.\"
.\" Copyright (c) 2025 Robert Clausecker <fuz@FreeBSD.org>
.\"
.\" SPDX-License-Identifier: BSD-2-Clause
.\"
.Dd November 9, 2025
.Dt STDC_COUNT_ONES 3
.Os
.Sh NAME
.Nm stdc_count_ones
.Nd count the ones in an integer
.Sh SYNOPSIS
.Lb libc
.In stdbit.h
.Ft unsigned int
.Fn stdc_count_ones_uc "unsigned char value"
.Ft unsigned int
.Fn stdc_count_ones_us "unsigned short value"
.Ft unsigned int
.Fn stdc_count_ones_ui "unsigned int value"
.Ft unsigned int
.Fn stdc_count_ones_ul "unsigned long value"
.Ft unsigned int
.Fn stdc_count_ones_ull "unsigned long long value"
.Ft unsigned int
.Fn stdc_count_ones "value"
.Sh DESCRIPTION
The
.Nm stdc_count_ones_ Ns Em type Ns ()
family of functions returns the number of bits set to\~1 in
.Fa value ,
where 
.Fa value
is of type
.Va unsigned char ,
.Va unsigned short ,
.Va unsigned int ,
.Va unsigned long ,
or
.Va unsigned long long
for
.Em type
being
.Sy uc ,
.Sy us ,
.Sy ui ,
.Sy ul ,
or
.Sy ull
respectively.
This is also known as the
.Dq population count
(popcount) or
.Dq Hamming weight
operation.
The
.Fn stdc_count_ones "value"
type-generic macro picks the appropriate
.Nm stdc_count_ones_ Ns Em type Ns ()
function based on the type of
.Fa value .
.Sh RETURN VALUES
Returns the number of bits set to\~1 in
.Fa value .
.Sh SEE ALSO
.Xr bit_count 3 ,
.Xr stdbit 3 ,
.Xr stdc_count_zeros 3 ,
.Xr stdc_has_single_bit 3
.Sh STANDARDS
The
.Nm stdc_count_ones_ Ns Em type Ns ()
family of functions and the
.Fn stdc_count_ones
type-generic macro conform to
.St -isoC-2023 .
.Sh HISTORY
The
.Nm stdc_count_ones_ Ns Em type Ns ()
family of functions and the
.Fn stdc_count_ones
type-generic macro were added in
.Fx 15.1.
.Sh AUTHOR
.An Robert Clausecker Aq Mt fuz@FreeBSD.org