IEEE Std 1003.1-2008, 2016 Edition

Copyright © 2001-2016 The IEEE and The Open Group

ilogb, ilogbf, ilogbl - return an unbiased exponent

#include <math.h>

int ilogb(doublex);

int ilogbf(floatx);

int ilogbl(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.1-2008 defers to the ISO C standard.These functions shall return the exponent part of their argument

x. Formally, the return value is the integral part of log_{r}|x| as a signed integral value, for non-zerox, whereris the radix of the machine's floating-point arithmetic, which is the value of FLT_RADIX defined in<float.h>.An 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 part of

xas a signed integer value. They are equivalent to calling the correspondinglogb() function and casting the returned value to typeint.If

xis 0, the value FP_ILOGB0 shall be returned.^{[XSI]}On XSI-conformant systems, a domain error shall occur;

otherwise, a^{[CX]}domain error may occur.If

xis ±Inf, the value {INT_MAX} shall be returned.^{[XSI]}On XSI-conformant systems, a domain error shall occur;

otherwise, a^{[CX]}domain error may occur.If

xis a NaN, the value FP_ILOGBNAN shall be returned.^{[XSI]}On XSI-conformant systems, a domain error shall occur;

otherwise, a^{[CX]}domain error may occur.If the correct value is greater than {INT_MAX},

^{[MX]}a domain error shall occur and an unspecified value shall be returned.^{[XSI]}On XSI-conformant systems, a domain error shall occur and {INT_MAX} shall be returned.If the correct value is less than {INT_MIN},

^{[MX]}a domain error shall occur and an unspecified value shall be returned.^{[XSI]}On XSI-conformant systems, a domain error shall occur and {INT_MIN} shall be returned.

These functions shall fail if:

- Domain Error
^{[XSI|MX]}The correct value is not representable as an integer.

^{[XSI]}Thexargument is zero, NaN, or ±Inf.

^{[XSI|MX]}If the integer expression (math_errhandling& MATH_ERRNO) is non-zero, thenerrnoshall be set to [EDOM]. If the integer expression (math_errhandling& MATH_ERREXCEPT) is non-zero, then the invalid floating-point exception shall be raised.These functions may fail if:

- Domain Error
- The
xargument is zero, NaN, or ±Inf.If the integer expression (

math_errhandling& MATH_ERRNO) is non-zero, thenerrnoshall be set to [EDOM]. If the integer expression (math_errhandling& MATH_ERREXCEPT) is non-zero, then the invalid 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.

The errors come from taking the expected floating-point value and converting it to

int, which is an invalid operation in IEEE Std 754-1985 (since overflow, infinity, and NaN are not representable in a typeint), so should be a domain error.There are no known implementations that overflow. For overflow to happen, {INT_MAX} must be less than LDBL_MAX_EXP*

log2(FLT_RADIX) or {INT_MIN} must be greater than LDBL_MIN_EXP*log2(FLT_RADIX) if subnormals are not supported, or {INT_MIN} must be greater than (LDBL_MIN_EXP-LDBL_MANT_DIG)*log2(FLT_RADIX) if subnormals are supported.

None.

feclearexcept,fetestexcept,logb,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

ilogb() function is no longer marked as an extension.The

ilogbf() andilogbl() functions are added for alignment with the ISO/IEC 9899:1999 standard.The RETURN VALUE section is revised for alignment with the ISO/IEC 9899:1999 standard.

Functionality relating to the XSI option is marked.

ISO/IEC 9899:1999 standard, Technical Corrigendum 2 #48 (SD5-XSH-ERN-71), #49, and #79 (SD5-XSH-ERN-72) are applied.

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