#include <math.h>
double fmod(double x, double y);
float fmodf(float x, float y);
long double fmodl(long double x, long double y);
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, and is not representable, a range error may occur, and either 0.0 (if supported), or an implementation-defined value shall be returned.
If x or y is NaN, a NaN shall be returned.
If y is zero, a domain error shall occur, and either a NaN (if supported), or an implementation-defined value shall be returned.
If x is infinite, a domain error shall occur, and either a NaN (if supported), or an implementation-defined value 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.
These functions shall fail if:
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:
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.
The following sections are informative.
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() , the Base Definitions volume of IEEE Std 1003.1-2001, Section 4.18, Treatment of Error Conditions for Mathematical Functions, <math.h>