fork()ing hell!!

theultimatechuf · #1 (**permalink**) 01-22-2002

Hello I am having serious trouble with the fork command, i basically want to create 9 or 10 child processes and store their pid numbers in array while the children stay resident until i kill() them later , i cannot seem to control how many are made as they all seem to create their own children.

I have managed to stop crashing my system, can somebody help please, none of my books are any use.

aniruddha · #2 (**permalink**) 01-22-2002

This is a very common problem observed with fork.
You can have some flag variable which will prevent the child process to fork again when the parent forks.

A very primitive code may be somthing like this:-

#include <stdio.h>
#include <unistd.h>
#include <errno.h>

void main()
{
int child1, child2, val, if_child;
char err[1000];

if_child = 1;

memset(err,'\0',strlen(err));

printf("\nParent process ID is %d \n",getpid());

child1 = fork();
if (child1 == -1)
{
strcpy(err, strerror(errno));
}
else
{
if(child1 > 0) /* If fork command is successful child PID will be greater than zero. */
{
printf("\nThe child1 process ID is %d \n", child1);
if_child = 0;
/* Set the flag here to prevent child process from forking */
}
}

memset(err,'\0',strlen(err));

if (if_child == 0)
{
child2 = fork();
if (child2 == -1)
{
strcpy(err, strerror(errno));
}

printf("\nThe child2 process ID is %d \n", child2);

kill(child2);
}

kill(child1);

exit(0);
}

theultimatechuf · #3 (**permalink**) 01-22-2002

Thanks a lot, looks promising, i look forward to the day when i can answer someone else's *NIX programmig question. Looks like it's gonna take longer than with VB.

bb666 · #4 (**permalink**) 01-23-2002

I had the same problem with one of my projects (you can see it if the thread called 'making a process tree') and getting more child processes than i expected was really a nasty problem.
Still haven't figured out how it works exactly but i hope aniruddha's advice will help me too.

theultimatechuf · #5 (**permalink**) 01-24-2002

Thanks annibuddha (was that how you spell it?) seeing bb666's post i thought i'd post this...

I had a bit of trouble as i added more children but decided to store the parent id at the start:

pid=getpid()

than later i just checked the variable which i understand would be in all the children against against getpid()

i.e. to make a child process stop and wait forever after it was created:

if(pid!=getpid()) //this is not the parent
for(;;)

then the processes just sit there until they are killed (hopefully )

bb666 · #6 (**permalink**) 01-25-2002

Yep. That's one way to do it or you could just send a stop signal so the child processes won't waste your memory for nothing. Just replace for(;;)
with kill(getpid(),SIGSTOP).
That's all fine and dandy but what if you want the child process to actually do something? Cause if you stop it or send it in an infinite loop then you can't quite work within the child can you?
That's actually the part i'm interested in: working within the child but prevent it from forking once the parent forks again.

Perderabo · #7 (**permalink**) 01-26-2002

You guys are making this much harder than it needs to be. Let's start by forking one process and storing its pid:

Code:

#include &lt;stdio.h>
#include &lt;unistd.h>

void main()
{
        int pid, parentpid, childpid;

        parentpid=getpid();
        printf("I am the parent process and my pid is %d\n", getpid());

        if (pid=fork()) {
                childpid=pid;
        } else {
                printf("I am a child process and my pid is %d\n", getpid());
                exit(0);
        }

        printf("I am still the parent process and my pid is %d\n", getpid());
        exit(0);
}

Here the child process just displays its pid then exits. You will probably want to do more with your child processes, but after your children take care of business they must exit so they participate in any further forking. Also in my example the parent process exits fairly quickly. This means that init will inherit the child process and will reap it when it dies. If I wanted to keep the parent around, I would need to insure that it issues wait() calls for each child who dies. If I didn't do this, the children would become zombies. I usually just let the parent die.

Once we have some code that does what we want, if we want to do it n times, we use a loop:

Code:

#include &lt;stdio.h>
#include &lt;unistd.h>

void main()
{
        int n, pid, parentpid, childpids[10];

        parentpid=getpid();
        printf("I am the parent process and my pid is %d\n", getpid());

        for(n=0; n<5; n++) {
                if (pid=fork()) {
                        childpids[n]=pid;
                } else {
                        printf("I am a child process and my pid is %d\n", 
                                        getpid());
                        exit(0);
                }
        }

        printf("I am still the parent process and my pid is %d\n", getpid());
        exit(0);
}

As requested by the OP, the children's pids are recorded in an array. But I still just let the parent die.

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