LOG
Section: POSIX Programmer's Manual (3P)
Updated: 2017
Page Index
PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
NAME
log,
logf,
logl
 natural logarithm function
SYNOPSIS
#include <math.h>
double log(double x);
float logf(float x);
long double logl(long double x);
DESCRIPTION
The functionality described on this reference page is aligned with the
ISO C standard. Any conflict between the requirements described here and the
ISO C standard is unintentional. This volume of POSIX.12017 defers to the ISO C standard.
These functions shall compute the natural logarithm of their argument
x,
loge(x).
An application wishing to check for error situations should set
errno
to zero and call
feclearexcept(FE_ALL_EXCEPT)
before calling these functions. On return, if
errno
is nonzero or fetestexcept(FE_INVALID  FE_DIVBYZERO 
FE_OVERFLOW  FE_UNDERFLOW) is nonzero, an error has occurred.
RETURN VALUE
Upon successful completion, these functions shall return the natural
logarithm of
x.
If
x
is ±0, a pole error shall occur and
log(),
logf(),
and
logl()
shall return HUGE_VAL, HUGE_VALF, and HUGE_VALL,
respectively.
For finite values of
x
that are less than 0,
or if
x
is Inf,
a domain error shall occur, and
either a NaN (if supported), or
an implementationdefined value shall be returned.
If
x
is NaN, a NaN shall be returned.
If
x
is 1, +0 shall be returned.
If
x
is +Inf,
x
shall be returned.
ERRORS
These functions shall fail if:
 Domain Error

The finite value of
x
is negative,
or
x
is Inf.

If the integer expression (math_errhandling & MATH_ERRNO) is
nonzero, then
errno
shall be set to
[EDOM].
If the integer expression (math_errhandling & MATH_ERREXCEPT) is
nonzero, then the invalid floatingpoint exception shall be raised.
 Pole Error

The value of
x
is zero.

If the integer expression (math_errhandling & MATH_ERRNO) is
nonzero, then
errno
shall be set to
[ERANGE].
If the integer expression (math_errhandling & MATH_ERREXCEPT) is
nonzero, then the dividebyzero floatingpoint exception shall be
raised.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
On error, the expressions (
math_errhandling & MATH_ERRNO) and
(
math_errhandling & MATH_ERREXCEPT) are independent of each
other, but at least one of them must be nonzero.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
exp(),
feclearexcept(),
fetestexcept(),
isnan(),
log10(),
log1p()
The Base Definitions volume of POSIX.12017,
Section 4.20, Treatment of Error Conditions for Mathematical Functions,
<math.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.12017, Standard for Information Technology
 Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/manpages/reporting_bugs.html .