Re: [PATCH] IB/mlx4: Avoid implicit enumerated type conversion

[Jason Gunthorpe]

On Thu, Sep 27, 2018 at 03:42:24PM -0700, Nick Desaulniers wrote:
> On Thu, Sep 27, 2018 at 3:33 PM Bart Van Assche <bvanassche@xxxxxxx> wrote:
> >
> > On Thu, 2018-09-27 at 16:28 -0600, Jason Gunthorpe wrote:
> > > On Thu, Sep 27, 2018 at 01:34:16PM -0700, Nick Desaulniers wrote:
> > >
> > > > > Neither ib_qp_create_flags nor mlx4_ib_qp_flags have negative values, is
> > > > > signedness necessary?
> > > >
> > > > enums are by default restricted to the range of ints.
> > >
> > > That's not quite right, the compiler sizes the enum to be able to fit
> > > the largest value contained within, today that is int, but if we added
> > > 1<<31, then it would become larger.
> >
> > Hi Jason,
> >
> > Are you perhaps confusing C and C++? For C++, an enumeration whose underlying
> > type is not fixed, the underlying type is an integral type that can represent
> > all the enumerator values defined in the enumeration. For C however I think
> > that enumeration values are restricted to what fits in an int.
> >
> > Bart.
> >
> 
> To quote the sacred texts (ANSIIISO9899-1990):

> 6.5.2.2 Enumeration specifiers
> The expression that defines the value of an enumeration constant shall
> be an integral constant
> expression that has a value representable as an int.

This is the wrong part of the standard to quote it is talking about
*enumeration constants* not the 'enum X' itself.

Anyhow, the standard is hard to read in this area, but reality is
this:

#include <stdio.h>

enum a
{
	A1 = 1,
	A2 = 1ULL<<40,
};

int main(int argc, const char *argv[])
{
	printf("%zu\n", sizeof(enum a));
	return 0;
}

$ gcc -Wall -std=c11 test.c && ./a.out
8

I forget if this a common compiler extension, unclear standard, or was
formally revised in C11 or what, but it is the real world the Linux
kernel lives in.

It is even more confusing if you wonder what types A1 and A2 are!

Jason