websocket主要用于浏览器与服务器之间建立长连接，服务器需要主动给浏览器发送数据时。当然，这种情况，websocket不是唯一解决方案。websocket基本流程如下：

这里注意，在①②中，客户端与服务器建立连接请求网页数据和服务器返回网页数据时，还是使用的http协议，从③开始，才是使用websocket。

websocket重点由两部分组成。

握手

websocket握手用于检验连接数据是否合法，主要流程是浏览器发送一个随机base64编码的值，服务端接收后加上一段全局变量，再进行SHA-1和base64编码返回给客户端。客户端检验，正确后建立连接。

int readline(char* allbuf,int level,char* linebuf)

{

int len = strlen(allbuf);

for (;level < len; ++level) //level相当于下标

{

if(allbuf[level]=='\r' && allbuf[level+1]==' ')

return level+2;

else

*(linebuf++) = allbuf[level];

}

return -1;

}

int handshark(struct sockitem *si, struct reactor *mainloop)

{

//si是每个socket独有的参数，包括接收和发送数据的缓冲区。reactor是全局变量，存储epoll相关数据。

char linebuf[256];

char sec_accept[32];

int level = 0;

unsigned char sha1_data[SHA_DIGEST_LENGTH+1] = {0};

char head[BUFFER_LENGTH] = {0}; //宏定义

do

{

memset(linebuf, 0, sizeof(linebuf));

level = readline(si->recvbuffer, level, linebuf);

if (strstr(linebuf,"Sec-WebSocket-Key") != NULL)

{

strcat(linebuf, GUID);

SHA1((unsigned char*)&linebuf+19,strlen(linebuf+19),(unsigned char*)&sha1_data);

base64_encode(sha1_data,strlen(sha1_data),sec_accept);

sprintf(head, "HTTP/1.1 101 Switching Protocols\r " \

"Upgrade: websocket\r " \

"Connection: Upgrade\r " \

"Sec-WebSocket-Accept: %s\r " \

"\r ", sec_accept);

printf("response ");

printf("%s ", head);

memset(si->recvbuffer, 0, BUFFER_LENGTH);

memcpy(si->sendbuffer, head, strlen(head)); // to send

si->slength = strlen(head);

// to set epollout events

struct epoll_event ev;

ev.events = EPOLLOUT | EPOLLET;

//ev.data.fd = clientfd;

si->sockfd = si->sockfd;

si->callback = send_cb;

si->status = WS_DATATRANSFORM;

ev.data.ptr = si;

epoll_ctl(mainloop->epfd, EPOLL_CTL_MOD, si->sockfd, &ev);

break;

}

} while((si->recvbuffer[level] != '\r' || si->recvbuffer[level+1] != ' ') && level != -1);

return 0;

}

通信

握手完成后，开始正常的通信，这里贴出websocket协议封装的代码，协议结构图就不贴了。

char* decode_packet(char *stream, char *mask, int length, int *ret)

{

nty_ophdr *hdr = (nty_ophdr*)stream; //stream为输入字符串

unsigned char *data = stream + sizeof(nty_ophdr);

int size = 0;

int start = 0;

//char mask[4] = {0};

int i = 0;

//if (hdr->fin == 1) return NULL;

if ((hdr->mask & 0x7F) == 126)

{

nty_websocket_head_126 *hdr126 = (nty_websocket_head_126*)data;

size = hdr126->payload_length;

for (i = 0;i < 4;i ++)

{

mask[i] = hdr126->mask_key[i]; //掩码

}

start = 8;

} else if ((hdr->mask & 0x7F) == 127)

{

nty_websocket_head_127 *hdr127 = (nty_websocket_head_127*)data;

size = hdr127->payload_length;

for (i = 0;i < 4;i ++)

{

mask[i] = hdr127->mask_key[i]; //掩码

}

start = 14;

} else {

size = hdr->payload_length;

memcpy(mask, data, 4);

start = 6;

}

*ret = size;

umask(stream+start, size, mask);

return stream + start; //返回解后字符串

}

int encode_packet(char *buffer,char *mask, char *stream, int length)

{

//buffer为输出的封装后字符串，stream为输入的待封装数据

nty_ophdr head = {0};

head.fin = 1;

head.opcode = 1;

int size = 0;

if (length < 126)

{

head.payload_length = length;

memcpy(buffer, &head, sizeof(nty_ophdr));

size = 2;

} else if (length < 0xffff)

{

nty_websocket_head_126 hdr = {0};

hdr.payload_length = length;

memcpy(hdr.mask_key, mask, 4);

memcpy(buffer, &head, sizeof(nty_ophdr));

memcpy(buffer+sizeof(nty_ophdr), &hdr, sizeof(nty_websocket_head_126));

size = sizeof(nty_websocket_head_126);

} else

{

nty_websocket_head_127 hdr = {0};

hdr.payload_length = length;

memcpy(hdr.mask_key, mask, 4);

memcpy(buffer, &head, sizeof(nty_ophdr));

memcpy(buffer+sizeof(nty_ophdr), &hdr, sizeof(nty_websocket_head_127));

size = sizeof(nty_websocket_head_127);

}

memcpy(buffer+2, stream, length);

return length + 2;

}

再贴出通信的代码

int transform(struct sockitem *si, struct reactor *mainloop)

{

int ret = 0;

char mask[4] = {0};

char *data = decode_packet(si->recvbuffer, mask, si->rlength, &ret);

printf("data : %s , length : %d ", data, ret);

ret = encode_packet(si->sendbuffer, mask, data, ret);

si->slength = ret;

memset(si->recvbuffer, 0, BUFFER_LENGTH);

struct epoll_event ev;

ev.events = EPOLLOUT | EPOLLET;

//ev.data.fd = clientfd;

si->sockfd = si->sockfd;

si->callback = send_cb;

si->status = WS_DATATRANSFORM;

ev.data.ptr = si;

epoll_ctl(mainloop->epfd, EPOLL_CTL_MOD, si->sockfd, &ev);

return 0;

}

这里再提示一下，握手、通信等阶段使用状态机实现。注意使用epoll的话，在每个状态里写epoll_ctl，不要再最后总体结构中写，因为如果数据量大的话，不是一次recv/send就能搞定。

另外，从websocket协议中可以获得启示。今后，在自定义开发基于TCP的协议时，要注意三个方面：操作码、包长度、掩码（非必须）。当然，数据的话，可以使用xml、json等协议。

长连接

最后，关于长连接，再啰嗦一下。TCP中也有保持长连接的keepalive机制，但是不建议使用，不要使用TCP的keepalive去代替应用层的心跳包。因为keepalive超时，TCP会主动回收连接相关资源，并且应用层无法得到任何反馈，可控性不强，只知道超时回收了，没法对超时做出一些判断。例如，TCP的keepalive机制，⽆法判断进程阻塞或者死锁的情况。所以，在应用层设计心跳包去维持长连接。