From 86f79af827729f3968d8b3b8f860ac29d200da0d Mon Sep 17 00:00:00 2001 From: Erik Bray Date: Thu, 11 Aug 2016 15:57:53 +0200 Subject: [PATCH] kill(pid, sig) before waitpid() returns -1 for sig != 0 This is a followup to a report back in 2011 about essentially the same issue: https://cygwin.com/ml/cygwin/2011-04/msg00031.html The same test program in that report demonstrates the issue, but with kill sending any non-zero signal. To reiterate, the problem here is POSIX compliance with respect to sending signals to zombie processes. http://pubs.opengroup.org/onlinepubs/9699919799/functions/kill.html claims: Existing implementations vary on the result of a kill() with pid indicating an inactive process (a terminated process that has not been waited for by its parent). Some indicate success on such a call (subject to permission checking), while others give an error of [ESRCH]. Since the definition of process lifetime in this volume of POSIX.1-2008 covers inactive processes, the [ESRCH] error as described is inappropriate in this case. In particular, this means that an application cannot have a parent process check for termination of a particular child with kill(). (Usually this is done with the null signal; this can be done reliably with waitpid().) In response to the originally issue, this was fixed *specifically* for the case of kill(pid, 0). But my reading of the above is that kill() should return 0 in this case regardless of the signal (modulo permissions, etc.). On Linux, for example, when calling kill with pid of a zombie process the kernel will happily deliver the signal to the relevant task_struct; it will just never be acted on since the task will never run again. Signed-off-by: Corinna Vinschen --- winsup/cygwin/signal.cc | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/winsup/cygwin/signal.cc b/winsup/cygwin/signal.cc index ff101e368..d819e779b 100644 --- a/winsup/cygwin/signal.cc +++ b/winsup/cygwin/signal.cc @@ -260,7 +260,7 @@ _pinfo::kill (siginfo_t& si) } this_pid = pid; } - else if (si.si_signo == 0 && this && process_state == PID_EXITED) + else if (this && process_state == PID_EXITED) { this_process_state = process_state; this_pid = pid;