A simple dynamic array in c

dynamic.c

#include <stdio.h>
#include <stdlib.h>
int main(void) {  
  int allocated_size = 10;
  int used_size = 0;
  char c, *input, *tmp_input;
  
  // allocate our buffer
  input = (char*)malloc(allocated_size);
  if (input == NULL) {
    printf("Memory allocation error");
    return 1;
  }
  
  printf("Input a string, press enter when done: ");

  while((c = getchar()) != '\n') {

    // make sure there's an empty one at the end to avoid
    // having to do this check after the loop
    if (used_size == allocated_size-1) {

      // see http://stackoverflow.com/questions/19146037/efficiency-of-growing-a-dynamic-array-by-a-fixed-constant-each-time
      allocated_size *= 2;
      tmp_input = (char*)realloc(input, allocated_size);
      if (tmp_input == NULL) {
        free (input);
        printf("Memory allocation error");
        return 1;
      }
      input = tmp_input;
    }

    input[used_size++] = c;
  }

  // we are sure that there's a spot for last one
  // because of if (used_size == allocated_size-1)
  input[used_size] = '\0';

  printf("\nYou entered the string: %s\n", input);

  // free our buffer
  free (input);
  return 0;
}