How to use a function through header files to enter/display data in an array in C?

I created two files for displaying and reading data separately and included them in my main file which is for sorting it. But when I execute the main program the data is not displayed but the function is being called.

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

#include "arrdisp.c"
#include "arrread.c"
void selsort(int arr[]);
int random(int min, int max);
int n;
void main() {
  int a[10], i;
  char ch;
  printf("Do you want to insert student marks manually?(y/n):");
  scanf("%c", &ch);
  if (ch == 'y') {
    read(a, n);

    for (i = 0; i < n; i++) printf("%d, ", a[i]);

  } else {
    printf("How many random marks do you want to insert:");
    scanf("%d", &n);

    for (i = 0; i < n; i++) a[i] = random(0, 100);
  }

  printf("before sorting:\n");
  disp(a, n);
  printf("\nAfter sorting:\n");
  selsort(a);
  disp(a, n);

  printf("Array elements");

  for (i = 0; i < n; i++) printf("%d, ", a[i]);
}
int random(int min, int max) {
  return min + rand() / (RAND_MAX / (max - min + 1) + 1);
}

void swap(int *xp, int *yp) {
  int temp = *xp;
  *xp = *yp;
  *yp = temp;
}
void selsort(int arr[]) {
  int i, j, min_idx;

  for (i = 0; i < n - 1; i++) {
    min_idx = i;
    for (j = i + 1; j < n; j++)
      if (arr[j] < arr[min_idx]) min_idx = j;

    if (min_idx != i) swap(&arr[min_idx], &arr[i]);
  }
}

Can anyone explain what's going wrong?

I tried making the array extern variable and got some error in the main program itself saying that the array is undefined. So I tried defining normal functions in the same program and still had the same output, so I don't think it's a problem with the header file but I'm still clueless.

You're not initializing n, so your display loops will not work because a non initialized int variable will get zero.

The only place I see you initialize n is in your else statement, which will not be accessed if the user chooses y.

Ask the user to input 'n':

printf("Do you want to insert student marks manually?(y/n):");
scanf("%c",&ch);

if(ch == 'y')
{
    printf("How many students:");
    scanf("%d", &n);
    read(a, n);

    for (i = 0; i < n; i++)
        printf("%d, ", a[i]);
}