C语言简单实现WebSocket协议

如果不了解什么是 WebSocket 协议，请先阅读这篇帖子WebSocket 协议

创建 WebSocket 连接

通过阅读WebSocket 协议这篇帖子我们可以得知:

• WebSocket 是建立在 TCP 连接上的协议
• WebSocket 需要发送一次基于 HTTP 的握手请求

那我们可以编写这样的C语言代码（环境Windows10，编译器TDM-GCC，编译时需加入参数 -lwsock32）

#include <WinSock2.h>

// 创建 WebSocket 连接
SOCKET ws_init(const char *host, u_short port)
{
    
    // 创建 TCP Socekt
    WSADATA data;
    WORD sockVersion = MAKEWORD(2, 2);
    WSAStartup(sockVersion, &data);
    SOCKET ws_sock = socket(AF_INET, SOCK_STREAM, 0);

    // 向指定 Server 端发起连接
    struct sockaddr_in addr;
    memset(&addr, 0, sizeof(struct sockaddr_in));
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = inet_addr(host);
    addr.sin_port = htons(port);
    connect(ws_sock, (struct sockaddr *)&addr, sizeof(struct sockaddr));

    // 发送 WebSocket 握手请求
    char handShake[] = "GET /ws HTTP/1.1\n\
Host: 127.0.0.1:6700\n\
Connection: Upgrade\n\
Upgrade: websocket\n\
Sec-WebSocket-Version: 13\n\
Sec-WebSocket-Key: Bt4+Nfq12qxyxHslV2iFFg==\n\n";
    send(ws_sock, handShake, strlen(handShake), 0);

    // 忽略 Server 端响应数据
    char response[1024 * 2];
    recv(ws_sock, response, 1024 * 2, 0);

    return ws_sock;
}

收发 WebSocket 数据包

                     WebSocket 数据包解析

0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-------+-+-------------+-------------------------------+
|F|R|R|R| opcode|M| Payload len |    Extended payload length    |
|I|S|S|S|  (4)  |A|     (7)     |             (16/64)           |
|N|V|V|V|       |S|             |   (if payload len==126/127)   |
| |1|2|3|       |K|             |                               |
+-+-+-+-+-------+-+-------------+ - - - - - - - - - - - - - - - +
|     Extended payload length continued, if payload len == 127  |
+ - - - - - - - - - - - - - - - +-------------------------------+
|                               |Masking-key, if MASK set to 1  |
+-------------------------------+-------------------------------+
| Masking-key (continued)       |          Payload Data         |
+-------------------------------- - - - - - - - - - - - - - - - +
:                     Payload Data continued ...                :
+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +
|                     Payload Data continued ...                |
+---------------------------------------------------------------+

我们知道通过位运算可以进行bit位的操作，结合WebSocket 协议中对数据帧讲解，可得

// 发送 WebSocket 数据包
void ws_send(SOCKET ws_sock, char data[])
{
    short head = 0;

    head = head | 1;  // 设置 FIN 值为 1
    head = head << 3; // 设置 RSV1/2/3 默认值均为 0

    // 设置 opcode 值为 OPCODE_TEXT
    head = head << 4;
    head = head | (1 << 0);

    // 设置 MASK 值为 1 代表需要掩码计算
    head = head << 1;
    head = head | (1 << 0);

    // 设置 Payload len 和 Extended payload length
    head = head << 7;
    size_t size = strlen(data);

    if (size < 126)
    {
        head = head | size;
        char headlen[2];
        char payload[size + 6];

        memcpy(headlen, (char *)&head, sizeof(short));
        payload[0] = headlen[1];
        payload[1] = headlen[0];

        // Masking-key
        // 设成重复的值编码起来比较方便
        payload[2] = 33;
        payload[3] = 33;
        payload[4] = 33;
        payload[5] = 33;

        for (size_t i = 0; i < size; i++)
        {
            payload[i + 6] = data[i] ^ 33;
        }
        send(ws_sock, payload, size + 6, 0);
    }
    else if (size < 65536) // 1 << 16
    {
        head = head | 126;
        char headlen[2];
        char datalen[2];
        char payload[size + 8];

        memcpy(headlen, (char *)&head, sizeof(short));
        payload[0] = headlen[1];
        payload[1] = headlen[0];

        memcpy(datalen, (char *)&size, sizeof(short));
        payload[2] = datalen[1];
        payload[3] = datalen[0];

        // Masking-key
        // 设成重复的值编码起来比较方便
        payload[4] = 33;
        payload[5] = 33;
        payload[6] = 33;
        payload[7] = 33;

        for (size_t i = 0; i < size; i++)
        {
            payload[i + 8] = data[i] ^ 33;
        }
        send(ws_sock, payload, size + 8, 0);
    }
    else // 1 << 64
    {
        head = head | 127;
        char headlen[2];
        char datalen[8];
        char payload[size + 14];

        memcpy(headlen, (char *)&head, sizeof(short));
        payload[0] = headlen[1];
        payload[1] = headlen[0];

        memcpy(datalen, (char *)&size, sizeof(long long));
        for (size_t i = 0; i < 8; i++)
        {
            payload[i + 2] = datalen[7 - i];
        }

        // Masking-key
        // 设成重复的值编码起来比较方便
        payload[10] = 33;
        payload[11] = 33;
        payload[12] = 33;
        payload[13] = 33;

        for (size_t i = 0; i < size; i++)
        {
            payload[i + 14] = data[i] ^ 33;
        }
        send(ws_sock, payload, size + 14, 0);
    }
}

// 接收 WebSocket 数据包
void ws_recv(SOCKET ws_sock, char *data, int datalen)
{
    size_t size = recv(ws_sock, data, datalen, 0);

    if (size < 126)
    {
        for (size_t i = 0; i < size; i++)
        {
            data[i] = data[i + 2];
        }
    }
    else if (size < 65536) // 1 << 16
    {
        for (size_t i = 0; i < size; i++)
        {
            data[i] = data[i + 4];
        }
    }
    else // 1 << 64
    {
        for (size_t i = 0; i < size; i++)
        {
            data[i] = data[i + 10];
        }
    }
}