Re: A lighter read function

Re: A lighter read function

[DaviD W. Sanderson]

[Apologies to the list; I am unable to reply to "golem!david"]

> exactly the same functionality as the example source, except it
> uses line(1) and doesn't do the assignment if the read fails.

line(1) is not available everywhere.  At least not on the 4.3BSD-utah
systems I use.  Perhaps rephrasing in terms of `sed 1q' would be better.

DaviD W. Sanderson (dws@cs.wisc.edu)

Re: A lighter read function

[David J. Fiander]

>> exactly the same functionality as the example source, except it
>> uses line(1) and doesn't do the assignment if the read fails.
>
>line(1) is not available everywhere.  At least not on the 4.3BSD-utah
>systems I use.  Perhaps rephrasing in terms of `sed 1q' would be better.

Try 'david%golem@lsuc.on.ca'.  True, but the problem with sed
is that it doesn't fail on EOF, which is true of line.

line is available on all SCO UNIX systems, and I first saw it
on a BSD 4.2 system (I think).  Besides it is easily written:

#include <stdio.h>

main(argc, argv)
int argc;
char **argv;
{
	char	buf[256];
	int	len;

	if (fgets(buf, sizeof(buf), stdin) == (char *)NULL) {
		fputs(buf, stdout);
		exit(0);
	} else
		exit(1);
}

Re: A lighter read function

[Chris Siebenmann]

 This has come up before on the mailing list. One cannot use sed, awk,
or handgrown alternatives that use stdio when you are not reading from
a terminal, because their buffering will eat too much of the input
stream and then silently discard it when they exit. You can convince
yourself by defining read in terms of one of them and then doing:
	{echo A; echo B} | {read a; echo a $a; read b; echo b $b}
and noticing what comes out.

 line is safe, and trivial to write; I will happily mail my version
to anyone who wants it. It even comes with a manual page.

 Here is my current version of read along with a couple of functions
that it relies on:

# A real read function, as in 'read var var var'. Returns failure
# on EOF.  relies on the 'line' program.
nl='
'
fn read { _v=() _i=() {
	_v=`` $nl {line; echo $status}
	if (! ~ $_v(2) 0) return $_v(2);
	_v=`{echo $_v(1)}
	for (_i in $*) {
		$_i=$_v(1); sshift _v 1
	}
	$_i=($$_i $_v)
}; return 0; }

# 'supershift' function for rc.
# usage:
#	sshift varname
#	sshift varname number
# shifts the list varname by number (default 1) positions.
fn sshift { if (~ $#* 1 ) { _sshift $1 1 $$1 } else { _sshift $1 $2 $$1 } }

# this function does most of the work.
# _sshift NAME COUNT LISTELEMS
# shift LISTELEMS COUNT items left, and assign the result to NAME.
fn _sshift { _vn=() { _vn=$1; shift; shift $1; shift; $_vn=$* } }

	- cks

Re: A lighter read function

[David J. Fiander]

>From: 	Chris Siebenmann <hawkwind.utcs.toronto.edu!cks@rutgers.uucp>
> Here is my current version of read along with a couple of functions
>that it relies on:

It looks pretty good.

>
># A real read function, as in 'read var var var'. Returns failure
># on EOF.  relies on the 'line' program.
>nl='
>'
>fn read { _v=() _i=() {
>	_v=`` $nl {line; echo $status}

This is why byron added the "``ifs{}" extension, but the Plan 9
way is probably

	ifs=$nl { _v=`{line; echo $status}}

which sets ifs for only the one command, but sets _v globally.

>	if (! ~ $_v(2) 0) return $_v(2);

There's a small problem with this.  If line sees an EOF, then
it will print a newline, and exit with status 1.  _v is thus
set to "1", since there is nothing before the first nl.  There
is no $_v(2), so the ~ fails, and read returns success.

