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stddef.h: add unreachable() for C23 compliance

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unreachable() is a hint to the compiler that it is unreachable.
Add a new man page unreachable(3) to document this macro.

Reviewed by:	imp
Approved by:	markj (mentor)
MFC after:	1 month
Differential Revision:	https://reviews.freebsd.org/D53967
This commit is contained in:
Robert Clausecker 2025-11-27 21:19:49 +01:00
parent 04d000a378
commit b381d09802
4 changed files with 98 additions and 3 deletions
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4
include/stddef.h
View file

@ -61,6 +61,10 @@ typedef __max_align_t max_align_t;
#endif
#endif
#if __ISO_C_VISIBLE >= 2023
#define unreachable(x) __unreachable(x)
#endif
#ifndef offsetof
#define offsetof(type, field) __builtin_offsetof(type, field)
#endif

3
share/man/man3/Makefile
View file

@ -35,7 +35,8 @@ MAN= alloca.3 \
sysexits.3 \
tgmath.3 \
timeradd.3 \
tree.3
tree.3 \
unreachable.3
MLINKS= arb.3 ARB8_ENTRY.3 \
arb.3 ARB8_HEAD.3 \

5
share/man/man3/assert.3
View file

@ -25,7 +25,7 @@
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.Dd April 20, 2021
.Dd November 27, 2025
.Dt ASSERT 3
.Os
.Sh NAME
@ -118,7 +118,8 @@ constraint and includes the provided string literal:
If none is provided, it only points at the constraint.
.Sh SEE ALSO
.Xr abort2 2 ,
.Xr abort 3
.Xr abort 3 ,
.Xr unreachable 3
.Sh STANDARDS
The
.Fn assert

89
share/man/man3/unreachable.3 Normal file
View file

@ -0,0 +1,89 @@
.\"
.\" Copyright (c) 2025 Robert Clausecker <fuz@FreeBSD.org>
.\"
.\" SPDX-License-Identifier: BSD-2-Clause
.\"
.Dd November 27, 2025
.Dt UNREACHABLE 3
.Os
.Sh NAME
.Nm unreachable
.Nd the unreachable macro
.Sh SYNOPSIS
.In stddef.h
.Fd #define unreachable()
.Sh DESCRIPTION
If the
.Fn unreachable
macro is reached during execution, behavior is undefined.
This can be useful to hint to the compiler that some invariant is guaranteed to
hold or that some case cannot occur.
.Sh EXAMPLES
Suppose a floating-point number
.Va x
is to be classified using the
.Xr fpclassify 3
macro and a different action is to be taken based on the result of the
classification.
As the set of possible return values is known, the
.Fn unreachable
macro can be used to hint to the compiler that it can omit checks for
other possible return values:
.Bd -literal -offset 3n
#include <math.h>
#include <stddef.h>
#include <stdio.h>
void print_classification(double x)
{
printf("%f: ", x);
switch (fpclassify(x)) {
case FP_INFINITE:
puts("infinite");
break;
case FP_NAN:
puts("not a number");
break;
case FP_NORMAL:
puts("normal");
break;
case FP_SUBNORMAL:
puts("subnormal");
break;
case FP_ZERO:
puts("zero");
break;
default:
unreachable();
}
.Ed
.Sh SEE ALSO
.Xr assert 3
.Sh STANDARDS
The
.Fn unreachable
macro conforms to
.St -isoC-2023 .
.Sh HISTORY
A
.Dv /*NOTREACHED*/
conventional comment was supported by the historical
.Xr lint 1
utility to suppress warnings about unreachable statements during static
analysis.
The
.Fn unreachable
macro was added in
.Fx 15.1
based on the earlier private
.Fn __unreachable
macro for compliance with
.St -isoC-2023 .
.Sh AUTHOR
.Ah Robert Clausecker Aq Mt fuz@FreeBSD.org