System call to count number of children in a process

Question

I've done all the steps to create a system call, and then created a user program and run it:

proc.c

int countChildren (int pid) {

    struct proc *p;
    int count = 0;

    acquire(&ptable.lock);

    for(p = ptable.proc; p < &ptable.proc[NPROC]; p++)
      if(p->parent->pid == pid) count++;

    release(&ptable.lock);

    return count;
}

sysproc.c

int
sys_getchildren(void)
{ 
    int pid;
    argint(0, &pid);
    return countChildren(pid);

}

userProgram.c

...
#include "types.h"
#include "user.h"

int main (void) {

    int n1 = fork(); 

    int n2 = fork();

    int n3 = fork();

    int n4 = fork(); 

    if (n1 > 0 && n2 > 0 && n3 > 0 && n4 > 0) { 
        printf(1,"parent\n"); 
        printf(1," getchildren = %d \n", getchildren()); 
    } 
    exit();
}

But the result is not what I expected, below is the result:

enter image description here

Mathieu · Accepted Answer · 2019-12-10T13:41:58.097

I think that your kernel code is corrected, your problem comes from the user code: you create processes but you do not take care of them so they become zombies and could not be counted.

When a process exit and is not waited by its parent, it become a zombie:

a zombie is a process is adopted by the init process (see exit definition in file proc.c) and cannot be counted in child.

To correct your testing code, make the processes to sleep for some time and to wait their child:

#include "types.h"
#include "user.h"

int main (void) {

    int n1 = fork(); 
    int n2 = fork();
    int n3 = fork();
    int n4 = fork(); 

    if (n1 > 0 && n2 > 0 && n3 > 0 && n4 > 0) { 
        printf(1,"parent\n"); 
        printf(1," getchildren = %d \n", getchildren()); 
    } 

    /* wait for all child to terminate */
    while(wait() != -1) { }

    /* give time to parent to reach wait clause */
    sleep(1);

    exit();
}

Edit: you have a little typo in syscall, instead of getint you should get pid from myproc:

int
sys_getchildren(void)
{ 
    int pid;
    pid = myproc()->pid;
    return countChildren(pid);
}

or shorter:

int
sys_getchildren(void)
{ 
    return countChildren(myproc()->pid);
}

Even when I use the above code, it says that `getchildren = 1` whereas the parent process should have 4 direct children, am I wrong? — mgh, Dec 10 '19 at 12:55
@mahdigh: indeed, the `pid` was not correctly set in syscall. see my update — Mathieu, Dec 10 '19 at 13:42

System call to count number of children in a process

1 Answers1