CURLOPT_READDATA (3) - Linux Man Pages
CURLOPT_READDATA: custom pointer passed to the read callback
NAMECURLOPT_READDATA - custom pointer passed to the read callback
DESCRIPTIONData pointer to pass to the file read function. If you use the CURLOPT_READFUNCTION(3) option, this is the pointer you'll get as input in the 4th argument to the callback.
If you don't specify a read callback but instead rely on the default internal read function, this data must be a valid readable FILE * (cast to 'void *').
If you're using libcurl as a win32 DLL, you MUST use a CURLOPT_READFUNCTION(3) if you set this option.