hi Lal, thanks for the analysis and clearing the doubt. so, parent process returns while child is added in to runqueue. right?
Regards, mitul modi On Sun, Oct 5, 2008 at 1:55 PM, Lal <[EMAIL PROTECTED]> wrote: > On Thu, Oct 2, 2008 at 3:55 PM, Mitul Modi <[EMAIL PROTECTED]> wrote: > > > > > > On Wed, Oct 1, 2008 at 6:12 PM, srimugunthan dhandapani > > <[EMAIL PROTECTED]> wrote: > >> > >> hi all, > >> I want to understand how the fork call return 0 in child and 'pid of > >> child' in the parent. > >> Presently my (naive)understanding is that the %eax value is stored > >> differently for the child and the parent. > >> Both the child and the parent returns from fork to the same instruction > >> address, but will have different return values according to %eax. > > The parent process and child process get their return values > differently. As Peter Teoh explained, kernel writes the child > process's pid directly to parent process user space pointer. > In function copy_process > if (clone_flags & CLONE_PARENT_SETTID) > if (put_user(p->pid, parent_tidptr)) > > While child process gets return value from %eax. > In function copy_thread > childregs = task_pt_regs(p); > *childregs = *regs; > childregs->eax = 0; > childregs->esp = esp; > > Therefore, after fork system call in application, parent process > returns with child process' pid while child process returns with 0. > > -Lal >
