fmod(), fmodf(), fmodl()

QNX SDP8.0C Library ReferenceAPIDeveloper

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: 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 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:

Call feclearexcept(FE_ALL_EXCEPT) before calling fmod(), fmodf(), or fmodl().
On return, if fetestexcept(FE_ALL_EXCEPT) is nonzero, then an error has occurred.

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:

Classification:

C11, POSIX 1003.1


Safety:
Cancellation point	No
Signal handler	Yes
Thread	Yes

Page updated: August 11, 2025