log()函数用于求自然对数的数学函数，按照函数说明，如果传递非法参数作为函数参数，可能会引发异常。

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SYNOPSIS
    \#include <math.h>
    double log(double x);
    float logf(float x);
    long double logl(long double x);
 
RETURN VALUE
    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
    The following errors can occur:
    Domain error: x is negative
        errno is set to EDOM. An invalid floating-point exception (FE_INVALID) is raised.
    Pole error: x is zero
        errno is set to ERANGE. A divide-by-zero floating-point exception (FE_DIVBYZERO) is raised.