curl : curl_easy_setopt

       CURLOPT_READFUNCTION
              Function pointer that should match the following prototype: size_t function( void *ptr,size_t size, size_t nmemb, void *stream); This function gets called by libcurl as  soonas  it  needs to read data in order to send it to the peer. The data area pointed at bythe pointer ptr may be filled with at most size multiplied with nmemb number of  bytes.Your  function  must  return  the actual number of bytes that you stored in that memoryarea. Returning 0 will signal end-of-file to the library and cause it to stop the  curârent transfer.

       CURLOPT_READDATA
              Data  pointer  to  pass  to the file read function. If you use the CURLOPT_READFUNCTIONoption, this is the pointer you'll get as input. If you don't specify a  read  callbackbut instead rely on the default internal read function, this data must be a valid readâable FILE *.