I got around this by writing it as:

	x = `{line || echo 1^$nl^1; echo 0}

Then, if line fails, we get

	x = (1 1)

 otherwise we get

	x = ('the line' 0)

--
David J. Fiander   |email [Fr., = enamel] Used attrib. in `email ink', ink
<david@golem.uucp> |used on glass, porcelain, etc.

Re: A lighter read function

[Chris Siebenmann]

 David's quite right; obviously I haven't tested my read function enough.
I'd make it
	_v=`` $nl {line; echo $status $nl $status}
since I don't want to count on line returning just 0 or 1 status.
And it should check $#_v too, just in case.

	- cks

Re: A lighter read function

[David Hogan]

"David J. Fiander"  writes:

> 	x = `{line || echo 1^$nl^1; echo 0}
>
> Then, if line fails, we get
>
> 	x = (1 1)
>
>  otherwise we get
>
> 	x = ('the line' 0)

Well, actually, if line fails, x is set to (1 1 0).  While it is true that
the value of the 3rd element of the list (if one exists) is ignored by the
rest of the function, this is rather sloppy coding.  Why not use an if
statement?  Ok, so it's more lines of code, but at least you can see at a
glance what is going on (and you avoid making silly logic mistakes like the
one above).

Now if you'll excuse me, I have to clean out my in box.

Re: A lighter read function

[Ronald S H Khoo]

In article <22832.692454506@golem.UUCP> David J Fiander wrote:

> line is available on all SCO UNIX systems, and I first saw it
> on a BSD 4.2 system (I think).  Besides it is easily written:

> #include <stdio.h>

This looks wrong to me.  line(1) does not use stdio.  Where available,
line(1) should preserve the seek position of the stream so that anyone
who comes afterwards can read the rest.  Trying to fseek back there
ain't much good either, since input might come from a non-seekable
device or pipe.  Hence, line(1) must use single character reads, just
like sh(1) does (what does rc(1L) do ?).

Here is a demonstration of line(1)'s behaviour on a SCO UNIX machine:
script 'typescript' started on Wed Dec 11 21:31:34 1991
$ grep LSEEK /usr/include/sys.s
#define	LSEEK	19
$ (line > /dev/null
>  perl -e 'print syscall(19,0,0,1)."\n"'  # ie lseek(0, 0L, SEEK_CUR)
> ) < /etc/passwd
24
$ (sed 1q > /dev/null
>  perl -e 'print syscall(19,0,0,1)."\n"'
> ) < /etc/passwd
512
$ script 'typescript' ended on Wed Dec 11 21:33:10 1991

Please excuse the use of sh(1) -- I haven't had time to learn rc.
Please excuse the use of perl(1L) -- I couldn't think of another
way of executing lseek(2) from the command line ...

(BTW, David, in case anyone you know wants an "alternative" approach to
 script(1) on SCO, mine's 83 lines of perl :-)

-- 
Ronald Khoo <ronald@robobar.co.uk> +44 81 991 1142 (O) +44 71 229 7741 (H)

Re: A lighter read function

[David J. Fiander]

>From: 	ronald@robobar.co.uk (Ronald S H Khoo)
>
>This looks wrong to me.  line(1) does not use stdio.  Where available,
>line(1) should preserve the seek position of the stream so that anyone
>who comes afterwards can read the rest.  Trying to fseek back there

You're right.  I thought of this during the trip to work on the
subway.  But then, that will make the line binary even smaller.

- David

A lighter read function

[David J. Fiander]

Having seen, and been shocked at, the read example that comes
with the rc sources, I decided to try to write one that doesn't
require all of awk to run.  The following function provides
exactly the same functionality as the example source, except it
uses line(1) and doesn't do the assignment if the read fails.

nl='
'
fn read {
	x=() ifs=$nl {
		x = `{line || echo 1^$nl^1; echo 0} 
		if (~ $x(2) 0) $1 = $x(1)
		return $x(2)
	}
}

--
David J. Fiander   |email [Fr., = enamel] Used attrib. in `email ink', ink
<david@golem.uucp> |used on glass, porcelain, etc.