frexp(), frexpf(), frexpl()

QNX SDP8.0C Library ReferenceAPIDeveloper

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: 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:

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:

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

Returns:

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


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
Signal handler	Yes
Thread	Yes

Page updated: August 11, 2025