scalbn(), scalbnf(), scalbnl()

QNX SDP8.0C Library ReferenceAPIDeveloper

Load the exponent of a radix-independent floating point number

Synopsis:

#include <math.h>

double scalbn ( double x,
                int n );

float scalbnf ( float x,
                int n );

long double scalbnl ( long double x,
                      int 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:

The scalbn(), scalbnf(), and scalbnl() functions compute x × rⁿ, where r is the radix of the machine's floating-point arithmetic. The difference between the scalbn* and scalbln* functions is the type of the second argument.

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

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

Returns:

x × rⁿ


If:	These functions return:	Errors:
`x` is NaN	NaN	—
`x` is ±0.0 or ±Inf	`x`	—
`n` is 0	`x`	—
The correct value would cause underflow	The correct value, after rounding	FE_UNDERFLOW
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 = scalbn(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:

C11, POSIX 1003.1


Safety:
Cancellation point	No
Signal handler	Yes
Thread	Yes

Page updated: August 09, 2024