libevent实现websocket客户端代码

下面是一个基于libevent库实现的WebSocket客户端的示例代码：

#include <event2/event.h>
#include <event2/buffer.h>
#include <event2/http.h>
#include <event2/http_struct.h>
#include <event2/keyvalq_struct.h>
#include <event2/bufferevent_ssl.h>
#include <openssl/ssl.h>
#include <openssl/err.h>

static void websocket_readcb(struct bufferevent *bev, void *ctx);
static void websocket_eventcb(struct bufferevent *bev, short events, void *ctx);

int main(int argc, char **argv)
{
    struct event_base *base;
    struct bufferevent *bev;
    struct sockaddr_in sin;
    SSL_CTX *ssl_ctx;
    SSL *ssl;

    base = event_base_new();

    memset(&sin, 0, sizeof(sin));
    sin.sin_family = AF_INET;
    sin.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
    sin.sin_port = htons(8080);

    bev = bufferevent_socket_new(base, -1, BEV_OPT_CLOSE_ON_FREE);
    bufferevent_setcb(bev, websocket_readcb, NULL, websocket_eventcb, NULL);
    bufferevent_enable(bev, EV_READ | EV_WRITE);
    bufferevent_socket_connect(bev, (struct sockaddr *)&sin, sizeof(sin));

    ssl_ctx = SSL_CTX_new(TLSv1_2_client_method());
    ssl = SSL_new(ssl_ctx);
    bufferevent_openssl_set_ssl(bev, ssl, BUFFEREVENT_SSL_CONNECTING);
    bufferevent_openssl_set_allow_dirty_shutdown(bev, 1);
    SSL_set_tlsext_host_name(ssl, "localhost");
    
    // 发送WebSocket握手请求
    char request[1024];
    sprintf(request, "GET /websocket HTTP/1.1\r\n"
                      "Host: localhost:8080\r\n"
                      "Upgrade: websocket\r\n"
                      "Connection: Upgrade\r\n"
                      "Sec-WebSocket-Key: x3JJHMbDL1EzLkh9GBhXDw==\r\n"
                      "Sec-WebSocket-Version: 13\r\n"
                      "\r\n");
    bufferevent_write(bev, request, strlen(request));

    event_base_dispatch(base);

    SSL_shutdown(ssl);
    SSL_free(ssl);
    SSL_CTX_free(ssl_ctx);
    bufferevent_free(bev);
    event_base_free(base);

    return 0;
}

static void websocket_readcb(struct bufferevent *bev, void *ctx)
{
    struct evbuffer *input = bufferevent_get_input(bev);
    char *line = NULL;
    size_t len = 0;
    ssize_t nread;

    while ((nread = evbuffer_get_line(input, &line, &len, EVBUFFER_EOL_CRLF)) > 0) {
        printf("%s\n", line);
        free(line);
    }
}

static void websocket_eventcb(struct bufferevent *bev, short events, void *ctx)
{
    if (events & BEV_EVENT_CONNECTED) {
        printf("Connected.\n");
    } else if (events & BEV_EVENT_ERROR) {
        printf("Error.\n");
    } else if (events & BEV_EVENT_EOF) {
        printf("Connection closed.\n");
        event_base_loopbreak(bufferevent_get_base(bev));
    }
}

这个示例代码中，首先创建了一个event_base对象，然后创建了一个bufferevent对象，并设置了回调函数。接着，使用bufferevent_socket_connect函数连接到WebSocket服务器，然后创建了一个SSL_CTX对象和一个SSL对象，并使用bufferevent_openssl_set_ssl函数将SSL对象关联到bufferevent对象上。接下来，发送了一个WebSocket握手请求。最后，调用event_base_dispatch函数进入事件循环。

在事件循环中，如果接收到了数据，就调用websocket_readcb函数进行处理。如果连接建立成功，就输出Connected。如果发生错误，就输出Error。如果连接关闭，就输出Connection closed，并调用event_base_loopbreak函数退出事件循环。

需要注意的是，这个示例代码中使用了OpenSSL库，所以需要链接libssl和libcrypto库。在Linux系统中，可以使用以下命令编译：

$ gcc -o websocket-client websocket-client.c -levent -lssl -lcrypto