Called TlsFree but it's still there!

I called TlsFree after storing something in the Thread Local Storage and when I call TlsGetValue(...) it returns the object! Why is this?

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Originally Posted by 6tr6tr

I called TlsFree after storing something in the Thread Local Storage and when I call TlsGetValue(...) it returns the object! Why is this?

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Deallocating a resource doesn't necessarily destroy the "bits" of that resource. So it still might appear to be there, even though it isn't. Accessing it is still wrong.

As an analogy, imagine that you've been banned from campus. That doesn't mean you can't go on campus, it just means you'll get in trouble if you get caught :)

The real question of the day is why are you trying to use a freed buffer? Or were you just curious.

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Originally Posted by master5001

The real question of the day is why are you trying to use a freed buffer? Or were you just curious.

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Mostly just curious but I also want people using the code to not accidently be able to call get again and have it work during their tests and then have it lead to issues later. How would I be sure to free/delete that data?

Quote:

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Originally Posted by 6tr6tr

Mostly just curious but I also want people using the code to not accidently be able to call get again and have it work during their tests and then have it lead to issues later. How would I be sure to free/delete that data?

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It *is* freed. The values of the bits are still there, however. If you want, you could clear the buffer to zero (or some other value indicating "been freed"), BEFORE freeing it.

Accessing freed storage is a common error, which often goes undetected for exactly this reason. Some runtimes will mark storage with special byte values before freeing it, when operating in debug mode, to try to catch these errors.

As explained above, the free function doesn't actually remove the data in the entry, it just says "this index can be used again".

Something like this seems like a good idea:

Code:

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void myTlsFree(long &index)
{
    TlsSetValue(index, 0);
    TlsFree(index);
    index = 0;
}

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Mats

Quote:

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Originally Posted by brewbuck

Accessing freed storage is a common error

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really? how common? Never seen it happen except on forums like this where people are trying to learn the language

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Originally Posted by KIBO

really? how common? Never seen it happen except on forums like this where people are trying to learn the language

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I've found bugs like that in commercial code. Perhaps not frequently, but it certainly happens.

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Mats

There were many programs that broke on the transition to Win95 because it was far less forgiving about accessing freed memory than Win 3.x.