使用libcurl的正确姿势

libcurl支持访问http、ftp等各种服务器，下载图片AV什么的不在话下。但其存在多种接口，异步接口也很难以理解，到底什么样的使用姿势才是正确滴？我们来看看可用的体位：

• easy interface：最简单的同步接口，容易理解，但同步访问实在不是性能之选。至于引入多线程，那是简单问题复杂化。注意异步访问也是以easy interface为基础，所以还是要学习一下：《libcurl教程》。
• multi interface：异步访问接口，性能杠杠滴，但是。。。真的很难理解啊。。。官方文档：《multi interface overview》。

  • curl_multi_perform() + select()：select()性能不够好，还受到file descriptors不能大于1024的限制。参考《使用libcurl进行异步并发访问与文件上传》。

  • curl_multi_socket_action()：使用epoll模型，性能最好，但更难懂。。。参考范例。

  • curl_multi_perform() + curl_multi_wait()：这个据说是facebook做出的伟大贡献（参见《Introducing curl_multi_wait》），保证性能的同时也相对容易使用，强力推荐的姿势。抄录示例代码如下：

    /* curl_multi_test.c
    Clemens Gruber, 2013
    clemens.gruber@pqgruber.com
    Code description:
    Requests 4 Web pages via the CURL multi interface
    and checks if the HTTP status code is 200.
    Update: Fixed! The check for !numfds was the problem.
    */

    include <stdio.h>

    include <stdlib.h>

    ifndef WIN32

    include <unistd.h>

    endif

    include <curl/multi.h>

    define MAX_WAIT_MSECS 301000 / Wait max. 30 seconds */

    static const char *urls[] = {
    "http://www.microsoft.com",
    "http://www.yahoo.com",
    "http://www.wikipedia.org",
    "http://slashdot.org"
    };
    #define CNT 4

    static size_t cb(char *d, size_t n, size_t l, void p)
    {
    / take care of the data here, ignored in this example /
    (void)d;
    (void)p;
    return nl;
    }

    static void init(CURLM *cm, int i)
    {
    CURL *eh = curl_easy_init();
    curl_easy_setopt(eh, CURLOPT_WRITEFUNCTION, cb);
    curl_easy_setopt(eh, CURLOPT_HEADER, 0L);
    curl_easy_setopt(eh, CURLOPT_URL, urls[i]);
    curl_easy_setopt(eh, CURLOPT_PRIVATE, urls[i]);
    curl_easy_setopt(eh, CURLOPT_VERBOSE, 0L);
    curl_multi_add_handle(cm, eh);
    }

    int main(void)
    {
    CURLM *cm=NULL;
    CURL *eh=NULL;
    CURLMsg *msg=NULL;
    CURLcode return_code=0;
    int still_running=0, i=0, msgs_left=0;
    int http_status_code;
    const char *szUrl;

    curl_global_init(CURL_GLOBAL_ALL);

    cm = curl_multi_init();

    for (i = 0; i < CNT; ++i) {
    init(cm, i);
    }

    curl_multi_perform(cm, &still_running);

    do {
    int numfds=0;
    int res = curl_multi_wait(cm, NULL, 0, MAX_WAIT_MSECS, &numfds);
    if(res != CURLM_OK) {
    fprintf(stderr, "error: curl_multi_wait() returned %d\n", res);
    return EXIT_FAILURE;
    }
    /*
    if(!numfds) {
    fprintf(stderr, "error: curl_multi_wait() numfds=%d\n", numfds);
    return EXIT_FAILURE;
    }
    */
    curl_multi_perform(cm, &still_running);

    } while(still_running);

    while ((msg = curl_multi_info_read(cm, &msgs_left))) {
    if (msg->msg == CURLMSG_DONE) {
    eh = msg->easy_handle;

    return_code = msg->data.result;
    if(return_code!=CURLE_OK) {
    fprintf(stderr, "CURL error code: %d\n", msg->data.result);
    continue;
    }

    // Get HTTP status code
    http_status_code=0;
    szUrl = NULL;

    curl_easy_getinfo(eh, CURLINFO_RESPONSE_CODE, &http_status_code);
    curl_easy_getinfo(eh, CURLINFO_PRIVATE, &szUrl);

    if(http_status_code==200) {
    printf("200 OK for %s\n", szUrl);
    } else {
    fprintf(stderr, "GET of %s returned http status code %d\n", szUrl, http_status_code);
    }

    curl_multi_remove_handle(cm, eh);
    curl_easy_cleanup(eh);
    }
    else {
    fprintf(stderr, "error: after curl_multi_info_read(), CURLMsg=%d\n", msg->msg);
    }
    }

    curl_multi_cleanup(cm);

    return EXIT_SUCCESS;
    }