[libvirt] [PATCH 4/4] virFree: Check const correctness

[Martin Kletzander]

On Tue, Jul 15, 2014 at 02:38:36PM +0200, Michal Privoznik wrote:
>Up to now it's possible to do something like this:
>
>const char *ptr;
>
>ptr = strdup("my example string");
>
>VIR_FREE(ptr);
>
>The problem is, const char * pointers should not be modified (and
>freeing them is kind of modification). We should avoid this. A little
>trick is used: assigning a const pointer into 'void *' triggers
>compiler warning about discarding 'const' qualifier from pointer. So
>the virFree() function gains new dummy argument, that is not touched
>anyhow, just fulfills the const correctness check duty.
>
>Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
>---
> src/util/viralloc.c       |  6 ++++--
> src/util/viralloc.h       | 20 ++++++++++++++++----
> src/xenapi/xenapi_utils.c |  2 +-
> 3 files changed, 21 insertions(+), 7 deletions(-)
>
>diff --git a/src/util/viralloc.c b/src/util/viralloc.c
>index be9f0fe..0134e67 100644
>--- a/src/util/viralloc.c
>+++ b/src/util/viralloc.c
[...]
>@@ -569,13 +569,15 @@ int virAllocVar(void *ptrptr,
>
> /**
>  * virFree:
>+ * @ptr: dummy pointer to check const correctness
>  * @ptrptr: pointer to pointer for address of memory to be freed
>  *
>  * Release the chunk of memory in the pointer pointed to by
>  * the 'ptrptr' variable. After release, 'ptrptr' will be
>  * updated to point to NULL.
>  */
>-void virFree(void *ptrptr)
>+void virFree(void *ptr ATTRIBUTE_UNUSED,
>+             void *ptrptr)

What if you don't add another argument, but just change the void
*ptrptr to void **ptrptr.  Compiler shouldn't be mad about not knowing
the size resulting of de-referencing ptrptr, you get the check you
want and keep the macro without side-effects.

Martin
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