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FreeBSD 11.0 - man page for atexit (freebsd section 3)

ATEXIT(3)						   BSD Library Functions Manual 						 ATEXIT(3)

atexit -- register a function to be called on exit
Standard C Library (libc, -lc)
#include <stdlib.h> int atexit(void (*function)(void)); int atexit_b(void (^function)(void));
The atexit() function registers the given function to be called at program exit, whether via exit(3) or via return from the program's main(). Functions so registered are called in reverse order; no arguments are passed. These functions must not call exit(); if it should be necessary to terminate the process while in such a function, the _exit(2) function should be used. (Alternatively, the function may cause abnormal process termination, for example by calling abort(3).) At least 32 functions can always be registered, and more are allowed as long as sufficient memory can be allocated. The atexit_b() function behaves identically to atexit(), except that it takes a block, rather than a function pointer.
The atexit() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.
[ENOMEM] No memory was available to add the function to the list. The existing list of functions is unmodified. [ENOSYS] The atexit_b() function was called by a program that did not supply a _Block_copy() implementation.
at_quick_exit(3), exit(3)
The atexit() function conforms to ISO/IEC 9899:1990 (``ISO C90'').
September 6, 2002 BSD