Writing LARGE FILES

I am trying to compile a C program that can read and write LARGE FILEs  
(> 2GB) to IFS.  I am using the fopen/fread/fwrite/fclose functions and
compiled with the *IFS64IO option.  The fread() can read the large file,
but fwrite() still fails when it reaches 2GB.  I even tried explicitly
using fopen64(), etc.  Still No Joy.  All that happens if I specify
'#define __IFS64_IO__' is a warning that I can't redefine the macro.  

I even called IBM Software Support, and they currently have no ideas
about why this is happening.

I'd like this to work on V4R5, but would be satisfied with V5R1+.  

Has ANYONE gotten a C routine to write an IFS file larger than 2GB, and
how did you do it?

Thanks