c语言实现根据http提供的Content-Length提取消息体信息

可以使用C语言中的socket库来进行HTTP通信，并通过解析HTTP响应头中的Content-Length字段来获取消息体的长度，然后从socket中读取相应长度的数据即可。

以下是一个示例代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netdb.h>

#define MAX_BUF_SIZE 1024

int main(int argc, char *argv[])
{
    if (argc != 2) {
        printf("Usage: %s <url>\n", argv[0]);
        return 1;
    }
    char *url = argv[1];

    // 解析URL
    struct hostent *server = gethostbyname(url);
    if (server == NULL) {
        printf("Error: cannot resolve hostname %s\n", url);
        return 1;
    }
    struct sockaddr_in server_addr = {0};
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(80);
    server_addr.sin_addr = *((struct in_addr *) server->h_addr);

    // 创建socket
    int sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) {
        printf("Error: cannot open socket\n");
        return 1;
    }

    // 连接服务器
    if (connect(sockfd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
        printf("Error: cannot connect to server\n");
        return 1;
    }

    // 发送HTTP请求
    char request[MAX_BUF_SIZE] = {0};
    snprintf(request, MAX_BUF_SIZE, "GET / HTTP/1.1\r\nHost: %s\r\n\r\n", url);
    int n = write(sockfd, request, strlen(request));
    if (n < 0) {
        printf("Error: cannot send request\n");
        return 1;
    }

    // 解析HTTP响应头
    char response[MAX_BUF_SIZE] = {0};
    n = read(sockfd, response, MAX_BUF_SIZE);
    if (n < 0) {
        printf("Error: cannot receive response\n");
        return 1;
    }
    char *body_start = strstr(response, "\r\n\r\n") + 4;
    int body_len = atoi(strstr(response, "Content-Length: ") + strlen("Content-Length: "));
    char *body_end = body_start + body_len;

    // 读取消息体
    char body[MAX_BUF_SIZE] = {0};
    memcpy(body, body_start, body_len);
    while (body_end > body_start) {
        n = read(sockfd, body + body_len - (body_end - body_start), body_end - body_start);
        if (n < 0) {
            printf("Error: cannot receive body\n");
            return 1;
        }
        body_end -= n;
    }

    printf("Response body: %s\n", body);

    close(sockfd);
    return 0;
}

注意，以上代码仅供参考，并未进行错误处理和完整性校验，请勿直接用于生产环境。