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# fmod, fmodf, fmodl

From cppreference.com
< c‎ | numeric‎ | math

Defined in header `<math.h>` |  |
float fmodf( float x, float y ); | (1) | (since C99)
double fmod( double x, double y ); | (2) |
long double fmodl( long double x, long double y ); | (3) | (since C99)
Defined in header `<tgmath.h>` |  |
#define fmod( x, y ) | (4) | (since C99)

1-3) Computes the floating-point remainder of the division operation x/y.
4) Type-generic macro: If any argument has type long double, `fmodl` is called. Otherwise, if any argument has integer type or has type double, `fmod` is called. Otherwise, `fmodf` is called.

The floating-point remainder of the division operation x/y calculated by this function is exactly the value x - n*y, where `n` is x/y with its fractional part truncated.

The returned value has the same sign as `x` and is less or equal to `y` in magnitude.

### [edit] Parameters

x, y | - | floating point values

### [edit] Return value

If successful, returns the floating-point remainder of the division x/y as defined above.

If a domain error occurs, an implementation-defined value is returned (NaN where supported).

If a range error occurs due to underflow, the correct result (after rounding) is returned.

### [edit] Error handling

Errors are reported as specified in math_errhandling.

Domain error may occur if `y` is zero.

If the implementation supports IEEE floating-point arithmetic (IEC 60559),

- If `x` is ±0 and `y` is not zero, ±0 is returned
- If `x` is ±∞ and `y` is not NaN, NaN is returned and FE_INVALID is raised
- If `y` is ±0 and `x` is not NaN, NaN is returned and FE_INVALID is raised
- If `y` is ±∞ and `x` is finite, `x` is returned.
- If either argument is NaN, NaN is returned

### [edit] Notes

POSIX requires that a domain error occurs if `x` is infinite or `y` is zero.

`fmod`, but not remainder is useful for doing silent wrapping of floating-point types to unsigned integer types: (0.0 <= (y = fmod(rint(x), 65536.0 )) ? y : 65536.0 + y) is in the range `[-0.0 .. 65535.0]`, which corresponds to unsigned short, but remainder(rint(x), 65536.0 is in the range `[-32767.0, +32768.0]`, which is outside of the range of signed short.

The double version of fmod behaves as if implemented as follows:

    double fmod(double x, double y)
    {
    #pragma STDC FENV_ACCESS ON
        double result = remainder(fabs(x), (y = fabs(y)));
        if (signbit(result)) result += y;
        return copysign(result, x);
    }

### [edit] Example

Run this code

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

    #pragma STDC FENV_ACCESS ON
    int main(void)
    {
        printf("fmod(+5.1, +3.0) = %.1f\n", fmod(5.1,3));
        printf("fmod(-5.1, +3.0) = %.1f\n", fmod(-5.1,3));
        printf("fmod(+5.1, -3.0) = %.1f\n", fmod(5.1,-3));
        printf("fmod(-5.1, -3.0) = %.1f\n", fmod(-5.1,-3));

        // special values
        printf("fmod(+0.0, 1.0) = %.1f\n", fmod(0, 1));
        printf("fmod(-0.0, 1.0) = %.1f\n", fmod(-0.0, 1));
        printf("fmod(+5.1, Inf) = %.1f\n", fmod(5.1, INFINITY));

        // error handling
        feclearexcept(FE_ALL_EXCEPT);
        printf("fmod(+5.1, 0) = %.1f\n", fmod(5.1, 0));
        if(fetestexcept(FE_INVALID)) puts("    FE_INVALID raised");
    }

Possible output:

    fmod(+5.1, +3.0) = 2.1
    fmod(-5.1, +3.0) = -2.1
    fmod(+5.1, -3.0) = 2.1
    fmod(-5.1, -3.0) = -2.1
    fmod(+0.0, 1.0) = 0.0
    fmod(-0.0, 1.0) = -0.0
    fmod(+5.1, Inf) = 5.1
    fmod(+5.1, 0) = nan
        FE_INVALID raised

### [edit] References

- C11 standard (ISO/IEC 9899:2011):

  - 7.12.10.1 The fmod functions (p: 254)

  - 7.25 Type-generic math <tgmath.h> (p: 373-375)

  - F.10.7.1 The fmod functions (p: 528)

- C99 standard (ISO/IEC 9899:1999):

  - 7.12.10.1 The fmod functions (p: 235)

  - 7.22 Type-generic math <tgmath.h> (p: 335-337)

  - F.9.7.1 The fmod functions (p: 465)

- C89/C90 standard (ISO/IEC 9899:1990):

  - 4.5.6.4 The fmod function

### [edit] See also

div ldiv lldiv (C99) | computes quotient and remainder of integer division (function) [edit]
remainder remainderf remainderl (C99)(C99)(C99) | computes signed remainder of the floating-point division operation (function) [edit]
remquo remquof remquol (C99)(C99)(C99) | computes signed remainder as well as the three last bits of the division operation (function) [edit]
C++ documentation for fmod

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