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NAME

logb, logbf, logbl - radix-independent exponent

SYNOPSIS

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

DESCRIPTION

^[CX] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between therequirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2017 defers to the ISO Cstandard.
These functions shall compute the exponent ofx, which is the integral part of log_r|x|, as a signed floating-point value, for non-zerox, wherer is the radix of the machine's floating-pointarithmetic, which is the value of FLT_RADIX defined in the<float.h>header.
Ifx is subnormal it is treated as though it were normalized; thus for finite positivex:
1 <=x* FLT_RADIX^-logb(x) < FLT_RADIX
An application wishing to check for error situations should seterrno to zero and callfeclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, iferrno is non-zero orfetestexcept(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 ofx.
Ifx is ±0,logb(),logbf(), andlogbl() shall return -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL,respectively.
^[MX] Onsystems that support the IEC 60559 Floating-Point option, a pole error shall occur;
otherwise, a^[CX] pole error may occur.
^[MX] Ifx is NaN, a NaN shall be returned.
^[MX] Ifx is ±Inf, +Inf shall be returned.

ERRORS

These functions shall fail if:
Pole Error
^[MX]The value ofx is ±0.
If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, thenerrno shall be set to [ERANGE]. Ifthe integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the divide-by-zero floating-point exceptionshall be raised.
These functions may fail if:
Pole Error
The value ofx is 0.
If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, thenerrno shall be set to [ERANGE]. Ifthe integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the divide-by-zero floating-point exceptionshall be raised.

The following sections are informative.

EXAMPLES

None.

APPLICATION USAGE

On error, the expressions (math_errhandling & MATH_ERRNO) and (math_errhandling & MATH_ERREXCEPT) areindependent of each other, but at least one of them must be non-zero.

RATIONALE

None.

FUTURE DIRECTIONS

None.

CHANGE HISTORY

First released in Issue 4, Version 2.

Issue 5

Moved from X/OPEN UNIX extension to BASE.

Issue 6

Thelogb() function is no longer marked as an extension.
Thelogbf() andlogbl() 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 9899:1999standard.
IEC 60559:1989 standard floating-point extensions over the ISO/IEC 9899:1999 standard are marked.