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fmod(3) Library Functions Manual fmod(3)


fmod, fmodf, fmodl - floating-point remainder function


Math library (libm, -lm)


#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():

|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE


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.


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.


See math_error(7) for information on how to determine whether an error has occurred when calling these functions.

The following errors can occur:

errno is set to EDOM (but see BUGS). An invalid floating-point exception (FE_INVALID) is raised.
errno is set to EDOM. An invalid floating-point exception (FE_INVALID) is raised.


For an explanation of the terms used in this section, see attributes(7).

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


C11, POSIX.1-2008.


C99, POSIX.1-2001.

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


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


The call fmod(372, 360) returns 348.

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

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



2024-05-02 Linux man-pages 6.8