Возврат из функции int main() в С++

/*
 * Copyright (c) 1987, 1993
 *      The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *      This product includes software developed by the University of
 *      California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *      @(#)sysexits.h  8.1 (Berkeley) 6/2/93
 */
 
#ifndef _SYSEXITS_H_
#define _SYSEXITS_H_
 
/*
 *  SYSEXITS.H -- Exit status codes for system programs.
 *
 *      This include file attempts to categorize possible error
 *      exit statuses for system programs, notably delivermail
 *      and the Berkeley network.
 *
 *      Error numbers begin at EX__BASE to reduce the possibility of
 *      clashing with other exit statuses that random programs may
 *      already return.  The meaning of the codes is approximately
 *      as follows:
 *
 *      EX_USAGE -- The command was used incorrectly, e.g., with
 *              the wrong number of arguments, a bad flag, a bad
 *              syntax in a parameter, or whatever.
 *      EX_DATAERR -- The input data was incorrect in some way.
 *              This should only be used for user's data & not
 *              system files.
 *      EX_NOINPUT -- An input file (not a system file) did not
 *              exist or was not readable.  This could also include
 *              errors like "No message" to a mailer (if it cared
 *              to catch it).
 *      EX_NOUSER -- The user specified did not exist.  This might
 *              be used for mail addresses or remote logins.
 *      EX_NOHOST -- The host specified did not exist.  This is used
 *              in mail addresses or network requests.
 *      EX_UNAVAILABLE -- A service is unavailable.  This can occur
 *              if a support program or file does not exist.  This
 *              can also be used as a catchall message when something
 *              you wanted to do doesn't work, but you don't know
 *              why.
 *      EX_SOFTWARE -- An internal software error has been detected.
 *              This should be limited to non-operating system related
 *              errors as possible.
 *      EX_OSERR -- An operating system error has been detected.
 *              This is intended to be used for such things as "cannot
 *              fork", "cannot create pipe", or the like.  It includes
 *              things like getuid returning a user that does not
 *              exist in the passwd file.
 *      EX_OSFILE -- Some system file (e.g., /etc/passwd, /etc/utmp,
 *              etc.) does not exist, cannot be opened, or has some
 *              sort of error (e.g., syntax error).
 *      EX_CANTCREAT -- A (user specified) output file cannot be
 *              created.
 *      EX_IOERR -- An error occurred while doing I/O on some file.
 *      EX_TEMPFAIL -- temporary failure, indicating something that
 *              is not really an error.  In sendmail, this means
 *              that a mailer (e.g.) could not create a connection,
 *              and the request should be reattempted later.
 *      EX_PROTOCOL -- the remote system returned something that
 *              was "not possible" during a protocol exchange.
 *      EX_NOPERM -- You did not have sufficient permission to
 *              perform the operation.  This is not intended for
 *              file system problems, which should use NOINPUT or
 *              CANTCREAT, but rather for higher level permissions.
 */
 
#define EX_OK           0       /* successful termination */
 
#define EX__BASE        64      /* base value for error messages */
 
#define EX_USAGE        64      /* command line usage error */
#define EX_DATAERR      65      /* data format error */
#define EX_NOINPUT      66      /* cannot open input */
#define EX_NOUSER       67      /* addressee unknown */
#define EX_NOHOST       68      /* host name unknown */
#define EX_UNAVAILABLE  69      /* service unavailable */
#define EX_SOFTWARE     70      /* internal software error */
#define EX_OSERR        71      /* system error (e.g., can't fork) */
#define EX_OSFILE       72      /* critical OS file missing */
#define EX_CANTCREAT    73      /* can't create (user) output file */
#define EX_IOERR        74      /* input/output error */
#define EX_TEMPFAIL     75      /* temp failure; user is invited to retry */
#define EX_PROTOCOL     76      /* remote error in protocol */
#define EX_NOPERM       77      /* permission denied */
#define EX_CONFIG       78      /* configuration error */
 
#define EX__MAX 78      /* maximum listed value */
 
#endif /* !_SYSEXITS_H_ */