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From: John Baldwin <jhb@freebsd.org>
To: Warner Losh <imp@bsdimp.com>
Cc: src-committers <src-committers@freebsd.org>, svn-src-head@freebsd.org,
 svn-src-all@freebsd.org, Warner Losh <imp@freebsd.org>
Subject: Re: svn commit: r292809 - head/lib/libc/stdio
Date: Tue, 29 Dec 2015 11:37:42 -0800
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On Monday, December 28, 2015 01:01:26 PM Warner Losh wrote:
> I'll look at that, but I don't think posix_memalign is the right way to go.
> The alignment of FILE is more strict than posix_memalign will return. Ian's
> idea of __alignof__ is the way to go. We allocate them in one block on
> purpose for performance, and posix_memalign would be a one at a time affair.

posix_memalign gives you whatever alignment you ask for.  Using __alignof__
to determine the alignment instead of hardcoding sizeof(int64_t) would
certainly be an improvement.  If you move the glue after the FILE objects
then you can use posix_memalign() directly as so:

	void *mem;
	int error;

	error = posix_memalign(&mem, MAX(ALIGNBYTES, __alignof__(mbstate_t)),
	    n * sizeof(FILE) + sizeof(*g));
	if (error)
		return (NULL);
	p = (FILE *)mem;
	g = (struct glue *)(p + n);
	g->next = NULL;
	g->niobs = n;
	g->iobs = p;
	...

(This presumes that the requested alignment of 'struct glue' is less than
the alignment needed by FILE which should be true.)

-- 
John Baldwin