float logf(float x);
logf computes the base-e logarithm of x.
If x = 0, errno is set to ERANGE and logf returns −HUGE_VALF. If x < 0, errno is set to EDOM and logf returns −HUGE_VALF.
If x < 0 or x = −∞, logf returns NaN.
If x = 0, logf returns −∞.
If x = ∞, logf returns ∞.
If x = NaN, logf returns x.