atexit

This is a discussion on atexit within the C Programming forums, part of the General Programming Boards category; can anyone how atexit works coz i couldnt rlly understand how it goes thanks its lil bit confusing...

  1. #1
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    atexit

    can anyone how atexit works coz i couldnt rlly understand how it goes thanks
    its lil bit confusing

  2. #2
    spurious conceit MK27's Avatar
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    Quote Originally Posted by lolguy View Post
    can anyone how atexit works coz i couldnt rlly understand how it goes thanks
    its lil bit confusing
    Hey I just copied this from the GNU Ref Manual:
    Code:
         #include <stdio.h>
         #include <stdlib.h>
         
         void
         bye (void)
         {
           puts ("Goodbye, cruel world....");
         }
         
         int
         main (void)
         {
           atexit (bye);
           exit (EXIT_SUCCESS);
         }
    I imagine the purpose is to close files, network connections, child processes, save data, and take care of anything else that might otherwise be left running or in an undefined state after the main process ends. You can do something similar (and perhaps more effectively) with a signal handler.
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

  3. #3
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    Basically, it works like this:
    Code:
    // inside your C-library:
    ...
        main();
    ...
        if (atexit_function) atexit_function();
    ...
    atexit itself looks something like this:
    Code:
    void (*atexit_function)(void) = NULL;
    void atexit(void (*func)(void))
    {
       atexit_function = func;
    }
    In real life it's a bit more complex, because there can be several atexit functions, so it's more like an array than a single pointer - but in essence that's how it works.

    The function exit() will make the program jump to the point just after main (or something that has that effect).

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  4. #4
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    so u just call it inside a function and it exits that function ?
    but why does it run in reverse mode ?

  5. #5
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    Code:
    #include <stdio.h>
    #include <stdlib.h>
    static void double_it()
    {
    	int x=2;
    	x*=x;
    	printf("%d",x);
    }
    int main(void)
    {
    atexit(double_it);
    return getchar();
    }
    so it just exit by that function ? or there is more to it ?

  6. #6
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    sorry 1 more question
    main returns getchar first ? or atexit doubles first ?

  7. #7
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    Quote Originally Posted by lolguy View Post
    sorry 1 more question
    main returns getchar first ? or atexit doubles first ?
    atexit() is called AFTER main returns.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  8. #8
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    oh okay thanks so it the last thing the program runs ty for help i understand it now

  9. #9
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    Quote Originally Posted by lolguy View Post
    oh okay thanks so it the last thing the program runs ty for help i understand it now
    Ok, so technically there may well be stuff that happens AFTER atexit(), but that is PURELY part of the C runtime, not something you can change. Just like the C runtime runs code BEFORE main() that you also do not have any way to influence [unless you take the C runtime source code and modify it].

    As an example, after atexit(), any memory allocated through malloc() and friends may be given back to the OS (assuming the OS doesn't do this by itself).

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

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