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### 2.31 `on_exit`—request execution of function with argument at program exit

Synopsis

    #include <stdlib.h>
    int on_exit (void (*function)(int, void *), void *arg);

Description
You can use `on_exit` to enroll functions in a list of functions that will be called when your program terminates normally. The argument is a pointer to a user-defined function which takes two arguments. The first is the status code passed to exit and the second argument is of type pointer to void. The function must not return a result. The value of arg is registered and passed as the argument to function.

The functions are kept in a LIFO stack; that is, the last function enrolled by `atexit` or `on_exit` will be the first to execute when your program exits. You can intermix functions using `atexit` and `on_exit`.

There is no built-in limit to the number of functions you can enroll in this list; however, after every group of 32 functions is enrolled, `atexit`/`on_exit` will call `malloc` to get space for the next part of the list. The initial list of 32 functions is statically allocated, so you can always count on at least that many slots available.

Returns
`on_exit` returns `0` if it succeeds in enrolling your function, `-1` if it fails (possible only if no space was available for `malloc` to extend the list of functions).

Portability
`on_exit` is a non-standard glibc extension

Supporting OS subroutines required: None