Cygwin 1.7.x on Windows 7: Exit statuses of Win32 executables are sometimes wrong

[John Dong]

Hi,

Cygwin on Windows 7, seems to exhibit a rather peculiar behavior: Sometimes the exit status of a Win32 process is incorrectly captured by Cygwin.

I'm running Cygwin 1.7.9(0.237/5/3) on Windows 7 64-bit, but I've reproduced this behavior with every release of Cygwin 1.7 on both 32-bit and 64-bit Windows 7. It does not seem to happen in XP 32-bit, and I've not tried any other environments.


To reproduce, first I wrote a Win32 console application (using Visual Studio 2010 / cl.exe version 16 as my compiler) that exits with the status the user passes in:

> int _tmain(int argc, _TCHAR* argv[])
> {
> 	int ret = _ttoi(argv[1]);
> 	_tprintf(_T("Exiting with %i\n"), ret); 
> 	
> 	return ret;
> }


Then, I wrote a shell script that called this executable ("exiter.exe") with argument 0 in an infinite loop:

> #!/bin/sh
> set -e
> while true; do
>  /cygdrive/c/exiter.exe 0
> echo $?
> done
> 

I expect this script to run forever, as the exit code should always be zero. However, after running this overnight, I see the script terminate:

> Exiting with 0
> 0
> Exiting with 0
> 0
> Exiting with 0
> 0
> Exiting with 0
> 0
> Exiting with 0
> 
> $ echo $?
> 1
> 



The last line of output implies that exiter.exe executed "return 0", but /bin/sh saw a nonzero exit status (of 1) and thus stopped execution due to -e.

Reproducing this seems nondeterministic -- sometimes I can get it to happen in 5 minutes, other times it takes overnight. I've tried using a different shell (like dash), but it doesn't make a difference, leading me to suspect this to be a lower-level issue within the Cygwin DLL. It also seems to not happen for non-zero exit codes (e.g. checking that exiter.exe 1 returns 1 always seems to succeed), though I'm not 100% confident that I've tested this thoroughly enough.

Again, I've not been able to reproduce this under Windows XP using any version of Cygwin, but I have been able to reproduce it on both 32-bit and 64-bit Windows 7. I'm not running anything special on this machine -- it's a fresh install of Windows 7 Professional, just with Cygwin installed.


Thanks in advance,

John


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