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.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk
.\" <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" References consulted:
.\" Linux libc source code
.\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\" 386BSD man pages
.\" Modified 1993-07-24 by Rik Faith (faith@cs.unc.edu)
.\" Modified 1995-08-14 by Arnt Gulbrandsen <agulbra@troll.no>
.\" Modified 2002-07-27 by Walter Harms
.\" (walter.harms@informatik.uni-oldenburg.de)
.\"
.TH LOG2 3 2021-03-22 "" "Linux Programmer's Manual"
.SH NAME
log2, log2f, log2l \- base-2 logarithmic function
.SH SYNOPSIS
.nf
.B #include <math.h>
.PP
.BI "double log2(double " x );
.BI "float log2f(float " x );
.BI "long double log2l(long double " x );
.fi
.PP
Link with \fI\-lm\fP.
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR log2 (),
.BR log2f (),
.BR log2l ():
.nf
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
.fi
.SH DESCRIPTION
These functions return the base 2 logarithm of
.IR x .
.SH RETURN VALUE
On success, these functions return the base 2 logarithm of
.IR x .
.PP
For special cases, including where
.I x
is 0, 1, negative, infinity, or NaN, see
.BR log (3).
.SH ERRORS
See
.BR math_error (7)
for information on how to determine whether an error has occurred
when calling these functions.
.PP
For a discussion of the errors that can occur for these functions, see
.BR log (3).
.SH VERSIONS
These functions first appeared in glibc in version 2.1.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface Attribute Value
T{
.BR log2 (),
.BR log2f (),
.BR log2l ()
T} Thread safety MT-Safe
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
C99, POSIX.1-2001, POSIX.1-2008.
.PP
The variant returning
.I double
also conforms to
SVr4, 4.3BSD.
.SH SEE ALSO
.BR cbrt (3),
.BR clog2 (3),
.BR log (3),
.BR log10 (3),
.BR sqrt (3)
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