C function returns 0 when shouldn't

Question

I'm making a set of functions in C for simulate a microcanonial ensamble (a periodic-bounded box with N particles with fixed Energy and Volume). The point is that one of my main functions is to get the distance between 2 particles, and sometimes it returns 0 even if the particles are not that close.

The function is

// Determinar la mínima distancia dadas dos coordenadasy una longitud de caja periodica
float pdist(float x1, float y1, float x2, float y2, float a) {
   float Dx = abs(x2-x1);
   float Dy = abs(y2-y1);
   if(Dx >a/2) Dx = a-Dx;
   if(Dy >a/2) Dy = a-Dy;
   return sqrt(pow(Dx,2)+pow(Dy,2));
}

If you want to see all the code, you can see it in https://gitlab.com/jarr.tecn/statistical_mechanics

in the file box.h and one example in ejemplo.h

Answer 1

The problem comes from abs function which will return an integer:

#include <math.h>
#include <stdio.h>
#include <stdlib.h>

float pdist(float x1, float y1, float x2, float y2, float a) {
    float Dx = abs(x2-x1);
    float Dy = abs(y2-y1);
    if(Dx >a/2) Dx = a-Dx;
    if(Dy >a/2) Dy = a-Dy;
    return sqrt(pow(Dx,2)+pow(Dy,2));
}

float pdist2(float x1, float y1, float x2, float y2, float a) {
    float Dx = fabsf(x2-x1);
    float Dy = fabsf(y2-y1);
    if(Dx >a/2) Dx = a-Dx;
    if(Dy >a/2) Dy = a-Dy;
    return sqrt(pow(Dx,2)+pow(Dy,2));
 }

int main(void)
{
    printf("first form: %f\n", pdist(.1, .1, .2, .2, .5));
    printf("second form: %f\n", pdist2(.1, .1, .2, .2, .5));
}

Gives:

first form: 0.000000
second form: 0.141421