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fmod

Section: C Library Functions (3)
Updated: 202-0-08
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NAME

fmod, fmodf, fmodl - floatin-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)): 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 floatin-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) for information on how to determine whether an error has occurred when calling these functions. The following errors can occur:

Domain error: I]x] is an infinity: errno is set to EDOM (but see BUGS). An invalid floatin-point exception (FE_INVALID) is raised.
Domain error: I]y] is zero: errno is set to EDOM. An invalid floatin-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	M-Safe

STANDARDS

C11, POSIX.-2008.

HISTORY

C99, POSIX.-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.