Re: [PATCH] correct FLEXPTR_* example in comment

From: René Scharfe <l.s.r@web.de>
Date: Aug 13, 2016, 17:26 UTC
Message-ID: <57AF582A.3050406@web.de>
In-Reply-To: <20160813090947.uj6hurji5o6cwxge@sigill.intra.peff.net>

Am 13.08.2016 um 11:09 schrieb Jeff King:

Show 12 quoted lines

> On Sat, Aug 13, 2016 at 11:01:21AM +0200, René Scharfe wrote:
>
>> This section is about "The FLEXPTR_* variants", so use FLEXPTR_ALLOC_STR
>> in the example.
>
> Oops, yeah. Your patch is clearly an improvement.
>
> Since this is obviously an easy mistake to make (using one form rather
> than the other), I wondered if the compiler would catch it.
>
> I think it would catch an accidental use of FLEXPTR instead of FLEX,
> because it involves an attempted assignment of an array.

Gcc 5.4 reports "invalid use of flexible array member".

Show 5 quoted lines

> But I don't
> think we would catch the reverse; we'd just write the data directly on
> top of the pointer. That would probably crash immediately at runtime, so
> if you exercise the code at all in tests, it is OK. But something to be
> aware of.

No hint at compile time, segfaults at runtime.

> I suppose it could assert(sizeof((x)->flexname) == FLEX_ALLOC) or
> something, but I'm not sure if it is worth worrying about.

You can't use sizeof with an actual flexible array. It only works if FLEX_ARRAY is defined as 1 (for platforms without native support), and perhaps also if it's 0.

offsetof(struct x, arr) == sizeof(struct x) won't work either because of padding.

I have no idea what you can do with a flexible array that would throw a compile error when done with a pointer.

René