Question

This is a homework question. I have to write a program forking itself 20 times. Each new process is adding +1 to a variable (integer) which is shared between all of them. The thing is, I have to use semaphores (IPC). This piece of code is ‘working’ – giving the value of 20 in the end.

*buf = 0;
for(i=1; i<=20; ++i) 
{
    if(fork()!=0)
    {
        *buf += 1;
        exit(0);
    }
}

EDIT:
Based on this code I am trying to get output like :
I am child 1…
I am child 2…
.
.
.
I am child 20…

It worked once (first time), and then the order became random. But I did not change any code. What am I doing wrong?

Answer 1

Well your major problem is this:

    if (fork()!=0)   //<-- this

fork() will return -1 on error, the parent pid, OR ZERO for the child. So you are actually doing everything in the parent. Change to (fork() ==0) and it does what you want.

Also you should wait on your children and detach shared memory. (I added some output of the process ids to make it a little clearer.)

printf("I AM THE PARENT pid = %d\n", getpid());

*buf = 0;
for(i=1; i<=20; ++i)
{
    if((pid = fork()) == -1)
    {
        perror("fork");
        exit(1);
    }

    if (pid == 0)
    {
        v(semid, 0);
        *buf += 1;
        p(semid, 0);
        printf("I am child %d with pid = %d\n", i, getpid());
        shmdt(buf);
        exit(0);
    }
}


for (i = 1; i <= 20; ++i)
{
    pid = wait(&status);
    printf("child pid = %d reaped\n", pid);
}

printf("buf: %p\n", buf);
printf("*buf: %d\n", *buf);

shmdt(buf);

return 0;