Read file in reverse

read-in-reverse.c

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

int main(int argc, char **argv) {
  // This is set to be really small since my test file is only 10 bytes long.
  size_t bufferSize = 4;

  char * buffer = (char *)calloc(bufferSize, sizeof(char));

  if (!buffer) {
    printf("Could not allocate buffer.\n");

    return 1;
  }

  FILE *file = fopen("n.txt", "rb");

  if (!file) {
    printf("Could not open file.\n");

   return 1;
  }

  fseeko(file, 0, SEEK_END);

  off_t fileSize = ftello(file);

  int i = 1;

  int bytesRead = 0;

  size_t offset;

  if (bufferSize >= fileSize) {
    offset = 0;
  } else {
    offset = fileSize - (bufferSize * i);
  }

  do {
    fseeko(file, offset, SEEK_SET);

    bytesRead = fread(buffer, 1, bufferSize, file);

    for (size_t j = 0, k = bytesRead - 1; j < k; j += 1, k -= 1) {
      buffer[j] = buffer[j] ^ buffer[k];
      buffer[k] = buffer[j] ^ buffer[k];
      buffer[j] = buffer[j] ^ buffer[k];
    }

    printf("%.*s", bytesRead, buffer);

    if (offset == 0) {
      break;
    }

    i += 1;

    if (bufferSize * i > fileSize) {
      bufferSize = offset;

      offset = 0;
    } else {
      offset = fileSize - (bufferSize * i);
    }
  } while (bytesRead > 0);

  fclose(file);

  free(buffer);

  printf("\n");

  return 0;
}