Thread: Need help with fork and exit

Hello Everyone,

I am trying to write a code where parent process will increment a variable and then child will increment the same variable(its own copy). However, I see that after child process returns to parent by calling exit, it also executes rest of the parents code in its context while using parents copy of the variable. The program is given below.

Code:

#include <stdio.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <semaphore.h>
sem_t mutex;

int main(void)
{
int i, j, fd;
pid_t pid;
int count = 0;
sem_init(&mutex, 1, 1);
/* fork a new process */
if (fork() == 0) {
/* The child will run this section of code */
for (j=0;j<5;j++)
{
sem_wait(&mutex);
printf("Child count %d %d\n", ++count, getpid());
sem_post(&mutex);
}
/* exit the child process */
printf("Child PID(%d): exiting...\n", getpid());
exit;
}

/* The parent will run this section of code */
/* give the child a chance to start running */
sleep(2);
for (i=0;i<5;i++)
{
sem_wait(&mutex);
printf("Parent count %d %d\n", ++count, getpid());
sem_post(&mutex);
}
/* exit the parent process */
printf("Parent PID(%d): exiting...\n", getpid());
return 0;
}

Here is the output:

./a.out
Child count 1 29825
Child count 2 29825
Child count 3 29825
Child count 4 29825
Child count 5 29825
Child PID(29825): exiting...
Parent count 1 29824
Parent count 2 29824
Parent count 3 29824
Parent count 4 29824
Parent count 5 29824
Parent PID(29824): exiting...
Parent count 6 29825
Parent count 7 29825
Parent count 8 29825
Parent count 9 29825
Parent count 10 29825
Parent PID(29825): exiting...


I see that if I use 'return' instead of 'exit' then this problem does not appear. If I use an "if-else" to segregate child and parent code then also this problem does not appear.

Could anyone please help me to understand what is going on here?

Thanks,
With Regards,
Indranil

Hello,

When i compile you program I have a warning "statement with no effect" line 27. It goes away if you put exit(1), and then the program works correctly.

Hope this helps,

hint: exit is a function, not a keyword.

is this an OS class?^^

ok, so the special thing about fork() is its return value. When in the child process it returns 0 and when in the parent process it doesn't. So this way it can be used to determine in which process we currently are.

Suppose there is first a parent with PID 29824. It runs until fork(). At this moment an exact same child process is created, which will continue running from that line. Right now you have two processes which run at the same time. However the parent process will skip the if and go directly to sleep(). That's why you see the "Child count" print first. If you remove sleep() they should be in mixed order. (If I'm not mistaken. I can't compile your code...). Then child exits in the if clause, and parent should be done sleeping. Then the parent continues after sleep, printing it's count. Of course, it will also start from 1, since it's variable is independent of that of the child process.
I don't know where the second parent comes from though, didn't appear in your code.

I don't know what you mean by the child executing the parents code in its context. After the child exits, it is gone, away. The main thing to realize is that after fork() the child and parent process both exist and execute at the same time.

but I thought the child process stops after "exit" ?

it would if exit were being called as a function, but in this program it is not. the OP is attempting to use exit in the same way as return is used, and this is incorrect. refer to my previous post above.