Fwd: [Xquartz-dev] ALLOCATE_LOCAL / crashes
Jeremy Huddleston
jeremyhu at freedesktop.org
Sat Dec 8 12:29:32 PST 2007
Hi Keith,
We've been having some trouble due to alloca(BIG) in the Xquartz
server. Could you comment on the acceptability of something like Greg
suggests below? Should we put it inside __APPLE__ ifdefs, or do you
think it would be good across the board? Or do you have a better
solution we should think about?
Thanks,
Jeremy
Begin forwarded message:
From: Greg Parker <gparker at apple.com>
Date: December 7, 2007 15:27:17 PST
To: Developer talk about Xquartz <xquartz-dev at lists.macosforge.org>
Subject: Re: [Xquartz-dev] ALLOCATE_LOCAL / crashes
Reply-To: Developer talk about Xquartz <xquartz-
dev at lists.macosforge.org>
On Dec 7, 2007, at 1:48 PM, Jeremy Huddleston wrote:
> So... I wonder if we should do this across the board when compiling
> X. Other things than the server use Xalloca.h. I'm thinking I
> should put it in my standard CFLAGS. Am I being overly cautious and
> paranoid, or is that something worth doing?
We should do something like this. It uses alloca() if the allocation
is small enough, otherwise it uses the fallback allocator (probably
malloc or Xalloc). The start of the block holds the allocated size so
DEALLOCATE_LOCAL knows whether to call free.
(16KB might be too small on i386 and/or too big on ppc. There's lots
of gcc-isms here; it should be protected by __GNUC__ or something. I
think it works in C++ too. I have only tested it trivially.)
#ifndef ALLOCATE_LOCAL_MAX_ALLOCA
#define ALLOCATE_LOCAL_MAX_ALLOCA (16*1024)
#endif
#define
ALLOCATE_LOCAL(size) \
({ \
__SIZE_TYPE__ _x_size
= \
sizeof(__SIZE_TYPE__) + (__SIZE_TYPE__)
(size); \
__SIZE_TYPE__
*_x_p; \
if (_x_size > ALLOCATE_LOCAL_MAX_ALLOCA)
{ \
_x_p = (__SIZE_TYPE__
*)ALLOCATE_LOCAL_FALLBACK(_x_size); \
} else
{ \
_x_p = (__SIZE_TYPE__
*)__builtin_alloca(_x_size); \
} \
if (_x_p) *_x_p =
_x_size; \
(void *)(_x_p +
1); \
})
#define DEALLOCATE_LOCAL(ptr) \
({ \
__SIZE_TYPE__ *_x_p = (__SIZE_TYPE__ *)(ptr); \
if (_x_p) { \
_x_p = _x_p - 1; \
__SIZE_TYPE__ _x_size = *_x_p; \
if (_x_size > ALLOCATE_LOCAL_MAX_ALLOCA) { \
DEALLOCATE_LOCAL_FALLBACK(_x_p); \
} \
} \
})
--
Greg Parker gparker at apple.com Runtime Wrangler
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