On 4/17/06, Gaurav Dhiman <gauravd.chd@xxxxxxxxx> wrote:
> On 4/17/06, Asim Kadav <linkasim@xxxxxxxxx> wrote:
> > Hi,
> >
> > Process A has a memory allocated and memory pointer x pointing to that memory.
> > I fork process B from a running process A.B gets a new copy of the
> > memory and the pointer.However, printing x in A and B points to the
> > same memory location(though actually differnt physical memory
> > locations).What is the concept of VM here.Can anyone kindly explain ?
>
> The address you are seeing here are the virtual address not the
> physical address. Virtual address are mapped to physical address
> through page table and page directory structures which are different
> for every process. The virtual address you are seeing is 32 bit
> address, this is divided into three parts, first 10 bits tells the
> index into page directory whose base address is stored in CR3 register
> of processor in i386 architecture. From this index we get the base
> address of concerned page table. Next 10 bits of virtual address is
> the index into page table whose base address we just got from page
> directory. Now at this index in page table we get the base address of
> the physical page we are concerned with. Now last 12 bits of virtual
> address is an offset into this physical address. This is how the
> virtual address is converted to physical address.
>
If the virtual address are same , then how can the physical address be
different. Can you give some examples please. I am not clear about
that.
> So when you are printing the address of a varaible in your parent and
> child process, both have same virtual address, but are actually mapped
> to two different physical addresses through their own page table.
>
> Hope this explanation helps you.
>
> regards,
> Gaurav
>
>
> >
> > =======================
> > For eg. :
> >
> > [root@XXXXX]# cat test.c
> > #include <stdio.h>
> >
> > main ()
> > {
> > char a;
> > a='b';
> > fork();
> > a='c';
> > printf ("%x\n",&a);
> > }
> >
> >
> > [root@XXXXXX]# ./a.out
> > bfed3237
> > bfed3237
> >
> > Thanks,
> > Link
> >
> > --
> > Kernelnewbies: Help each other learn about the Linux kernel.
> > Archive: http://mail.nl.linux.org/kernelnewbies/
> > FAQ: http://kernelnewbies.org/faq/
> >
> >
>
>
> --
> --
> -Gaurav
> Email: gauravd.chd@xxxxxxxxx
> ---------------------------------
> Read my blog at: http://lkdp.blogspot.com/
> ---------------------------------
>
> --
> Kernelnewbies: Help each other learn about the Linux kernel.
> Archive: http://mail.nl.linux.org/kernelnewbies/
> FAQ: http://kernelnewbies.org/faq/
>
>
--
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Archive: http://mail.nl.linux.org/kernelnewbies/
FAQ: http://kernelnewbies.org/faq/
