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From: chet@odin.INS.CWRU.Edu (Chet Ramey)
Newsgroups: comp.unix.wizards
Subject: Re: dup2
Message-ID: <1991Feb8.002436.21328@usenet.ins.cwru.edu>
Date: 8 Feb 91 00:24:36 GMT
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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:

dup2 (fd1, fd2)
     int fd1, fd2;
{
  if (fcntl (fd1, F_GETFL, 0) == -1)    /* fd1 is an invalid fd */
    return (-1);
  if (fd2 < 0 || fd2 >= getdtablesize ())
    {
      errno = EBADF;
      return (-1);
    }
  if (fd1 == fd2)
    return (0);
  (void) close (fd2);
  return (fcntl (fd1, F_DUPFD, fd2));
}

(getdtablesize() can be replaced with:

	sysconf(_SC_OPEN_MAX)	Posix

	ulimit(4, 0L)		System V.3 and up

	NOFILE			just about anything else

and of course the value can be fetched once into a static variable and
cached.)

Chet
-- 
Chet Ramey				``There's just no surf in
Network Services Group			  Cleveland, U.S.A. ...''
Case Western Reserve University
chet@ins.CWRU.Edu		My opinions are just those, and mine alone.