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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.

You can use the (non-ANSI) function matherr to specify error handling for these functions.
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. You can control the error behavior via matherr.
Portability
log is ANSI. logf is an extension.