#include <math.h>
double log(double x);
log computes the base-e logarithm of x.
If x = 0, errno is set to ERANGE
and log returns -HUGE_VAL.
If x < 0, errno is set to EDOM
and log returns -HUGE_VAL.
If x < 0 or x = -?, log returns
NaN with signal.
If x = 0, log returns -? with signal.
If x = ?, log returns ?.
If x = NaN, log returns x
with no signal.
log conforms to ISO/IEC 9899:1990 (C90) and ISO/IEC 9899:1999 (C99).