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;

}