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.\"
.\" Copyright (c) 2025 Robert Clausecker <fuz@FreeBSD.org>
.\"
.\" SPDX-License-Identifier: BSD-2-Clause
.\"
.Dd November 9, 2025
.Dt STDC_FIRST_TRAILING_ONE 3
.Os
.Sh NAME
.Nm stdc_first_trailing_one
.Nd find the first trailing one in an integer
.Sh SYNOPSIS
.Lb libc
.In stdbit.h
.Ft unsigned int
.Fn stdc_first_trailing_one_uc "unsigned char value"
.Ft unsigned int
.Fn stdc_first_trailing_one_us "unsigned short value"
.Ft unsigned int
.Fn stdc_first_trailing_one_ui "unsigned int value"
.Ft unsigned int
.Fn stdc_first_trailing_one_ul "unsigned long value"
.Ft unsigned int
.Fn stdc_first_trailing_one_ull "unsigned long long value"
.Ft unsigned int
.Fn stdc_first_trailing_one "value"
.Sh DESCRIPTION
The
.Nm stdc_first_trailing_one_ Ns Em type Ns ()
family of functions returns the index of the least significant set bit 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.
The
.Fn stdc_first_trailing_one "value"
type-generic macro picks the appropriate
.Nm stdc_first_trailing_one_ Ns Em type Ns ()
function based on the type of
.Fa value .
.Pp
Functions
.Fn stdc_first_trailing_one_ui ,
.Fn stdc_first_trailing_one_ul ,
and
.Fn stdc_first_trailing_one_ull
are identical to
.Bx 4.3
functions
.Xr ffs 3 ,
.Xr ffsl 3 ,
and
.Xr ffsll 3
respectively, except for operating on unsigned instead of signed values.
.Sh RETURN VALUES
Returns the index of the least significant set bit in
.Fa value .
The bits are numbered such that the least significant bit has number\~1,
and the most significant bit has number
.Ms w
where
.Ms w
is the number of bits in the type of
.Fa value .
If no bits are set in
.Fa value
(i.\^e.\&
.Fa value
is zero), one is returned.
.Sh SEE ALSO
.Xr bit_ffs 3 ,
.Xr ffs 3 ,
.Xr ffsl 3 ,
.Xr ffsll 3 ,
.Xr stdbit 3 ,
.Xr stdc_trailing_zeros 3 ,
.Xr stdc_first_trailing_zero 3 ,
.Xr stdc_first_leading_one 3
.Sh STANDARDS
The
.Nm stdc_first_trailing_one_ Ns Em type Ns ()
family of functions and the
.Fn stdc_first_trailing_one
type-generic macro conform to
.St -isoC-2023 .
.Sh HISTORY
The
.Nm stdc_first_trailing_one_ Ns Em type Ns ()
family of functions and the
.Fn stdc_first_trailing_one
type-generic macro were added in
.Fx 15.1.
.Sh AUTHOR
.An Robert Clausecker Aq Mt fuz@FreeBSD.org
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