mfs_filesys.c

实用的程序代码

    if (new_dir_block != first_dir_block)
      mfs_file_system[first_dir_block].u.dir_data.num_entries += 1;
    mfs_file_system[new_dir_block].u.dir_data.dir_ent[new_dir_index].index = new_entry_index;
    set_filename(mfs_file_system[new_dir_block].u.dir_data.dir_ent[new_dir_index].name, get_basename(filename));
    mfs_file_system[new_dir_block].u.dir_data.dir_ent[new_dir_index].deleted = 'n';
    return new_entry_index;
  }
}

/**
 * move a list of blocks starting and start_index and chained
 * through next_block pointers and ending at end_index
 * to the free list
 * the blocks are all already marked as MFS_BLOCK_TYPE_EMPTY
 * the prev_block of start_index is 0, and the next_block of end_index = 0
 */
static int move_to_free_list(int start_index, int end_index) {
  if (mfs_free_block_list != 0) { /* free list exists and is non empty */
    /* prepend this list to the existing free list */
    mfs_file_system[mfs_free_block_list].prev_block = end_index;
    mfs_file_system[end_index].next_block = mfs_free_block_list;
  }
  else { /* free list is empty - no need to prepend */
  }
  mfs_free_block_list = start_index;
  return 1; /* always succeeds */
}

/**
 * mark all the data blocks associated with a file as free and add them
 * to the free list
 * return 1 for success and 0 for failure
 * delete dir blocks only if the directory is empty
 */
static int delete_data_in_file(int file_index) {
  int next_block;
  int current_block;
  if (mfs_file_system[file_index].block_type == MFS_BLOCK_TYPE_FILE) {
    /* OK to delete */
  }
  else if (mfs_file_system[file_index].block_type == MFS_BLOCK_TYPE_DIR) {
    /* first verify whether dir is actually empty */
    if (mfs_file_system[file_index].u.dir_data.num_entries - mfs_file_system[file_index].u.dir_data.num_deleted != 2) {
      /* dir is not empty so cannot delete */
      return 0;
    }
  }
  else { /* don't know what this is; cannot delete */
    return 0;
  }
  /* at this point we know that it is OK to delete */

  current_block = file_index;
  while((next_block = mfs_file_system[current_block].next_block) != 0) {
    mfs_file_system[current_block].block_type = MFS_BLOCK_TYPE_EMPTY;
    current_block = next_block;
  }
  mfs_file_system[current_block].block_type = MFS_BLOCK_TYPE_EMPTY;
  move_to_free_list(file_index, current_block);
  return 1;
}

/**
 * delete the data blocks corresponding to the file and then delete the
 * file entry from its directory
 * return 1 on success, 0 on failure
 * delete will not work on a directory unless the directory is empty
 */
int mfs_delete_file (char *filename) {
  int dir_block;
  int dir_index;
  int entry_index;
  int new_block;
  int first_dir_block;
  if (!get_dir_ent(filename, &dir_block, &dir_index)) { 
    /* file does not exist */
    return 0 ; /* cannot delete file if it does not exist */
  }
  entry_index = mfs_file_system[dir_block].u.dir_data.dir_ent[dir_index].index;
  if (delete_data_in_file(entry_index)) {
    /* now delete the file entry from the directory */
    mfs_file_system[dir_block].u.dir_data.dir_ent[dir_index].deleted = 'y';
    mfs_file_system[dir_block].u.dir_data.num_deleted += 1;
    first_dir_block = get_first_dir_block(dir_block);
    if (dir_block != first_dir_block)
      mfs_file_system[first_dir_block].u.dir_data.num_deleted += 1;
  }
  return 1;
}

/**
 * create a new empty directory inside the current directory
 * return index of new directory in file system if success, 0 if failure 
 */
int mfs_create_dir(char *newdir) {
  return create_file(newdir, MFS_BLOCK_TYPE_DIR);
}

