基本環境

使用版本為libevent-2.1.5，目前為beta版，其中evhttp和舊版區別在于新增了如下接口

// 設置回調函數，在包頭讀取完成后回調 void evhttp_request_set_header_cb (struct evhttp_request *, int(*cb)(struct evhttp_request *, void *))// 設置回調函數，在body有數據返回后回調 void evhttp_request_set_chunked_cb (struct evhttp_request *, void(*cb)(struct evhttp_request *, void *))

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這樣的好處是可以在合適的時機回調我們注冊的回調函數，比如下載1G的文件，在之前的版本只有下載完成后才會回調，現在每下載一部分數據就會回調一次，讓上層應用更加靈活，尤其在http代理時，可以做到邊下載邊回復

2.1.5版本的完整接口文檔詳見http://www.wangafu.net/~nickm/libevent-2.1/doxygen/html/http_8h.html

請求流程

http客戶端使用到的接口函數及請求流程如下

初始化event_base和evdns_base

struct event_base *event_base_new(void); struct evdns_base * evdns_base_new(struct event_base *event_base, int initialize_nameservers);

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創建evhttp_request對象，并設置回調函數，這里的回調函數是和數據接收相關的

struct evhttp_request *evhttp_request_new(void (*cb)(struct evhttp_request *, void *), void *arg); void evhttp_request_set_header_cb(struct evhttp_request *, int (*cb)(struct evhttp_request *, void *)); void evhttp_request_set_chunked_cb(struct evhttp_request *, void (*cb)(struct evhttp_request *, void *)); void evhttp_request_set_error_cb(struct evhttp_request *, void (*)(enum evhttp_request_error, void *));

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創建evhttp_connection對象，并設置回調函數，這里的回調函數是和連接狀態相關的

struct evhttp_connection *evhttp_connection_base_new(struct event_base *base, struct evdns_base *dnsbase, const char *address, unsigned short port); void evhttp_connection_set_closecb(struct evhttp_connection *evcon,void (*)(struct evhttp_connection *, void *), void *);

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有選擇的向evhttp_request添加包頭字段

int evhttp_add_header(struct evkeyvalq *headers, const char *key, const char *value);

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發送請求

int evhttp_make_request(struct evhttp_connection *evcon,struct evhttp_request *req,enum evhttp_cmd_type type, const char *uri);

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派發事件

int event_base_dispatch(struct event_base *);

完整代碼

#include "event2/http.h" #include "event2/http_struct.h" #include "event2/event.h" #include "event2/buffer.h" #include "event2/dns.h" #include "event2/thread.h"#include <stdio.h> #include <string.h> #include <assert.h> #include <sys/queue.h> #include <event.h>void RemoteReadCallback(struct evhttp_request* remote_rsp, void* arg) {event_base_loopexit((struct event_base*)arg, NULL); } int ReadHeaderDoneCallback(struct evhttp_request* remote_rsp, void* arg) {fprintf(stderr, "< HTTP/1.1 %d %s\n", evhttp_request_get_response_code(remote_rsp), evhttp_request_get_response_code_line(remote_rsp));struct evkeyvalq* headers = evhttp_request_get_input_headers(remote_rsp);struct evkeyval* header;TAILQ_FOREACH(header, headers, next){fprintf(stderr, "< %s: %s\n", header->key, header->value);}fprintf(stderr, "< \n");return 0; }void ReadChunkCallback(struct evhttp_request* remote_rsp, void* arg) {char buf[4096];struct evbuffer* evbuf = evhttp_request_get_input_buffer(remote_rsp);int n = 0;while ((n = evbuffer_remove(evbuf, buf, 4096)) > 0){fwrite(buf, n, 1, stdout);} }void RemoteRequestErrorCallback(enum evhttp_request_error error, void* arg) {fprintf(stderr, "request failed\n");event_base_loopexit((struct event_base*)arg, NULL); }void RemoteConnectionCloseCallback(struct evhttp_connection* connection, void* arg) {fprintf(stderr, "remote connection closed\n");event_base_loopexit((struct event_base*)arg, NULL); }int main(int argc, char** argv) {if (argc != 2){printf("usage:%s url", argv[1]);return 1;}char* url = argv[1];struct evhttp_uri* uri = evhttp_uri_parse(url);if (!uri){fprintf(stderr, "parse url failed!\n");return 1;}struct event_base* base = event_base_new();if (!base){fprintf(stderr, "create event base failed!\n");return 1;}struct evdns_base* dnsbase = evdns_base_new(base, 1);if (!dnsbase){fprintf(stderr, "create dns base failed!\n");}assert(dnsbase);struct evhttp_request* request = evhttp_request_new(RemoteReadCallback, base);evhttp_request_set_header_cb(request, ReadHeaderDoneCallback);evhttp_request_set_chunked_cb(request, ReadChunkCallback);evhttp_request_set_error_cb(request, RemoteRequestErrorCallback);const char* host = evhttp_uri_get_host(uri);if (!host){fprintf(stderr, "parse host failed!\n");return 1;}int port = evhttp_uri_get_port(uri);if (port < 0) port = 80;const char* request_url = url;const char* path = evhttp_uri_get_path(uri);if (path == NULL || strlen(path) == 0){request_url = "/";}printf("url:%s host:%s port:%d path:%s request_url:%s\n", url, host, port, path, request_url);struct evhttp_connection* connection = evhttp_connection_base_new(base, dnsbase, host, port);if (!connection){fprintf(stderr, "create evhttp connection failed!\n");return 1;}evhttp_connection_set_closecb(connection, RemoteConnectionCloseCallback, base);evhttp_add_header(evhttp_request_get_output_headers(request), "Host", host);evhttp_make_request(connection, request, EVHTTP_REQ_GET, request_url);event_base_dispatch(base);return 0; }

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編譯

g++ http_client.cpp -I/opt/local/libevent-2.1.5/include -L/opt/local/libevent-2.1.5/lib -levent -g -o http_client

運行示例，這里只打印了包頭字段

$ ./http_client http://www.qq.com >/dev/null < HTTP/1.1 200 OK < Server: squid/3.4.3 < Content-Type: text/html; charset=GB2312 < Cache-Control: max-age=60 < Expires: Fri, 05 Aug 2016 08:48:31 GMT < Date: Fri, 05 Aug 2016 08:47:31 GMT < Transfer-Encoding: chunked < Connection: keep-alive < Connection: Transfer-Encoding <

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