Date: Sun, 5 Jul 2009 18:28:56 +0200 From: Fabian Keil <freebsd-listen@fabiankeil.de> To: freebsd-hackers@freebsd.org Subject: Zero-length allocation with posix_memalign() Message-ID: <20090705182856.799b6b07@fabiankeil.de>
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I recently submitted a patch to the vlc developers that prevents
a crash on FreeBSD 8.0 by not calling posix_memalign() with a
size argument of zero.
A simplified test case would be:
#include <stdlib.h>
int main(int argc, char **argv) {
void *ptr;
posix_memalign(&ptr, 16, 0);
return (0);
}
which triggers:
Assertion failed: (size !=3D 0), function arena_malloc, file /usr/src/lib/l=
ibc/stdlib/malloc.c, line 3349.
R=E9mi Denis-Courmont, one of the vlc developers, pointed out
that passing a zero size to posix_memalign() should actually
work, though:
| In principle, while useless, there is no reason why allocating an empty=20
| picture should not be possible. posix_memalign() does support zero-length=
=20
| allocation anyway:
| http://www.opengroup.org/onlinepubs/9699919799/functions/posix_memalign.h=
tml
| | If the size of the space requested is 0, the behavior is
| | implementation-defined; the value returned in memptr shall be either a
| | null pointer or a unique pointer.
http://mailman.videolan.org/pipermail/vlc-devel/2009-July/062299.html
I get the impression that this deviation from the standard could be
easily fixed with something similar to the following, which is mostly
copy and pasted from malloc():
index 5404798..a078d07 100644
--- a/malloc.c
+++ b/malloc.c
@@ -5303,6 +5303,15 @@ posix_memalign(void **memptr, size_t alignment, size=
_t size)
int ret;
void *result;
=20
+ if (size =3D=3D 0) {
+ if (opt_sysv =3D=3D false)
+ size =3D 1;
+ else {
+ ret =3D 0;
+ *memptr =3D result =3D NULL;
+ goto RETURN;
+ }
+ }
if (malloc_init())
result =3D NULL;
else {
I assume the "goto RETURN" isn't entirely compliant either as
it skips the alignment check, but so does the malloc_init()
failure branch.
Fabian
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