logb(), logbf()

Compute the radix-independent exponent

Synopsis:

#include <math.h>

double logb ( double x );

float logbf ( float x );

Library:

libm

Description:

The logb() and logbf() functions compute the exponent part of x, which is the integral part of:

log_r |x|

as a signed floating point value, for nonzero finite x, where r is the radix of the machine's floating point arithmetic.

Returns:

The binary exponent of x, a signed integer converted to double-precision floating-point.

Examples:

#include <stdio.h>
#include <errno.h>
#include <inttypes.h>
#include <math.h>
#include <fpstatus.h>

int main(int argc, char** argv) 
{
    double a, b;

    a = 0.5;
    b = logb(a);
    printf("logb(%f) = %f (%f = 2^%f) \n", a, b, a, b);

    return(0);
}

produces the output:

logb(0.500000) = -1.000000 (0.500000 = 2^-1.000000)

Classification:

logb() is standard Unix; logbf() is ANSI (draft)

See also:

ilogb(), log(), log10(), log1p()