From: John P. Linderman <jpl@research.att.com>
Date: Sun, 10 Aug 2003 15:44:33 +0000 (-0400)
Subject: Re: killing for vital signs [PATCH]
X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=commitdiff_plain;h=1e9c1022d083eab04a1d88cf3f92a6f2fc2b0791;p=p5sagit%2Fp5-mst-13.2.git

Re: killing for vital signs [PATCH]
From: "John P. Linderman" <jpl@research.att.com>
Message-Id: <200308101944.PAA96547@raptor.research.att.com>

p4raw-id: //depot/perl@20607
---

diff --git a/pod/perlfunc.pod b/pod/perlfunc.pod
index f47b6a5..f10927e 100644
--- a/pod/perlfunc.pod
+++ b/pod/perlfunc.pod
@@ -2378,7 +2378,7 @@ same as the number actually killed).
     kill 9, @goners;
 
 If SIGNAL is zero, no signal is sent to the process.  This is a
-useful way to check that the process is alive and hasn't changed
+useful way to check that a child process is alive and hasn't changed
 its UID.  See L<perlport> for notes on the portability of this
 construct.
 
@@ -2386,7 +2386,9 @@ Unlike in the shell, if SIGNAL is negative, it kills
 process groups instead of processes.  (On System V, a negative I<PROCESS>
 number will also kill process groups, but that's not portable.)  That
 means you usually want to use positive not negative signals.  You may also
-use a signal name in quotes.  See L<perlipc/"Signals"> for details.
+use a signal name in quotes.
+
+See L<perlipc/"Signals"> for more details.
 
 =item last LABEL
 
diff --git a/pod/perlipc.pod b/pod/perlipc.pod
index 637030c..8412bfd 100644
--- a/pod/perlipc.pod
+++ b/pod/perlipc.pod
@@ -95,13 +95,22 @@ it doesn't kill itself):
     }
 
 Another interesting signal to send is signal number zero.  This doesn't
-actually affect another process, but instead checks whether it's alive
+actually affect a child process, but instead checks whether it's alive
 or has changed its UID.
 
     unless (kill 0 => $kid_pid) {
 	warn "something wicked happened to $kid_pid";
     }
 
+When directed at a process whose UID is not identical to that
+of the sending process, signal number zero may fail because
+you lack permission to send the signal, even though the process is alive.
+You may be able to determine the cause of failure using C<$!>.
+
+    unless (kill 0 => $pid or $! == $!{EPERM}) {
+	warn "$pid looks dead";
+    }
+
 You might also want to employ anonymous functions for simple signal
 handlers: