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From: maart@cs.vu.nl (Maarten Litmaath)
Newsgroups: comp.lang.c
Subject: Re: Using Macros
Message-ID: <7263@star.cs.vu.nl>
Date: 9 Aug 90 11:33:18 GMT
References: <362.26be9dcc@astro.pc.ab.com> <21057@grebyn.com> <642@travis.csd.harris.com>
Sender: news@cs.vu.nl
Reply-To: maart@cs.vu.nl (Maarten Litmaath)
Organization: VU Dept. of Computer Science, Amsterdam, The Netherlands
Lines: 56

In article <642@travis.csd.harris.com>,
	brad@SSD.CSD.HARRIS.COM (Brad Appleton) writes:
)...
)As another example, if you didnt have the dup2() system call on your Unix
)system and wanted to write a macro for it, you could use the following:
)
)    #define  dup2(to,from)  ( (close(from) || (to = dup()) < 0) ? -1 : 0 )
)
)If there is some reason why this would not give the same results as dup2()
)(and fail for the same reasons) then let me know. The main problem I foresee
)is that I cant have a file-descriptor numbered lower than `from' available
)before I make the dup2() call.

The main problem is your definition being wrong.  Try the following instead:

	#define	dup2(from, to)	(close(to), dup(from))

It still fails if to == from.  This probably gets it right:

	#include	<sys/errno.h>

	#define	MAXFD	19	/* archaic */

	int	dup2(from, to)
	int	from, to;
	{
		int	fds[MAXFD + 1];
		register	int	*fdp = fds, fd;
		extern	int	errno;

		if (from < 0 || from > MAXFD || to < 0 || to > MAXFD) {
			errno = EBADF;
			return -1;
		}

		if (from == to)
			return to;

		while ((fd = *fdp++ = dup(from)) < to && fd >= 0)
			;

		if (fd != to) {
			close(to);
			(void) dup(from);
			if (fd < 0)
				--fdp;
		}

		while (fdp > fds)
			close(*--fdp);

		return to;
	}
--
   "UNIX was never designed to keep people from doing stupid things, because
    that policy would also keep them from doing clever things."  (Doug Gwyn)