Use with [curl->set]. The parameter which should be the full data to post in a HTTP POST operation. You must make sure that the data is formatted the way you want the server to receive it. [curl] will not convert or encode it for you. Most web servers will assume this data to be url-encoded.

This POST is a normal application/x-www-form-urlencoded kind (and [curl] will set that Content-Type by default when this option is used), which is the most commonly used one by HTML forms. See also the [CURLOPT_POST]. Using [CURLOPT_POSTFIELDS] implies [CURLOPT_POST].

If you want to do a zero-byte POST, you need to set [CURLOPT_POSTFIELDSIZE] explicitly to zero, as simply setting [CURLOPT_POSTFIELDS] to NULL or "" just effectively disables the sending of the specified string. [curl] will instead assume that you'll send the POST data using the read callback.

To make multipart/formdata posts (aka RFC 2388-posts), check out the [CURLOPT_HTTPPOST] option.