How to use file system in C

fs.c

void wrt_file(char * dat, char * filename) { 
  FILE *file = fopen(filename, "w");
    fprintf(file, dat);
  fclose(file);
}

void rd_file(char * filename, char * file_str) {
  int c;
  int idx = 0;
  FILE *file;
  printf("filename: %s\n", filename);
  file = fopen(filename, "r");
  if (file) {
    while ((c = getc(file)) != EOF) {
      file_str[idx] = c;
      idx++;
    }
    fclose(file);
  }
  printf("filedat: %s\n", file_str);
}