'\" t
.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\" and Copyright 2008, Linux Foundation, written by Michael Kerrisk
.\"     <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified 1993-07-24 by Rik Faith (faith@cs.unc.edu)
.\" Modified 2002-07-27 by Walter Harms
.\" 	(walter.harms@informatik.uni-oldenburg.de)
.\"
.TH fmod 3 2023-10-31 "Linux man-pages 6.06"
.SH NAME
fmod, fmodf, fmodl \- floating-point remainder function
.SH LIBRARY
Math library
.RI ( libm ", " \-lm )
.SH SYNOPSIS
.nf
.B #include <math.h>
.P
.BI "double fmod(double " x ", double " y );
.BI "float fmodf(float " x ", float " y );
.BI "long double fmodl(long double " x ", long double " y );
.fi
.P
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.P
.BR fmodf (),
.BR fmodl ():
.nf
    _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
        || /* Since glibc 2.19: */ _DEFAULT_SOURCE
        || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
.fi
.SH DESCRIPTION
These functions compute the floating-point remainder of dividing
.I x
by
.IR y .
The return value is
.I x
\-
.I n
*
.IR y ,
where
.I n
is the quotient of
.I x
/
.IR y ,
rounded toward zero to an integer.
.P
To obtain the modulus, more specifically, the Least Positive Residue,
you will need to adjust the result from fmod like so:
.P
.in +4n
.nf
z = fmod(x, y);
if (z < 0)
	z += y;
.fi
.in
.P
An alternate way to express this is with
.IR "fmod(fmod(x, y) + y, y)" ,
but the second
.BR fmod ()
usually costs way more than the one branch.
.SH RETURN VALUE
On success, these
functions return the value \fIx\fP\ \-\ \fIn\fP*\fIy\fP,
for some integer
.IR n ,
such that the returned value has the same sign as
.I x
and a magnitude less than the magnitude of
.IR y .
.P
If
.I x
or
.I y
is a NaN, a NaN is returned.
.P
If
.I x
is an infinity,
a domain error occurs, and
a NaN is returned.
.P
If
.I y
is zero,
a domain error occurs, and
a NaN is returned.
.P
If
.I x
is +0 (\-0), and
.I y
is not zero, +0 (\-0) is returned.
.SH ERRORS
See
.BR math_error (7)
for information on how to determine whether an error has occurred
when calling these functions.
.P
The following errors can occur:
.TP
Domain error: \fIx\fP is an infinity
.I errno
is set to
.B EDOM
(but see BUGS).
An invalid floating-point exception
.RB ( FE_INVALID )
is raised.
.TP
Domain error: \fIy\fP is zero
.I errno
is set to
.BR EDOM .
An invalid floating-point exception
.RB ( FE_INVALID )
is raised.
.\" POSIX.1 documents an optional underflow error, but AFAICT it doesn't
.\" (can't?) occur -- mtk, Jul 2008
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR fmod (),
.BR fmodf (),
.BR fmodl ()
T}	Thread safety	MT-Safe
.TE
.SH STANDARDS
C11, POSIX.1-2008.
.SH HISTORY
C99, POSIX.1-2001.
.P
The variant returning
.I double
also conforms to
SVr4, 4.3BSD, C89.
.SH BUGS
Before glibc 2.10, the glibc implementation did not set
.\" https://www.sourceware.org/bugzilla/show_bug.cgi?id=6784
.I errno
to
.B EDOM
when a domain error occurred for an infinite
.IR x .
.SH EXAMPLES
The call
.I fmod(372, 360)
returns 348.
.P
The call
.I fmod(-372, 360)
returns -12.
.P
The call
.I fmod(-372, -360)
also returns -12.
.SH SEE ALSO
.BR remainder (3)