fork(), variable values between child and parent process????

dearvivekkumar · 02-24-2010, 04:37 AM

Code:

int main()
{
int pi;
int i=10;
pid=fork();
if(pid==0)
{
printf("The initial value of the variable i in the child process is %d\n"i);
 i+=10;
printf("value of varible  in child process after incrementation is %d\n",i);
printf('cheil terminated\n");
}
else
{
wait(0)
printf("value of i in parent process is %d\n',i);
}
}

Q 1. The value of the variable pid returned by the fork() function will be greater than 0 in the parent process and equal to zero in the child process?
but during forking, there values are exactly copied so what's went wrong here?

Q 2. "changes to the variable in one process is not reflected in the other process" why it is so???
>> Even if we have variable i declared as a pointer or a global it wont make a difference.

Code:

int main()
{
int i, pid;
i=10;
printf("before fork i is %d\n",i);
pid=fork();
if(pid==0)
{
printf("in child i's address is %u\n",&i);
i=20;
printf("i is 5d\n",i);
}
else
{
printf("In pareent i's address is %u\n",&i);
printf("i is %d\n",i);
}
}

Q. Through this program it is clear that both process is using the data from the same location, so where's the original value is residing when the child process is in execution.?

what is the reason, please reply. I am trying of learning unix progrmming.So, it may be a easy one for the proficient ones. so please help me out.

thanking in anticipation!

Aquarius_Girl · 02-24-2010, 04:49 AM

Quote:

The value of the variable pid returned by the fork() function will be greater than 0 in the parent process and equal to zero in the child process?
but during forking, there values are exactly copied so what's went wrong here?

I tried your program like this:

Code:

#include <stdio.h>
int main()
{
	int pi;
	int i   = 10;
	int pid = fork();
	
	if (pid == 0)
	{
		printf ("\npid : child %d\n", pid);
	}
	else
	{
		wait(0);
		printf ("\npid : parent %d\n", pid);
	}
}

I compiled the program and got following output:
pid : child 0

pid : parent 14415

So they are different, what's the problem ?

Have I understood your question wrongly ?

Quote:

"changes to the variable in one process is not reflected in the other process" why it is so???

The child process now has a different address space so the variable changes will not be reflected !

Quote:

Through this program it is clear that both process is using the data from the same location, so where's the original value is residing when the child process is in execution.?

when the child process is created, 2 copies of the variable are created, parent can modify his own copy and child can modify his own copy ..

dearvivekkumar · 02-24-2010, 04:57 AM

When we create any process any process it is exact duplicate of its parent. I want to know then why pid in child and parent process is different. it's different I knew after running the code but why it is so??

dearvivekkumar · 02-24-2010, 05:03 AM

Quote:

when the child process is created, 2 copies of the variable are created, parent can modify his own copy and child can modify his own copy ..

but they are using the same address location., i am trying to understand if they are operating on the same address then then where the parent process varible is while child is executing.

What is different "address space"?
I appreciate if you please explain this.

thank you anisha kaul

Aquarius_Girl · 02-24-2010, 05:06 AM

Quote:

When we create any process any process it is exact duplicate of its parent. I want to know then why pid in child and parent process is different. it's different I knew after running the code but why it is so??

Now I execute the following program with getpid() function:

Code:

#include <stdio.h>
int main()
{
	int pi;
	int i   = 10;
	int pid = fork();
	
	if (pid == 0)
	{
		printf ("\npid : child %d %d\n", pid, 	getpid());
	
	}
	else
	{
		wait(0);
		printf ("\npid : parent %d %d\n", pid, 	getpid());
	}
}

Output:
pid : child 0 14563

pid : parent 14563 14562

See closely,

Parent has received child's pid, i.e. 14563
and Parent's own pid is : 14562

dearvivekkumar · 02-24-2010, 05:11 AM

thaks mam!
i have got it.

Aquarius_Girl · 02-24-2010, 05:18 AM

Quote:

but they are using the same address location., i am trying to understand if they are operating on the same address then then where the parent process varible is while child is executing.

What is different "address space"?

They are not operating on the same address space, When a child process is created it copies the parents address space and thus now there exist 2 different address spaces.

For a very very basic understanding of address space see this link : http://www.uniforum.chi.il.us/slides/oracle/sld021.htm