1.\" $NetBSD: ilog2.3,v 1.6 2021/11/05 22:39:35 riastradh Exp $ $ 2.\" 3.\" Copyright (c) 2010 The NetBSD Foundation, Inc. 4.\" All rights reserved. 5.\" 6.\" This code is derived from software contributed to The NetBSD Foundation 7.\" by Jukka Ruohonen. 8.\" 9.\" Redistribution and use in source and binary forms, with or without 10.\" modification, are permitted provided that the following conditions 11.\" are met: 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in the 16.\" documentation and/or other materials provided with the distribution. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 19.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 20.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 21.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 22.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 23.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 24.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 25.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 26.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 27.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 28.\" POSSIBILITY OF SUCH DAMAGE. 29.\" 30.Dd November 5, 2021 31.Dt ILOG2 3 32.Os 33.Sh NAME 34.Nm ilog2 35.Nd integer base-2 logarithm 36.Sh SYNOPSIS 37.In sys/bitops.h 38.Ft int 39.Fn ilog2 "size x" 40.Sh DESCRIPTION 41For positive 42.Fa x , 43the 44.Fn ilog2 45macro returns the integer part of the base-2 logarithm of 46.Fa x ; 47that is, 48.Fo floor 49.Fn log_2 x 50.Fc . 51If 52.Fa n Li = Fn ilog2 x , 53then 54.Fa n 55is the largest integer such that 56.No 2** Ns Fa n Li <= Fa x ; 57in other words, 58.Fn ilog2 59returns the largest integer to which 2 can be raised to obtain a value 60at most 61.Fa x . 62.Pp 63The type of the input parameter must be unsigned. 64.Pp 65If 66.Fa x 67is a constant expression, then so is 68.Fn ilog2 x . 69.Sh ERRORS 70.Fn ilog2 71returns \-1 when 72.Fa x 73is zero. 74.Sh SEE ALSO 75.Xr bitops 3 , 76.Xr ffs 3 , 77.Xr ilogb 3 , 78.Xr log2 3 , 79.Xr imax 9 , 80.Xr powerof2 9 81.Sh HISTORY 82The 83.Fn ilog2 84macro first appeared in 85.Nx 5.0 . 86