scalb()

Updated: April 19, 2023

Load the exponent of a radix-independent floating point number

Synopsis:

#include <math.h>

double scalb( double x,
              double n );

Arguments:

x
The floating point number that you want to multiply by the exponent.
n
The exponent to apply to the radix of the machine's floating-point arithmetic.

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:

This function computes x × rn, where r is the radix of the machine's floating point arithmetic and n is a finite number. When r is 2, scalb() is equivalent to ldexp().

Note: We recommend that you use scalbn() because it computes by manipulating exponents, instead of using mock multiplications or additions.

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

Returns:

x * rn

If: These functions return: Errors:
n isn't an integer NaN FE_INVALID
x is Inf, and n is Inf Inf
x is Inf, and n isn't Inf NaN FE_INVALID
x isn't Inf, and n is Inf Inf, with the sign matching x
x isn't Inf, and n is -Inf 0.0, with the sign matching x
x or n is NaN NaN
The correct value would cause overflow Inf FE_OVERFLOW

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 )
{
    double a, b, c, d;
    int except_flags;

    feclearexcept(FE_ALL_EXCEPT);
    a = 10;
    b = 2;
    c = scalb(a, b);

    except_flags = fetestexcept(FE_ALL_EXCEPT);
    if(except_flags) {
        /* An error occurred; handle it appropriately. */
    }

    feclearexcept(FE_ALL_EXCEPT);

    d = sqrt(c/a);

    except_flags = fetestexcept(FE_ALL_EXCEPT);
    if(except_flags) {
        /* An error occurred; handle it appropriately. */
    }

    printf("Radix of machine's fp arithmetic is %f \n", d);
    printf("So %f = %f * (%f ^ %f) \n", c, a, d, b);

    return EXIT_SUCCESS;
}

produces the output:

Radix of machine's fp arithmetic is 2.000000
So 40.000000 = 10.000000 * (2.000000 ^ 2.000000)

Classification:

Standard Unix; removed from POSIX.1-2008

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