Constant SYS_PROC_WAITPID 

pub const SYS_PROC_WAITPID: usize = 310;

Wait for a specific child process to change state.

• pid: child PID to wait for (-1 = any child)
• status_ptr: pointer to receive exit status
• options: WNOHANG, WUNTRACED, WCONTINUED flags

Returns the child PID, or -ECHILD if no children exist.