Clean-up functions can't fail because, well, how do you clean up from a failed clean-up?

Raymond Chen

Commenter Matt asks how you’re supposed to handle failures in functions like fclose or CloseHandle. Obviously, you can’t. If a clean-up function fails, there’s not much you can do because, well, how do you clean up from a failed clean-up? These clean-up functions fall into the category of “Must not fail for reasons beyond the program’s control.” If a program tries to close a file and it gets an error back, what can it do? Practically speaking, nothing. The only way a clean-up function can fail is if the program fundamentally screws up, say by attempting to close something that was never open or otherwise passing an invalid parameter. It’s not like a program can try to close the handle again (or worse go into loop closing the handle repeatedly until it finally closes). Remember this when writing your own clean-up functions. Assuming the parameters are valid, a clean-up function must succeed. (I will now ruin my rhetorical flourish by yammering about the fclose function because if I don’t, people will bring it up in the comments anyway. The fclose function does extra work before closing, and that extra work may indeed run into problems, but the important thing is that when fclose returns, the stream is well and truly closed.)

Addendum: Once again I wish to emphasize that while it may be possible for functions like fclose to run into errors while they are closing the stream, the point is that the result of the call to fclose is always a closed stream. I think most of the comments are losing sight of the point of my article and rat-holding on the various ways fclose can run into errors. That’s not the same as failing. It never fails; it always succeeds, but possibly with errors.

Raymond Chen

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