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Re: [wg14/wg21 liaison] Semantics of angelic nondeterminism

From: Martin Uecker <ma.uecker_at_[hidden]>
Date: Fri, 12 Apr 2024 22:33:50 +0200
Am Freitag, dem 12.04.2024 um 20:27 +0200 schrieb Jens Maurer via Liaison:
> On 12/04/2024 10.27, Jā‚‘ā‚™ā‚› Gustedt via Liaison wrote
> > If it is the case that C++ supposes that a `malloc`ed storage instance
> > has a fixed type from the start (we might just not know it yet) then
> > it is not modeling C's behavior correctly, and calling C functions
> > with `malloc`ed storage from C++ becomes hazardous. You may produce
> > non-value representations for the type that C++ is expecting, or even
> > introduce aliasing where the compiler wouldn't expect it.
> Well, calling compiler-invisible (non-inline) functions is fairly
> safe, because C++ allows to change the type of memory using
> placement-new or a rather new function std::start_lifetime_as.
> The compiler can't know whether a function call does that or not,
> so it has to assume the worst.

It is very much an essential feature of C that one can
reason locally. Compilers know how to reason locally,
because in many cases they do not see the full program.
Almost all C programs are also not strictly conforming,
but have undefined behavior. Their practical meaning is
inferred by reasoning about individual parts.

A specification strategy that fundamentally relies on
quantifying over all executations of a program to decide
whether the whole program has defined behavior or not,
seems not very useful to me.


> Jens
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Received on 2024-04-12 20:33:55