'\" t
.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH log 3 (date) "Linux man-pages (unreleased)"
.SH NAME
log, logf, logl \- natural logarithmic function
.SH LIBRARY
Math library
.RI ( libm ,\~ \-lm )
.SH SYNOPSIS
.nf
.B #include <math.h>
.P
.BI "double log(double " x );
.BI "float logf(float " x );
.BI "long double logl(long double " x );
.fi
.P
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.P
.BR logf (),
.BR logl ():
.nf
    _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
        || /* Since glibc 2.19: */ _DEFAULT_SOURCE
        || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
.fi
.SH DESCRIPTION
These functions return the natural logarithm of
.IR x .
.SH RETURN VALUE
On success, these functions return the natural logarithm of
.IR x .
.P
If
.I x
is a NaN,
a NaN is returned.
.P
If
.I x
is 1, the result is +0.
.P
If
.I x
is positive infinity,
positive infinity is returned.
.P
If
.I x
is zero,
then a pole error occurs, and the functions return
.RB \- HUGE_VAL ,
.RB \- HUGE_VALF ,
or
.RB \- HUGE_VALL ,
respectively.
.P
If
.I x
is negative (including negative infinity), then
a domain error occurs, and a NaN (not a number) is returned.
.SH ERRORS
See
.BR math_error (7)
for information on how to determine whether an error has occurred
when calling these functions.
.P
The following errors can occur:
.TP
Domain error: \f[I]x\f[] is negative
.I errno
is set to
.BR EDOM .
An invalid floating-point exception
.RB ( FE_INVALID )
is raised.
.TP
Pole error: \f[I]x\f[] is zero
.I errno
is set to
.BR ERANGE .
A divide-by-zero floating-point exception
.RB ( FE_DIVBYZERO )
is raised.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR log (),
.BR logf (),
.BR logl ()
T}	Thread safety	MT-Safe
.TE
.SH STANDARDS
C11, POSIX.1-2008.
.SH HISTORY
C99, POSIX.1-2001.
.P
The variant returning
.I double
also conforms to
SVr4, 4.3BSD, C89.
.SH BUGS
In glibc 2.5 and earlier,
taking the
.BR log ()
of a NaN produces a bogus invalid floating-point
.RB ( FE_INVALID )
exception.
.SH SEE ALSO
.BR cbrt (3),
.BR clog (3),
.BR log10 (3),
.BR log1p (3),
.BR log2 (3),
.BR sqrt (3)