ilogb, ilogbf, ilogbl

 

C

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Type support

Dynamic memory management

Error handling

Program utilities

Date and time utilities

Strings library

Algorithms

Numerics

Input/output support

Localization support

Thread support (C11)

Atomic operations (C11)

 

Numerics

Common mathematical functions

Floating-point environment

Complex number arithmetics

Random number generation

Type-generic math

 

Common mathematical functions

Functions
Basic operations
abslabsllabsimaxabs (C99) fabs divldivlldivimaxdiv (C99) fmod	remainder (C99) remquo (C99) fma (C99) fmax (C99) fmin (C99) fdim (C99) nannanfnanl (C99)(C99)(C99)
Exponential functions
exp exp2 (C99) expm1 (C99)	log log10 log1p (C99) log2 (C99)
Power functions
sqrt cbrt (C99)	hypot (C99) pow
Trigonometric and hyperbolic functions
sin cos tan asin acos atan atan2	sinh cosh tanh asinh (C99) acosh (C99) atanh (C99)
Error and gamma functions
erf (C99) erfc (C99)	lgamma (C99) tgamma (C99)
Nearest integer floating point operations
ceil floor roundlroundllround (C99)(C99)(C99)	trunc (C99) nearbyint (C99) rintlrintllrint (C99)(C99)(C99)
Floating point manipulation functions
ldexp scalbnscalbln (C99)(C99) ilogb (C99) logb (C99)	frexp modf nextafternexttoward (C99)(C99) copysign (C99)
Classification
fpclassify (C99)
isfinite (C99)
isinf (C99)
isnan (C99)
isnormal (C99)
signbit (C99)
Macro constants
HUGE_VALFHUGE_VALHUGE_VALL (C99)(C99)	FP_NORMALFP_SUBNORMALFP_ZEROFP_INFINITEFP_NAN (C99)(C99)(C99)(C99)(C99)

 

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 /*implementation-defined*/	(5)	(since C99)
#define FP_ILOGBNAN /*implementation-defined*/	(6)	(since C99)

1-3) Extracts the value of the unbiased exponent from the floating-point argument arg, and returns it as a signed integer value.

4) Type-generic macros: If 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.

5) Expands to integer constant expression whose value is either INT_MIN or -INT_MAX.

6) Expands to integer constant expression whose value is either INT_MIN or +INT_MAX.

Formally, the unbiased exponent is the integral part of log
r|arg| as a signed integral value, for non-zero arg, where r is FLT_RADIX.

Parameters

arg

-

floating point value

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.

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 floating-point 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

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 XSI-conformant 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_EXP-LDBL_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*r-e
| is between 1 and r (typically between 1 and 2), but for the exponent e returned by frexp, |arg*2-e
| is between 0.5 and 1.

Example

Run this code

#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

See also

frexpfrexpffrexpl (C99)(C99)	breaks a number into significand and a power of 2 (function)
logblogbflogbl (C99)(C99)(C99)	extracts exponent of the given number (function)
scalbnscalbnfscalbnlscalblnscalblnfscalblnl (C99)(C99)(C99)(C99)(C99)(C99)	computes efficiently a number times FLT_RADIX raised to a power (function)
C++ documentation for ilogb