log(3) Linux Man Page - System Grab Bag

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: 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). allbox; lbx lb lb T{ log()logf()logl()

Interface	Attribute	Value
T}	Thread safety	MT-Safe

Standards

C11, POSIX.1-2008.

History

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.