[FFmpeg-devel] [PATCH 3/5] lavf/matroskadec: support standard (non-WebM) WebVTT formatting

[Andreas Rheinhardt]

Nicolas George:
> Andreas Rheinhardt (12020-06-20):
>> Unfortunately all pointer arithmetic involving NULL is undefined
>> behaviour in C, even NULL + 0.
> 
> I am pretty sure the UB is not there, and that NULL+0 is valid. Do you
> have the reference?
> 
C99, 6.5.6 (the part about additive operators) says that pointer
arithmetic is only defined in two cases: First, if it involves a pointer
to an array (an object that is not an element of an array is treated as
an array with exactly one element for the purposes of pointer
arithmetic) and an offset so small so that it contains an element offset
places away from the pointer. And secondly, if the pointer points to the
last element of an array and the offset is 1.
(For C90, this stuff is contained in 6.3.6.)

A nullpointer does not point to any object, hence it is undefined.

Notice that UBSan in Clang 10 started warning about this (see ticket
8592 or see
https://github.com/llvm/llvm-project/commit/536b0ee40ab97f2878dc124a321cf9108ee3d233).

> Anyway, even if it is valid, it is not good style, I agree on that.

While it would be very bad style to write NULL + 0 or something like
that when it is known at compile-time that this is the only case that
will happen, I don't consider an expression that might lead to NULL + 0
to be bad style. I actually think that this is one of the insanities of
the C standard (C++ btw explicitly allows nullptr + 0).

- Andreas