编写一个基于mbedtls建立tls连接通信的代码示例

以下是一个基于mbedtls建立TLS连接通信的代码示例，示例中使用了mbedtls的客户端模式：

#include "mbedtls/net_sockets.h"
#include "mbedtls/ssl.h"
#include "mbedtls/entropy.h"
#include "mbedtls/ctr_drbg.h"
#include "mbedtls/error.h"

#include <stdio.h>
#include <string.h>

#define SERVER_NAME "example.com"
#define SERVER_PORT "443"

int main()
{
    int ret;
    mbedtls_net_context server_fd;
    mbedtls_ssl_context ssl;
    mbedtls_ssl_config conf;
    mbedtls_entropy_context entropy;
    mbedtls_ctr_drbg_context ctr_drbg;
    char buf[1024];

    // 初始化各种上下文
    mbedtls_net_init(&server_fd);
    mbedtls_ssl_init(&ssl);
    mbedtls_ssl_config_init(&conf);
    mbedtls_ctr_drbg_init(&ctr_drbg);
    mbedtls_entropy_init(&entropy);

    // 生成随机数种子
    ret = mbedtls_ctr_drbg_seed(&ctr_drbg, mbedtls_entropy_func, &entropy, NULL, 0);
    if (ret != 0) {
        printf("mbedtls_ctr_drbg_seed failed: %d\n", ret);
        goto cleanup;
    }

    // 连接服务器
    ret = mbedtls_net_connect(&server_fd, SERVER_NAME, SERVER_PORT, MBEDTLS_NET_PROTO_TCP);
    if (ret != 0) {
        printf("mbedtls_net_connect failed: %d\n", ret);
        goto cleanup;
    }

    // 初始化 SSL 上下文
    ret = mbedtls_ssl_config_defaults(&conf, MBEDTLS_SSL_IS_CLIENT, MBEDTLS_SSL_TRANSPORT_STREAM, MBEDTLS_SSL_PRESET_DEFAULT);
    if (ret != 0) {
        printf("mbedtls_ssl_config_defaults failed: %d\n", ret);
        goto cleanup;
    }

    // 设置主机名和端口号
    ret = mbedtls_ssl_set_hostname(&ssl, SERVER_NAME);
    if (ret != 0) {
        printf("mbedtls_ssl_set_hostname failed: %d\n", ret);
        goto cleanup;
    }

    // 设置 SSL 上下文
    mbedtls_ssl_set_bio(&ssl, &server_fd, mbedtls_net_send, mbedtls_net_recv, NULL);
    mbedtls_ssl_set_rng(&ssl, mbedtls_ctr_drbg_random, &ctr_drbg);
    mbedtls_ssl_conf_authmode(&conf, MBEDTLS_SSL_VERIFY_REQUIRED);

    // 建立 TLS 连接
    ret = mbedtls_ssl_handshake(&ssl);
    if (ret != 0) {
        printf("mbedtls_ssl_handshake failed: %d\n", ret);
        goto cleanup;
    }

    // 发送数据
    ret = mbedtls_ssl_write(&ssl, "Hello, world!", strlen("Hello, world!"));
    if (ret < 0) {
        printf("mbedtls_ssl_write failed: %d\n", ret);
        goto cleanup;
    }

    // 接收数据
    do {
        ret = mbedtls_ssl_read(&ssl, buf, sizeof(buf) - 1);
        if (ret == MBEDTLS_ERR_SSL_WANT_READ || ret == MBEDTLS_ERR_SSL_WANT_WRITE) {
            continue;
        } else if (ret <= 0) {
            printf("mbedtls_ssl_read failed: %d\n", ret);
            goto cleanup;
        }
        buf[ret] = '\0';
        printf("%s", buf);
    } while (ret > 0);

cleanup:
    mbedtls_ssl_close_notify(&ssl);
    mbedtls_ssl_session_reset(&ssl);
    mbedtls_net_free(&server_fd);
    mbedtls_ssl_free(&ssl);
    mbedtls_ssl_config_free(&conf);
    mbedtls_ctr_drbg_free(&ctr_drbg);
    mbedtls_entropy_free(&entropy);

    return ret;
}

在此示例中，我们首先初始化各种上下文，然后生成随机数种子。接着，我们连接到服务器并初始化SSL上下文。在设置主机名和端口号之后，我们设置SSL上下文并建立TLS连接。最后，我们发送一条消息并接收来自服务器的响应。最后，我们清理并释放所有的上下文。