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Unix Shell background processing

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# 22  
Old 09-28-2010
yes, I edited this like 9 times, so reread again please Smilie

---------- Post updated at 04:03 PM ---------- Previous update was at 04:01 PM ----------
Quote:
Originally Posted by Mercfh
Ah that makes sense. I guess it's not a big deal, i might go back through and make some changes. I mean.....the way I have it isn't "wrong" is it?
It is wrong. Try 'echo a &', then 'sleep 60'. It won't wait for sleep to finish because it waited for echo's return instead of sleep's. Just redo wait() with waitpid() and it will wait for the correct process. It will still be incomplete, because you'll still be leaving around zombie processes whenever you background something, but adding a SIGCHLD handler can be done later to complete it.
# 23  
Old 09-28-2010
Ya I noticed literally just now with using sleep 60 right after a ls &......
it still let me type it in.

darnit.
so to use waitpid i'd replace wait with waitpid( pid, &status, WNOHANG);
? or should I use an actual integer for pid

btw I made an integer called status in my execute function, so it now looks like this.
Code:
void Execute(char* cmd[],bool BG)
{
    int status;
    pid_t pid;
    pid = fork();
    switch(pid)
    {
        case -1:  
//Debug     cout << "Fork Failure" << endl;
            exit(-1);
        case  0:
            execvp(cmd[0], cmd);
    
            if(execvp(cmd[0], cmd) == -1)
            {
                cout << "Command Not Found" << endl;
                exit(0);
            }
   
          default:
            if(BG == 0)
            {

                    waitpid( pid, &status, WNOHANG);
//Debug             cout << "DEBUG:Child Finished" << endl;
            }


    }
    
}

the man. pages for waitpid are a bit confusing....
Last edited by Mercfh; 09-28-2010 at 07:21 PM..
# 24  
Old 09-28-2010
From man waitpid:
Code:
WNOHANG     return immediately if no child has exited.

...which doesn't sound like what you want. Give it 0 instead.

Otherwise, looks OK.
# 25  
Old 09-28-2010
It seemed to have worked. The terminal is still messed up, but im ok with that. I tried the sleep and even when it was off kilter it still made it wait.

So as to improve my learning, cause you explain why exactly this worked. wait() as I understand waits for any process to finish before moving on....so waitpid waits for the specific pid process to finish.......but aren't background processes specific to this pid too.

Also...I assume status really has no point except maybe in the sighandler.

I have no clue how you figured this out? i never would've thought of that. But then again im new to processes and such.
# 26  
Old 09-28-2010
Quote:
Originally Posted by Mercfh
So as to improve my learning, cause you explain why exactly this worked. wait() as I understand waits for any process to finish before moving on....so waitpid waits for the specific pid process to finish.......but aren't background processes specific to this pid too.
Every process has its own unique pid. By waiting for the PID you can wait for one specific process and no others.
Quote:
Also...I assume status really has no point except maybe in the sighandler.
You can get the function's return value with WEXITSTATUS(status). This is important to find out if a process returned success(0) or failure(anything else). You might want to make it the return value of your Execute function.
Quote:
I have no clue how you figured this out?
I remembered that this was leaving zombie processes around, so that what you were waiting for probably wasn't what you were getting.
# 27  
Old 09-28-2010
Quote:
Every process has its own unique pid. By waiting for the PID you can wait for one specific process and no others.
Oh so it's waiting for them in order? as they come then, instead of just whatever gets done first? sorta...?

either way you've been a HUGE help. and I can say I've honestly learned alot by doing this....it certainly was interesting..
# 28  
Old 09-28-2010
Quote:
Originally Posted by Mercfh
Oh so it's waiting for them in order as they come then, instead of just whatever gets done first? sorta...?
I'm not sure there's any particular order you can count on if you don't give the pid.
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