[libvirt] [PATCH]: Fix VIR_ALLOC_N for 0 byte arrays

Daniel Veillard veillard at redhat.com
Thu Jun 19 09:51:07 UTC 2008

On Thu, Jun 19, 2008 at 10:49:59AM +0200, Chris Lalancette wrote:
> Hello,
>      For 0.4.3, danpb's new memory management scheme went into libvirt.  This is
> fine, except that is subtly alters the semantics of malloc(), calloc(), and
> realloc().  In particular, if you say:
> foo = malloc(0);
> glibc will happily return a non-NULL pointer to you.  However, with the new
> memory management stuff, if you say:
> foo = VIR_ALLOC(0);
> you will actually get a NULL pointer back.  Personally, I think this is a
> dangerous deviation from malloc() semantics that everyone is used to, and is
> indeed causing problems with the remote driver.  The short of it is that the
> remote driver allocates memory on behalf of the remote side using VIR_ALLOC_N,
> and this call is returning NULL so that the NULL checks elsewhere in the code
> fire and return failure.
> The attached patch fixes this situation by removing the 0 checks from the memory
> allocation paths, and just lets them fall through to the normal malloc(),
> calloc(), or realloc() routines, restoring old semantics.
> Signed-off-by: Chris Lalancette <clalance at redhat.com>

  Agreed, it's a problem, +1, but
since Dan explicitely made the == 0 test to return NULL he probably 
had a purpose about this (I suspect detecting 0 sized memory allocations).
So i would not apply the patch before he had a chance to comment on it,
but yes I personally thing we should reverse that bit. A zero lenght
allocation is legal, and may be later extended with realloc. This can
lead to vastly simplified code (to the expense of a seemingly useless 
initial allocation).


Red Hat Virtualization group http://redhat.com/virtualization/
Daniel Veillard      | virtualization library  http://libvirt.org/
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