clone and the CLONE_CHILD_CLEARTID flag

Question

I need some help with the clone() system call. I'm trying to use it with the flag CLONE_CHILD_CLEARTID but I cant see any change in the value of the field I specify as argument. Here it is a simple code:

int the_child(){
   exit(0);
}
int main(int argc, char * argv[])
{
    pid_t child_id = 99;
    printf("child %p\n",child_id);

    clone((int (*)(void *))the_child,
          NULL,
          CLONE_VM | CLONE_CHILD_CLEARTID | SIGCHLD,
          NULL, NULL,NULL, child_id);

    sleep(1);
    printf("child %p\n",child_id);
 }

However when the two printf display always 99, What am I doing wrong?

Pavan Manjunath · Answer 1 · 2012-03-24T15:52:18.747

1

You are passing child_id through value. You should pass it as a pointer.

This is where the clearing of tid takes place inside the kernel

/*
 * We don't check the error code - if userspace has
 * not set up a proper pointer then tough luck.
 */
put_user(0, tsk->clear_child_tid);

Can you see the comment warning of the same ? :)

See this blog where they are tracking the arguments to clone system call through strace

EDIT : Adding from our discussion in comments, child_stack can be zero for sys_clone system call but not for the library function clone()

edited Mar 24 '12 at 15:52

answered Mar 24 '12 at 13:25

Pavan Manjunath

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I edited the code and used `&child_id` in the `clone` but nothing changed :( – Federico Mar 24 '12 at 14:09
My bad. Seams that I can not `NULL` value for the child stack. – Federico Mar 24 '12 at 14:33
@Federico Actually for the `sys_clone` system call, on top of which is the library function `clone` is implemented, `child_stack` can be zero. May be not when you are using the library function. After you supplied the stack appropriately for the child, what happened to the `tid` clearing functionality? – Pavan Manjunath Mar 24 '12 at 15:49

score 0 · Answer 2 · edited Mar 25 '12 at 10:26

0

Shouldn't you allocate memory (malloc()) before creating clone?

edited Mar 25 '12 at 10:26

arrowd

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answered Mar 24 '12 at 17:39

Kimboo Rasta

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clone and the CLONE_CHILD_CLEARTID flag

2 Answers2