logf (3)
Leading comments
Copyright 1993 David Metcalfe (david@prism.demon.co.uk) and Copyright 2008, Linux Foundation, written by Michael Kerrisk <mtk.manpages@gmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting ...
NAME
log, logf, logl  natural logarithmic functionSYNOPSIS
#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 
_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 floatingpoint exception (FE_INVALID) is raised.
 Pole error: x is zero
 errno is set to ERANGE. A dividebyzero floatingpoint 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  MTSafe 
CONFORMING TO
C99, POSIX.12001, POSIX.12008.The variant returning double also conforms to SVr4, 4.3BSD, C89.