用C语言实现websocket服务器

Websocket Echo Server Demo

背景

嵌入式设备的应用开发大都依靠C语言来完成，我去研究如何用C语言实现websocket服务器也是为了在嵌入式设备中实现一个ip camera的功能，用户通过网页访问到嵌入式设备的摄像头以及音频，在学习的过程中先实现echo server是最基本的。

主要参考资源

• 编写 WebSocket 服务器——MDN
• Linux下用C编写WebSocet服务以响应HTML5的WebSocket请求

具体实现

整个websocket从握手到数据传输帧头的格式不在这里展开，具体参考编写 WebSocket 服务器——MDN，在这里只介绍一下websocket echo server的实现。

• 头文件及宏定义

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
/*在握手时需要进行sha1编码和base64编码，
在这里用openssl的库来实现*/
#include <openssl/sha.h>
#include <openssl/pem.h>
#include <openssl/bio.h>
#include <openssl/evp.h>

#define BUFFER_SIZE 1024
#define RESPONSE_HEADER_LEN_MAX 1024
#define GUID "258EAFA5-E914-47DA-95CA-C5AB0DC85B11"

• 数据帧头

/*-----------为了便于理解，在这里吧数据帧格式粘出来-------------------
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 ...                |
+---------------------------------------------------------------+
--------------------------------------------------------------------*/
typedef struct _frame_head {
    char fin;
    char opcode;
    char mask;
    unsigned long long payload_length;
    char masking_key[4];
} frame_head;

• 封装套接字函数
  为了使套接字使用看起来简洁一些，封装一个被动套接字函数，只需要传入监听端口和监听队列个数就可以返回套接字描述符，调用者可以直接用这个描述符accept去接收客户端连接。

int passive_server(int port,int queue)
{
    ///定义sockfd
    int server_sockfd = socket(AF_INET,SOCK_STREAM, 0);

    ///定义sockaddr_in
    struct sockaddr_in server_sockaddr;
    server_sockaddr.sin_family = AF_INET;
    server_sockaddr.sin_port = htons(port);
    server_sockaddr.sin_addr.s_addr = htonl(INADDR_ANY);

    ///bind，成功返回0，出错返回-1
    if(bind(server_sockfd,(struct sockaddr *)&server_sockaddr,sizeof(server_sockaddr))==-1)
    {
        perror("bind");
        exit(1);
    }
    ///listen，成功返回0，出错返回-1
    if(listen(server_sockfd,queue) == -1)
    {
        perror("listen");
        exit(1);
    }
    printf("监听%d端口\n",port);
    return server_sockfd;
}

• Base64编码函数
  握手函数会用到

int base64_encode(char *in_str, int in_len, char *out_str)
{
    BIO *b64, *bio;
    BUF_MEM *bptr = NULL;
    size_t size = 0;

    if (in_str == NULL || out_str == NULL)
        return -1;

    b64 = BIO_new(BIO_f_base64());
    bio = BIO_new(BIO_s_mem());
    bio = BIO_push(b64, bio);

    BIO_write(bio, in_str, in_len);
    BIO_flush(bio);

    BIO_get_mem_ptr(bio, &bptr);
    memcpy(out_str, bptr->data, bptr->length);
    out_str[bptr->length-1] = '\0';
    size = bptr->length;

    BIO_free_all(bio);
    return size;
}

• 逐行读取函数
  握手函数循环调用，每次获得一行字符串，返回下一行开始位置

/**
 * @brief _readline
 * read a line string from all buffer
 * @param allbuf
 * @param level
 * @param linebuf
 * @return
 */
int _readline(char* allbuf,int level,char* linebuf)
{
    int len = strlen(allbuf);
    for (;level<len;++level)
    {
        if(allbuf[level]=='\r' && allbuf[level+1]=='\n')
            return level+2;
        else
            *(linebuf++) = allbuf[level];
    }
    return -1;
}

