floor() is a Mathematical Function that returns the largest integer that can be represented as a floating point value which is not greater then the given number "num". In other words , it returns the Round Down value of a given number.

    float floor( float num);
    double floor( double num);
    long double floor( long double num);

   #include <stdio.h>
   #include <math.h>
   int main ()
    {
       printf ("floor of 3.2 is %.1lf\n", floor (3.2) );
       printf ("floor of 3.8 is %.1lf\n", floor (3.8) );
       printf ("floor of -4.3 is %.1lf\n", floor (-4.3) );
       printf ("floor of -7.7 is %.1lf\n", floor (-7.7) );
       return 0;
    }

   floor of 3.2 is 3.0
   floor of 3.8 is 3.0
   floor of -4.3 is -5.0
   floor of -7.7 is -8.0

In the above example floor() function is used to return the round down value of the number specified.