The Open Group Base Specifications Issue 7
IEEE Std 1003.1-2008, 2016 Edition
Copyright © 2001-2016 The IEEE and The Open Group

#### NAME

> logb, logbf, logbl - radix-independent exponent

#### SYNOPSIS

> ``#include <math.h>
> double logb(double` x`);
> float logbf(float` x`);
> long double logbl(long double` x`);
> ``

#### DESCRIPTION

> [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 POSIX.1-2008 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.

#### RETURN VALUE

> Upon successful completion, these functions shall return the exponent of x.

> If x is ±0, logb(), logbf(), and logbl() shall return -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL, respectively.

> [MX] On systems that support the IEC 60559 Floating-Point option, a pole error shall occur;
> otherwise, a [CX] pole error may occur.

> [MX] If x is NaN, a NaN shall be returned.

> [MX] If x is ±Inf, +Inf shall be returned.

#### ERRORS

> These functions shall fail if:

> Pole Error
>   [MX] 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.

> These functions may 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.

The following sections are informative.

#### EXAMPLES

> None.

#### APPLICATION USAGE

> 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.

#### RATIONALE

> None.

#### FUTURE DIRECTIONS

> None.

#### SEE ALSO

> feclearexcept, fetestexcept, ilogb, scalbln

> XBD Treatment of Error Conditions for Mathematical Functions, <float.h>, <math.h>

#### CHANGE HISTORY

> First released in Issue 4, Version 2.

#### Issue 5

> Moved from X/OPEN UNIX extension to BASE.

#### Issue 6

> 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.

#### Issue 7

> ISO/IEC 9899:1999 standard, Technical Corrigendum 2 #50 (SD5-XSH-ERN-76) is applied.

End of informative text.

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