scalb()

QNX SDP8.0C Library ReferenceAPIDeveloper

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: 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:

If you want only selected processes to run with the SVE version, you can include both libraries in your OS image and use the -l m or -l m-sve option to qcc to link explicitly against the appropriate one.
If you want all processes to use the SVE version, include libm-sve.so in your OS image and set up a symbolic link from libm.so to libm-sve.so. Use the -l m option to qcc to link against the library.

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:

Call feclearexcept(FE_ALL_EXCEPT) before calling scalb().
On return, if fetestexcept(FE_ALL_EXCEPT) is nonzero, then an error has occurred.

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
Signal handler	Yes
Thread	Yes

Page updated: August 11, 2025