Re: [Xen-devel] [PATCH 24/29] libxl: Introduce FILLZERO

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Tue, 2015-02-10 at 20:10 +0000, Ian Jackson wrote:
> FILLZERO is a macro for memset(&foo,0,sizeof(foo)).  It eliminates the
> possiblity to make the error memset(&foo,0,sizeof(&foo)).

but not:
        foo *p = allocate_a_foo()
     memset(p, 0, sizeof(p))
although that's probably less likely to go wrong and I don't think it
can be avoided by the sorts of tricks used here.

> No callers yet, but document it in CODING_STYLE.  (In accordance with
> existing libxl policy, I haven't gone through all existing possible
> call sites.)

We don't usually expose such helpers in the public API, but I suppose
you have a good reason to do so here, could you mention it in the commit
log please.

>  
> +
> +#define LIBXL_FILLZERO(object) (memset(&(object), 0, sizeof((object))))

Evaluates object twice, so LIBXL_FILEZERO(*(p++)), would behave
surprisingly. I'm not sure if this can be resolved though, so this might
be a Don't Do That Then situation.

Only answer I can think of is to require the type be passed to the macro
and validating it matches the object via a pointer comparison.

Ian.


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