\" $NetBSD: ilog2.3,v 1.6 2021/11/05 22:39:35 riastradh Exp $ $

\" $NetBSD: ilog2.3,v 1.6 2021/11/05 22:39:35 riastradh Exp $ $
\"
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\" 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 .