(autoconf.info)Exiting Portably

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12.8 Exiting Portably

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

   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 (Note: 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

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