child/child.go
func (c *Child) Pid() int {
	c.RLock()
	defer c.RUnlock()
	return c.pid()
}
func (c *Child) Pid() int {
	c.RLock()
	defer c.RUnlock()
	return c.pid()
}
// Pid returns the pid of the child process. If no child process exists, 0 is
// returned.