What is sizeof(std::streamsize) ?

Hi,

  I was trying to work around the #define _LARGEFILE_SOURCE/
_FILE_OFFSET_BITS... nightmare to get large file sup****t in C,
thinking that things were nicely done in C++. But at least on my
compiler the size of std::streamsize is 4 !
  How am I supposed to deal with large file in this case ?

Thanks,
-Mathieu
Ps: for some reason a google search did not reveal anything
interesting...