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From: gwyn@smoke.brl.mil (Doug Gwyn)
Newsgroups: comp.unix.wizards
Subject: Re: dup2
Message-ID: <15136@smoke.brl.mil>
Date: 8 Feb 91 21:40:31 GMT
References: <27B1CA1C.22559@ics.uci.edu>
Organization: U.S. Army Ballistic Research Laboratory, APG, MD.
Lines: 41

In article <27B1CA1C.22559@ics.uci.edu> schmidt@ics.uci.edu (Doug Schmidt) writes:
>	I'm curious, is it possible to implement the dup2() system
>call using only routines available in the standard C library and other
>existing system calls?

/*
	dup2 -- 7th Edition UNIX system call emulation for UNIX System V

	last edit:	11-Feb-1987	D A Gwyn
*/

#include	<errno.h>
#include	<fcntl.h>

extern int	close(), fcntl();

int
dup2( oldfd, newfd )
	int		oldfd;		/* already-open file descriptor */
	int		newfd;		/* desired duplicate descriptor */
{
	register int	ret;		/* for fcntl() return value */
	register int	save;		/* for saving entry errno */

	if ( oldfd == newfd )
		return oldfd;		/* be careful not to close() */

	save = errno;			/* save entry errno */
	(void) close( newfd );		/* in case newfd is open */
	/* (may have just clobbered the original errno value) */

	ret = fcntl( oldfd, F_DUPFD, newfd );	/* dupe it */

	if ( ret >= 0 )
		errno = save;		/* restore entry errno */
	else				/* fcntl() returned error */
		if ( errno == EINVAL )
			errno = EBADF;	/* we think of everything */

	return ret;			/* return file descriptor */
}