logf (3)  Linux Manuals
logf: natural logarithmic function
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 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.
BUGS
In glibc 2.5 and earlier, taking the log() of a NaN produces a bogus invalid floatingpoint (FE_INVALID) exception.COLOPHON
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