Thread: atexit

Originally Posted by lolguy

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.

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

Originally Posted by lolguy

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

atexit() is called AFTER main returns.

--
Mats

Originally Posted by lolguy

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