ilogb, ilogbf, ilogbl
Defined in header
<math.h>


int ilogbf( float arg );

(1)  (since C99) 
int ilogb( double arg );

(2)  (since C99) 
int ilogbl( long double arg );

(3)  (since C99) 
Defined in header
<tgmath.h>


#define ilogb( arg )

(4)  (since C99) 
Defined in header
<math.h>


#define FP_ILOGB0 /*implementationdefined*/

(5)  (since C99) 
#define FP_ILOGBNAN /*implementationdefined*/

(6)  (since C99) 
arg
, and returns it as a signed integer value. arg
has type long double, ilogbl
is called. Otherwise, if arg
has integer type or the type double, ilogb
is called. Otherwise, ilogbf
is called.Formally, the unbiased exponent is the integral part of log
rarg as a signed integral value, for nonzero arg, where r
is FLT_RADIX.
Contents 
[edit] Parameters
arg    floating point value 
[edit] Return value
If no errors occur, the unbiased exponent of arg
is returned as a signed int value.
If arg
is zero, FP_ILOGB0 is returned.
If arg
is infinite, INT_MAX is returned.
If arg
is a NaN, FP_ILOGBNAN is returned.
If the correct result is greater than INT_MAX or smaller than INT_MIN, the return value is unspecified.
[edit] Error handling
Errors are reported as specified in math_errhandling
A domain error or range error may occur if arg
is zero, infinite, or NaN.
If the implementation supports IEEE floatingpoint arithmetic (IEC 60559),
 If the correct result is greater than INT_MAX or smaller than INT_MIN, FE_INVALID is raised.
 If
arg
is ±0, ±∞, or NaN, FE_INVALID is raised.  In all other cases, the result is exact (FE_INEXACT is never raised) and the current rounding mode is ignored
[edit] Notes
If arg
is not zero, infinite, or NaN, the value returned is exactly equivalent to (int)logb(arg)
POSIX requires that a domain error occurs if arg
is zero, infinite, NaN, or if the correct result is outside of the range of int.
POSIX also requires that, on XSIconformant systems, the value returned when the correct result is greater than INT_MAX is INT_MAX and the value returned when the correct result is less than INT_MIN is INT_MIN.
The correct result can be represented as int on all known implementations. For overflow to occur, INT_MAX must be less than LDBL_MAX_EXP*log2(FLT_RADIX) or INT_MIN must be greater than LDBL_MIN_EXPLDBL_MANT_DIG)*log2(FLT_RADIX).
The value of the exponent returned by std::ilogb
is always 1 less than the exponent retuned by frexp because of the different normalization requirements: for the exponent e
returned by ilogb
, arg*re
 is between 1 and r
(typically between 1 and 2), but for the exponent e
returned by frexp, arg*2e
 is between 0.5 and 1.
[edit] Example
#include <stdio.h> #include <math.h> #include <float.h> #include <fenv.h> #pragma STDC FENV_ACCESS ON int main() { double f = 123.45; printf("Given the number %.2f or %a in hex,\n", f, f); double f3; double f2 = modf(f, &f3); printf("modf() makes %.0f + %.2f\n", f3, f2); int i; f2 = frexp(f, &i); printf("frexp() makes %f * 2^%d\n", f2, i); i = ilogb(f); printf("logb()/ilogb() make %f * %d^%d\n", f/scalbn(1.0, i), FLT_RADIX, i); // error handling feclearexcept(FE_ALL_EXCEPT); printf("ilogb(0) = %d\n", ilogb(0)); if(fetestexcept(FE_INVALID)) puts(" FE_INVALID raised"); }
Possible output:
Given the number 123.45 or 0x1.edccccccccccdp+6 in hex, modf() makes 123 + 0.45 frexp() makes 0.964453 * 2^7 logb()/ilogb() make 1.92891 * 2^6 ilogb(0) = 2147483648 FE_INVALID raised
[edit] See also
(C99)(C99)

breaks a number into significand and a power of 2 (function) 
(C99)(C99)(C99)

extracts exponent of the given number (function) 
(C99)(C99)(C99)(C99)(C99)(C99)

computes efficiently a number times FLT_RADIX raised to a power (function) 
C++ documentation for ilogb
