double fmod(double, double );

returns the remainder of a statement

compile in linux with: gcc fmod.c -o fmod -lm -Wall

#include <stdio.h> /* including standard library */
//#include <windows.h> /* uncomment this for Windows */
 
#include <math.h> /* including math library */
 
int main ( void ) 
{
double y,x;
x = 9;
y = 4.2;
printf("rest of %f / %f is %f\n", x, y, fmod(x,y) );
return 0;
}

 output:
 user@host:~$ ./fmod 
 rest of 9.000000 / 4.200000 is 0.600000