CURLcode curl_easy_setopt(CURL *handle, CURLOPT_POSTFIELDS, char *postdata);
The data pointed to is NOT copied by the library: as a consequence, it must be preserved by the calling application until the associated transfer finishes. This behavior can be changed (so libcurl does copy the data) by setting the CURLOPT_COPYPOSTFIELDS(3) option.
This POST is a normal application/x-www-form-urlencoded kind (and libcurl will set that Content-Type by default when this option is used), which is commonly used by HTML forms. Change Content-Type with CURLOPT_HTTPHEADER(3).
You can use curl_easy_escape(3) to url-encode your data, if necessary. It returns a pointer to an encoded string that can be passed as postdata.
Using CURLOPT_POSTFIELDS(3) implies setting CURLOPT_POST(3) to 1.
If CURLOPT_POSTFIELDS(3) is explicitly set to NULL then libcurl will get the POST data from the read callback. If you want to send a zero-byte POST set CURLOPT_POSTFIELDS(3) to an empty string, or set CURLOPT_POST(3) to 1 and CURLOPT_POSTFIELDSIZE(3) to 0.
Using POST with HTTP 1.1 implies the use of a "Expect: 100-continue" header, and libcurl will add that header automatically if the POST is either known to be larger than 1MB or if the expected size is unknown. You can disable this header with CURLOPT_HTTPHEADER(3) as usual.
To make multipart/formdata posts (aka RFC2388-posts), check out the CURLOPT_HTTPPOST(3) option combined with curl_formadd(3).
CURL *curl = curl_easy_init();
if(curl) {
const char *data = "data to send";
curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
/* size of the POST data */
curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE, 12L);
/* pass in a pointer to the data - libcurl will not copy */
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, data);
curl_easy_perform(curl);
}