What's the difference between _Exit() and _exit()?

Question

In answer to this question, some people have said to use _Exit() and others have said to use _exit(). Could someone explain the difference (if any) between the two, and the origins of both?

Answer 1

_exit() and _Exit() are absolutely identical. To understand the differences between exit() and _exit(), you would need to know about the functions on_exit(3) and atexit(3). These functions are used to register functions that are called automatically when the process exits normally, that is, via a return from main(), or via a call to exit(). on_exit() and atexit() differ only in that they allow functions to be registered with different signatures (think of these registered functions as destructors (as in object-oriented programming) for the processes).

exit(), upon being called attempts to execute all the functions registered using atexit() or onexit(). Upon executing them, it calls _exit(). _exit() does the normal process termination stuff - closing file-descriptors, releasing memory, re-parenting orphaned children to init, et cetera. Think of _exit() as the bare-bones system call used by a process to terminate itself.

Since a whole lot of applications do not use atexit() or on_exit(), for these, _exit(), _Exit(), and exit() behave identically.

Answer 2

Right from the man page here:

The function _Exit() is equivalent to _exit().

Although in C++11, it is standardized as either std::_Exit or std::quick_exit. According to Mike Seymour here.

Answer 3

_Exit(2) is from C99. _exit(2) is from POSIX. At least, according to the manpage I have installed here.

They are entirely equivalent.