modf(), modff(), modfl()

Updated: October 26, 2022

Break a number into integral and fractional parts

Synopsis:

#include <math.h>

double modf( double value,
             double* iptr );

float modff( float value,
             float* iptr );

long double modfl( long double value,
                   long double* iptr );

Arguments:

value
The value that you want to break into parts.
iptr
A pointer to a location where the function can store the integral part of the number.

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:

The modf(), modff(), and modfl() functions break the given value into integral and fractional parts, each of which has the same sign as the argument. They store the integral part in the object pointed to by iptr.

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

Returns:

The signed fractional part of value.

If value is: These functions return: And set *iptr to: Errors:
±Inf 0.0, with the same sign as value Inf, with the same sign as value
NaN NaN NaN

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>

int main( void )
{
    double integral_value, fractional_part;

    fractional_part = modf( 4.5, &integral_value );
    printf( "%f %f\n", fractional_part, integral_value );
    
    fractional_part = modf( -4.5, &integral_value );
    printf( "%f %f\n", fractional_part, integral_value );
    
    return EXIT_SUCCESS;
}

produces the output:

0.500000 4.000000
-0.500000 -4.000000

Classification:

C11, POSIX 1003.1

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