/**
 * delete the directory named newdir if it exists, and is empty
 * return 1 on success, 0 on failure
 * cannot delete . or ..
 */
int mfs_delete_dir (char *newdir) {
  if (!strcmp(newdir,"..") || !strcmp(newdir,"."))
    return 0;
  return mfs_delete_file(newdir);
}

/**
 * rename from_file to to_file
 * works for dirs as well as files
 * cannot rename to something that already exists
 * return 1 on success, 0 on failure
 */
int mfs_rename_file(char *from_file, char *to_file) {
  int from_dir_block;
  int to_dir_block;
  int from_dir_index;
  int to_dir_index;
  if (get_dir_ent(from_file, &from_dir_block, &from_dir_index) &&
      !get_dir_ent(to_file, &to_dir_block, &to_dir_index)) {
    set_filename(mfs_file_system[from_dir_block].u.dir_data.dir_ent[from_dir_index].name, get_basename(to_file));
    return 1;
  }
  return 0;
}

/**
 * return 0 if filename is not a file in the current directory
 * return 1 if filename is a file in the current directory
 * return 2 if filename is a directory in the current directory
 */
int mfs_exists_file(char *filename) {
  int dir_block;
  int dir_index;
  int file_block;
  if (get_dir_ent(filename, &dir_block, &dir_index)) {
    file_block = mfs_file_system[dir_block].u.dir_data.dir_ent[dir_index].index;
    if (mfs_file_system[file_block].block_type == MFS_BLOCK_TYPE_DIR)
      return 2;
    else if (mfs_file_system[file_block].block_type == MFS_BLOCK_TYPE_FILE)
      return 1;
    else return 0;
  }
  return 0;
}
/**
 * return the name of the current directory 
 * in dirname =  pre_allocated buffer of at least 16 chars
 * return 1 if success, 0 if failure
 */
int mfs_get_current_dir_name(char *dirname){
  int dir_block;
  int dir_index;
  int file_block;
  file_block = mfs_current_dir;
  if (file_block == 0) {
     strcpy(dirname, "/");
     return 1;
  }
  if(!get_dir_ent_by_index(file_block, &dir_block, &dir_index)) {
     return 0; /* cannot find current dir in its parent */
  }
  strcpy(dirname, mfs_file_system[dir_block].u.dir_data.dir_ent[dir_index].name);
  return 1;
}

/**
 * get the number of used blocks and the number of free blocks in the file system through pointers
 * the return value is  1 (for success) and 0 for failure to obtain the numbers
 */
int mfs_get_usage(int *num_blocks_used, int *num_blocks_free) {
  int i;
  *num_blocks_used = 0;
  *num_blocks_free = 0;
  for ( i = 0; i < MFS_MAX_FILE_BLOCKS; i++) {
    if (mfs_file_system[i].block_type == MFS_BLOCK_TYPE_EMPTY)
      *num_blocks_free += 1;
    else
      *num_blocks_used += 1;
  }
  return 1;
}


/**
 * returns the index of the first free entry in the mfs_open_files array
 * if there is no free entry, returns -1
 */
static int get_first_free_ftab_index() {
  int i;
  for (i=0; i < MFS_MAX_OPEN_FILES; i++) {
    if (mfs_open_files[i].mode == MFS_MODE_FREE)
      return i;
  }
  return -1;
}

