Path: utzoo!attcan!uunet!etnibsd!vsh
From: vsh@etnibsd.UUCP (Steve Harris)
Newsgroups: comp.lang.perl
Subject: (repost, and possible fix):  is ( -e _ ) always true??
Message-ID: <1180@etnibsd.UUCP>
Date: 26 Oct 90 20:08:56 GMT
Organization: Eaton Corp, Semiconductor Equipment Div., Beverly, MA
Lines: 107

Nobody answered my first query, so I "fixed" it myself.

(I'm still running patchlevel 18, if this has been fixed in a
more recent release, say so and I'll shut up!  But, looking through
the patches, I don't see anything that looks like this problem has
been addressed.)

Bug:  the -e operator (test for file existence) always returns 1 (true)
	when applied to the magic argument _

For example:

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

print "-e _ returns ", -e _ ? "true" : "false", "\n";	# returns true (error)

$f = ".";						# file does exist
print "-e $f returns ", -e $f ? "true" : "false", "\n";	# returns true
print "-e _ returns ", -e _ ? "true" : "false", "\n";	# returns true

$f = "....";						# file doesn't exist
print "-e $f returns ", -e $f ? "true" : "false", "\n";	# returns false
print "-e _ returns ", -e _ ? "true" : "false", "\n";	# returns true (error)

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

Here's what **SEEMS** to be going on:

The return value of the -e operator is computed from the return value
of the stat system call.  This value is passed back by the mystat()
function.  However, when mystat() is called for the magic argument _,
it simply returns 0.

The fix is to have it return the previous return value.

Here are some diffs that work on my system, but there's an awful lot I
don't know about that's going on here (in particular, I don't understand
why ( -e _ ) returns true BEFORE any other filetest/stat operations have
been performed).  Also, remember that these diffs are made against
patchlevel 18 files.  Anyway, I'd wait till Larry has given his
blessing before installing them.

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

*** doio.c.BAK	Fri Oct 26 14:13:44 1990
--- doio.c	Fri Oct 26 15:02:26 1990
***************
*** 752,757 ****
--- 752,758 ----
  STR *str;
  {
      STIO *stio;
+     static int retcd = -1;
  
      if (arg[1].arg_type & A_DONT) {
  	stio = stab_io(arg[1].arg_ptr.arg_stab);
***************
*** 758,780 ****
  	if (stio && stio->ifp) {
  	    statstab = arg[1].arg_ptr.arg_stab;
  	    str_set(statname,"");
! 	    return fstat(fileno(stio->ifp), &statcache);
  	}
  	else {
  	    if (arg[1].arg_ptr.arg_stab == defstab)
! 		return 0;
  	    if (dowarn)
  		warn("Stat on unopened file <%s>",
  		  stab_name(arg[1].arg_ptr.arg_stab));
  	    statstab = Nullstab;
  	    str_set(statname,"");
! 	    return -1;
  	}
      }
      else {
  	statstab = Nullstab;
  	str_sset(statname,str);
! 	return stat(str_get(str),&statcache);
      }
  }
  
--- 759,781 ----
  	if (stio && stio->ifp) {
  	    statstab = arg[1].arg_ptr.arg_stab;
  	    str_set(statname,"");
! 	    return (retcd = fstat(fileno(stio->ifp), &statcache));
  	}
  	else {
  	    if (arg[1].arg_ptr.arg_stab == defstab)
! 		return retcd;
  	    if (dowarn)
  		warn("Stat on unopened file <%s>",
  		  stab_name(arg[1].arg_ptr.arg_stab));
  	    statstab = Nullstab;
  	    str_set(statname,"");
! 	    return (retcd = -1);
  	}
      }
      else {
  	statstab = Nullstab;
  	str_sset(statname,str);
! 	return (retcd = stat(str_get(str),&statcache));
      }
  }

-- 
Steve Harris - Eaton Corp. - Beverly, MA - uunet!etnibsd!vsh