frexp(), frexpf(), frexpl()

Updated: April 19, 2023

Break a floating-point number into a normalized fraction and an integral power of 2

Synopsis:

#include <math.h>

double frexp( double value, 
              int* exp );

float frexpf( float value, 
              int* exp );

long double frexp( long double value, 
                   int* exp );

Arguments:

value
The value you want to break into a normalized fraction.
exp
A pointer to a location where the function can store the integral power of 2.

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:

These functions break a floating-point number into a normalized fraction and an integral power of 2. It stores the integral power of 2 in the int pointed to by exp.

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

Returns:

A number, x, that has a magnitude in the interval [0.5, 1.0] or is 0.0, such that value = x * 2exp.

If value is: These functions return: *exp Errors:
±0.0 0.0, with the same sign as value 0
±Inf value Unspecified
NaN value Unspecified

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

int main( void )
{
    int expon;
    double value;
    int except_flags;

    feclearexcept(FE_ALL_EXCEPT);
    value = frexp(  4.25, &expon );
    printf( "%f %d\n", value, expon );
    except_flags = fetestexcept(FE_ALL_EXCEPT);
    if(except_flags) {
        /* An error occurred; handle it appropriately. */
    }

    feclearexcept(FE_ALL_EXCEPT);
    value = frexp( -4.25, &expon );
    printf( "%f %d\n", value, expon );
    except_flags = fetestexcept(FE_ALL_EXCEPT);
    if(except_flags) {
        /* An error occurred; handle it appropriately. */
    }

    return EXIT_SUCCESS;
}

produces the output:

0.531250 3
-0.531250 3

Classification:

C11, POSIX 1003.1

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