1.34 log, logf—natural logarithms

Synopsis

#include <math.h>
double log(double x);
float logf(float x);

Description
Return the natural logarithm of x, that is, its logarithm base e (where e is the base of the natural system of logarithms, 2.71828…). log and logf are identical save for the return and argument types.

Returns
Normally, returns the calculated value. When x is zero, the returned value is -HUGE_VAL and errno is set to ERANGE. When x is negative, the returned value is NaN (not a number) and errno is set to EDOM.

Portability
log is ANSI. logf is an extension.