IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)

Copyright © 2001-2018 IEEE and The Open Group

logb, logbf, logbl - radix-independent exponent

#include <math.h>

double logb(doublex);

float logbf(floatx);

long double logbl(long doublex);

^{[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.1-2017 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-zerox, whereris the radix of the machine's floating-point arithmetic, which is the value of FLT_RADIX defined in the<float.h>header.If

xis subnormal it is treated as though it were normalized; thus for finite positivex:1 <=x* FLT_RADIX^{-logb(x)}< FLT_RADIXAn application wishing to check for error situations should set

errnoto zero and callfeclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, iferrnois non-zero orfetestexcept(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

xis ±0,logb(),logbf(), andlogbl() 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]}Ifxis NaN, a NaN shall be returned.

^{[MX]}Ifxis ±Inf, +Inf shall be returned.

These functions shall fail if:

- Pole Error
^{[MX]}The value ofxis ±0.If the integer expression (

math_errhandling& MATH_ERRNO) is non-zero, thenerrnoshall 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
xis 0.If the integer expression (

math_errhandling& MATH_ERRNO) is non-zero, thenerrnoshall 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.

None.

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.

None.

None.

feclearexcept,fetestexcept,ilogb,scalblnXBD

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() 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 9899:1999 standard.

IEC 60559:1989 standard floating-point extensions over the ISO/IEC 9899:1999 standard are marked.

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

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