Re: CCriticalSection - does my thread already have a lock?

"Doug Harrison [MVP]" <>
Wed, 19 Jul 2006 16:10:29 -0500
On Wed, 19 Jul 2006 13:32:04 -0600, "Dan Baker" <dbmail> wrote:

I have a thread pool waiting on an IOCP. Several threads can wake-up at the
same time, and have received data from the exact same socket. I have the
data associated with the socket locked via a CCriticalSection. But, during
shutdown, I want to manipulate the data -- if it isn't locked down. I am
sharing a routine between these two thread-types (worker and main). This
routine can delete the object, IF the current thread has the lock -- if the
current thread does NOT have the lock, then it needs to flag the object for
later deletion.

So, the question: Can I query a CCriticalSection object to determine if the
current thread has a lock?

No, but you can query a CSingleLock object. You can do this because a
CSingleLock object is only to be used within a single thread, so thread
safety isn't an issue. The only way to query a CCriticalSection is to try
to acquire it, but you can't tell if you already have it, which sorta makes
the whole thing moot from your perspective. There is a function
TryEnterCriticalSection which may be of interest, but it is NT4 and above
only, and it doesn't allow you to distinguish between already having the
critical section and acquiring it anew. Note that CCriticalSection does
provide a Lock(timeout) function, but the only value for timeout it accepts
is INFINITE. A value of 0 would be the Lock equivalent to TryEnter, but the
Lock function will assert on it in debug builds, and in release builds,
zero is the same(!) as INFINITE.

But all is not lost. You could implement your design by using thread-local
storage. This is what I think your question boils down to: How do I
associate information with a thread ID? One way is thread-local storage;
another is a map<thread-id, info> protected by a mutex.

P.S. There may well be an undocumented way to determine the owner of a
CRITICAL_SECTION, but I'll let you research that on your own if you're
interested. :)

Doug Harrison
Visual C++ MVP

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