[libmicrohttpd] How much upload_data is uploaded to MHD_AccessHandlerCallback()?

[Bob Furber]

Can someone tell me how much upload_data is uploaded to 
MHD_AccessHandlerCallback()?

Can the upload_data uploaded to MHD_AccessHandlerCallback() ever be less 
than the lesser of all the data sent by the client or the size of the 
upload_data buffer?

In other words, if a POST request were to contain, say, 64 bytes and the 
upload_data buffer were larger, say, 256 bytes, could the upload_data 
buffer contain anything less than 64 bytes? Could one expect a short 
POST request to arrive in more than one packet?