/C

# fmod, fmodf, fmodl

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.

### Parameters

 x, y - floating point values

### 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.

### 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

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);
}```

### Example

```#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```
• 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