On 6/14/06, Raseel Bhagat <raseelbhagat@xxxxxxxxx> wrote:
Hi Abu,
On 6/14/06, Jinesh K J <jineshkj.newsletters@xxxxxxxxx> wrote:
> On 6/14/06, Abu M. Muttalib <abum@xxxxxxxxx> wrote:
>
> > As mentioned in the description of vfork call, it is said that child is
not
> > allowed to write to the address space, but in the following example its
not
> > so. The child is able to write to the process address space. This
program
> > was tested with Linux Kernel 2.6.9. Why is it so?
>
> i think you might probably have misread it. the child is not
> disallowed to write to the address space, but its just not healthy.
> since child is using the same address space as that of the parent,
> whatever global variables accessed by the child and parent will both
> be the same.
Exactly, it does not mention in the man page of vfork that the child is NOT
ALLOWED to write to the address space.
> u can refer to the arch/xxx/kernel/process.c and see the sys_vfork
> implementation. the child's virtual memory is a clone of that of the
> parent. also note that the parent is put on hold till the child exits
> or execvs, so that this technique cannot be used for truly parallel
> IPC between parent and child. its intention is clearly mentioned in
> the man page itself.
Exactly, the vfork is primarily used in cases when it is expected that the
child will execve immediately (before writing anything) after it is born.
> >
> > fork.c
> >
----------------------------------------------------------------------------
> > ---------------------------------------
> > #include <stdio.h>
> >
> > unsigned char *glob_var = NULL;
> >
> > void main()
> > {
> > int pid = -8,i;
> > pid = vfork();
> >
> > if(pid < 0)
> > printf("\n FORK ERROR \n");
> >
> > if(pid == 0)
> > {
> > unsigned char * local_var = NULL;
> > local_var = (unsigned char *)malloc(5);
> > strcpy(local_var,"ABCD");
> > glob_var = local_var;
> > printf("\nCHILD :Value of glob_var is %X local_var is
%X glob_var is %c
> > \n",glob_var,local_var,*glob_var);
> > for(i=0;i<4;i++)
> > {
> > printf("\n CHAR is %c \n",glob_var[i]);
> > }
> > printf("\nCHILD1 :Value of glob_var is %X
%c\n",glob_var,*(glob_var));
> > }
> >
> > if(pid > 0)
> > {
> > printf("\nParent : Value of glob_var is %X
%c\n",glob_var,*(glob_var));
> > free(glob_var);
> > printf("\nParent : Value of glob_var is %X
%c\n",glob_var,*(glob_var));
> > exit(0);
> > }
> > }
> > ------------------------------
Btw, the same code gives Segmentation Fault on my Solaris 8 machine !!!!
i think its because the vfork implementation varies across platforms.
may be in solaris, the child would be sharing parents VM but could be
in write protected mode.
--
Raseel.
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