Is it possible to change the exit code in a function registered with atexit()?

Question

The man page for atexit(3) says the following:

POSIX.1-2001 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().

However, I'm interested in modifying the exit code in a finalizer for my program. The only way I've managed to do this is by calling exit() from within my finalization function, but the man page explicitly warns against that.

Is there any practical danger against doing this? Are there any implementations where that approach might cause problems? Even better, is there another way of doing this?

Answer 1

You can call _exit() instead.

Within the Notes section of the man page:

The function _exit() is like exit(), but does not call any functions registered with atexit() or on_exit().

This should avoid the "recursive" issue that is being warned about in the POSIX spec. If you are somehow able to guarantee that your "exit code changing" exit handler runs last, this should work perfectly, modulo the caveats also listed in the Notes:

Whether it flushes standard I/O buffers and removes temporary files created with tmpfile(3) is implementation-dependent. On the other hand, _exit() does close open file descriptors, and this may cause an unknown delay, waiting for pending output to finish. If the delay is undesired, it may be useful to call functions like tcflush(3) before calling _exit(). Whether any pending I/O is canceled, and which pending I/O may be canceled upon _exit(), is implementation-dependent.