Killing A Process

tommyb05 · 05-31-2009, 08:49 AM

The purpose of the following code is to complete this goal: launch a process, get its status, and then kill it.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include <sys/wait.h>
#include <unistd.h>

int main()
{

int g, s;
pid_t c;

if((c = fork()) == 0)
{

     execl("/usr/bin/vlc", "vlc", NULL);

}

     wait(&s);

     kill(c, SIGKILL);

     return 0;

}

Now, what the code does do it launch the process, gets its status, and then wait for the user to exit off the process manually and then kill the process id. I want it to automatically (sleep for 5 seconds) and kill the process without requiring user input to close the process.

My question is how can I get the status of my child process, wait 5 seconds, and then kill the process?

I hope I explained what I need the code to do well, if not, ask me questions and I will answer them to the best of my ability.

Thank you!

MK27 · 05-31-2009, 09:03 AM

Since you are not in control of the process, AFAICT the only way to get status on it would be with a system call (eg, "ps --pid XXXX").

sleep(5) will delay for 5 seconds.

You should use SIGTERM before you use SIGKILL and only use SIGKILL if SIGTERM fails. SIGTERM is the actual default used by the *nix "kill" command.

tommyb05 · 05-31-2009, 10:01 AM

I'm pretty sure I wait() takes in an int for the child process's status (s here), but that isn't my problem. I can sleep() all I want but it still kills the process AFTER I close it manually, not before.

Any other ideas?

cas · 05-31-2009, 10:29 AM

It depends on what you're trying to do. This:

Quote:

what the code does do it launch the process, gets its status, and then wait for the user to exit off the process manually and then kill the process id

leads me to believe that you're slightly confused about what wait() and kill() do.

When you say "get its status", what do you mean? You appear to believe that wait() gets information on a running process, but it does not. You don't wait() on a process that's running, you wait on a process that's dead (or stopped, but that's not an issue here). Or, at least, when you wait() on a running process, it waits until the process dies; hence its name.

Once a process exits, there's no point in killing it, either. It's already dead. So if you've successfully waited on a process, you shouldn't kill it, because you don't own that pid anymore.

So when you're asking for the status of a process, what exactly are you looking for?

Also: the NULL you pass to execl() should be cast to char*, because the compiler can't properly convert NULL in a variable argument list. One of the rare occasions where a cast is correct.

tommyb05 · 05-31-2009, 10:48 AM

Alright, I believe I understand a little better now. Heres what I see when I run this code:

The program launches (and I want it to die 5 seconds after its launched)

I have to manually click "X" on the program for it to close, and then lau quits.

How can I make it automatically close after 5 seconds?

cas · 05-31-2009, 12:41 PM

MK27's post has the relevant information, so:

Code:

pid_t pid;
int s;
pid = fork();
if(pid == -1) err(1, "fork");
if(pid == 0)
{
  execl("/usr/bin/vlc", "vlc", (char *)NULL);
  _exit(1);
}
sleep(5);
kill(pid, SIGTERM);
sleep(1);
kill(pid, SIGKILL); /* not strictly necessary, but SIGTERM won't necessarily kill off a process  */
if(wait(&s) == -1) err(1, "wait");
if(WIFSIGNALED(s) && (WTERMSIG(s) == SIGTERM || WTERMSIG(s) == SIGKILL)) puts("killed by us");
else if(WIFEXITED(s) && WEXITSTATUS(s) == 0) puts("exited normally");
else puts("something else happened");

The last bit there tries to figure out how the process died, but it's not necessarily accurate. If vlc catches SIGTERM and exits by itself, then WIFSIGNALED(s) will be false. You can circumvent this by always just sending SIGKILL but that's usually a bad idea.

Brafil · 06-01-2009, 06:11 AM

_exit will hopefully never execute. execl _replaces_ the process image.

matsp · 06-01-2009, 06:16 AM

Quote:

Originally Posted by Brafil

_exit will hopefully never execute. execl _replaces_ the process image.

But it will exit if execl() can't find the executable (or some such).

--
Mats

Brafil · 06-01-2009, 06:41 AM

I said hopefully. Else you're right. But I think it'd be better to print out an error message and then quit.

05-31-2009, 08:49 AM	#1
tommyb05 Registered User Join Date: May 2009 Posts: 11	Killing A Process The purpose of the following code is to complete this goal: launch a process, get its status, and then kill it. Code: #include <stdio.h> #include <stdlib.h> #include <string.h> #include <signal.h> #include <sys/wait.h> #include <unistd.h> int main() { int g, s; pid_t c; if((c = fork()) == 0) { execl("/usr/bin/vlc", "vlc", NULL); } wait(&s); kill(c, SIGKILL); return 0; } Now, what the code does do it launch the process, gets its status, and then wait for the user to exit off the process manually and then kill the process id. I want it to automatically (sleep for 5 seconds) and kill the process without requiring user input to close the process. My question is how can I get the status of my child process, wait 5 seconds, and then kill the process? I hope I explained what I need the code to do well, if not, ask me questions and I will answer them to the best of my ability. Thank you!
tommyb05 is offline

05-31-2009, 09:03 AM	#2
MK27 critical genius Join Date: Jul 2008 Location: SE Queens Posts: 5,228	Since you are not in control of the process, AFAICT the only way to get status on it would be with a system call (eg, "ps --pid XXXX"). sleep(5) will delay for 5 seconds. You should use SIGTERM before you use SIGKILL and only use SIGKILL if SIGTERM fails. SIGTERM is the actual default used by the nix "kill" command. __________________ "A man can't just sit around." --* Larry Walters
MK27 is offline

05-31-2009, 10:01 AM	#3
tommyb05 Registered User Join Date: May 2009 Posts: 11	I'm pretty sure I wait() takes in an int for the child process's status (s here), but that isn't my problem. I can sleep() all I want but it still kills the process AFTER I close it manually, not before. Any other ideas?
tommyb05 is offline

05-31-2009, 10:48 AM	#5
tommyb05 Registered User Join Date: May 2009 Posts: 11	Alright, I believe I understand a little better now. Heres what I see when I run this code: The program launches (and I want it to die 5 seconds after its launched) I have to manually click "X" on the program for it to close, and then lau quits. How can I make it automatically close after 5 seconds?
tommyb05 is offline

06-01-2009, 06:11 AM	#7
Brafil Making mistakes Join Date: Dec 2008 Posts: 347	_exit will hopefully never execute. execl _replaces_ the process image. __________________ Look at this: Community Project And this: Ascent - Programmer needed
Brafil is offline

06-01-2009, 06:41 AM	#9
Brafil Making mistakes Join Date: Dec 2008 Posts: 347	I said hopefully. Else you're right. But I think it'd be better to print out an error message and then quit. __________________ Look at this: Community Project And this: Ascent - Programmer needed
Brafil is offline

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