These functions shall compute the exponent of x, which is the integral
part of log _r | x|, as a signed floating-point value, for
non-zero x, where r is the radix of the machine's floating-point
arithmetic, which is the value of FLT_RADIX defined in the
If x is subnormal it is treated as though it were normalized; thus for
finite positive x:
1 <=x* FLT_RADIX**-logb(x) < FLT_RADIX
An application wishing to check for error situations should set errno to
zero and call feclearexcept(FE_ALL_EXCEPT) before calling these
functions. On return, if errno is non-zero or
fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is
non-zero, an error has occurred.
Upon successful completion, these functions shall return the exponent of
If x is ±0, a pole error shall occur and logb(),
logbf(), and logbl() shall return -HUGE_VAL, -HUGE_VALF, and
If x is NaN, a NaN shall be returned.
If x is ±Inf, +Inf shall be returned.
If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then
errno shall be set to [ERANGE]. If the integer expression
(math_errhandling & MATH_ERREXCEPT) is non-zero, then the divide-by-zero
floating-point exception shall be raised.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html