一、什么是 WebSocket
WebSocket 是 HTML5 开始提供的一种，建立在单个 TCP 连接上的全双工的网络通信协议。WebSocket 协议在2008年诞生，2011年成为国际标准。现在最新版本浏览器都已经支持了。
它的最大特点就是，服务器可以主动向客户端推送信息，客户端也可以主动向服务器发送信息，是真正的双向平等对话，属于服务器推送技术的一种。
熟悉计算机网络协议的人都知道：HTTP 协议是一种无状态的、无连接的、单向的应用层协议。它采用了请求/响应模型。这种通信模型有一个弊端：HTTP 协议无法实现服务器主动向客户端发起消息。
二、WebSocket 应用场景
- 即时聊天通信
- 在线协同编辑/编辑
- 实时数据流的拉取与推送
- 实时地图位置
三、WebSocket 协议原理及与 HTTP 的关系
WebSocket 协议（以下简称，ws协议），与http协议一样，需要通过已建立的TCP连接来传输数据。具体实现上是先通过http协议建立通道，然后在此基础上用真正ws协议进行通信，所以ws协议和http协议是有一定的交叉关系的。
3.1 ws和Http区别：
相对于 HTTP 这种非持久的协议来说， ws协议是一个持久化协议。举个例子：
HTTP 的生命周期通过 Request 来界定，也就是一个 Request 一个 Response ，那么在 HTTP1.0 中，这次 HTTP 请求就结束了。
在 HTTP1.1 中进行了改进，加了一个 keep-alive。也就是说，在一个 HTTP 连接中，可以发送多个 Request，接收多个 Response。但是，时刻扣住一个点：在 HTTP 中永远是一个 Request 对应一个 Response。而且这个 Response 也是被动的，服务端不能主动发起。
3.2 ws和Http联系：
WebSocket 是基于 HTTP 协议的，或者说借用了 HTTP 协议来完成一部分握手。
四、WebSocket 通信过程
首先我们来看个典型的 WebSocket 握手
4.1 客户端发送ws握手的请求信息如下：
熟悉 HTTP 的道友可能发现了，这段类似 HTTP 协议的握手请求中，多了Upgrade：websocket 和 Sec-WebSocket-Version、Sec-WebSocket-Key等。这些就是ws协议的核心部分，告诉服务器我发起的请求要用ws协议。其中：
- Upgrade：websocket：表示我要用ws协议通信
- Sec-WebSocket-Version：是ws版本号。
- Sec-WebSocket-Key 是一个 Base64 encode 的值，由浏览器随机生成。
4.2 服务器响应报文示例：
我们先来了解一下，服务器在收到ws握手报文要干什么吧：
4.3 小结：
客户端发起http请求，经过3次握手后，建立起TCP连接（注意：ws在发送握手前，Tcp的连接已经建立，也就是说Tcp的三次握手已经成功）。 ws握手报文里存放ws支持的版本号等信息，如：Upgrade、Connection、WebSocket-Version等；然后，服务器收到客户端的握手请求后，同样采用HTTP协议回馈数据；最后，客户端收到连接成功的消息后，开始借助于TCP传输信道进行全双工通信。
五、Websocket的优缺点
优点：
WebSocket协议一旦建议后，互相沟通所消耗的请求头是很小的
服务器可以向客户端推送消息了
缺点：
少部分浏览器不支持，浏览器支持的程度与方式有区别（IE10）
六、总结
WebSocket 是为了在 web 应用上进行双通道通信而产生的协议，相比于轮询HTTP请求的方式，WebSocket 有节省服务器资源，效率高等优点。
WebSocket 中的掩码是为了防止早期版本中存在中间缓存污染攻击等问题而设置的，客户端向服务端发送数据需要掩码，服务端向客户端发送数据不需要掩码。
WebSocket 中 Sec-WebSocket-Key 的生成算法是拼接服务端和客户端生成的字符串，进行SHA1哈希算法，再用base64编码。WebSocket 协议握手是依靠 HTTP 协议的，依靠于 HTTP 响应101进行协议升级转换。
七、完整代码实现
#include <stdio.h> #include <stdlib.h> #include <string.h> #include <sys/socket.h> #include <sys/epoll.h> #include <arpa/inet.h> #include <fcntl.h> #include <unistd.h> #include <errno.h> #include <openssl/sha.h> #include <openssl/pem.h> #include <openssl/bio.h> #include <openssl/evp.h> #define GUID "258EAFA5-E914-47DA-95CA-C5AB0DC85B11" enum { WS_HANDSHARK = 0, WS_TRANMISSION = 1, WS_END = 2, WS_COUNT }; struct ws_ophdr { unsigned char opcode:4, rsv3:1, rsv2:1, rsv1:1, fin:1; unsigned char pl_len:7, mask:1; }; #define BUFFER_LENGTH 1024 #define ACCEPT_KEY_LENGTH 64 #define MAX_EPOLL_EVENTS 1024 #define SERVER_PORT 8888 #define PORT_COUNT 1 typedef int NCALLBACK(int ,int, void*); struct ntyevent { int fd; int events; void *arg; int (*callback)(int fd, int events, void *arg); int status; char buffer[BUFFER_LENGTH]; //request int length; //long last_active; char wbuffer[BUFFER_LENGTH]; //response int wlength; char sec_accept[ACCEPT_KEY_LENGTH]; int wsstatus; //0, 1, 2, 3 }; struct eventblock { struct eventblock *next; struct ntyevent *events; }; struct ntyreactor { int epfd; int blkcnt; struct eventblock *evblks; }; int recv_cb(int fd, int events, void *arg); int send_cb(int fd, int events, void *arg); struct ntyevent *ntyreactor_idx(struct