#include <math.h>
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 with signal.
If x = 0, logf returns -? with signal.
If x = ?, logf returns ?.
If x = NaN, logf returns x
with no signal.
logf conforms to ISO/IEC 9899:1990 (C90) and ISO/IEC 9899:1999 (C99).