.\" $NetBSD: ilog2.3,v 1.6 2021/11/05 22:39:35 riastradh Exp $ $
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.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Jukka Ruohonen.
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.Dd November 5, 2021
.Dt ILOG2 3
.Os
.Sh NAME
.Nm ilog2
.Nd integer base-2 logarithm
.Sh SYNOPSIS
.In sys/bitops.h
.Ft int
.Fn ilog2 "size x"
.Sh DESCRIPTION
For positive
.Fa x ,
the
.Fn ilog2
macro returns the integer part of the base-2 logarithm of
.Fa x ;
that is,
.Fo floor
.Fn log_2 x
.Fc .
If
.Fa n Li = Fn ilog2 x ,
then
.Fa n
is the largest integer such that
.No 2** Ns Fa n Li <= Fa x ;
in other words,
.Fn ilog2
returns the largest integer to which 2 can be raised to obtain a value
at most
.Fa x .
.Pp
The type of the input parameter must be unsigned.
.Pp
If
.Fa x
is a constant expression, then so is
.Fn ilog2 x .
.Sh ERRORS
.Fn ilog2
returns \-1 when
.Fa x
is zero.
.Sh SEE ALSO
.Xr bitops 3 ,
.Xr ffs 3 ,
.Xr ilogb 3 ,
.Xr log2 3 ,
.Xr imax 9 ,
.Xr powerof2 9
.Sh HISTORY
The
.Fn ilog2
macro first appeared in
.Nx 5.0 .