svn commit: r292809 - head/lib/libc/stdio

Ian Lepore ian at freebsd.org
Tue Dec 29 22:51:37 UTC 2015


On Tue, 2015-12-29 at 11:37 -0800, John Baldwin wrote:
> 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.)
> 

If there's going to be an assumption that __alignof__(glue) <=
__alignof__(FILE), it might be nice to have a static_assert() of that
to prevent a future time bomb similar to the one that exploded on arm
when it turned out the opposite assumption was wrong.

-- Ian



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