#!/usr/bin/perl
print "effective: $>, real: $<\n";
$> = 111;
open(O,">foo") or die "Can't write foo: $!\n";
close O or die "Can't close O: $!\n";
$> = 0;
open(O,">bar") or die "Can't write bar: $!\n";
close O or die "Can't close O: $!\n";
$> = 111 ;
# this must fail.
open(O,">bar") or warn "Can't write bar: $!\n";
# change real uid
$< = 111; # oops, forgot to set $> to 0
print "effective: $>, real: $<\n";
$< = 0;   # no effect
print "effective: $>, real: $<\n";
[download]

This outputs:

effective: 0, real: 0
Can't write bar: Permission denied
effective: 111, real: 111
effective: 111, real: 111
[download]

As you see, the second change of the real uid had no effect. Let's see what's in here:

quux [gm] /tmp/foo # ls -l
total 4
-rw-r--r-- 1 root root      0 2006-06-26 02:31 bar
-rw-r--r-- 1  111 root      0 2006-06-26 02:31 foo
-rw-r--r-- 1 root root    338 2006-06-26 02:29 setuid.pl
[download]

Where do you get the function setuid from? can't find that in my perlfunc...

update: ah, POSIX.

   setuid  Sets the real user identifier and the effective user identi
+-
           fier for this process.  Similar to assigning a value to the
           Perl's builtin $< variable, see "$UID" in perlvar, except
           that the latter will change only the real user identifier.
[download]

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}