CURLcode curl_easy_setopt(CURL *handle, CURLOPT_POST, long post);
DESCRIPTIONA parameter set to 1 tells libcurl to do a regular HTTP post. This will also make the library use a "Content-Type: application/x-www-form-urlencoded" header. (This is by far the most commonly used POST method).
Optionally, you can provide data to POST using the CURLOPT_READFUNCTION(3) and CURLOPT_READDATA(3) options but then you must make sure to not set CURLOPT_POSTFIELDS(3) to anything but NULL. When providing data with a callback, you must transmit it using chunked transfer-encoding or you must set the size of the data with the CURLOPT_POSTFIELDSIZE(3) or CURLOPT_POSTFIELDSIZE_LARGE(3) options. To enable chunked encoding, you simply pass in the appropriate Transfer-Encoding header, see the post-callback.c example.
You can override the default POST Content-Type: header by setting your own with CURLOPT_HTTPHEADER(3).
Using POST with HTTP 1.1 implies the use of a "Expect: 100-continue" header. You can disable this header with CURLOPT_HTTPHEADER(3) as usual.
If you use POST to a HTTP 1.1 server, you can send data without knowing the size before starting the POST if you use chunked encoding. You enable this by adding a header like "Transfer-Encoding: chunked" with CURLOPT_HTTPHEADER(3). With HTTP 1.0 or without chunked transfer, you must specify the size in the request.