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I am following the examples from "Android NDK Game Development Cookbook" for creating a cross-platform thread wrapper for use in my own Android NDK game engine. In the example's Thread class, at a certain point, a check is done to see if a thread handle corresponds to the current thread. This is done as follows:

if ( GetCurrentThread() != FThreadHandle )

The FThreadHandle handle was assigned using the follwing call earlier on:

#if defined(_WIN32)
    FThreadHandle = ( uintptr_t )_beginthreadex( NULL, 0, &ThreadStaticEntryPoint, ThreadParam, 0, &ThreadID );
#else
    pthread_create( &FThreadHandle, NULL, ThreadStaticEntryPoint, ThreadParam );
#endif

And the getCurrentThread function is defined as follows:

native_thread_handle_t iThread::GetCurrentThread()
{
#if defined( _WIN32)
    return GetCurrentThreadId();
#elif defined( ANDROID )
    return gettid();
#else
    return pthread_self();
#endif
}

My question is: Is this function implementation correct? I have 3 major concerns about this implementation:

  • I would expect that for _WIN32, the GetCurrentThread function would be used instead of GetCurrentThreadId, since we want the handle of the thread and not its id.
  • Furthermore, why is gettid() used for android instead of pthread_self()? Wouldn't I get the wrong id using gettid (since this is a system wide thread id)?
  • And lastly, is it legal to just use the != operator to compare pthread ids? Shouldn't pthread_equal be used instead?
ChrisV
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    This does seem broken -- there's no reason for it to be using gettid() rather than pthread_self() on Android. The pthread_equal() man page is pretty unequivocal on your third point, so I think the code you've shown is just wrong. – fadden Sep 25 '14 at 17:43
  • I thought as much. I think I'm going to write a not so positive review on Amazon about this book. They just give you the code, an example of how to use the classes, and then they just tell you: Well, you go figure it out for yourself. – ChrisV Sep 26 '14 at 07:45
  • Related to `gettid()` vs. `pthread_self()`: http://stackoverflow.com/q/6372102/694576 – alk Sep 30 '14 at 10:36

1 Answers1

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I will just comment on the GetCurrentThread (Win32) part.

It says:

"Retrieves a pseudo handle for the calling thread."

In other words, if you call it from different threads, each thread may receive the same, bogus, handle. Or, in other words, it is possible that the returned value is simply a constant value that is recognized by the Win32 API as a placeholder that represents the current thread.

I'm not sure what needs to be done in order to get a genuine, unique handle for the thread.

I am looking at OpenThread (Win32) function, but I haven't tried it myself.

One thing to remember is that on Windows, "handles" are resources that needs to be cleaned up after use. For each "OpenSomething" function, read the documentation carefully to see if you need to call a "CloseSomething" after use. Also, don't assume that the handle release function to always have the same name as the creation/duplication function. Failure to do so may destabilize the system.

Last but not the least: try search on The Old New Thing.

rwong
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