CURLOPT_READDATA (3) - Linux Manuals
CURLOPT_READDATA: custom pointer passed to the read callback
NAME
CURLOPT_READDATA - custom pointer passed to the read callbackSYNOPSIS
#include <curl/curl.h>CURLcode curl_easy_setopt(CURL *handle, CURLOPT_READDATA, void *pointer);
DESCRIPTION
Data 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.