ilogb, ilogbf, ilogbl

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

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 and a domain error or range error may occur.

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 correct result is greater than INT_MAX or smaller than INT_MIN , a domain error or a range error may occur

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 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

#include <fenv.h>
#include <float.h>
#include <math.h>
#include <stdio.h>
// #pragma STDC FENV_ACCESS ON
 
int main(void)
{
    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");
}

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

References

See also