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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.