# fmod(3) — Linux manual page

```fmod(3)                 Library Functions Manual                 fmod(3)
```

## NAME         top

```       fmod, fmodf, fmodl - floating-point remainder function
```

## LIBRARY         top

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

## SYNOPSIS         top

```       #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)):

fmodf(), fmodl():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
```

## DESCRIPTION         top

```       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         top

```       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         top

```       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         top

```       For an explanation of the terms used in this section, see
attributes(7).
┌─────────────────────────────────────┬───────────────┬─────────┐
│ Interface                           │ Attribute     │ Value   │
├─────────────────────────────────────┼───────────────┼─────────┤
│ fmod(), fmodf(), fmodl()            │ Thread safety │ MT-Safe │
└─────────────────────────────────────┴───────────────┴─────────┘
```

## STANDARDS         top

```       C11, POSIX.1-2008.
```

## HISTORY         top

```       C99, POSIX.1-2001.

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

## BUGS         top

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

## EXAMPLES         top

```       The call fmod(372, 360) returns 348.

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

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

```       remainder(3)
```

## COLOPHON         top

```       This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project.  Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩.  If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.