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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 MINIX

preap

preap(1)                                                           User Commands                                                          preap(1)

NAME

       preap - force a defunct process to be reaped by its parent

SYNOPSIS

       preap [-F] pid...

DESCRIPTION

       A  defunct  (or  zombie)  process  is one whose exit status has yet to be reaped by its parent. The exit status is reaped via the wait(3C),
       waitid(2), or waitpid(3C) system call. In the normal course of system operation, zombies may occur, but are typically short-lived. This may
       happen  if a parent exits without having reaped the exit status of some or all of its children. In that case, those children are reparented
       to PID 1. See init(1M), which periodically reaps such processes.

       An irresponsible parent process may not exit for a very long time and thus leave zombies on the system. Since the operating system destroys
       nearly all components of a process before it becomes defunct, such defunct processes do not normally impact system operation. However, they
       do consume a small amount of system memory.

       preap forces the parent of the process specified by pid to waitid(3C) for pid, if pid represents a defunct process.

       preap will attempt to prevent the administrator from unwisely reaping a child process which might soon be reaped by the parent, if:

         o  The process is a child of init(1M).

         o  The parent process is stopped and might wait on the child when it is again allowed to run.

         o  The process has been defunct for less than one minute.

OPTIONS

       The following option is supported:

       -F       Forces the parent to reap the child, overriding safety checks.

OPERANDS

       The following operand is supported:

       pid      Process ID list.

EXIT STATUS

       The following exit values are returned by preap, which prints the exit status of each target process reaped:

       0        Successfully operation.

       non-zero Failure, such as no such process, permission denied, or invalid option.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE         |      ATTRIBUTE VALUE        |
       +-----------------------------+-----------------------------+
       |Availability                 |SUNWesu (32-bit)             |
       +-----------------------------+-----------------------------+
       |                             |SUNWesxu (64-bit)            |
       +-----------------------------+-----------------------------+

SEE ALSO

       proc(1), init(1M), waitid(2), wait(3C), waitpid(3C), proc(4), attributes(5)

WARNINGS

       preap should be applied sparingly and only in situations in which the administrator or developer has confirmed that defunct processes  will
       not be reaped by the parent process. Otherwise, applying preap may damage the parent process in unpredictable ways.

SunOS 5.10                                                          26 Mar 2001                                                           preap(1)
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