.\" $NetBSD: log.3,v 1.6.8.1 2019/09/05 08:19:40 martin Exp $
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.\" Copyright (c) 2011 Jukka Ruohonen <jruohonen@iki.fi>
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.Dd September 2, 2019
.Dt LOG 3
.Os
.Sh NAME
.Nm log ,
.Nm logf ,
.Nm log10 ,
.Nm log10f ,
.Nm log1p ,
.Nm log1pf ,
.Nm log2 ,
.Nm log2f
.Nd logarithm functions
.Sh LIBRARY
.Lb libm
.Sh SYNOPSIS
.In math.h
.Ft double
.Fn log "double x"
.Ft float
.Fn logf "float x"
.Ft double
.Fn log10 "double x"
.Ft float
.Fn log10f "float x"
.Ft double
.Fn log1p "double x"
.Ft float
.Fn log1pf "float x"
.Ft double
.Fn log2 "double x"
.Ft float
.Fn log2f "float x"
.Sh DESCRIPTION
The following functions compute logarithms:
.Bl -bullet -offset 2n
.It
The
.Fn log
and
.Fn logf
functions return the natural logarithm.
.It
The
.Fn log10
and
.Fn log10f
functions return the base 10 logarithm.
.It
The
.Fn log1p
and
.Fn log1pf
functions return the natural logarithm of (1.0 +
.Fa x )
accurately even for very small values of
.Fa x .
.It
The
.Fn log2
and
.Fn log2f
functions return the base 2 logarithm.
.El
.Sh RETURN VALUES
Upon successful completion, the functions return the logarithm of
.Fa x
as described above.
Otherwise the following may occur:
.Bl -enum -offset indent
.It
If
.Fa x
is \*(Na, all functions return \*(Na.
.It
If
.Fa x
is positive infinity, all functions return
.Fa x .
If
.Fa x
is negative infinity, all functions return \*(Na.
.It
If
.Fa x
is +0.0 or -0.0, the
.Fn log ,
.Fn log10 ,
and
.Fn log2
families return either
.Dv -HUGE_VAL ,
.Dv -HUGE_VALF ,
or
.Dv -HUGE_VALL ,
whereas the
.Fn log1p
family returns
.Fa x .
.It
If
.Fa x
is +1.0, the
.Fn log ,
.Fn log10 ,
and
.Fn log2
families return +0.0.
If
.Fa x
is -1.0, the
.Fn log1p
family returns
.Dv -HUGE_VAL ,
.Dv -HUGE_VALF ,
or
.Dv -HUGE_VALL .
.El
.Pp
In addition, on a
.Tn VAX ,
.Va errno
is set to
.Er EDOM
and the reserved operand is returned
by
.Fn log
unless
.Fa x
> 0, by
.Fn log1p
unless
.Fa x
> \-1.
.Sh SEE ALSO
.Xr exp 3 ,
.Xr ilogb 3 ,
.Xr math 3
.Sh STANDARDS
The described functions conform to
.St -isoC-99 .
.Sh HISTORY
A
.Fn log
function appeared in
.At v1 .