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Re: Intermittent failures retrieving process exit codes

I spent most of the week debugging this issue. This appears to be a defect in Windows. I can reproduce the issue without Cygwin. I can't rule out other third party kernel mode software possibly contributing to the issue. A simple change to Cygwin works around the problem for me.

I don't know which Windows releases are affected by this. I've only reproduced the problem (outside of Cygwin) with Wow64 processes running on 64-bit Windows 7. I haven't yet tried elsewhere.

The problem appears to be a race condition involving concurrent calls to TerminateProcess() and ExitThread(). The example code below minimally mimics the threads created and exit process/thread calls that are performed when running Cygwin's false.exe. The primary thread exits the process via TerminateProcess() ala pinfo::exit() in winsup/cygwin/ The secondary thread exits itself via ExitThread() ala Cygwin's signal processing thread function, wait_sig(), in winsup/cygwin/

When the race condition results in the undesirable outcome, the exit code for the process is set to the exit code for the secondary thread's call to ExitThread(). I can only speculate at this point, but my guess is that the TerminateProcess() code disassociates the calling thread from the process before other threads are stopped such that ExitThread(), concurrently running in another thread, may determine that the calling thread is the last thread of the process and overwrite the process exit code.

The issue also reproduces if ExitProcess() is called in place of TerminateProcess(). The test case below only uses TerminateProcess() because that is what Cygwin does.

Source code to reproduce the issue follows. Again, Cygwin is not required to reproduce the problem. For my own testing, I compiled the code using Microsoft's Visual Studio 2010 x86 compiler with the command 'cl /Fetest-exit-code.exe test-exit-code.cpp'


#include <windows.h>
#include <stdio.h>
#include <stdlib.h>

DWORD WINAPI SecondaryThread(
    LPVOID lpParameter)

int main() {
    HANDLE hSecondaryThread = CreateThread(
        NULL,                               // lpThreadAttributes
        0,                                  // dwStackSize
        SecondaryThread,                    // lpStartAddress
        (LPVOID)0,                          // lpParameter
        0,                                  // dwCreationFlags
        NULL);                              // lpThreadId
    if (!hSecondaryThread) {
        fprintf(stderr, "CreateThread failed.  GLE=%lu\n",
            (unsigned long)GetLastError());


    if (!TerminateProcess(GetCurrentProcess(), 1)) {
        fprintf(stderr, "TerminateProcess failed.  GLE=%lu\n",
            (unsigned long)GetLastError());

    return 0;

To run the test, a simple .bat file is used:


@echo off

echo test...
    echo test-exit-code.exe returned %ERRORLEVEL%
    exit /B 1
goto loop

test.bat should run indefinitely. The amount of time it takes to fail on my machine (64-bit Windows 7 running in a VMware Workstation 8 VM under Kubuntu 12.04 on a Lenovo T420 Intel i7-2640M 2 processor laptop) varies considerably. I had one run fail in less than 10 iterations, but most of the time it has taken upwards of 5 minutes to get a failure.

The workaround I implemented within Cygwin was simple and sloppy. I added a call to Sleep(1000) immediately before the call to ExitThread() in wait_sig() in winsup/cygwin/ Since this thread (probably) doesn't exit until the process is exiting anyway, the call to Sleep() does not adversely affect shutdown. The thread just gets terminated while in the call to Sleep() instead of exiting before the process is terminated or getting terminated while still in the call to ExitThread(). A better solution might be to avoid the thread exiting at all (so long as it can't get terminated while holding critical resources), or to have the process exiting thread wait on it. Neither of these is ideal. Orderly shutdown of multi-threaded processes is really hard to do correctly on Windows.

Since the exit code for the signal processing thread is not used, having the wait_sig() thread (and any other threads that could potentially concurrently exit with another thread) exit with a special status value such as STATUS_THREAD_IS_TERMINATING (0xC000004BL) would enable diagnosis of this issue as any process exit code matching this would be a likely indicator that this issue was encountered.

As is, when this race condition results in the undesirable outcome, since the signal processing thread exits with a status of 0, the exit status of the process is 0. This explains why false.exe works so well to reproduce the issue. It would be impossible to produce a negative test using true.exe.


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