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2017-09-15
Aliases: logf(3), logf(3), logf(3), logf(3), logf(3), logf(3), logf(3), logf(3), logl(3), logl(3), logl(3), logl(3), logl(3), logl(3), logl(3), logl(3)

manpages-dev

Manual pages about using GNU/Linux for development

man-pages

Linux kernel and C library user-space interface documentation

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():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 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).
Interface Attribute Value
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.

COLOPHON

This page is part of release 5.00 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.
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