CURLcode curl_easy_setopt(CURL *handle, CURLOPT_POST, long post);
Use one of CURLOPT_POSTFIELDS(3) or CURLOPT_COPYPOSTFIELDS(3) options to specify what data to post and CURLOPT_POSTFIELDSIZE(3) or CURLOPT_POSTFIELDSIZE_LARGE(3) to set the data size.
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 an 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. (Since 7.66.0, libcurl will automatically use chunked encoding for POSTs if the size is unknown.)
When setting CURLOPT_POST(3) to 1, libcurl will automatically set CURLOPT_NOBODY(3) and CURLOPT_HTTPGET(3) to 0.
If you issue a POST request and then want to make a HEAD or GET using the same re-used handle, you must explicitly set the new request type using CURLOPT_NOBODY(3) or CURLOPT_HTTPGET(3) or similar.
CURL *curl = curl_easy_init();
if(curl) {
curl_easy_setopt(curl, CURLOPT_URL, "https://example.com/foo.bin");
curl_easy_setopt(curl, CURLOPT_POST, 1L);
/* set up the read callback with CURLOPT_READFUNCTION */
ret = curl_easy_perform(curl);
curl_easy_cleanup(curl);
}