linux-kernel - Re: [PATCH 1/4] stringbuf: A string buffer implementation

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Message-Id: <0533759C-81D0-45CB-A663-088A6D4E56A9@mac.com>
Date:	Wed, 24 Oct 2007 16:59:48 -0400
From:	Kyle Moffett <mrmacman_g4@....com>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Linus Torvalds <torvalds@...l.org>, Andrew Morton <akpm@...l.org>,
	linux-kernel@...r.kernel.org,
	Matthew Wilcox <willy@...ux.intel.com>
Subject: Re: [PATCH 1/4] stringbuf: A string buffer implementation

On Oct 24, 2007, at 15:59:49, Matthew Wilcox wrote:
> +static void sb_vprintf(struct stringbuf *sb, gfp_t gfp, const char  
> *format, va_list args)
> +{

[...]

> +	s = sb->buf + sb->len;
> +	size = vsnprintf(s, sb->alloc - sb->len, format, args);

[...]

> +	/* Point to the end of the old string since we already updated - 
> >len */
> +	s += sb->len - size;
> +	vsprintf(s, format, args);

[...]

> +void sb_printf(struct stringbuf *sb, gfp_t gfp, const char  
> *format, ...)
> +{
> +	va_list args;
> +
> +	va_start(args, format);
> +	sb_vprintf(sb, gfp, format, args);
> +	va_end(args);
> +}

This seems unlikely to work reliably as the various "v*printf"  
functions modify the va_list argument they are passed.  It may happen  
to work on your particular architecture depending on how that  
argument data is passed and stored, but you probably actually want to  
make a copy of the varargs list for the first vsnprintf call.   
Example below:

> va_list argscopy;
> va_copy(argscopy, args);
>
> [...]
>
> size = vsnprintf(s, sb->alloc - sb->len, format, argscopy)
>
> [...]
>
> s += sb->len - size;
> vsprintf(s, format, args);
>
> [...]
>
> va_end(argscopy);

Cheers,
Kyle Moffett
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