Re: Linux process...

[stephan <fuku.riu@xxxxxxxxx>]

Initialize the stack variable, when you do its retained thoughout 
execution of both processes. I think when a variable is uninitialized  
its given null value, and doesnt really get copied in a sense, I have 
also read that threads can have their own local thread storage areas.

try a strace on the executable to see what flags clone() is being used 
by glib.


beginner_h4x3r wrote:
> Okay, so the child process was not actually access it's parent
> variable, the child given a copy (i have learned about Copy On Write
> mechanism too).  But it is like a C language issue: we can access any
> variable which declared in that function in this case main() function.
> So in my code, when i try to access stack_int variable in child
> process, it's not wrong, compiler even recognize this as 'valid'
> approach... How about my conclusion?
>
> On Mon, Mar 30, 2009 at 6:36 PM, Fabian Ischia <fischia@xxxxxxxxxxxxxxxx> wrote:
>   
> > From the example code, I think the answer is a bit simpler than it looks
> > like. The address space is "duplicated" not "shared". Whatever you do in one
> > process "after" the fork will not affect the other process.
> > The Child process has not initialized the variable, so being a stack
> > variable it just contains garbage.
> >
> > Fabian
> >
> > beginner_h4x3r wrote:
> >     
> > > Hi All..
> > >
> > > I am a beginner hacker, i want to learn Linux from scratch. I read
> > > some resources on Linux's process management. Process duplicates it's
> > > page table to it's child process, right? so i wrote demonstrate code
> > > to prove this.
> > >
> > > #include <stdio.h>
> > > #include <stdlib.h>
> > > #include <unistd.h>
> > > #include <sys/types.h>
> > > #include <sys/wait.h>
> > >
> > > int main (void) {
> > >  pid_t child;
> > >  int stack_int;
> > >
> > >  child = fork ();
> > >  if (child == 0) {
> > >      sleep (1); /* ;p */
> > >      printf ("child process stack_int value %i, address: %p\n",
> > > stack_int, &stack_int);
> > >      exit (0);
> > >    }
> > >  if (child == -1) {
> > >      perror ("fork");
> > >      return -1;
> > >    }
> > >  stack_int = 32;
> > >  printf ("main process stack_int value %i, address: %p\n", stack_int,
> > > &stack_int);
> > >  waitpid (child, NULL, 0);
> > >
> > >  return 0;
> > > }
> > >
> > > The output is:
> > > main process stack_int value 32, address: 0xbf9c66ec
> > > child process stack_int value 8495092, address: 0xbf9c66ec
> > >
> > > stack_int value is different from parent and it's child.
> > >
> > > My question: why the stack_int has a same address between parent and
> > > it's child ?, but confusedly... they have a different value, i was
> > > though it should be different, since process duplicate it's page to
> > > child, please explain me. ;)
> > >
> > > Thanks before.
> > >
> > > --- curious_hacker
> > > --
> > > To unsubscribe from this list: send the line "unsubscribe
> > > linux-c-programming" in
> > > the body of a message to majordomo@xxxxxxxxxxxxxxx
> > > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > >
> > >       
> --
> To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in
> the body of a message to majordomo@xxxxxxxxxxxxxxx
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>
>   

--
To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html