LOG

Section: Linux Programmer's Manual (3)
Updated: 2008-08-10
Index Return to Main Contents

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);

Link with -lm.

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

logf(), logl(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE; or cc -std=c99

DESCRIPTION

The log() function returns 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.

CONFORMING TO

C99, POSIX.1-2001. 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 floating-point (FE_INVALID) exception.

COLOPHON

This page is part of release 3.22 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

LOG