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Full Discussion: kill a process which run out of time
Top Forums Programming kill a process which run out of time Post 302273850 by aaronwong on Tuesday 6th of January 2009 01:51:44 AM
Old 01-06-2009
Quote:
Originally Posted by nicos
hello everybody!!
i want ur help! it is urgent!!

...
pid=fork();
if(pid==0)
{
execl(a program);
exit(1);}
else if (pid>0)
{
timer(5); //(command 1)timer is a function that count up to 5sec
if(kill(pid,0)==0)kill(pid,9);//(command 2)
wait(&status);
....
}
else
perror("error");
....
The purpose of command1 and command2 is, if the execution of child process
does not finish in 5sec(timer(5)), i want to kill it (kill(pid,9)).
Can anybody pleasee help why this thing does not work properly!!!
THANX in advance!:-)
It's because before calling wait() to wait child exits, the child won't actually disappear in the system, i.e. if a child exits, but the parent does not immediately call wait() or waitpid() to handle the SIGCHLD from its child process, the exited child process would keep as a zombie in the system, until the parent exits or the parent call wait() or waitpid(). And kill -SIGNAL to a zombie will always succeed, so kill() returns 0.
You can try the following code zombie.c:
Quote:
include<stdio.h>
#include<unistd.h>
#include<sys/types.h>

int main()
{
pid_t pid;

pid=fork();

if(pid==0) {
printf("child process, sleep 5s and exit!");
sleep(5);
exit(0);
}
else if(pid>0) {
printf("parent process, sleep 10s, then kill and wait!");
sleep(10);
printf("parent process, kill!");
if(kill(pid, 0)==0) {
printf("child still alive!");
kill(pid, 9);
}
waitpid(pid, NULL);
exit(0);
}
}
you will see after 5s, child exits, and before parent kill it and wait it, by "pf -ef | grep zombie", a <defunct> process could be found.
 

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LEARN ABOUT ULTRIX

kill

kill(2) 							System Calls Manual							   kill(2)

Name
       kill - send signal to a process

Syntax
       #include <sys/types.h>
       #include <signal.h>

       kill(pid, sig)
       pid_t pid;
       int sig;

Description
       The  system  call sends the signal sig to a process specified by the process number pid.  The sig can be a signal specified in a call or it
       can be 0.  If the sig is 0, error checking is performed, but a signal is not sent.  This call can be used to check the validity of pid.

       The sending and receiving processes must have the same effective user ID, otherwise this call is  restricted  to  the  superuser  with  the
       exception of the signal SIGCONT.  The signal SIGCONT can always be sent to a child or grandchild of the current process.

       If the process number is 0, the signal is sent to all other processes in the sender's process group.

       If  the process number is negative but not -1, the signal is sent to all processes whose process-group-id is equal to the absolute value of
       the process number.

       The above two options are variants of

       If the process number is -1, and the user is the superuser, the signal is broadcast for all processes except to system  processes  and  the
       process sending the signal.

       Processes may send signals to themselves.

Environment
       System Five
       POSIX

       When  your  program  is	compiled in the System V or POSIX environment, a signal is sent if either the real or effective uid of the sending
       process matches the real or saved-set-uid (as described in ) of the receiving process. In addition, any process can use a pid  of  -1,  and
       the signal is sent to all processes subject to these permission checks.

       In POSIX mode, the pid argument is of type pid_t.

Return Values
       Upon successful completion, a value of 0 is returned.  Otherwise, a value of -1 is returned, and is set to indicate the error.

Diagnostics
       The system call fails under the following conditions:

       [EINVAL]       The sig is not a valid signal number.

       [EPERM]	      The  sending  process  is not the superuser, and its effective user ID does not match the effective user ID of the receiving
		      process.

       [ESRCH]	      No process can be found corresponding to that specified by pid.

See Also
       execve(2), getpgrp(2), getpid(2), killpg(2), sigvec(2), pause(3)

																	   kill(2)
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