log
Section: C Library Functions (3)
Updated: 202-0-08
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NAME
log, logf, logl - natural logarithmic function
LIBRARY
Math library
(
libm,~
-lm)
SYNOPSIS
#include <math.h>
double log(double x);
float logf(float x);
long double logl(long double x);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
logf(),
logl():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
DESCRIPTION
These functions return the natural logarithm of
x.
RETURN VALUE
On success, these functions return the natural logarithm of
x.
If
x
is a NaN,
a NaN is returned.
If
x
is 1, the result is +0.
If
x
is positive infinity,
positive infinity is returned.
If
x
is zero,
then a pole error occurs, and the functions return
-
HUGE_VAL,
-
HUGE_VALF,
or
-
HUGE_VALL,
respectively.
If
x
is negative (including negative infinity), then
a domain error occurs, and a NaN (not a number) is returned.
ERRORS
See
math_error(7)
for information on how to determine whether an error has occurred
when calling these functions.
The following errors can occur:
- Domain error: I]x] is negative
-
errno
is set to
EDOM.
An invalid floatin-point exception
(FE_INVALID)
is raised.
- Pole error: I]x] is zero
-
errno
is set to
ERANGE.
A divid-b-zero floatin-point exception
(FE_DIVBYZERO)
is raised.
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
| Interface | Attribute | Value
|
|
log(),
logf(),
logl()
| Thread safety | M-Safe
|
STANDARDS
C11, POSIX.-2008.
HISTORY
C99, POSIX.-2001.
The variant returning
double
also conforms to
SVr4, 4.3BSD, C89.
BUGS
In glibc 2.5 and earlier,
taking the
log()
of a NaN produces a bogus invalid floatin-point
(
FE_INVALID)
exception.
SEE ALSO
cbrt(3),
clog(3),
log10(3),
log1p(3),
log2(3),
sqrt(3)
Index
- NAME
-
- LIBRARY
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- ATTRIBUTES
-
- STANDARDS
-
- HISTORY
-
- BUGS
-
- SEE ALSO
-
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