Re: How to write a proper fork hook code?

[Samuel Thibault]

Svante Signell, le Sat 28 Feb 2015 15:32:47 +0100, a écrit :
> * how come?
> - code calling fcntl() works.
> - code calling the (RPC) file_record_lock() does not!

Which code? Without showing what you are doing, it's difficult for us to
understand what you mean.

> * the code below iterates over the file descriptor table entries. I
> would like to iterate over open file descriptors in the chile. How to do
> that?

It looks almost right.

> for (i = 0; i < _hurd_dtablesize; ++i)

You should enter the critical section before that, by using
HURD_CRITICAL_BEGIN and HURD_CRITICAL_END

> {      struct hurd_fd *d = _hurd_dtable[i];
>   if (d == NULL)
>     /* Nothing to do for an unused descriptor cell.  */
>     continue;
> 
>   /* Check if the FD is open */
>   // Not working??
>   //if (d->flags == O_RDONLY ||d->flags == O_WRONLY)

I don't think you need to test for that, d == NULL should be enough.

> err = HURD_FD_PORT_USE (d, __file_record_lock (port, F_SETLK64, &fl64));

You can probably use _hurd_port_get directly instead, see ctty_new_pgrp
for instance.

> No difference when using _hurd_fork_child_hook or
> _hurd_atfork_child_hook:
> //text_set_element (_hurd_fork_child_hook,fork_child_rlock);
> text_set_element (_hurd_atfork_child_hook, fork_child_rlock);

As I told you, the difference is that the former is run inside the
critical section (and various locks), while the latter is run outside of
it.

Samuel