dup2() system call

[Forrie Aldrich]

Brandon S. Allbery writes:
| 
| As quoted from <2180 at estevax.UUCP> by iain at estevax.UUCP (Hr Iain Lea):
| +---------------
| | I am porting a program from BSD to a Sys5r2 ish derivative and need the
| | dup2() function call.
| +---------------
| 
| Is this one in the FAQ?
| 
| /*
|  * Near-duplicate for dup2().  ("Near"?  We discussed this for a whole bloody
|  * month --- I don't want to discuss it any more.  BSD and System V have
|  * enough differing errno values that that part is pointless anyway.)
|  *
|  * Caveats: doesn't necessarily return the same errno values on failure; does
|  * not leave f2 open if the dup fails.
|  */
| 
| #include <ioctl.h>
| #include <errno.h>
| 
| #define dup2(f1,f2) (close(f2),fcntl(f1,F_DUPFD,f2))
| 
| ++Brandon
[...]

Well, for reference's sake, here's another version that I caught off the net
a little while ago... hope it helps.

/*
	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 */
}

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Forrest Aldrich, Jr.|   ...uunet!eci!morwyn!forrie       |forrie at morywn.UUCP
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Forrest Aldrich, Jr.|   ...uunet!eci!morwyn!forrie       |forrie at morywn.UUCP
                    |           <email paths>            | 
CREATIVE CONNECTIONS|  ...uunet!zinn!eci!morwyn!forrie   |Graphic Illustration
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                   \___ PO Box 1541 - Dover, NH  03820 ___/