[R-pkg-devel] Canonical way to Rprintf R_xlen_t

[Ivan Krylov]

On Wed, 29 Nov 2023 06:11:23 +1100
Hugh Parsonage <hugh.parsonage using gmail.com> wrote:

> Rprintf("%lld", (long long) xlength(x));

This is fine. long longs are guaranteed to be at least 64 bits in size
and are signed, just like lengths in R.

> Rprintf("%td, xlength(x));

Maybe if you cast it to ptrdiff_t first. Otherwise I would expect this
to fail on an (increasingly rare) 32-bit system where R_xlen_t is int
(which is an implementation detail).

In my opinion, ptrdiff_t is just the right type for array lengths if
they have to be signed (which is useful for Fortran interoperability),
so Rprintf("%td", (ptrdiff_t)xlength(x)) would be my preferred option
for now. By definition of ptrdiff_t, you can be sure [*] that there
won't be any vectors on your system longer than PTRDIFF_MAX.

> using the string macro found in Mr Kalibera's commit of r85641:
> R_PRIdXLEN_T

I think this will be the best solution once we can afford
having our packages depend on R >= 4.4.

-- 
Best regards,
Ivan

[*] https://en.cppreference.com/w/c/types/ptrdiff_t posits that there
may exist long vectors that fit in SIZE_MAX (unsigned) elements but not
PTRDIFF_MAX (signed) elements. If such vector exists, subtracting two
pointers to its insides may result in undefined behaviour. This may be
already possible in a 32-bit process on Linux running with a 3G
user-space / 1G kernel-space split. The only way around the problem is
to use unsigned types for lengths, but that would preclude Fortran
compatibility.