fmod(), fmodf(), fmodl()

Compute a residue, using floating-point modular arithmetic

Synopsis:

#include <math.h>

double fmod( double x, 
             double y );

float fmodf( float x, 
             float y );

long double fmodl( long double x,
                   long double y );

Arguments:

x
An arbitrary number.
y
The modulus.

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 fmod() and fmodf() functions compute the floating-point residue of x (mod y), which is the remainder of x / y, even if the quotient x / y isn't representable.

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

Returns:

The residue, x - (i × y), for some integer i such that, if y is nonzero, the result has the same sign as x and a magnitude less than the magnitude of y.

If: These functions return: Errors:
x is Inf NaN FE_INVALID
y is 0.0 NaN FE_INVALID
x or y is NaN NaN
The correct value would cause underflow 0.0

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

int main( void )
{
    int except_flags;

    feclearexcept(FE_ALL_EXCEPT);
    printf( "%f\n", fmod(  4.5,  2.0 ) );
    except_flags = fetestexcept(FE_ALL_EXCEPT);
    if(except_flags) {
        /* An error occurred; handle it appropriately. */
    }

    feclearexcept(FE_ALL_EXCEPT);
    printf( "%f\n", fmod( -4.5,  2.0 ) );
    except_flags = fetestexcept(FE_ALL_EXCEPT);
    if(except_flags) {
        /* An error occurred; handle it appropriately. */
    }

    feclearexcept(FE_ALL_EXCEPT);
    printf( "%f\n", fmod(  4.5, -2.0 ) );
    except_flags = fetestexcept(FE_ALL_EXCEPT);
    if(except_flags) {
        /* An error occurred; handle it appropriately. */
    }

    feclearexcept(FE_ALL_EXCEPT);
    printf( "%f\n", fmod( -4.5, -2.0 ) );
    except_flags = fetestexcept(FE_ALL_EXCEPT);
    if(except_flags) {
        /* An error occurred; handle it appropriately. */
    }

    return EXIT_SUCCESS;
}

produces the output:

0.500000
-0.500000
0.500000
-0.500000

Classification:

C11, POSIX 1003.1

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