#include <stdlib.h> int atexit(void (*function)(void));
The same function may be registered multiple times: it is called once for each registration.
POSIX.1 requires that an implementation allow at least ATEXIT_MAX (32) such functions to be registered. The actual limit supported by an implementation can be obtained using sysconf(3).
When a child process is created via fork(2), it inherits copies of its parent's registrations. Upon a successful call to one of the exec(3) functions, all registrations are removed.
| Interface | Attribute | Value |
| atexit() | Thread safety | MT-Safe |
If one of the registered functions calls _exit(2), then any remaining functions are not invoked, and the other process termination steps performed by exit(3) are not performed.
POSIX.1 says that the result of calling exit(3) more than once (i.e., calling exit(3) within a function registered using atexit()) is undefined. On some systems (but not Linux), this can result in an infinite recursion; portable programs should not invoke exit(3) inside a function registered using atexit().
The atexit() and on_exit(3) functions register functions on the same list: at normal process termination, the registered functions are invoked in reverse order of their registration by these two functions.
According to POSIX.1, the result is undefined if longjmp(3) is used to terminate execution of one of the functions registered using atexit().
void
bye(void)
{
printf("That was all, folks\n");
}
int
main(void)
{
long a;
int i;
a = sysconf(_SC_ATEXIT_MAX);
printf("ATEXIT_MAX = %ld\n", a);
i = atexit(bye);
if (i != 0) {
fprintf(stderr, "cannot set exit function\n");
exit(EXIT_FAILURE);
}