Compiling with MinGW won't help, this is a problem with the I/O routines. You 
might
be able to recompile with -DXD3_STDIO=1 and -DXD3_WIN32=0 to use the stdio.h I/O
routines, which might handle stdout better. I will investigate and fix.

Interestingly, it seems to work fine under MSYS's bash shell for Windows. It's 
not
cygwin. I'm using the official binary, I didn't compile my own yet.

FWIW, it's really easy to build & run on Cygin, and the stdout stuff will work.

[deleted comment]

The way to fix this issue is to replace the following lines in 
xdelta3-main.h:680

#define XSTDOUT_XF(f) { (f)->file = winStartupInfo.hStdOutput; (f)->filename =
"(stdout)"; }
#define XSTDIN_XF(f)  { (f)->file = winStartupInfo.hStdInput;  (f)->filename =
"(stdin)"; }

with

#define XSTDOUT_XF(f) { (f)->file = GetStdHandle(STD_OUTPUT_HANDLE); 
(f)->filename =
"(stdout)"; }
#define XSTDIN_XF(f)  { (f)->file = GetStdHandle(STD_INPUT_HANDLE);  
(f)->filename =
"(stdin)"; }

The STARTUPINFO structure is not the correct place to get the stdin or stdout 
handles
as the calling process is not required to provide any and the console or cmd 
shell do
not.

Fixed in SVN 247.  I haven't been able to build/verify this, as my windows 
laptop
broke a few months ago and I haven't replaced it.

3.0u