log, logf, logl - natural logarithmic function
Math library (
libm,
-lm)
#include <math.h>
double log(double x);
float logf(float x);
long double logl(long double x);
logf(),
logl():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
These functions return the natural logarithm of
x.
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.
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: x is negative
-
errno is set to EDOM. An invalid
floating-point exception (FE_INVALID) is raised.
- Pole error: x is zero
-
errno is set to ERANGE. A divide-by-zero
floating-point exception (FE_DIVBYZERO) is raised.
For an explanation of the terms used in this section, see
attributes(7).
Interface |
Attribute |
Value |
log (), logf (), logl () |
Thread safety |
MT-Safe |
C99, POSIX.1-2001, POSIX.1-2008.
The variant returning
double also conforms to SVr4, 4.3BSD.
In glibc 2.5 and earlier, taking the
log() of a NaN produces a bogus
invalid floating-point (
FE_INVALID) exception.
cbrt(3),
clog(3),
log10(3),
log1p(3),
log2(3),
sqrt(3)