Before going on, assume you've read my previous post on pid management

Thanks to the struct process, our work is much simplified. Quoting Eric S.Raymond here.

Smart data structures and dumb code works a lot better than the other way around.


At first glance, the logic of waitpid is trivial. Yes, it's indeed in terms of the "core code": Just acquire the exitlock and then see if the process has exited, then wait it exit using cv_wait on exitcv and get it's exitcode. Here I use cv to coordinate child and parent process. Or you can use semaphore with initial count 0: child will V the semaphore when it exits, and parent will P the semaphore on waitpid.

But it turns out that most the code of waitpid is argument checking! More arguments means more potential risks from user space. Sigh~ Anyway, we are doing kernel programming. And just take a look at $OS161_SRC/user/testbin/badcall/bad_waitpid.c and you'll know what I mean.

So basically, we need to check:

  • Is the status pointer properly aligned (by 4) ?
  • Is the status pointer a valid pointer anyway (NULL, point to kernel, ...)?
  • Is options valid? (More flags than WNOHANG | WUNTRACED )
  • Does the waited pid exist/valid?
  • If exist, are we allowed to wait it ? (Is it our child?)

And also, after successfully get the exitcode, don't forget to destroy the child's process structure and free its slot in the procs array. Since one child has only one parent, and after we wait for it, no one will care for it any more!


This part is easy. (Mostly because exit only take on integer argument!) All we need to do is find our struct process entry using curthread->t_pid. And then indicate that "I've exited" and fill the exitcode. The only thing to note that the exitcode must be maked using the MACROs in $OS161_SRC/kern/include/kern/wait.h. Suppose user passing in _exitcode, then we need to set the real exitcode as _MKWAIT_EXIT(_exitcode).

And if we are smarter, we can first check if parent exist or if parent has exited, then we even don't bother fill the exitcode since no one cares! Anyway, it's just a tiny shortcut, and totally optional.