This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       On  error,  the  expressions  (math_errhandling & MATH_ERRNO) and (math_errhandling & MATH_ERREXCEPT) are
       independent of each other, but at least one of them must be non-zero.

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
       for Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group  Base
       Specifications  Issue  7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
       Engineers, Inc and The Open Group.  In the event of any discrepancy between this version and the original
       IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee  document.
       The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

       Any  typographical  or formatting errors that appear in this page are most likely to have been introduced
       during  the  conversion  of  the  source  files  to  man  page  format.  To  report  such   errors,   see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                                   2017                                          FMOD(3POSIX)

       The  functionality  described  on  this  reference  page is aligned with the ISO C standard. Any conflict
       between the requirements described  here  and  the  ISO C  standard  is  unintentional.  This  volume  of
       POSIX.1‐2017 defers to the ISO C standard.

       These functions shall return the floating-point remainder of the division of x by y.

       An   application   wishing   to   check   for  error  situations  should  set  errno  to  zero  and  call
       feclearexcept(FE_ALL_EXCEPT) before  calling  these  functions.  On  return,  if  errno  is  non-zero  or
       fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error has occurred.

       These functions shall fail if:

       Domain Error
                   The x argument is infinite or y is zero.

                   If  the  integer  expression (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be
                   set to [EDOM].  If the integer expression (math_errhandling &  MATH_ERREXCEPT)  is  non-zero,
                   then the invalid floating-point exception shall be raised.

       These functions may fail if:

       Range Error The result underflows.

                   If  the  integer  expression (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be
                   set to [ERANGE].  If the integer expression (math_errhandling & MATH_ERREXCEPT) is  non-zero,
                   then the underflow floating-point exception shall be raised.

       Thefollowingsectionsareinformative.

       None.

       None.

       fmod, fmodf, fmodl — floating-point remainder value function

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult the corresponding Linux manual page for details of Linux behavior), or the  interface
       may not be implemented on Linux.

       None.

       These  functions shall return the value x-i*y, for some integer i such that, if y is non-zero, the result
       has the same sign as x and magnitude less than the magnitude of y.

       If the correct value would cause underflow, and is not  representable,  a  range  error  may  occur,  and
       fmod(),  modf(),  and  fmodl()  shall  return  0.0,  or  (if  the  IEC 60559 Floating-Point option is not
       supported) an implementation-defined value no greater in magnitude than DBL_MIN, FLT_MIN,  and  LDBL_MIN,
       respectively.

       If x or y is NaN, a NaN shall be returned, and none of the conditions below shall be considered.

       If y is zero, a domain error shall occur, and a NaN shall be returned.

       If x is infinite, a domain error shall occur, and a NaN shall be returned.

       If x is ±0 and y is not zero, ±0 shall be returned.

       If x is not infinite and y is ±Inf, x shall be returned.

       If the correct value would cause underflow, and is representable, a range error may occur and the correct
       value shall be returned.

feclearexcept(), fetestexcept(), isnan()

       Section4.20, TreatmentofErrorConditionsforMathematicalFunctions, <math.h>

       #include <math.h>

       double fmod(double x, double y);
       float fmodf(float x, float y);
       long double fmodl(long double x, long double y);