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From: mouse@mcgill-vision.UUCP
Newsgroups: comp.unix.wizards
Subject: Re: /dev/stdin for 4.3?
Message-ID: <792@mcgill-vision.UUCP>
Date: Thu, 4-Jun-87 18:54:28 EDT
Article-I.D.: mcgill-v.792
Posted: Thu Jun  4 18:54:28 1987
Date-Received: Sat, 13-Jun-87 11:24:59 EDT
References: <7359@brl-adm.ARPA> <5856@brl-smoke.ARPA> <15318@onfcanim.UUCP>
Organization: McGill University, Montreal
Lines: 76

In article <15318@onfcanim.UUCP>, dave@onfcanim.UUCP (Dave Martindale) writes:
> In article <5856@brl-smoke.ARPA> gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>) writes:
>> [ discussing how to implement /dev/fd*, /dev/std{in,out,err} ]

>> This is almost trivial:
>> 1) Implement /dev/fd instead (general file descriptor device), using
>> minor device to select the fd #.  Make /dev/stdin a link to
>> /dev/fd/0, similarly for /dev/stdout etc.

Is this a SysV thing?  I was not aware of any way to make a device
which could be treated like a directory like that (/dev/fd/0).

> I believe it's better to do the equivalent of a full dup(), returning
> a newly-opened descriptor.  You still need only an open() entry, but
> you have to alloc an ofile table slot and increment the reference
> count on the file struct.

Here's how I did it:

% ls -l /dev/std*
crw-rw-rw-  1 mouse     64,   2 Mar 23 05:31 /dev/stderr
crw-rw-rw-  1 mouse     64,   0 Mar 23 05:31 /dev/stdin
crw-rw-rw-  1 mouse     64,   1 Mar 23 05:31 /dev/stdout
% 

machine/conf.c entry:

	stddevopen,	nodev,		nodev,		nodev,		/* 64 */
	nodev,		nulldev,	nulldev,	0,
	nodev,		nodev,

(notice, only the "open" slot has anything in it).

stddevopen(dev,mode)
dev_t dev;
int mode;
{
 int fd = minor(dev);
 int rfd = u.u_r.r_val1;

 if ((fd < 0) || (fd >= NOFILE))
  { return(EBADF);
  }
 if (u.u_ofile[fd] == NULL)
  { return(ENXIO);
  }
 if ((mode&FREAD) && !(u.u_ofile[fd]->f_flag&FREAD))
  { return(EACCES);
  }
 if ((mode&FWRITE) && !(u.u_ofile[fd]->f_flag&FWRITE))
  { return(EACCES);
  }
 u.u_ofile[rfd]->f_count --;
 u.u_ofile[rfd] = u.u_ofile[fd];
 u.u_pofile[rfd] = u.u_pofile[fd] & ~UF_EXCLOSE;
 u.u_ofile[fd]->f_count ++;
 return(0);
}

So there.  (stddevopen() is, functionally, just another interface to dup().)

Another use for /dev/std*:  in a csh script, how do you generate a
fixed message on the script's standard error?

sh -c 'echo this is the message 1>&2'

Ugly.  Unreadable by someone who doesn't know sh syntax.  Forks an
unnecessary sh process.

echo this is the message > /dev/stderr

Much better.  (Yes, I know sh is better for most scripts.)

					der Mouse

				(mouse@mcgill-vision.uucp)