Why does my thread handle suddenly go bad? All I did was wait on it!
A customer reported that they had a very strange bug, where waiting on a thread handle causes it to become invalid. Here’s a code fragment:
DWORD waitResult = WaitForSingleObject(hThread, INFINITE); assert(waitResult == WAIT_OBJECT_0); // assertion passes DoSomeCleanup(); CloseHandle(hThread);
That final call to
CloseHandle results in a
HANDLE exception when run in the debugger. How did the handle become invalid? We sucessfully waited on it just a few lines earlier.
There wasn’t enough information to go on, so we had to make some guesses. Perhaps
hThread was already closed, and it got recycled to refer to some unrelated kernel object, and it’s that unrelated object that you’re waiting on when you call
WaitForSingleObject. And then when you do some cleanup, that causes the unrelated handle to be closed, which means that the numeric value of
hThread now refers to an invalid handle.
The customer did some investigation and discovered that they were obtaining the thread handle from the
_beginthread function. The handle returned by the
_beginthread function is explicitly documented as being closed by the
_endthreadautomatically closes the thread handle, whereas
_endthreadexdoes not. Therefore, when you use
_endthread, do not explicitly close the thread handle by calling the Win32
CloseHandleAPI. This behavior differs from the Win32
Basically, the deal is that the
_beginthread function returns a handle to the created thread, but does not give you ownership of the handle. Ownership of that handle remains with the thread itself, and the thread automatically closes the handle when it exits. (Not mentioned in the MSDN documentation for
_beginthread is that the runtime automatically calls
_endthread if the thread function returns normally. This detail is mentioned in the documentation for
_endthread, which is probably a better place for it anyway.)
Basically, the handle returned by the
_beginthread function is useless. You don’t know how long it’s valid, and it might even be invalid by the time you even receive it!
_endthreadex fixed the problem.