linux-kernel - Re: [PATCH v2] seq_file: Document seq_open_private(), seq_release

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Message-ID: <53F1DE1F.8050004@codethink.co.uk>
Date:	Mon, 18 Aug 2014 12:06:07 +0100
From:	Rob Jones <rob.jones@...ethink.co.uk>
To:	linux-fsdevel@...r.kernel.org
CC:	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-kernel@...ts.codethink.co.uk, viro@...IV.linux.org.uk,
	ebiederm@...ssion.com, swhiteho@...hat.com,
	ian.molton@...ethink.co.uk
Subject: Re: [PATCH v2] seq_file: Document seq_open_private(), seq_release_private()

Aargh! Scratch this patch. I got carried away with the error checking in
the examples. The "if (IS_ERR(p))" check is redundant. I'll remove it
and resubmit.

On 15/08/14 15:10, Rob Jones wrote:
> Despite the fact that these functions have been around for years, they are
> little used (only 15 uses in 13 files at the preseht time) even though
> many other files use work-arounds to achieve the same result.
>
> By documenting them, hopefully they will become more widely used.
>
> Signed-off-by: Rob Jones <rob.jones@...ethink.co.uk>
>
> ---
>   Documentation/filesystems/seq_file.txt |   36 ++++++++++++++++++++++++++++++++
>   1 file changed, 36 insertions(+)
>
> diff --git a/Documentation/filesystems/seq_file.txt b/Documentation/filesystems/seq_file.txt
> index a1e2e0d..c68308d8 100644
> --- a/Documentation/filesystems/seq_file.txt
> +++ b/Documentation/filesystems/seq_file.txt
> @@ -226,6 +226,42 @@ be used for more than one file, you can store an arbitrary pointer in the
>   private field of the seq_file structure; that value can then be retrieved
>   by the iterator functions.
>
> +There is also a wrapper function to seq_open() called seq_open_private(). It
> +kmallocs a zero filled block of memory and stores a pointer to it in the
> +private field of the seq_file structure, returning 0 on success. The
> +block size is specified in a third parameter to the function, e.g.:
> +
> +	static int ct_open(struct inode *inode, struct file *file)
> +	{
> +		return seq_open_private(file, &ct_seq_ops,
> +					sizeof(struct mystruct));
> +	}
> +
> +There is also a variant function, __seq_open_private(), which is functionally
> +identical except that, if successful, it returns the pointer to the allocated
> +memory block, allowing further initialisation e.g.:
> +
> +	static int ct_open(struct inode *inode, struct file *file)
> +	{
> +		struct mystruct *p =
> +			__seq_open_private(file, &ct_seq_ops, sizeof(*p));
> +
> +		if (!p)
> +			return -ENOMEM;
> +
> +		if (IS_ERR(p))
> +			return p;
> +
> +		p->foo = bar; /* initialize my stuff */
> +			...
> +		p->baz = true;
> +
> +		return 0;
> +	}
> +
> +A corresponding close function, seq_release_private() is available which
> +frees the memory allocated in the corresponding open.
> +
>   The other operations of interest - read(), llseek(), and release() - are
>   all implemented by the seq_file code itself. So a virtual file's
>   file_operations structure will look like:
>

-- 
Rob Jones
Codethink Ltd
mailto:rob.jones@...ethink.co.uk
tel:+44 161 236 5575
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