行結果如下：

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

<!doctype html>
<html>
<head>
    <title>Example Domain</title>

    <meta charset="utf-8" />
    <meta http-equiv="Content-type" content="text/html; charset=utf-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1" />
    <style type="text/css">
    body {
        background-color: #f0f0f2;
        margin: 0;
        padding: 0;
        font-family: "Open Sans", "Helvetica Neue", Helvetica, Arial, sans-serif;
        
    }
    div {
        width: 600px;
        margin: 5em auto;
        padding: 50px;
        background-color: #fff;
        border-radius: 1em;
    }
    a:link, a:visited {
        color: #38488f;
        text-decoration: none;
    }
    @media (max-width: 700px) {
        body {
            background-color: #fff;
        }
        div {
            width: auto;
            margin: 0 auto;
            border-radius: 0;
            padding: 1em;
        }
    }
    </style>    
</head>

<body>
<div>
    <h1>Example Domain</h1>
    <p>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.</p>
    <p><a href="http://www.iana.org/domains/example">More information...</a></p>
</div>
</body>
</html>
 

可以看到 HTTP GET 請求成功，返回頁面程式碼。 總結esp32上使用 http request 的方法：

1.使用 initialise_wifi，初始化WIFI，連線網際網路 2.使用 getaddrinfo 獲取HTTP伺服器地址資訊，並填充 struct addrinfo 結構體 3.使用 socket 建立套接字，HTTP 協議實質是 TCP 協議 4.使用 connect 建立 TCP 連線 5.使用 write 進行 TCP send，等價於傳送 GET 請求 6.使用 read 進行 TCP recv，等價於接收 RESPONSE 響應 7.使用 close 釋放套接字

所以使用 HTTP 請求協議是非常簡單的，因為其實質是 TCP 協議的呼叫，只是稍微封裝了一下 TCP 協議而已，而大部分介面還是一致的，需要注意的是 HTTP 協議請求格式問題，畢竟它是 HTTP 協議，得遵循一定的請求規則，例如本例中的請求：

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";

上面這字串就是遵循了 HTTP 協議要求使用 TCP 傳送而已

程式設計注意事項：字串中使用巨集的方法 " WEB_URL "