N2961
Literal Suffixes for size_t

1. Changelog

1.1. Revision 0 - April 12^th, 2022

• Initial Release 🎉

• Targeting C2Y/C3X (after C23).

2. Design

This is a (minor) compatibility issue with literal suffixes that may be potentially used in shared code in the future. C++ adopted suffixed literals for size_t in [p0330]. The design is as follows:

#include <stddef.h>

size_t ulit_val = 0zu;

The u, as with current literals, can be placed on either the left or the right of the z suffix to make it unsigned. For symmetry with existing suffix literal rules, it also has a signed variant. That type is the "signed integer type of size_t", which normally resolves to ptrdiff_t:

#include <stddef.h>

ptrdiff_t lit_val = 0z;

The signed variant lacks the u as a piece of the z suffix. This also matches the way printf adjusts specific codes to display size_t or ptrdiff_t-sized variables. The design is simple and, thankfully, compatible with C++. It also provides a way to avoid signed comparison warnings in compilers which implement more strict comparisons checks, e.g., when comparing a size_t value against some fixed constant value.

2.1. Even After Compatibility, Do We Really Need This?

Yes. The trip to put this paper into C++ was an extremely long one and came with a ton of reasons. All of it can be seen in [p0330] and 90% of that reasoning applies to C, especially in the face of _Generic.

3. Wording

Wording is relative to [N2731].

3.1. Intent

The goal of this wording is to provide:

• a literal suffix for both the signed (ptrdiff_t) or unsigned (size_t) variant of the type of the expression "sizeof(0)"; and,

• use the suffix z in either the form uz or zu to produce a size_t.

3.2. Specification

3.2.1. Add two new grammar productions to §6.4.4.1 Integer constants¸ Syntax, ¶1

6.4.4.1 Integer constants Syntax

…

integer-suffix:

unsigned-suffix long-suffix_opt

unsigned-suffix long-long-suffix

unsigned-suffix size-suffix

long-suffix unsigned-suffix_opt

long-long-suffix unsigned-suffix_opt

size-suffix unsigned-suffix_opt

…

long-long-suffix: one of

ll LL

size-suffix: one of

z Z

3.2.2. Add two new table rows to §6.4.4.1 Integer constants¸ Semantics, ¶6

Suffix	Decimal Constant	Octal, Binary, or Hexadecimal Constant
…	…	…
z or Z	the corresponding signed integer type of the expression sizeof(0)'s type (6.5.3.4)	the corresponding signed integer type of the expression sizeof(0)'s type
Both U or U and z or Z	the same type as the expression sizeof(0)	the same type as the expression sizeof(0)

…

Forward references: preprocessing numbers (6.4.8), numeric conversion functions (7.22.1) , The sizeof and _Alignof operators (6.5.3.4) .