xref: /netbsd-src/lib/libm/man/log.3 (revision 7690962a143b99b7634b1782f4680d8cffeb0a2f)

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.\" Copyright (c) 2011 Jukka Ruohonen <jruohonen@iki.fi>
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.Dd January 24, 2024
.Dt LOG 3
.Os
.Sh NAME
.Nm log ,
.Nm logf ,
.Nm logl ,
.Nm log10 ,
.Nm log10f ,
.Nm log10l ,
.Nm log1p ,
.Nm log1pf ,
.Nm log1pl ,
.Nm log2 ,
.Nm log2f
.Nm log2l
.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 log double
.Fn log10l "long double x"
.Ft double
.Fn log1p "double x"
.Ft float
.Fn log1pf "float x"
.Ft long double
.Fn log1pl "long double x"
.Ft double
.Fn log2 "double x"
.Ft float
.Fn log2f "float x"
.Ft long double
.Fn log2l "long double x"
.Sh DESCRIPTION
The following functions compute logarithms:
.Bl -bullet -offset 2n
.It
The
.Fn log ,
.Fn logf
and
.Fn logl
functions return the natural logarithm.
.It
The
.Fn log10 ,
.Fn log10f
and
.Fn log10l
functions return the base 10 logarithm.
.It
The
.Fn log1p ,
.Fn log1pf
and
.Fn log1pl
functions return the natural logarithm of (1.0 +
.Fa x )
accurately even for very small values of
.Fa x .
.It
The
.Fn log2 ,
.Fn log2f
and
.Fn log2l
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 .