A C or C++ program can exit with status N by returning
N from the main function. Portable programs are supposed
to exit either with status 0 or EXIT_SUCCESS to succeed, or with
status EXIT_FAILURE to fail, but in practice it is portable to
fail by exiting with status 1, and test programs that assume Posix can
fail by exiting with status values from 1 through 255. Programs on
SunOS 2.0 (1985) through 3.5.2 (1988) incorrectly exited with zero
status when main returned nonzero, but ancient systems like these
are no longer of practical concern.
A program can also exit with status N by passing N to the
exit function, and a program can fail by calling the abort
function. If a program is specialized to just some platforms, it can fail
by calling functions specific to those platforms, e.g., _exit
(Posix) and _Exit (C99). However, like other functions, an exit
function should be declared, typically by including a header. For
example, if a C program calls exit, it should include stdlib.h
either directly or via the default includes (see Default Includes).
A program can fail due to undefined behavior such as dereferencing a null pointer, but this is not recommended as undefined behavior allows an implementation to do whatever it pleases and this includes exiting successfully.