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.1-2024 defers to the ISO C standard.

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 <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.

^[MX] The returned value shall be exact and shall be independent of the current rounding direction mode.

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.

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 4.23 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.

Issue 8

Austin Group Defect 1302 is applied, aligning these functions with the ISO/IEC 9899:2018 standard.