#include <math.h>

double modf (
  double val,    /* value to calculate modulo for */
  double *ip);   /* integer portion of modulo */

The modf function splits the floating-point number val into integer and fractional components. The fractional part of val is returned as a signed floating-point number. The integer part is stored as a floating-point number at ip.

The modf function returns the signed fractional part of val.

#include <math.h>
#include <stdio.h> /* for printf */

void tst_modf (void) {
  float x;
  float int_part, frc_part;

  x = 123.456;

  frc_part = modf (x, &int_part);

  printf ("%f = %f + %f\n", x, int_part,frc_part);
}