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To:	rc@hawkwind.utcs.toronto.edu
Subject: Re: read 
Date:	Mon, 17 May 1993 22:13:30 -0400
From:	Alan Watson <alan@oldp.astro.wisc.edu>
X-Mts: smtp

Gosh, this stuff is tricky, isn't it?  Here are both external and
built-in implementations of read, following v7 semantics on EOF
(i.e., set the variable to '' and return 1), and a corrected trip.read.
None of the other reads in the list or the examples get this right
(including my previous offerings).

Of course, all of this will be made redundant by bqstatus.

I take it by the silence that no one objects to a built-in read by
default ("yeah, right").

Alan.

; cat addon/read.c
/* 
 b_reads -- read a single line, terminated by \n or EOF, from the standard
 input, and assign the line without the terminator to av[1].
 */

static int
readchar (int fd)
{
   unsigned char c;

   if (read (fd, &c, 1) == 1)
      return c;
   else
      return EOF;
}

void
b_read (char **av)
{
   char *name;
   int c;
   char *line;
   SIZE_T len;
   SIZE_T max_len;
   SIZE_T max_len_quantum;

   /* check usage is "read name" */
   if (av[1] == NULL)
      rc_error ("missing variable name");
   if (av[2] != NULL)
      rc_error ("too many arguments to read");
   name = av[1];

   /* read a single line from stdin */
   line = NULL;
   len = 0;
   max_len = 0;
   max_len_quantum = 256;
   do {
      c = readchar (0);
      if (len == max_len) 
	 line = erealloc (line, max_len += max_len_quantum);
      if (c == '\n' || c == EOF)
	 line[len] = 0;
      else
	 line[len] = c;
      len++;
   } while (c != '\n' && c != EOF);

   /* assign whatever we read to the variable */
   assign (word (name, NULL), word (line, NULL), FALSE);
   efree (line);

   /* return TRUE if we terminated with a \n, otherwise FALSE */
   set (c == '\n');

   return;
}
; cat read.fn
fn read {
	switch ( $#* ) {
	case 0
		echo >[1=2] missing variable
		return 1
	case 1
	case *
		echo >[1=2] too many arguments to read
		return 1
	}
	lines = () {
		ifs = $nl { lines = `{ line ; echo $status } }
		if ( ~ $#lines 1 ) {
			$1 = ''
			return $lines
		} else {
			$1 = $lines(1)
			return $lines(2)
		}
	}
}
; cat trip.read
# trip.read -- check a read implementation performs like a true v7 sh read

fn check {
   echo $1 $2
   ./rc -c $2
   if ( ! ~ $status $1 ) {
      echo failed: $2
   }
}

check 0 true
check 1 false

nl = '
'
ifs = $nl

file = /etc/passwd
first = `{ cat $file }
first = $first(1)

echo 'start of trip.read'

# check read X and read '*' return 0
check 0 'exec <$file ; read X'
check 0 'exec <$file ; read ''*'''

# check read, read 1, read '=', and read '' return 1
check 1 'exec <$file ; read'
check 1 'exec <$file ; read 1'
check 1 'exec <$file ; read ''='''
check 1 'exec <$file ; read '''''

# check read X correctly reads first line
check 0 'exec <$file; read X ; ~ $X $first'
check 0 'echo -n $first | { read X ; ~ $X $first }'

# check read '*' correctly reads first line, preserves $0
check 0 'exec <$file ; old0 = $0 ; read ''*'' ; ~ $0 $old0 && ~ $1 $first && ~ $#* 1'

# check read returns 1 on EOF and I/O error
check 1 'exec </dev/null ; read X'
check 1 'exec <[0=]      ; read X'

# check read sets variable to '' when line is empty and on EOF and I/O error.
check 0 '{ X = X ; echo | { read X ; ~ $X '''' } }'
check 0 '{ exec </dev/null ; X = X ; read X ; ~ $X '''' }'
check 0 '{ exec <[0=]      ; X = X ; read X ; ~ $X '''' }'

echo 'end of trip.read'

# end-of-file