-Original Message-
From: Christopher Faylor
Sent: Wednesday, December 17, 2003 6:27 PM
To: [EMAIL PROTECTED]
Subject: Re: kill(pid, 0) issue
On Wed, Dec 17, 2003 at 05:23:20PM +0100, Nowakowski
Maciej-AMN011 wrote:
My application creates additional process using fork() function.
Created child process listens on a socket and exits when it receives
anything. The main process checks the child PID using kill(pid, 0)
with child PID as a parameter. Even when the child has exited this
function call returns 0. When I have supplied any PID which hasn't
ever existed it's fine and kill(non-existentPID, 0) returns -1.
Has anyone experienced something like this?
This seems like a perfect place for a simple test case. For instance:
#include stdio.h
#include stdlib.h
#include signal.h
#include unistd.h
#include sys/wait.h
int
main (int argc, char **argv)
{
int pid;
if (argv[1])
pid = atoi(argv[1]);
else if ((pid = fork ()) == 0)
{
puts (forking a process and then exiting);
exit (0);
}
else
{
int dummy;
wait (dummy);
}
printf (%d = kill (%d, 0)\n, kill (pid, 0), pid);
exit (0);
}
I tried the above with no argument and with an argument of a
previously forked-and-exited process. Both cases produced
the expected result, as did trying this on a running process.
I suspect that you are not 'wait()'ing for the process to
exit before checking if it exists. kill(pid, 0) will succeed
on both linux and cygwin if the process is not reaped by
calling wait (or waitpid, etc.) first.
cgf
Christopher,
Thanks for a reply. I have run your test case and it really works.
So then I have modified it slightly to make it similar to what my
application is doing. Now it reproduces the behaviour of my app.
I'm running this test below in a Windows console and using Cygwin
console to display processes issuing 'ps' command. I have expected
the child process to be finished after some time(when main process
is blocked on 'getc()') but kill(pid, 0) returns 0.
Probably there is an issue lying beneath I'm not aware of.
#include stdio.h
#include stdlib.h
#include signal.h
#include unistd.h
#include sys/wait.h
static void ChildExitSigHandle(int sig)
{
printf (Child termination signal received...\n);
}
int
main (int argc, char **argv)
{
int pid;
signal(SIGCHLD, ChildExitSigHandle);
if (argv[1])
pid = atoi(argv[1]);
else if ((pid = fork ()) == 0)
{
puts (forking a process and then exiting);
exit (0);
}
else
{
//int dummy;
//wait (dummy);
getc(stdin);
}
printf (%d = kill (%d, 0)\n, kill (pid, 0), pid);
exit (0);
}
Regards,
Maciek
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