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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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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():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| /* 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 += 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.
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:
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.
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 12.
The call fmod(-372, 360) returns -12.
The call fmod(-372, -360) also returns -12.
remainder(3)
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⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 fmod(3)
Pages that refer to this page: remainder(3), remquo(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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