HTTP POSTing
We get many questions regarding how to issue HTTP POSTs with libcurl the proper way. This chapter will thus include examples using both different versions of HTTP POST that libcurl supports.
The first version is the simple POST, the most common version, that most HTML pages using the <form> tag uses. We provide a pointer to the data and tell libcurl to post it all to the remote site:
第一个版本是最简单的Post，最通用的版本，大多数HTML网页所用的<form>方式。
我们提供一个指向数据的指针，然后告诉Libcurl把这个指针中的数据post到远端。
char *data="name=daniel&project=curl";
curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDS, data);
curl_easy_setopt(easyhandle, CURLOPT_URL, "http://posthere.com/");
curl_easy_perform(easyhandle); /* post away! */
Simple enough, huh? Since you set the POST options with the CURLOPT_POSTFIELDS, this automatically switches the handle to use POST in the upcoming request.
当你使用 CURLOPT_POSTFIELDS 设置了post选项，接下来该句柄的所有请求都会使用这种post方式。
Ok, so what if you want to post binary data that also requires you to set the Content-Type: header of the post? Well, binary posts prevent libcurl from being able to do strlen() on the data to figure out the size, so therefore we must tell libcurl the size of the post data. Setting headers in libcurl requests are done in a generic way, by building a list of our own headers and then passing that list to libcurl.
如果想要post二进制数据，需要设置Content-Type，需要告诉Libcurl数据的长度，
struct curl_slist *headers=NULL;
headers = curl_slist_append(headers, "Content-Type: text/xml");
/* post binary data */
curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDS, binaryptr);
/* set the size of the postfields data */
curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDSIZE, 23L);
/* pass our list of custom made headers */
curl_easy_setopt(easyhandle, CURLOPT_HTTPHEADER, headers);
curl_easy_perform(easyhandle); /* post away! */
curl_slist_free_all(headers); /* free the header list */
While the simple examples above cover the majority of all cases where HTTP POST operations are required, they don‘t do multi-part formposts. Multi-part formposts were introduced as a better way to post (possibly large) binary data and were first documented in the RFC 1867 (updated in RFC2388). They‘re called multi-part because they‘re built by a chain of parts, each part being a single unit of data. Each part has its own name and contents. You can in fact create and post a multi-part formpost with the regular libcurl POST support described above, but that would require that you build a formpost yourself and provide to libcurl. To make that easier, libcurl provides curl_formadd. Using this function, you add parts to the form. When you‘re done adding parts, you post the whole form.
curl_easy方式的Post已经满足了大多数场景的需求，但是multi-part可以更好的post。
叫做multi-part是因为他们又一个链表组成，每个part是一个单独的数据单元。
每个part有自己的名字和内容。
你可以自己建立一个formpost，然后提供给Libcurl调用。
但为了让这个过程更简单，libcurl提供了curl_formadd接口。
使用这个函数，你可以添加part到form中，
添加完成后，再把整个form发送出去。
The following example sets two simple text parts with plain textual contents, and then a file with binary contents and uploads the whole thing.
struct curl_httppost *post=NULL;
struct curl_httppost *last=NULL;
curl_formadd(&post, &last,
CURLFORM_COPYNAME, "name",
CURLFORM_COPYCONTENTS, "daniel", CURLFORM_END);
curl_formadd(&post, &last,
CURLFORM_COPYNAME, "project",
CURLFORM_COPYCONTENTS, "curl", CURLFORM_END);
curl_formadd(&post, &last,
CURLFORM_COPYNAME, "logotype-image",
CURLFORM_FILECONTENT, "curl.png", CURLFORM_END);
/* Set the form info */
curl_easy_setopt(easyhandle, CURLOPT_HTTPPOST, post);
curl_easy_perform(easyhandle); /* post away! */
/* free the post data again */
curl_formfree(post);
Multipart formposts are chains of parts using MIME-style separators and headers. It means that each one of these separate parts get a few headers set that describe the individual content-type, size etc. To enable your application to handicraft this formpost even more, libcurl allows you to supply your own set of custom headers to such an individual form part. You can of course supply headers to as many parts as you like, but this little example will show how you set headers to one specific part when you add that to the post handle:
Multipart formposts是由使用MIME类型分隔和头部的多个部分组成的链表，
这意味着链表的每个部分都有自己的头部来描述自己的数据类型、长度等
libcurl允许你设置自定义的头部
下面的例子演示如何指定头部信息：
struct curl_slist *headers=NULL;
headers = curl_slist_append(headers, "Content-Type: text/xml");
curl_formadd(&post, &last,
CURLFORM_COPYNAME, "logotype-image",
CURLFORM_FILECONTENT, "curl.xml",
CURLFORM_CONTENTHEADER, headers,
CURLFORM_END);
curl_easy_perform(easyhandle); /* post away! */
curl_formfree(post); /* free post */
curl_slist_free_all(headers); /* free custom header list */
Since all options on an easyhandle are "sticky", they remain the same until changed even if you do call curl_easy_perform, you may need to tell curl to go back to a plain GET request if you intend to do one as your next request. You force an easyhandle to go back to GET by using the CURLOPT_HTTPGET option:
虽然所有easyhandle中的属性都是粘性的（指的是除非重新设置，否则不会变更）。
在以上的代码中，Http method已经被改为了post，如果想要重新使用get，可以使用如下代码重新设置：
curl_easy_setopt(easyhandle, CURLOPT_HTTPGET, 1L);
Just setting CURLOPT_POSTFIELDS to "" or NULL will *not* stop libcurl from doing a POST. It will just make it POST without any data to send!
如果只是设置 CURLOPT_POSTFIELDS 为 ""或者 NULL，Libcurl仍然会继续使用post方式。
只不过Post的时候不带任何数据