SIMPLE SOLUTIONS

# LOG, LOGF, LOGL - reference manual online

Natural logarithmic function.

Chapter
2015-04-19
```LOG(3)                              Linux Programmer's Manual                              LOG(3)

NAME
log, logf, logl - natural logarithmic function

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():
_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE ||
_POSIX_C_SOURCE >= 200112L;
or cc -std=c99

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: 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.

ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).

┌──────────────────────┬───────────────┬─────────┐
│Interface             │ Attribute     │ Value   │
├──────────────────────┼───────────────┼─────────┤
│log(), logf(), logl() │ Thread safety │ MT-Safe │
└──────────────────────┴───────────────┴─────────┘
CONFORMING TO
C99, POSIX.1-2001, POSIX.1-2008.

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 floating-
point (FE_INVALID) exception.