ntyreactor *reactor, int sockfd); void nty_event_set(struct ntyevent *ev, int fd, NCALLBACK callback, void *arg) { ev->fd = fd; ev->callback = callback; ev->events = 0; ev->arg = arg; //ev->last_active = time(NULL); return ; } int nty_event_add(int epfd, int events, struct ntyevent *ev) { struct epoll_event ep_ev = {0, {0}}; ep_ev.data.ptr = ev; ep_ev.events = ev->events = events; int op; if (ev->status == 1) { op = EPOLL_CTL_MOD; } else { op = EPOLL_CTL_ADD; ev->status = 1; } if (epoll_ctl(epfd, op, ev->fd, &ep_ev) < 0) { printf("event add failed [fd=%d], events[%d]\n", ev->fd, events); return -1; } return 0; } int nty_event_del(int epfd, struct ntyevent *ev) { struct epoll_event ep_ev = {0, {0}}; if (ev->status != 1) { return -1; } ep_ev.data.ptr = ev; ev->status = 0; epoll_ctl(epfd, EPOLL_CTL_DEL, ev->fd, &ep_ev); return 0; } // int readline(char* allbuf,int idx,char* linebuf) { int len = strlen(allbuf); for (;idx < len; ++idx) { if(allbuf[idx]=='\r' && allbuf[idx+1]=='\n') return idx+2; else *(linebuf++) = allbuf[idx]; } return -1; } 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; } // request int ws_handshark(struct ntyevent *ev) { int idx = 0; char sec_data[128] = {0}; char sec_accept[128] = {0}; do { char linebuf[1024] = {0}; idx = readline(ev->buffer, idx, linebuf); if (strstr(linebuf, "Sec-WebSocket-Key")) { strcat(linebuf, GUID); SHA1(linebuf+19, strlen(linebuf+19), sec_data); base64_encode(sec_data, strlen(sec_data), sec_accept); printf("idx: %d, line: %ld\n",idx, sizeof("Sec-WebSocket-Key: ")); memcpy(ev->sec_accept, sec_accept, ACCEPT_KEY_LENGTH); } } while((ev->buffer[idx] != '\r' || ev->buffer[idx+1] != '\n') && idx != -1); } void umask(char *payload, int length, char *mask_key) { int i = 0; for (i = 0;i < length;i ++) { payload[i] ^= mask_key[i%4]; } } int ws_tranmission(struct ntyevent *ev) { struct ws_ophdr *hdr = (struct ws_ophdr *)ev->buffer; if (hdr->pl_len < 126) { unsigned char *payload = NULL; if (hdr->mask) { payload = ev->buffer + 6; umask(payload, hdr->pl_len, ev->buffer + 2); } else { payload = ev->buffer + 2; } printf("payload: %s\n", payload); } else if (hdr->pl_len == 126) { } else if (hdr->pl_len == 127) { } else { //assert(0); } } int ws_request(struct ntyevent *ev) { if (ev->wsstatus == WS_HANDSHARK) { ws_handshark(ev); ev->wsstatus = WS_TRANMISSION; } else if (ev->wsstatus == WS_TRANMISSION) { ws_tranmission(ev); } } int ws_response(struct ntyevent *ev) { ev->wlength = sprintf(ev->wbuffer, "HTTP/1.1 101 Switching Protocols\r\n" "Upgrade: websocket\r\n" "Connection: Upgrade\r\n" "Sec-WebSocket-Accept: %s\r\n\r\n", ev->sec_accept); printf("response: %s\n", ev->wbuffer); return ev->wlength; } // connection int recv_cb(int fd, int events, void *arg) { struct ntyreactor *reactor = (struct ntyreactor*)arg; struct ntyevent *ev = ntyreactor_idx(reactor, fd); if (ev == NULL) return -1; int len = recv(fd, ev->buffer, BUFFER_LENGTH, 0); nty_event_del(reactor->epfd, ev); //if (ev->buffer[]) if (len > 0) { ev->length = len; ev->buffer[len] = '\0'; ws_request(ev); //printf("recv [%d]:%s\n", fd, ev->buffer); nty_event_set(ev, fd, send_cb, reactor); nty_event_add(reactor->epfd, EPOLLOUT, ev); } else if (len == 0) { nty_event_del(reactor->epfd, ev); printf("recv_cb --> disconnect\n"); close(ev->fd); } else { if (errno == EAGAIN && errno == EWOULDBLOCK) { // } else if (errno == ECONNRESET){ nty_event_del(reactor->epfd, ev); close(ev->fd); } printf("recv[fd=%d] error[%d]:%s\n", fd, errno, strerror(errno)); } return len; } int send_cb(int fd, int events, void *arg) { struct ntyreactor *reactor = (struct ntyreactor*)arg; struct ntyevent *ev = ntyreactor_idx(reactor, fd); if (ev == NULL) return -1; ws_response(ev); int len = send(fd, ev->wbuffer, ev->wlength, 0); if (len > 0) { printf("send[fd=%d], [%d]%s\n", fd, len, ev->wbuffer); nty_event_del(reactor->epfd, ev); nty_event_set(ev, fd, recv_cb, reactor); nty_event_add(reactor->epfd, EPOLLIN, ev); } else { nty_event_del(reactor->epfd, ev); close(ev->fd); printf("send[fd=%d] error %s\n", fd, strerror(errno)); } return len; } int accept_cb(int fd, int events, void *arg) { struct ntyreactor *reactor = (struct ntyreactor*)arg; if (reactor == NULL) return -1; struct sockaddr_in client_addr; socklen_t len = sizeof(client_addr); int clientfd; if ((clientfd = accept(fd, (struct sockaddr*)&client_addr, &len)) == -1) { if (errno != EAGAIN && errno != EINTR) { } printf("accept: %s\n", strerror(errno)); return -1; } int flag = 0; if ((flag = fcntl(clientfd, F_SETFL, O_NONBLOCK)) < 0) { printf("%s: fcntl nonblocking failed, %d\n", __func__, MAX_EPOLL_EVENTS); return -1; } struct ntyevent *event = ntyreactor_idx(reactor, clientfd); if (event == NULL) return -1; nty_event_set(event, clientfd, recv_cb, reactor); nty_event_add(reactor->epfd, EPOLLIN, event); printf("new connect [%s:%d], pos[%d]\n", inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port), clientfd); return 0; } int init_sock(short port) { int fd = socket(AF_INET, SOCK_STREAM, 0); fcntl(fd, F_SETFL, O_NONBLOCK); struct sockaddr_in server_addr; memset(&server_addr, 0, sizeof(server_addr)); server_addr.sin_family = AF_INET; server_addr.sin_addr.s_addr = htonl(INADDR_ANY); server_addr.sin_port = htons(port); bind(fd, (struct sockaddr*)&server_addr, sizeof(server_addr)); if (listen(fd, 20) < 0) { printf("listen failed : %s\n", strerror(errno)); return -1; } printf("listen server port : %d\n", port); return fd; } int ntyreactor_alloc(struct ntyreactor *reactor) { if (reactor == NULL) return -1; if (reactor->evblks == NULL) return -1; struct eventblock *blk = reactor->evblks; while (blk->next != NULL) { blk = blk->next; } struct ntyevent* evs = (struct ntyevent*)malloc((MAX_EPOLL_EVENTS) * sizeof(struct ntyevent)); if (evs == NULL) { printf("ntyreactor_alloc ntyevent failed\n"); return -2; } memset(evs, 0, (MAX_EPOLL_EVENTS) * sizeof(struct ntyevent)); struct eventblock *block = malloc(sizeof(struct eventblock)); if (block == NULL) { printf("ntyreactor_alloc eventblock failed\n"); return -3; } block->events = evs; block->next = NULL; blk->next = block; reactor->blkcnt ++; return 0; } struct ntyevent *ntyreactor_idx(struct ntyreactor *reactor, int sockfd) { if (reactor == NULL) return NULL; if (reactor->evblks == NULL) return NULL; int blkidx = sockfd / MAX_EPOLL_EVENTS; while (blkidx >= reactor->blkcnt) { ntyreactor_alloc(reactor); } int i = 0; struct eventblock *blk = reactor->evblks; while (i++ != blkidx && blk != NULL) { blk = blk->next; } return &blk->events[sockfd % MAX_EPOLL_EVENTS]; } int ntyreactor_init(struct ntyreactor *reactor) { if (reactor == NULL) return -1; memset(reactor, 0, sizeof(struct ntyreactor)); reactor->epfd = epoll_create(1); if (reactor->epfd <= 0) { printf("create epfd in %s err %s\n", __func__, strerror(errno)); return -2; } struct ntyevent* evs = (struct ntyevent*)malloc((MAX_EPOLL_EVENTS) * sizeof(struct ntyevent)); if (evs == NULL) { printf("create epfd in %s err %s\n", __func__, strerror(errno)); close(reactor->epfd); return -3; } memset(evs, 0, (MAX_EPOLL_EVENTS) * sizeof(struct ntyevent)); struct eventblock *block = malloc(sizeof(struct eventblock)); if (block == NULL) { free(evs); close(reactor->epfd); return -3; } block->events = evs; block->next = NULL; reactor->evblks = block; reactor->blkcnt = 1; return 0; } int ntyreactor_destory(struct ntyreactor *reactor) { close(reactor->epfd); struct eventblock *blk = reactor->evblks; struct eventblock *blk_next; while (blk != NULL) { blk_next = blk->next; free(blk->events); free(blk); blk = blk_next; } return 0; } int ntyreactor_addlistener(struct ntyreactor *reactor, int sockfd, NCALLBACK *acceptor) { if (reactor == NULL) return -1; if (reactor->evblks == NULL) return -1; struct ntyevent *event = ntyreactor_idx(reactor, sockfd); if (event == NULL) return -1; nty_event_set(event, sockfd, acceptor, reactor); nty_event_add(reactor->epfd, EPOLLIN, event); return 0; } int ntyreactor_run(struct ntyreactor *reactor) { if (reactor == NULL) return -1; if (reactor->epfd < 0) return -1; if (reactor->evblks == NULL) return -1; struct epoll_event events[MAX_EPOLL_EVENTS+1]; int checkpos = 0, i; while (1) { int nready = epoll_wait(reactor->epfd, events, MAX_EPOLL_EVENTS, 1000); if (nready < 0) { printf("epoll_wait error, exit\n"); continue; } for (i = 0;i < nready;i ++) { struct ntyevent *ev = (struct ntyevent*)events[i].data.ptr; if ((events[i].events & EPOLLIN) && (ev->events & EPOLLIN)) { ev->callback(ev->fd, events[i].events, ev->arg); } if ((events[i].events & EPOLLOUT) && (ev->events & EPOLLOUT)) { ev->callback(ev->fd, events[i].events, ev->arg); } } } } int main(int argc, char *argv[]) { struct ntyreactor *reactor = (struct ntyreactor*)malloc(sizeof(struct ntyreactor)); ntyreactor_init(reactor); unsigned short port = SERVER_PORT; if (argc == 2) { port = atoi(argv[1]); } int i = 0; int sockfds[PORT_COUNT] = {0}; for (i = 0;i < PORT_COUNT;i ++) { sockfds[i] = init_sock(port+i); ntyreactor_addlistener(reactor, sockfds[i], accept_cb); } ntyreactor_run(reactor); ntyreactor_destory(reactor); for (i = 0;i < PORT_COUNT;i ++) { close(sockfds[i]); } free(reactor); return 0; }
文章参考于<零声教育>的C/C++linux服务期高级架构。
