fmod, fmodf, fmodl - floating-point remainder value function
double fmod(double x, double y);
float fmodf(float x, float y);
long double fmodl(long double x, long double y);
[CX] 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.1-2008 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 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, [MXX] and is not representable, a range error may occur, and fmod(), modf(), and fmodl() shall return [MXX] 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.
[MX] 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.
[MXX] If the correct value would cause underflow, and is representable, a range error may occur and the correct value shall be returned.
These functions shall fail if:
- Domain Error
- [MX] 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.
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.
feclearexcept, fetestexcept, isnan
Treatment of Error Conditions for Mathematical Functions, <math.h>
First released in Issue 1. Derived from Issue 1 of the SVID.
The DESCRIPTION is updated to indicate how an application should check for an error. This text was previously published in the APPLICATION USAGE section.
The behavior for when the y argument is zero is now defined.
The fmodf() and fmodl() functions are added for alignment with the ISO/IEC 9899:1999 standard.
The DESCRIPTION, RETURN VALUE, ERRORS, and APPLICATION USAGE sections are revised to align with the ISO/IEC 9899:1999 standard.
IEC 60559:1989 standard floating-point extensions over the ISO/IEC 9899:1999 standard are marked.
POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0151 , XSH/TC1-2008/0152 , and XSH/TC1-2008/0153  are applied.
POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0120  is applied.
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