---
name: "fmod"
section: "3"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/fmod"
---

# fmod(3)

floating-point remainder function

## NAME

fmod, fmodf, fmodl - floating-point remainder function

## LIBRARY

Math library (*libm*, *-lm*)

## 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***);**

Feature Test Macro Requirements for glibc (see [**feature\_test\_macros**(7)](/feature_test_macros/7.md)):

**fmodf**(), **fmodl**():

```
    _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
        || /* Since glibc 2.19: */ _DEFAULT_SOURCE
        || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
```

## DESCRIPTION

These functions compute the floating-point remainder of dividing *x* by *y*. The return value is *x* - *n* \* *y*, where *n* is the quotient of *x* / *y*, rounded toward zero to an integer.

To obtain the modulus, more specifically, the Least Positive Residue, you will need to adjust the result from **fmod**() like so:

```
z = fmod(x, y);
if (z < 0)
	z += fabs(y);
```

An alternate way to express this is with *fmod(fmod(x, y) + y, y)*, but the second **fmod**() usually costs way more than the one branch.

## RETURN VALUE

On success, these functions return the value *x - n\*y*, for some integer *n*, such that the returned value has the same sign as *x* and a magnitude less than the magnitude of *y*.

If *x* or *y* is a NaN, a NaN is returned.

If *x* is an infinity, a domain error occurs, and a NaN is returned.

If *y* is zero, a domain error occurs, and a NaN is returned.

If *x* is +0 (-0), and *y* is not zero, +0 (-0) is returned.

## ERRORS

See [**math\_error**(7)](/math_error/7.md) for information on how to determine whether an error has occurred when calling these functions.

The following errors can occur:

- Domain error: *x* is an infinity

  *errno* is set to **EDOM** (but see BUGS). An invalid floating-point exception (**FE\_INVALID**) is raised.

- Domain error: *y* is zero

  *errno* is set to **EDOM**. An invalid floating-point exception (**FE\_INVALID**) is raised.

## ATTRIBUTES

For an explanation of the terms used in this section, see [**attributes**(7)](/attributes/7.md).

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **fmod**(), **fmodf**(), **fmodl**() | Thread safety | MT-Safe |

## STANDARDS

C11, POSIX.1-2008.

## HISTORY

C99, POSIX.1-2001.

The variant returning *double* also conforms to SVr4, 4.3BSD, C89.

## BUGS

Before glibc 2.10, the glibc implementation did not set *errno* to **EDOM** when a domain error occurred for an infinite *x*.

## EXAMPLES

The call *fmod(372, 360)* returns 12.

The call *fmod(-372, 360)* returns -12.

The call *fmod(-372, -360)* also returns -12.

## SEE ALSO

[**remainder**(3)](/remainder/3.md)