logb, logbf, logbl - radix-independent exponent
double logb(double x);
float logbf(float x);
long double logbl(long double x);
[CX] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of IEEE Std 1003.1-2001 defers to the ISO C standard.
These functions shall compute the exponent of x, which is the integral part of logr |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 <float.h> header.
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 x.
If x is ±0, a pole error shall occur and logb(), logbf(), and logbl() shall return -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL, respectively.
[MX] If x is NaN, a NaN shall be returned.
If x is ±Inf, +Inf shall be returned.
These functions shall fail if:
- Pole Error
- The value of x is ±0.
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.
On error, the expressions (math_errhandling & MATH_ERRNO) and (math_errhandling & MATH_ERREXCEPT) are independent of each other, but at least one of them must be non-zero.
feclearexcept(), fetestexcept(), ilogb(), scalb(), the Base Definitions volume of IEEE Std 1003.1-2001, Section 4.18, Treatment of Error Conditions for Mathematical Functions, <float.h>, <math.h>
First released in Issue 4, Version 2.
Moved from X/OPEN UNIX extension to BASE.
The logb() function is no longer marked as an extension.
The logbf() and logbl() functions are added for alignment with the ISO/IEC 9899:1999 standard.
The DESCRIPTION, RETURN VALUE, ERRORS, and APPLICATION USAGE sections are revised to align with the ISO/IEC 9899:1999 standard.
IEC 60559:1989 standard floating-point extensions over the ISO/IEC 9899:1999 standard are marked.