L5/6: Files and Directories Flashcards
Six Main Topics
in the
Systems Programming Class
Program Development
Files and Directories
Tool Building
Processes
Networking
OS Implementation
Shell Command Equivalents:
mv
From stdio.h library:
rename( old_filepath, new_filepath);
Equivalent to:
%mv old new
Shell Command Equivalents:
ln
From unistd.h library:
link( old_filepath, new_filepath);
Equivalent to:
%ln old new
Shell Command Equivalents:
rm
From unistd.h library:
unlink( “old_file” );
Equivalent to:
%rm old_file
Shell Command Equivalents:
rm -r
From stdio.h library:
remove( “old_file” );
Equivalent to:
%rm -r old_file
Shell Command Equivalents:
cp
No direct method.
Use read/write File I/O operations
to copy old file to new.
Equivalent to:
%cp old new
Shell Command Equivalents:
cd
From unistd.h library:
chdir( “path” );
Equivalent to:
%cd path
Shell Command Equivalents:
mkdir
From unistd.h library:
mkdir( “newpath”, 0777);
Note: Not sure what the 0777 flag does
Equivalent to:
%mkdir newpath
Shell Command Equivalents:
rmdir
From unistd.h library:
rmdir( “path”);
Equivalent to:
%rmdir path
unistd.h
Methods that are equivalent to
common shell commands (5)
- link()
- equivalent to “ln”
- unlink()
- equivalent to “rm”
- chdir()
- equivalent to “chdir”
- mkdir()
- equivalent to “mkdir”
- rmdir()
- equivalent to “rmdir”
Physical Disk Organization:
Partition Sections
- boot and superblocks
- ilist
- contains inodes
- datablocks
Physical Disk Organization:
inode structure
An inode consists of
- a status block
- addresses to multiple data blocks
Physical Disk Organization:
Key Points
- Disk is arranged into blocks
- some for data
- some for management
- Each inode corresponds to one file and the data blocks associated with that file
- Physical layout is not consecutive, file blocks can be all over the place
How many data blocks can an inode point to?
13
How do inodes handle larger file sizes?
- The inode only has 13 pointers, so an indirect approach is used
- File blocks can each contain many pointers
- Pointers are classified into levels L0, L1, L2, …
- This allows a hierarchy of blocks to be created to handle arbitrary file sizes
- Level L0 is a pointer that points directly to an actual datablock
- L1 points to a block of L0 pointers, L2 to a block of L1 pointers, and so on.
Directory Access Rules (3)
- Search Rule
- to open a file, the user or program must have execute permission on each directory in the full pathname
- Write Rule
- to create or delete a file, must have write access to directory
- Read Rule
- to read file names, must have read access on the directory
inodes:
Design Consequences
for
Move Operations
- Move speed is independent of the file size
- data blocks do not have to be moved around
- only the pointers change(unless moving partitions/drives)
- Moves are easy to implement as atomic operations
Structure of a
Directory
- The directory is a specific type of file
- Contains a table of inodes paired with filenames
- inodes are identified with inode numbers
- The table contains a reference to the directory itself (.)
- and to it’s parent directory( .. )
Directory Structure:
How do Link(ln) operations work?
- Works similarly to move operation, but old reference is kept
- The inode number is copied from the first directory’s table and added to the second specified directory’s table with a new filename
- Both directories now have a reference to the same inode, but may have different names for the same file.
- File data is not duplicated
- Example:
ln dir_1/myFile dir_2/myFile2
Directory Structure:
How Move (mv)
operations work
- Physical data is not moved around on the disk, and will keep the same inode
- The inode number is copied from the table in the source directory
- Then added to the table in the destination directory with a new filename
- The old reference is then deleted from the table in the source directory
- Example:
mv dir_1/derp.txt dir_2/samederp.txt
Physical Disk Organization:
Common Data Block Size
Pointer Size
Block Size: 512 bytes
Pointer Size: 4 bytes
Disk Structure:
How data is accessed by a programmer
- A programmer can get to data only through an inode.
- Data is “seen” as a simple, expandable array of bytes
- Physical and “housekeeping” details are invisible to the programmer
Which Operations( Access Modes)
are files protected for?
Each file is protected for:
- read
- write
- execute
3 Identities for File Access
Access (bits)permissions are defined for:
- user
- group
- other