• 握手函数
  负责处理新客户端的连接，接收客户端http格式的请求，从中获得Sec-WebSocket-Key对应的值，与魔法字符串 258EAFA5-E914-47DA-95CA-C5AB0DC85B11 进行连接后进行sha1 hash，再将结果（sha1的直接结果，不是转化为字符串后的结果）进行Base64编码。最后构造响应头部，发送响应，与客户端建立websocket连接。

int shakehands(int cli_fd)
{
    //next line's point num
    int level = 0;
    //all request data
    char buffer[BUFFER_SIZE];
    //a line data
    char linebuf[256];
    //Sec-WebSocket-Accept
    char sec_accept[32];
    //sha1 data
    unsigned char sha1_data[SHA_DIGEST_LENGTH+1]={0};
    //reponse head buffer
    char head[BUFFER_SIZE] = {0};

    if (read(cli_fd,buffer,sizeof(buffer))<=0)
        perror("read");
    printf("request\n");
    printf("%s\n",buffer);

    do {
        memset(linebuf,0,sizeof(linebuf));
        level = _readline(buffer,level,linebuf);
        //printf("line:%s\n",linebuf);

        if (strstr(linebuf,"Sec-WebSocket-Key")!=NULL)
        {
            strcat(linebuf,GUID);
//            printf("key:%s\nlen=%d\n",linebuf+19,strlen(linebuf+19));
            SHA1((unsigned char*)&linebuf+19,strlen(linebuf+19),(unsigned char*)&sha1_data);
//            printf("sha1:%s\n",sha1_data);
            base64_encode(sha1_data,strlen(sha1_data),sec_accept);
//            printf("base64:%s\n",sec_accept);
            /* write the response */
            sprintf(head, "HTTP/1.1 101 Switching Protocols\r\n" \
                          "Upgrade: websocket\r\n" \
                          "Connection: Upgrade\r\n" \
                          "Sec-WebSocket-Accept: %s\r\n" \
                          "\r\n",sec_accept);

            printf("response\n");
            printf("%s",head);
            if (write(cli_fd,head,strlen(head))<0)
                perror("write");

            break;
        }
    }while((buffer[level]!='\r' || buffer[level+1]!='\n') && level!=-1);
    return 0;
}

• 字符串反转函数
  用于解决大端小端问题

void inverted_string(char *str,int len)
{
    int i; char temp;
    for (i=0;i<len/2;++i)
    {
        temp = *(str+i);
        *(str+i) = *(str+len-i-1);
        *(str+len-i-1) = temp;
    }
}

• 接收及存储数据帧头
  调用者传一个数据帧头结构体指针用于获取解析后的帧头
  解析过程依照MDN中说的结构解析就好。

int recv_frame_head(int fd,frame_head* head)
{
    char one_char;
    /*read fin and op code*/
    if (read(fd,&one_char,1)<=0)
    {
        perror("read fin");
        return -1;
    }
    head->fin = (one_char & 0x80) == 0x80;
    head->opcode = one_char & 0x0F;
    if (read(fd,&one_char,1)<=0)
    {
        perror("read mask");
        return -1;
    }
    head->mask = (one_char & 0x80) == 0X80;

    /*get payload length*/
    head->payload_length = one_char & 0x7F;

    if (head->payload_length == 126)
    {
        char extern_len[2];
        if (read(fd,extern_len,2)<=0)
        {
            perror("read extern_len");
            return -1;
        }
        head->payload_length = (extern_len[0]&0xFF) << 8 | (extern_len[1]&0xFF);
    }
    else if (head->payload_length == 127)
    {
        char extern_len[8];
        if (read(fd,extern_len,8)<=0)
        {
            perror("read extern_len");
            return -1;
        }
        inverted_string(extern_len,8);
        memcpy(&(head->payload_length),extern_len,8);
    }

    /*read masking-key*/
    if (read(fd,head->masking_key,4)<=0)
    {
        perror("read masking-key");
        return -1;
    }

    return 0;
}

• 去掩码函数
  从客户端发来的数据是经过异或加密的，我们在解析帧头的时候获取到了掩码，我们通过掩码可以解码出原数据。

/**
 * @brief umask
 * xor decode
 * @param data 传过来时为密文，解码后的明文同样存储在这里
 * @param len data的长度
 * @param mask 掩码
 */
void umask(char *data,int len,char *mask)
{
    int i;
    for (i=0;i<len;++i)
        *(data+i) ^= *(mask+(i%4));
}

