websocket是什么，如何用C语言写一个简单的websocket通信

WebSocket是一种在单个TCP连接上进行全双工通信的协议，它可以使浏览器和服务器之间实现实时通信。WebSocket 在 Web 应用程序中使用越来越广泛，它被广泛应用于在线游戏、视频会议、网络直播等场景。

下面是一个使用C语言编写的简单的WebSocket通信示例：

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdbool.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#define BUF_SIZE 1024

int main(int argc, char *argv[]) {
    int server_socket, client_socket;
    struct sockaddr_in server_addr, client_addr;
    socklen_t client_addr_size;
    char buf[BUF_SIZE];
    bool is_handshake = false;

    // 创建服务器socket
    server_socket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (server_socket < 0) {
        perror("socket");
        exit(1);
    }

    // 设置服务器socket地址
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = inet_addr("127.0.0.1");
    server_addr.sin_port = htons(8080);

    // 绑定服务器socket和地址
    if (bind(server_socket, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
        perror("bind");
        exit(1);
    }

    // 监听客户端连接
    if (listen(server_socket, 5) < 0) {
        perror("listen");
        exit(1);
    }

    printf("Server is running on port 8080...\n");

    while (1) {
        // 接受客户端连接
        client_addr_size = sizeof(client_addr);
        client_socket = accept(server_socket, (struct sockaddr *)&client_addr, &client_addr_size);
        if (client_socket < 0) {
            perror("accept");
            continue;
        }

        printf("Client %s:%d connected\n", inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));

        while (1) {
            // 接收客户端发送的数据
            int len = recv(client_socket, buf, BUF_SIZE, 0);
            if (len < 0) {
                perror("recv");
                break;
            } else if (len == 0) {  // 客户端关闭连接
                printf("Client %s:%d closed\n", inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));
                break;
            }

            if (!is_handshake) {  // 如果还没有进行WebSocket握手
                // 进行WebSocket握手
                char *key = strstr(buf, "Sec-WebSocket-Key: ");
                if (key) {
                    char *end = strstr(key, "\r\n");
                    if (end) {
                        char accept_key[BUF_SIZE];
                        memset(accept_key, 0, sizeof(accept_key));
                        strncpy(accept_key, key + strlen("Sec-WebSocket-Key: "), end - key - strlen("Sec-WebSocket-Key: "));
                        strcat(accept_key, "258EAFA5-E914-47DA-95CA-C5AB0DC85B11");

                        unsigned char sha1_buf[BUF_SIZE];
                        memset(sha1_buf, 0, sizeof(sha1_buf));
                        sha1((unsigned char *)accept_key, strlen(accept_key), sha1_buf);

                        char accept_key_base64[BUF_SIZE];
                        memset(accept_key_base64, 0, sizeof(accept_key_base64));
                        base64_encode(sha1_buf, 20, accept_key_base64);

                        char response[BUF_SIZE];
                        memset(response, 0, sizeof(response));
                        sprintf(response, "HTTP/1.1 101 Switching Protocols\r\nUpgrade: websocket\r\nConnection: Upgrade\r\nSec-WebSocket-Accept: %s\r\n\r\n", accept_key_base64);

                        // 发送握手响应
                        send(client_socket, response, strlen(response), 0);
                        is_handshake = true;

                        printf("WebSocket handshake completed\n");
                    }
                }
            } else {  // 已经完成WebSocket握手
                // 解析WebSocket帧数据
                int payload_len = buf[1] & 0x7f;
                int masking_key_offset = 2;
                if (payload_len == 126) {
                    payload_len = (buf[2] << 8) | buf[3];
                    masking_key_offset = 4;
                } else if (payload_len == 127) {
                    printf("Payload length > 65535 is not supported\n");
                    break;
                }

                unsigned char masking_key[4];
                memcpy(masking_key, buf + masking_key_offset, sizeof(masking_key));

                unsigned char *payload = (unsigned char *)malloc(payload_len);
                memcpy(payload, buf + masking_key_offset + sizeof(masking_key), payload_len);

                for (int i = 0; i < payload_len; i++) {
                    payload[i] ^= masking_key[i % sizeof(masking_key)];
                }

                // 打印接收到的消息
                printf("Received message: %s\n", payload);

                // 发送消息给客户端
                char message[BUF_SIZE] = "Hello, WebSocket!";
                send_websocket_frame(client_socket, message, strlen(message));

                free(payload);
            }
        }

        close(client_socket);
    }

    close(server_socket);

    return 0;
}

这个示例程序实现了一个简单的WebSocket服务器，它可以接收客户端发送的WebSocket帧数据，并且向客户端发送一条固定的消息。在实际使用中，你需要根据具体的需求进行修改和扩展。