使用libcurl進行文件上傳

上篇博文講到了如何使用multicurl來進行http並發訪問，今天繼續有關curl的主題，來八一八如何使用curl來上傳文件，在介紹具體方法之前了解下目前http文件上傳的基本實現。

    rfc1867描述了如何使用http協議來上傳客戶端文件，目前基本上所有的瀏覽器和web服務器都支持http文件上傳，它的使用也十分的簡單，具體的來說就是在頁面上創建一個form表單，表單的enctype屬性為multipart/form-data，action為接收上傳文件的cgi url，請求方式為post，在表單中添加type屬性為file的input，file input里面選擇需要上傳的文件，選擇好后點擊submit，服務器端收到multipart post請求后，會根據相關協議解析請求，然后保存上傳的文件內容，Multipart表單示例：

<form enctype="multipart/form-data" action="http://host:port/UploadFile" method=post>
    Upload :<br>
    <input name="userfile" type="file"><br>
    text field :<input type="text" name="text" value="text"><br>
    <input type="submit" value="提交"><input type=reset>
</form>

    好了，現在來講一講curl的文件上傳，對於curl來講，其實它要完成的任務就是構建一個multipart/formdata HTTP POST請求。類似於往multipart form表單中添加type為file或者text的input item一樣，curl也需要我們構造表單中的input item，curl_formadd函數可以幫助我們完成這個任務，它即可以添加普通的name-value section，也可以添加file upload section，下面舉幾個具體例子：

1、添加name/content section

curl_formadd(&post, &last, CURLFORM_COPYNAME, "name",   CURLFORM_COPYCONTENTS, "content", CURLFORM_END);

2、添加name/content/contenttype section

curl_formadd(&post, &last, CURLFORM_COPYNAME, "name",   CURLFORM_COPYCONTENTS, "content",   CURLFORM_CONTENTTYPE, "type", CURLFORM_END);

3、添加 file/filename section

curl_formadd(&post, &last, CURLFORM_COPYNAME, "pic",   CURLFORM_FILE, "demo.jpg", CURLFORM_FILENAME, "upload.pic", CURLFORM_END);

4、添加file/contenttype section

curl_formadd(&post, &last, CURLFORM_COPYNAME, "pic",   CURLFORM_FILE, "demo.jpg", CURLFORM_FILENAME, "upload.pic",  CURLFORM_CONTENTTYPE, "image/jpeg", CURLFORM_END);

     上面的post 和 last都是指向curl_httppost對象的指針， post指向的就是一個由所有section組成的鏈表的開端，last是該鏈表的尾指針。當我們添加完所有的form section之后，使用curl_easy_setopt(curl, CURLOPT_HTTPPOST,post)函數設置curl的http post，最后就是調用curl_easy_perform執行請求。需要注意的是，當使用libcurl的POST方式時，如果POST數據的大小大於1024個字節，libcurl不會直接發送POST請求，而是會分為兩步執行請求：
    1、發送一個請求，該請求頭部包含一個Expect： 100-continue的字段，用來詢問server是否願意接受數據
    2、當接收到從server返回的100-continue的應答后，它才會真正的發起POST請求，將數據發送給server。
    對於文件上傳來說，文件大小往往會超過1024個字節，所以如果你確認你的服務器不會拒絕你的文件上傳請求的話，可以禁止curl的Expect請求頭，具體方法可以去看看我的另外一篇文章《libcurl的使用問題“Expect100-continue” 》。

    最后附上curl官網上提供的文件上傳例子:

/* This is an example application source code using the multi interface
 * to do a multipart formpost without "blocking". */
#include <stdio.h>
#include <string.h>
#include <sys/time.h>

#include <curl/curl.h>

int main(void)
{
  CURL *curl;

  CURLM *multi_handle;
  int still_running;

  struct curl_httppost *formpost=NULL;
  struct curl_httppost *lastptr=NULL;
  struct curl_slist *headerlist=NULL;
  static const char buf[] = "Expect:";

  /* Fill in the file upload field. This makes libcurl load data from
     the given file name when curl_easy_perform() is called. */
  curl_formadd(&formpost,
               &lastptr,
               CURLFORM_COPYNAME, "sendfile",
               CURLFORM_FILE, "postit2.c",
               CURLFORM_END);

  /* Fill in the filename field */
  curl_formadd(&formpost,
               &lastptr,
               CURLFORM_COPYNAME, "filename",
               CURLFORM_COPYCONTENTS, "postit2.c",
               CURLFORM_END);

  /* Fill in the submit field too, even if this is rarely needed */
  curl_formadd(&formpost,
               &lastptr,
               CURLFORM_COPYNAME, "submit",
               CURLFORM_COPYCONTENTS, "send",
               CURLFORM_END);

  curl = curl_easy_init();
  multi_handle = curl_multi_init();

  /* initalize custom header list (stating that Expect: 100-continue is not
     wanted */
  headerlist = curl_slist_append(headerlist, buf);
  if(curl && multi_handle) {

    /* what URL that receives this POST */
    curl_easy_setopt(curl, CURLOPT_URL, "http://www.example.com/upload.cgi");
    curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);

    curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headerlist);
    curl_easy_setopt(curl, CURLOPT_HTTPPOST, formpost);

    curl_multi_add_handle(multi_handle, curl);

    curl_multi_perform(multi_handle, &still_running);

    do {
      struct timeval timeout;
      int rc; /* select() return code */

      fd_set fdread;
      fd_set fdwrite;
      fd_set fdexcep;
      int maxfd = -1;

      long curl_timeo = -1;

      FD_ZERO(&fdread);
      FD_ZERO(&fdwrite);
      FD_ZERO(&fdexcep);

      /* set a suitable timeout to play around with */
      timeout.tv_sec = 1;
      timeout.tv_usec = 0;

      curl_multi_timeout(multi_handle, &curl_timeo);
      if(curl_timeo >= 0) {
        timeout.tv_sec = curl_timeo / 1000;
        if(timeout.tv_sec > 1)
          timeout.tv_sec = 1;
        else
          timeout.tv_usec = (curl_timeo % 1000) * 1000;
      }

      /* get file descriptors from the transfers */
      curl_multi_fdset(multi_handle, &fdread, &fdwrite, &fdexcep, &maxfd);

      /* In a real-world program you OF COURSE check the return code of the
         function calls.  On success, the value of maxfd is guaranteed to be
         greater or equal than -1.  We call select(maxfd + 1, ...), specially in
         case of (maxfd == -1), we call select(0, ...), which is basically equal
         to sleep. */

      rc = select(maxfd+1, &fdread, &fdwrite, &fdexcep, &timeout);

      switch(rc) {
      case -1:
        /* select error */
        break;
      case 0:
      default:
        /* timeout or readable/writable sockets */
        printf("perform!\n");
        curl_multi_perform(multi_handle, &still_running);
        printf("running: %d!\n", still_running);
        break;
      }
    } while(still_running);

    curl_multi_cleanup(multi_handle);

    /* always cleanup */
    curl_easy_cleanup(curl);

    /* then cleanup the formpost chain */
    curl_formfree(formpost);

    /* free slist */
    curl_slist_free_all (headerlist);
  }
  return 0;
}