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Date: Mon, 23 Dec 2013 16:29:57 +0100
From: Philippe Gerum <rpm@xenomai.org>
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Subject: Re: [Xenomai] file descriptors rework
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To: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>, Xenomai <xenomai@xenomai.org>

On 12/22/2013 06:00 PM, Gilles Chanteperdrix wrote:
>
> Here is the new proposed API for file descriptors:
>
> struct xnfd_ops {
> 	int (*destroy)(struct xnfd *fd);
> 	int (*select_bind)(struct xnfd *fd, struct xnselector *selector,
> 			unsigned type, unsigned index);
> };
>
> struct xnfd {
> 	unsigned magic;
> 	struct mm_struct *mm;
> 	int ufd;
> 	struct xnfd_ops *ops;
> 	unsigned refs;
> 	struct hlist_node hlink; /* Link in global hash */
> 	struct list_head link;	 /* Link in per-process queue */
> };
>
> int xnfd_enter(struct xnfd *xnfd, unsigned magic, int ufd,
> 	struct mm_struct *mm, struct xnfd_ops *ops);
>
> struct xnfd *xnfd_get(int ufd, struct mm_struct *mm, unsigned magic);
>
> int xnfd_put(struct xnfd *xnfd);
>
> int xnfd_close(struct xnfd *xnfd);
>
> The xnfd structure has to be put inside larger structure, and
> container_of used to access the larger structure. The magic is there to
> cope with the fact that different file descriptors users will have
> different structures, so the actual type has to be found before using
> container_of.
>
> refs is incrementer on get, decremented on put, when it reaches 0, the
> "destroy" callback is called, this in order to avoid destroying a file
> descriptor under another file descriptor user's feet.
>
> xnfd_close negates the magic in order to render the file descriptor invalid.
>
> the select_bind callbacks is needed to implement select uniformly.
>
> If nobody disagrees with this API, I will start implementing ASAP, and
> rebase the message queues code upon this.
>

Looks fairly straightforward to me. AFAICT, there may be an opportunity 
to merge the xnselect features with a broader fd-oriented support.

-- 
Philippe.