/**
 * mode can be MFS_MODE_READ/MFS_MODE_WRITE/MFS_MODE_CREATE
 * this function should be used for FILEs and not DIRs
 * no error checking (is this FILE and not DIR?) is done for MFS_MODE_READ
 * MFS_MODE_CREATE automatically creates a FILE and not a DIR
 * MFS_MODE_WRITE fails if the specified file is a DIR
 * return index of file in array mfs_open_files or -1 
 */
int mfs_file_open(const char *filename, int mode) {
  int dir_block;
  int dir_index;
  int current_index;
  if (mfs_num_open_files >= MFS_MAX_OPEN_FILES) {/* cannot open any more files */
    return -1;
  }
  if (mode == MFS_MODE_READ || mode == MFS_MODE_WRITE) { /* look for existing file */
    if (get_dir_ent(filename, &dir_block, &dir_index)) { /* found it */
      if (mode == MFS_MODE_WRITE && mfs_file_system[mfs_file_system[dir_block].u.dir_data.dir_ent[dir_index].index].block_type != MFS_BLOCK_TYPE_FILE) {
	/* cannot open anything other than FILE for write */
	return -1;
      }
      mfs_num_open_files++;
      current_index = get_first_free_ftab_index();
      mfs_open_files[current_index].first_block = mfs_file_system[dir_block].u.dir_data.dir_ent[dir_index].index;
      mfs_open_files[current_index].current_block = mfs_open_files[current_index].first_block;
      mfs_open_files[current_index].mode = mode;
      mfs_open_files[current_index].offset = 0;
      return current_index;
    }
    else {
       //file/dir not found, open it in create mode
       if (mode == MFS_MODE_WRITE)
          mode = MFS_MODE_CREATE;
    }
  }

  if (mode == MFS_MODE_CREATE) { /* create a new file */
     dir_block = create_file(filename, MFS_BLOCK_TYPE_FILE);
    if (dir_block == 0) { /* failed to create the file */
      return -1;
    }
    mfs_num_open_files++;
    current_index = get_first_free_ftab_index();
    mfs_open_files[current_index].first_block = dir_block;
    mfs_open_files[current_index].current_block = dir_block;
    mfs_open_files[current_index].mode = MFS_MODE_WRITE;
    mfs_open_files[current_index].offset = 0;
    return current_index;
  }
  return -1;
}

/**
 * fd should be a valid index in mfs_open_files array
 * Works only if fd points to a file and not a dir
 * buf should be a pointer to a pre-allocated buffer of size buflen or more
 * buflen chars are read and placed in buf
 * if fewer than buflen chars are available then only that many chars are read
 * return num bytes read or 0 for error=no bytes read
*/
int mfs_file_read(int fd, char *buf, int buflen) {
  int num_read = 0;
  char *from_ptr = &(mfs_file_system[mfs_open_files[fd].current_block].u.block_data[mfs_open_files[fd].offset]);
  int num_left ;
  num_left =  mfs_file_system[mfs_open_files[fd].current_block].block_size ;
  if (num_left > MFS_BLOCK_DATA_SIZE)
    num_left = MFS_BLOCK_DATA_SIZE;
  num_left -=  mfs_open_files[fd].offset ;
  while (buflen > 0) {
    if (num_left == 0) { /* see if there is a next_block */
      int next_block = mfs_file_system[mfs_open_files[fd].current_block].next_block;
      if (next_block == 0) { /* nothing more to read */
	break;
      }
      if (mfs_file_system[next_block].block_size == 0) { /* nothing more to read */
	break;
      }
      from_ptr = &(mfs_file_system[next_block].u.block_data[0]);
      num_left = mfs_file_system[next_block].block_size;
      mfs_open_files[fd].current_block = next_block;
      mfs_open_files[fd].offset = 0;
    }
   
