Re: Linux process...

[stephan <fuku.riu@gmail.com>]

 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@somanetworks.com> 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
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