Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!cica!ctrsol!ginosko!uunet!auspex!guy
From: guy@auspex.auspex.com (Guy Harris)
Newsgroups: comp.sources.d
Subject: Re: Perl on Microport 286: can't find library routines
Message-ID: <2432@auspex.auspex.com>
Date: 9 Sep 89 20:13:49 GMT
References: <2869@splut.conmicro.com> <9257@attctc.Dallas.TX.US> <1989Sep7.222720.3381@NCoast.ORG>
Reply-To: guy@auspex.auspex.com (Guy Harris)
Organization: Auspex Systems, Santa Clara
Lines: 58

>dup2() under System V unixes:

(System V prior to R3, anyway; "dup2()" under S5R3 and later is just
"dup2()", it's already in "libc".)

Actually, to be strictly correct, you should do a bit more (1003.1 says
that "dup2" shall be equivalent to the sequence you give, except for
some extra checks it lists, which are done here):

#include <fcntl.h>
#include <sys/param.h>
#include <errno.h>

int
dup2(fildes, fildes2)
	int fildes, fildes2;
{
	/*
	 * Check whether "fildes" is a valid file descriptor:
	 * "If 'fildes' is not a valid file descriptor, 'dup2()'
	 * shall fail and not close 'fildes2'."
	 */
	if (fcntl(fildes, F_GETFL, 0) == -1)
		return -1;	/* "fcntl" set "errno" correctly */

	/*
	 * Check whether "fildes2" is within the correct range:
	 * "If 'fildes2' is less than zero or greater than
	 * {OPEN_MAX}, the 'dup2()' function shall return -1 and
	 * 'errno' shall be set to [EBADF]."
	 * No, don't use "ulimit" to get the maximum valid file
	 * descriptor; that first appeared in S5R3, but S5R3
	 * already *has* a "dup2()".
	 */
	if (fildes2 < 0 || fildes2 >= NOFILE) {
		errno = EBADF;
		return -1;
	}

	/*
	 * Check whether "fildes" is equal to "fildes2":
	 * "If 'fildes' is a valid file descriptor and is equal
	 * to 'fildes2', the 'dup2()' function shall return
	 * 'fildes2' without closing it."
	 */
	if (fildes == fildes2)
		return 0;

	/*
	 * Now do the real work:
	 * "The call 'fid = dup2 (fildes, fildes2);' shall be equivalent
	 * to 'close (fildes2); fid = fcntl (fildes, F_DUPFD, fildes2);'
	 * except for the following:" where "the following" are the
	 * cases listed above.
	 */
	(void) close(fildes2);
	return fcntl(fildes, F_DUPFD, fildes2);
}