    *buf = *from_ptr;
    *buf++;
    *from_ptr++;
    mfs_open_files[fd].offset += 1;
    num_read++;
    num_left--;
    buflen--;
  }
  return num_read;
}
/**
 * fd should be a valid index in mfs_open_files array
 * buf should be a pointer to a pre-allocated buffer of size buflen or more
 * buflen chars are read from buf and written to 1 or more blocks of the file
 * return 1 for success or 0 for error=unable to write to file
*/ 
int mfs_file_write (int fd, const char *buf, int buflen) {
  char *to_ptr = &(mfs_file_system[mfs_open_files[fd].current_block].u.block_data[mfs_open_files[fd].offset]);
  int num_left = MFS_BLOCK_DATA_SIZE - mfs_open_files[fd].offset;
  while (buflen > 0) {
    *to_ptr = *buf;
    *buf++;
    *to_ptr++;
    mfs_open_files[fd].offset += 1;
    num_left--;
    mfs_file_system[mfs_open_files[fd].current_block].block_size +=1;
    if (mfs_open_files[fd].current_block != mfs_open_files[fd].first_block)
      mfs_file_system[mfs_open_files[fd].first_block].block_size += 1;
    buflen--;
    if (num_left == 0 && buflen > 0) { /* create next_block */
      int new_block;
      /* create a new file block linked from this one */
      if (get_next_free_block(&new_block)) { /* found a free block */
	mfs_file_system[new_block].prev_block = mfs_open_files[fd].current_block;
	mfs_file_system[new_block].next_block = 0;
	mfs_file_system[new_block].block_type = MFS_BLOCK_TYPE_FILE;
	mfs_file_system[new_block].block_size = 0;
	mfs_file_system[mfs_open_files[fd].current_block].next_block = new_block;
	mfs_open_files[fd].current_block = new_block;
	mfs_open_files[fd].offset = 0;
      }
      else { /* no space for new block  - return failure */
	return 0;
      }
     
      to_ptr = &(mfs_file_system[new_block].u.block_data[0]);
      num_left = MFS_BLOCK_DATA_SIZE;
    }
  }
  return 1;
}

/**
 * recover the file table entry in mfs_open_files corresponding to the fd
 * if the fd is not valid, return 0
 * fd is not valid if the index in mfs_open_files is out of range, or
 * if the corresponding entry is not an open file
 * return 1 on success
 */
int mfs_file_close(int fd) {
  if (fd <0 || fd >= MFS_MAX_OPEN_FILES)
    return 0;
  if (mfs_open_files[fd].mode != MFS_MODE_FREE) {
    mfs_open_files[fd].mode = MFS_MODE_FREE;
    mfs_num_open_files--;
    return 1;
  }
  return 0;
}

/**
 * seek to a given offset within the file
 * if MFS_SEEK_END is specified, the offset can be either 0 or negative
 * otherwise offset should be positive or 0
 * return 0 on failure, 1 on success
 * it is an error to seek before beginning of file or after the end of file
 */
long mfs_file_lseek(int fd, long offset, int whence) {
  if (fd <0 || fd >= MFS_MAX_OPEN_FILES || mfs_open_files[fd].mode == MFS_MODE_FREE)
    return 0;
  if (whence == MFS_SEEK_SET || whence == MFS_SEEK_CUR) {
    if (whence == MFS_SEEK_CUR)
      offset += mfs_open_files[fd].offset;
    else
      mfs_open_files[fd].current_block = mfs_open_files[fd].first_block;
    if (offset > mfs_file_system[mfs_open_files[fd].first_block].block_size) { /* attempting to seek beyond end of file */
      return 0;
    }
  }
  else { /* whence == MFS_SEEK_END */
    if (offset > 0) { /* cannot seek beyond end of file */
      return 0;
    }
    else {
      offset += mfs_file_system[mfs_open_files[fd].first_block].block_size;
      mfs_open_files[fd].current_block = mfs_open_files[fd].first_block;
    }
  }
  /* at this point offset is a positive value, guaranteed to be within the file 
   * mfs_open_files[fd].current_block is set to first_block for MFS_SEEK_SET and MFS_SEEK_END
   * and is assumed to be set correctly for MFS_SEEK_CUR
   */
  while(offset > MFS_BLOCK_DATA_SIZE) {
    mfs_open_files[fd].current_block = mfs_file_system[mfs_open_files[fd].current_block].next_block;
    offset -= MFS_BLOCK_DATA_SIZE;
  }
  mfs_open_files[fd].offset = offset;
  return 1;
}