Counting words in file with dynamic memory reallocation in C

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

int count_chars(char block[]) {
  // For with early return is faster than while for this case
  for (int unsigned i = 0; i < 45; i++) {
    if (block[i] == '\0') return --i;
  }

  return i;
}

int main() {
  const unsigned int INT_SIZE = sizeof(int unsigned);

  // Defining file pointer
  FILE *fp = fopen("C:\\forlab.txt", "r");

  if (!fp) { // Checking if file exists
    printf("File unavailable.\n", );
    return -1;
  }

  // Defining initial int array
  int unsigned *word_legths = (int unsigned *) malloc(INT_SIZE);

  // Defining initial longest word's length
  int unsigned longest_length = 1;

  // Defining int variable so as not to recreate it in cycle every time
  int unsigned current_size;

  // Ironic, word cannot be longer that this since it is a length of block to read.
  // May be worth to just define original array as unsigned int array of size 255
  char block[45];

  while (!feof(fp)) {
    if (!fscanf(fp, "%45s", block)) {
      // Check if only whitespace remains before EOF and explicitly break cycle
      break;
    }

    // Reading current word's size
    current_size = count_chars(block);

    // Reallocating array, if necessary
    if (current_size > longest_length) {
      word_legths = realloc(word_legths, current_size * INT_SIZE);
      longest_length = current_size;
    }

    // Increasing word count
    word_legths[--current_size]++;
  }

  fclose(fp);

  for (unsigned int i = 0; i < longest_length; i++) {
    if (word_legths[i]) { // Can use implicit compare since everything is unsigned
      printf("Found %d words containing %d letters in supplied file.\n", word_legths[i], i + 1);
    }
  }
}