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

[Eric Blake]

On 07/15/2014 06:38 AM, 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(-)

But if you take my suggestion in 2/4 about merely removing the
'cast-away-const' while still keeping type safety, then a
single-argument virFree() should still be noisy on attempts to VIR_FREE
a const pointer.


> @@ -543,11 +543,23 @@ void virFree(void *ptrptr) ATTRIBUTE_NONNULL(1);
>   * @ptr: pointer holding address to be freed
>   *
>   * Free the memory stored in 'ptr' and update to point
> - * to NULL.
> + * to NULL. Moreover, this macro has a side effect in
> + * form of evaluating passed argument multiple times.

NACK.  I think it is possible to use sizeof() to come up with a
construct that will only do side effects once, rather than having to
weaken the guarantee of VIR_FREE.  Please give me some time to propose
an alternative.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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