使用libcurl进行文件上传

上篇博文讲到了如何使用multicurl来进行http并发访问，今天继续有关curl的主题，来八一八如何使用curl来上传文件，在介绍具体方法之前了解下目前http文件上传的基本实现。

    rfc1867描述了如何使用http协议来上传客户端文件，目前基本上所有的浏览器和web服务器都支持http文件上传，它的使用也十分的简单，具体的来说就是在页面上创建一个form表单，表单的enctype属性为multipart/form-data，action为接收上传文件的cgi url，请求方式为post，在表单中添加type属性为file的input，file input里面选择需要上传的文件，选择好后点击submit，服务器端收到multipart post请求后，会根据相关协议解析请求，然后保存上传的文件内容，Multipart表单示例：

<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，现在来讲一讲curl的文件上传，对于curl来讲，其实它要完成的任务就是构建一个multipart/formdata HTTP POST请求。类似于往multipart form表单中添加type为file或者text的input item一样，curl也需要我们构造表单中的input item，curl_formadd函数可以帮助我们完成这个任务，它即可以添加普通的name-value section，也可以添加file upload section，下面举几个具体例子：

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对象的指针， post指向的就是一个由所有section组成的链表的开端，last是该链表的尾指针。当我们添加完所有的form section之后，使用curl_easy_setopt(curl, CURLOPT_HTTPPOST,post)函数设置curl的http post，最后就是调用curl_easy_perform执行请求。需要注意的是，当使用libcurl的POST方式时，如果POST数据的大小大于1024个字节，libcurl不会直接发送POST请求，而是会分为两步执行请求：
    1、发送一个请求，该请求头部包含一个Expect： 100-continue的字段，用来询问server是否愿意接受数据
    2、当接收到从server返回的100-continue的应答后，它才会真正的发起POST请求，将数据发送给server。
    对于文件上传来说，文件大小往往会超过1024个字节，所以如果你确认你的服务器不会拒绝你的文件上传请求的话，可以禁止curl的Expect请求头，具体方法可以去看看我的另外一篇文章《libcurl的使用问题“Expect100-continue” 》。

    最后附上curl官网上提供的文件上传例子:

/* 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;
}