Hi All,

Here is a simple program I ran -

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

unsigned a =90;

int main ()
{
int pid = fork();

if ( pid == 0 )
{
a = 44;
printf ( "Addres of child copy of a = %X and value of a = %u\n", &a, a );

}
else
{
sleep(4);
printf ( "Address of parent copy of a = %X and value of a = %u\n", &a, a );
}
while(1);
return 0;
}

The output I got was :
Addres of child copy of a = 601030 and value of a = 44
Address of parent copy of a = 601030 and value of a = 90

What is strange is that the address of 'a' is the same in both the parent and the child!! And the value of 'a' is different!! fork() will ideally make a copy of even the data segment. In order to make sure my output doesn't depend on systems which implement copy on write ( i.e. till some write modifications are made, both parent and child use same copy even after fork() ) I forcibly modified value of 'a' in child. Yet address of 'a' remains the same in both child and parent.

Can anyone shed some light on fork()'s behavior?

Thanks,
Pavan

What you've observed is completely correct. Consider what would happen if `a' had a new address in either the pointer or child - then any previously obtained address of `a' would become invalid, and you simply cannot have that happening.

The reason you see this behaviour is because of virtual memory. Basically, when you request a memory address from an application program, you're actually requesting a virtual address, which is converted to a physical address by the kernel (well, actually by the CPU under direction from the kernel) which is the actual byte of RAM used. This allows two processes to maintain addresses over things like a fork() call and yet both have different copies of the variable.

See the Wikipedia article on virtual memory.


Why do you think this matters? It shouldn't (and no offence, but if you don't know about virtual memory then you're probably not writing anything where it does matter...).

1 members found this post helpful.

Thanks a lot for the quick reply John!

Just curious, so, does that imply that systems without virtual memory support cannot fork()? How do they create multiple processes then?

Thanks,
Pavan

Most modern systems have virtual memory support. When you specify "systems without virtual memory support" you're limiting yourself to very low-powered microcontrollers and such. These are the sort of systems that generally won't have an operating system (or will have a very small, minimalistic one). If you're writing software for one of these systems you may only have one process, or use your own (or aforementioned very small OS's) minimalistic task-switching between a predefined number of tasks. Each will probably have their own fixed areas of memory they can work in, and no memory protection.