logb(), logbf(), logbl()

Updated: October 26, 2022

Compute the radix-independent exponent

Synopsis:

#include <math.h>

double logb ( double x );

float logbf ( float x );

long double logbl ( long double x );

Arguments:

x
The number that you want to compute the radix-independent exponent of.

Library:

libm
The general-purpose math library.
libm-sve
(QNX Neutrino 7.1 or later) A library that optimizes the code for ARMv8.2 chips that have Scalable Vector Extension hardware.

Your system requirements will determine how you should work with these libraries:

Note: Compile your program with the -fno-builtin option to prevent the compiler from using a built-in version of the function.

Description:

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

logr |x|

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

To check for error situations, use feclearexcept() and fetestexcept(). For example:

Returns:

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

If x is: These functions return: Errors:
x is ±0.0 -Inf FE_DIVBYZERO
±Inf Inf
NaN NaN

These functions raise FE_INEXACT if the FPU reports that the result can't be exactly represented as a floating-point number.

Examples:

#include <stdio.h>
#include <inttypes.h>
#include <math.h>
#include <fenv.h>
#include <stdlib.h>

int main( void )
{
    int except_flags;
    double a, b;

    a = 0.5;
    feclearexcept(FE_ALL_EXCEPT);
    b = logb(a);
    except_flags = fetestexcept(FE_ALL_EXCEPT);
    if(except_flags) {
        /* An error occurred; handle it appropriately. */
    }

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

    return EXIT_SUCCESS;
}

produces the output:

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

Classification:

C11, POSIX 1003.1

Safety:  
Cancellation point No
Interrupt handler Yes
Signal handler Yes
Thread Yes