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From: rbj@uunet.UU.NET (Root Boy Jim)
Newsgroups: comp.unix.wizards,comp.unix.internals,comp.lang.perl
Subject: Re: dup2
Message-ID: <121729@uunet.UU.NET>
Date: 8 Feb 91 09:46:24 GMT
References: <27B1CA1C.22559@ics.uci.edu> <1991Feb8.002436.21328@usenet.ins.cwru.edu>
Followup-To: comp.unix.wizards
Organization: UUNET Communications Services, Falls Church, VA
Lines: 52
Cc: rbj

In article <1991Feb8.002436.21328@usenet.ins.cwru.edu> chet@po.CWRU.Edu writes:
>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?
>
>When you really get down to it.  the kernel is going to have to do the
>nitty-gritty duplication for you, otherwise it gets tricky.  If you have
>an fcntl(..., F_DUPFD, ...), it's straightforward. 
>
>Here's how we do it in bash:

[You may see this twice. Sorry if you do]

Well, a clear case of two left feet not knowing what the right hand is
doing. The solution is trivial once you grasp the recursive nature of it.

I was gonna tell y'all to look in ~emacs/src/sysdep.c, but the
version there is broken! I feel like a fool, as I sent it off to
Larry Wall. Dup and dup2 can fail, altho people treat it as if it can't.
I just debugged the following routine. Appended is a test driver.

dup2(old,new)
{
#ifdef F_DUPFD
	close(new);
	return(fcntl(old, F_DUPFD, new));
#else
        register int fd, ret;
        close(new);
        fd = dup(old);
        if (fd == -1) return(-1);
        if (fd == new) return(new);
        ret = dup2(old,new);
        close(fd);
        return(ret);
#endif
}

main(c,v) char *v[];
{
        register int a,b;
        (c > 2) || (printf("usage: %s srcfd dstfd\n",v[0]),exit(1));
        a = atoi(v[1]); b = atoi(v[2]);
        printf("dup2(%d,%d) = %d\n",a,b,dup2(a,b));
}
-- 

	Root Boy Jim Cottrell <rbj@uunet.uu.net>
	I got a head full of ideas
	They're driving me insane