atexit(3) Library Functions Manual atexit(3)

atexit - register a function to be called at normal process termination

Standard C library (libc, -lc)

#include <stdlib.h>
int atexit(void (*function)(void));

The atexit() function registers the given function to be called at normal process termination, either via exit(3) or via return from the program's main(). Functions so registered are called in the reverse order of their registration; no arguments are passed.

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.

The atexit() function returns the value 0 if successful; otherwise it returns a nonzero value.

For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
atexit () Thread safety MT-Safe

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().

C11, POSIX.1-2008.

POSIX.1-2001, C89, C99, SVr4, 4.3BSD.

Functions registered using atexit() (and on_exit(3)) are not called if a process terminates abnormally because of the delivery of a signal.

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.

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().

Since glibc 2.2.3, atexit() (and on_exit(3)) can be used within a shared library to establish functions that are called when the shared library is unloaded.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

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(2), dlopen(3), exit(3), on_exit(3)

2023-07-20 Linux man-pages 6.05.01