# ilogb

## PROLOG



           This manual page is part of the POSIX Programmer's Manual.  The
           Linux implementation of this interface may differ (consult the
           corresponding Linux manual page for details of Linux behavior), or
           the interface may not be implemented on Linux.
    


## NAME



           ilogb, ilogbf, ilogbl — return an unbiased exponent
    


## SYNOPSIS



           #include <math.h>
    
           int ilogb(double _x_);
           int ilogbf(float _x_);
           int ilogbl(long double _x_);
    


## DESCRIPTION



           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 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-zero _x_ , where _r_ is 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
           _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 part of _x_ as a signed integer value. They are equivalent
           to calling the corresponding _logb_() function and casting the
           returned value to type **int**.
    
           If _x_ is 0, the value FP_ILOGB0 shall be returned.  On XSI-
           conformant systems, a domain error shall occur;
           otherwise, a domain error may occur.
    
           If _x_ is ±Inf, the value {INT_MAX} shall be returned.  On XSI-
           conformant systems, a domain error shall occur;
           otherwise, a domain error may occur.
    
           If _x_ is a NaN, the value FP_ILOGBNAN shall be returned.  On XSI-
           conformant systems, a domain error shall occur;
           otherwise, a domain error may occur.
    
           If the correct value is greater than {INT_MAX}, a domain error
           shall occur and an unspecified value shall be returned.  On XSI-
           conformant systems, a domain error shall occur and {INT_MAX} shall
           be returned.
    
           If the correct value is less than {INT_MIN}, a domain error shall
           occur and an unspecified value shall be returned.  On XSI-
           conformant systems, a domain error shall occur and {INT_MIN} shall
           be returned.
    


## ERRORS



           These functions shall fail if:
    
           Domain Error
                       The correct value is not representable as an integer.
    
                       The _x_ argument is zero, NaN, or ±Inf.
    
                       If the integer expression (_math_errhandling_ &
                       MATH_ERRNO) is non-zero, then _errno_ shall 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 _x_ argument is zero, NaN, or ±Inf.
    
                       If the integer expression (_math_errhandling_ &
                       MATH_ERRNO) is non-zero, then _errno_ shall be set to
                       **[EDOM]**.  If the integer expression (_math_errhandling_ &
                       MATH_ERREXCEPT) is non-zero, then the invalid
                       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



           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 type **int**), 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.
    


## FUTURE DIRECTIONS



           None.
    


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