• 发送数据帧头

int send_frame_head(int fd,frame_head* head)
{
    char *response_head;
    int head_length = 0;
    if(head->payload_length<126)
    {
        response_head = (char*)malloc(2);
        response_head[0] = 0x81;
        response_head[1] = head->payload_length;
        head_length = 2;
    }
    else if (head->payload_length<0xFFFF)
    {
        response_head = (char*)malloc(4);
        response_head[0] = 0x81;
        response_head[1] = 126;
        response_head[2] = (head->payload_length >> 8 & 0xFF);
        response_head[3] = (head->payload_length & 0xFF);
        head_length = 4;
    }
    else
    {
        response_head = (char*)malloc(12);
        response_head[0] = 0x81;
        response_head[1] = 127;
        memcpy(response_head+2,head->payload_length,sizeof(unsigned long long));
        inverted_string(response_head+2,sizeof(unsigned long long));
        head_length = 12;
    }

    if(write(fd,response_head,head_length)<=0)
    {
        perror("write head");
        return -1;
    }

    free(response_head);
    return 0;
}

• 主函数
  接收一个连接并循环回射。

int main()
{
    int ser_fd = passive_server(4444,20);


    struct sockaddr_in client_addr;
    socklen_t addr_length = sizeof(client_addr);
    int conn = accept(ser_fd,(struct sockaddr*)&client_addr, &addr_length);

    shakehands(conn);

    while (1)
    {
        frame_head head;
        int rul = recv_frame_head(conn,&head);
        if (rul < 0)
            break;
//        printf("fin=%d\nopcode=0x%X\nmask=%d\npayload_len=%llu\n",head.fin,head.opcode,head.mask,head.payload_length);

        //echo head
        send_frame_head(conn,&head);
        //read payload data
        char payload_data[1024] = {0};
        int size = 0;
        do {
            int rul;
            rul = read(conn,payload_data,1024);
            if (rul<=0)
                break;
            size+=rul;

            umask(payload_data,size,head.masking_key);
            printf("recive:%s",payload_data);

            //echo data
            if (write(conn,payload_data,rul)<=0)
                break;
        }while(size<head.payload_length);
        printf("\n-----------\n");

    }

    close(conn);
    close(ser_fd);
}

• 客户端测试用例：
  将以下代码保存为websocket_client.html，用Chrome浏览器打开测试。
  代码中console.log是在浏览器中按F12在控制台中查看输出

<button onclick="svc_connectPlatform()"> connect</button>
<button onclick="svc_send('hello web')"> send</button>
<script>

    function svc_connectPlatform() {
        //alert("");
        var wsServer = 'ws://192.168.25.157:4444/';
        try {
            svc_websocket = new WebSocket(wsServer);
        } catch (evt) {
            console.log("new WebSocket error:" + evt.data);
            svc_websocket = null;
            if (typeof(connCb) != "undefined" && connCb != null)
                connCb("-1", "connect error!");
            return;
        }
        //alert("");
        svc_websocket.onopen = svc_onOpen;
        svc_websocket.onclose = svc_onClose;
        svc_websocket.onmessage = svc_onMessage;
        svc_websocket.onerror = svc_onError;
    }


    function svc_onOpen(evt) {
        console.log("Connected to WebSocket server.");
    }


    function svc_onClose(evt) {
        console.log("Disconnected");
    }


    function svc_onMessage(evt) {
        console.log('Retrieved data from server: ' + evt.data);
    }


    function svc_onError(evt) {
        console.log('Error occured: ' + evt.data);
    }


    function svc_send(msg) {
        if (svc_websocket.readyState == WebSocket.OPEN) {
            svc_websocket.send(msg);
        } else {
            console.log("send failed. websocket not open. please check.");
        }
    }
</script>

开源代码：https://github.com/lhc3538/my-websocket-server