How slow is exit()?

This is a discussion on How slow is exit()? within the C Programming forums, part of the General Programming Boards category; How slow is exit()? Is it better to use exit() or return up the stack to main()?...

  1. #1
    Frequently Quite Prolix dwks's Avatar
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    How slow is exit()?

    How slow is exit()? Is it better to use exit() or return up the stack to main()?
    dwk

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  2. #2
    and the hat of int overfl Salem's Avatar
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    I can't see the point of optimising something which only happens once.

    There's quite a lot of work goes on, closing files, freeing memory, removing entries from process tables, updating accounting information (say) and informing parent processes, all of which take a lot longer than how you get there in the first place.
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  3. #3
    Frequently Quite Prolix dwks's Avatar
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    Okay, I was just wondering if I should unravel the stack or not.
    dwk

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    and the hat of int overfl Salem's Avatar
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    Well there's no absolute 'need' to, but you might consider that if you want your software to be more re-usable, having it call exit() from within it might make it a lot harder.
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  5. #5
    cwr
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    The C standard guarantees that exit() will have the same behaviour as the final return from main, so there isn't going to be much difference.

  6. #6
    Frequently Quite Prolix dwks's Avatar
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    Okay, thanks, maybe I'll start using it, then.
    dwk

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    Ah hem....

    The C standard requires that calling exit(some_value) has the same net effect as returning some_value from main. In other words, the environment in which your program is executed (eg the shell under unix) will receive some_value as a status value.

    That is not the same as saying that calling exit() from some function will be more (or less) efficient than returning "up the stack" to main(), which then returns the value. The relative performance will, in general, depend on your compiler and library --- although, instinctively, I'd guess that calling exit() would be faster as it is not necessary to do checks in each caller. It is also not something that I'd worry about: if your program needs to exit, then a few extra machine instructions is not significant.

    Although not relevant to C, calling exit() will cause problems for some C++ programs, as the process of stack unwinding has side effects in C++ (eg invoking destructors of objects that are local to every function in the call stack).

  8. #8
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    Or here's a nutty idea...TRY IT! Just write a little script to run your program a few dozen times (depending on the speed of your computer) and then run your script and time it. Then change your program to the other method and repeat. If you're using linux you can do 'time <your script name>'...under Windows, not sure.
    If you understand what you're doing, you're not learning anything.

  9. #9
    Frequently Quite Prolix dwks's Avatar
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    I wrote my own program to time other programs, but it uses time() and isn't too accurate.

    Okay . . . about three times should be good enough for my computer.
    dwk

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    dwks: these 'how fast is x' questions are completley radiculous. Please think about what you are asking. Why the heck does it matter hwo fast 'exit' is? It's exiting yoru dam program, who cares?

    Use a profiler to time a program, that is what they are there for. and time() A) is not precise B) does not tell you ho wmuch time was spent IN your code.

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    Quote Originally Posted by dwks
    How slow is exit()? Is it better to use exit() or return up the stack to main()?
    I agree with other posters about speed -- its irrelevent. As for which is better? Depends. if you do a lot of dynamic allocation in main() when you SHOULD deallocate them before exiting the program. Most operating systems, however, will clean up (such as wipe you nose) for you. But for those os that don't, the program will have to clean up itself. Otherwise, returning from main() is identical to calling exit() from someplace else. Myself, I never call exit() from main().

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