# fmod - Man Page

floating-point remainder function

## Library

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

## Synopsis

#include <math.h> double fmod(doublex, doubley); float fmodf(floatx, floaty); long double fmodl(long doublex, long doubley);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

**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 += 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) 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).

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 348.

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

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

## See Also

## Referenced By

remainder(3), remquo(3), unu-2op(1).

The man pages fmodf(3) and fmodl(3) are aliases of fmod(3).