.\" Copyright (c) 2008-2010 David Schultz <das@FreeBSD.org>
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.Dd March 22, 2020
.Dt LOG 3
.Os
.Sh NAME
.Nm log ,
.Nm logf ,
.Nm logl ,
.Nm log10 ,
.Nm log10f ,
.Nm log10l ,
.Nm log2 ,
.Nm log2f ,
.Nm log2l ,
.Nm log1p ,
.Nm log1pf ,
.Nm log1pl
.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 long double
.Fn logl "long double x"
.Ft double
.Fn log10 "double x"
.Ft float
.Fn log10f "float x"
.Ft long double
.Fn log10l "long double x"
.Ft double
.Fn log2 "double x"
.Ft float
.Fn log2f "float x"
.Ft long double
.Fn log2l "long double x"
.Ft double
.Fn log1p "double x"
.Ft float
.Fn log1pf "float x"
.Ft long double
.Fn log1pl "long double x"
.Sh DESCRIPTION
The
.Fn log ,
.Fn logf ,
and
.Fn logl
functions compute the natural logarithm of
.Fa x .
.Pp
The
.Fn log10 ,
.Fn log10f ,
and
.Fn log10l
functions compute the logarithm base 10 of
.Fa x ,
while
.Fn log2 ,
.Fn log2f ,
and
.Fn log2l
compute the logarithm base 2 of
.Fa x .
.Pp
The
.Fn log1p ,
.Fn log1pf ,
and
.Fn log1pl
functions compute the natural logarithm of
.No "1 + x" .
Computing the natural logarithm as
.Li log1p(x)
is more accurate than computing it as
.Li log(1 + x)
when
.Fa x
is close to zero.
.Sh RETURN VALUES
These functions return the requested logarithm; the logarithm of 1 is +0.
An attempt to take the logarithm of \*(Pm0 results in a divide-by-zero
exception, and -\*(If is returned.
Otherwise, attempting to take the logarithm of a negative number
results in an invalid exception and a return value of \*(Na.
.Sh SEE ALSO
.Xr exp 3 ,
.Xr ilogb 3 ,
.Xr math 3 ,
.Xr pow 3
.Sh STANDARDS
The
.Fn log ,
.Fn logf ,
.Fn logl ,
.Fn log10 ,
.Fn log10f ,
.Fn log10l ,
.Fn log2 ,
.Fn log2f ,
.Fn log2l ,
.Fn log1p ,
.Fn log1pf ,
and
.Fn log1pl
functions conform to
.St -isoC-99 .
.Sh HISTORY
The
.Fn log
function first appeared in
.At v1 ;
.Fn log10
in
.At v7 ;
.Fn log1p
in
.Bx 4.3 .