So far, you have built a shell environment and a multi-thread scheduler with process synchronization. Excellent job!
What is still missing for a “real” operating system? A file system! In this assignment, you will implement utilities that perform operations on a file system similar to Microsoft’s FAT file system with some improvement.
You will be given a test file system disk image for self-testing, but you can create your own image following the specification, and your submission may be tested against other disk images following the same specification.
You should get comfortable examining the raw, binary data in the file system images using the program xxd.
VERY IMPORTANT: since you are dealing with binary data, functions intended for string manipulation such as strcpy() do NOT work (since binary data may contain binary ‘0’ anywhere), and you should use functions intended for binary data such as memcpy().
In order to help you finish this programming assignment on time successfully, the schedule of the lectures and the tutorials has been adjusted. There are two tutorials and one help session arranged during the course of this 15 assignment. NOTE: Please do attend the tutorials and follow the tutorial schedule closely.
In part I, you will write a program that displays information about the file system. In order to complete part I, you will need to read the file system super block and use the information in the super block to read the FAT.
Your program for part I will be invoked as follows (output value here just for illustration purposes):
Super block information: Block size: 512 Block count: 5120 FAT starts: 1 FAT blocks: 40 Root directory start: 41 Root directory blocks: 8 FAT information: Free Blocks: 5071 Reserved Blocks: 41 Allocated Blocks: 8
Please be sure to use the exact same output format as shown above.
In part II, you will write a program, with the routines already implemented for part I, that displays the contents of the root directory or a given sub-directory in the file system.
Your program for part II will be invoked as follows:
./disklist test.img /sub_dir
The directory listing should be formatted as follows:
- The first column will contain:
- (a) F for regular files, or
- (b) D for directories; followed by a single space
- then 10 characters to show the file size, followed by a single space
- then 30 characters for the file name, followed by a single space
- then the file modification date (we will not display the file creation date).
For example: F 2560 foo.txt 2005/11/15 12:00:00 F 5120 foo2.txt 2005/11/15 12:00:00 F 48127 makefs 2005/11/15 12:00:00 F 8 foo3.txt 2005/11/15 12:00:00
In part III, you will write a program that copies a file from the file system to the current directory in Linux. If the specified file is not found in the root directory or a given sub-directory of the file system, you should output the message File not found. and exit.
Your program for part III will be invoked as follows:
./diskget test.img /sub_dir/foo2.txt foo.txt
In part IV, you will write a program that copies a file from the current Linux directory into the file system, at the root directory or a given sub-directory. If the specified file is not found, you should output the message File not found. on a single line and exit.
Your program for part IV will be invoked as follows:
./diskput test.img foo.txt /sub_dir/foo3.txt
From time to time, the disk image may get corrupted due to wrong or incomplete operations, including what happened to the test image, although it does not affect the first four parts. Your fifth part is to go through the disk image according to the file system specification, including the super block, FDT, FAT and data blocks, find inconsistent information among them and fix these issues when possible. For example, a block indicated as reserved (for FAT and root directory) in FAT might be mistakenly used as a data block. In this case, the data shall be relocated, and the FAT and possibly FDT are updated accordingly. Also, a block indicated as allocated in FAT does not belong to any files. In this case, the entry in FAT is fixed to available. Further, a block is the last block of a file according to its size, but it is not indicated by -1 in FAT. In this case, the FAT entry is updated to -1. On the other hand, a block is not the last block of a file according to its size, but it is indicated by -1 in FAT, so the file is truncated up to this last block. Your program needs to be able to handle at least these three cases.
Your program for part V will be invoked as follows:
and output the problems that you identified and possibly fixed, e.g.,
Block 5 indicated reserved in FAT but used by foo.txt; foo.txt relocated Block 1005 indicated allocated in FAT but not used by any files; fixed to available Block 2005 is the last block of foo2.txt but not indicated -1 in FAT; fixed to -1 Block 3005 is not the last block of foo3.txt but indicated -1 in FAT; foo3.txt truncated to 4096 bytes
The FAT file system has three major components:
- the super block,
- the directory structure.
- the File Allocation Table (informally referred to as the FAT),
Each of these three components is described in the subsections below.
The first block (512 bytes) is reserved to contain information about the file system. The layout of the superblock is as follows:
|File system identifier||8 bytes||CSC360FS|
|Block Size||2 bytes||0x200|
|File system size (in blocks)||4 bytes||0x00001400|
|Block where FAT starts||4 bytes||0x00000001|
|Number of blocks in FAT||4 bytes||0x00000028|
|Block where root directory starts||4 bytes||0x00000029|
|Number of blocks in root dir||4 bytes||0x00000008|
Note: Block number starts from 0 in the file system.
Each directory entry takes up 64 bytes, which implies there are 8 directory entries per 512 byte block.
Each directory entry has the following structure:
The description of each field follows:
|Starting Block||4 bytes|
|Number of Blocks||4 bytes|
|File Size (in bytes)||4 bytes|
|Create Time||7 bytes|
|Modify Time||7 bytes|
|File Name||31 bytes|
|unused (set to 0xFF)||6 bytes|
Status This is bit mask that is used to describe the status of the file. Currently only 3 of the bits are used.
It is implied that only one of bit 2 or bit 1 can be set to 1. That is, an entry is either a normal file or it is a directory, not both.
Starting Block This is the location on disk of the first block in the file
Number of Blocks The total number of blocks in this file
File Size The size of the file, in bytes. The size of this field implies that the largest file we can support is 232 bytes long.
Create Time The date and time when this file was created. The file system stores the system times as integer values in the format:
Modify Time The last time this file was modified. Stored in the same format as the Create Time shown above. File Name The file name, null terminated. Because of the null terminator, the maximum length of any filename is 30 bytes.
Valid characters are upper and lower case letters (a-z, A-Z), digits (0-9) and the underscore character ( ).
Each directory entry contains the starting block number for a file, let’s say it is block number X. To find the next block in the file, you should look at entry X in the FAT. If the value you find there does not indicate End-of-File (see below) then that value, call it Y, is the next block number in the file.
That is, the first block is at block number X, you look in the FAT table at entry X and find the value Y. The second data block is at block number Y. Then you look in the FAT at entry Y to find the next data block number… continue this until you find the special value in the FAT entry indicating that you are at the last FAT entry of the file.
The FAT is really just a linked list, which the head of the list being stored in the “Starting Block” field in the directory entry, and the ‘next pointers’ being stored in the FAT entries.
FAT entries are 4 bytes long (32 bits), which implies there are 128 FAT entries per block.
Special values for FAT entries are described in Figure 5.
|0x00000000||This block is available|
|0x00000001||This block is reserved|
|0x00000002-0xFFFFFF00||Allocated blocks as part of files|
|0xFFFFFFFF||This is the last block in a file|
Different hardware architectures store multi-byte data (like integers) in different orders. Consider the large integer: 0xDEADBEEF
On the Intel architecture (Little Endian), it would be stored in memory as:
EF BE AD DE
On the PowerPC (Big Endian), it would be stored in memory as:
DE AD BE EF
Our file system will use Big Endian for storage. This will make debugging the file system by examining the raw data much easier.
This will mean that you have to convert all your integer values to Big Endian before writing them to disk. There are utility functions in netinit/in.h that do exactly that. (When sending data over the network, it is expected the data is in Big Endian format.)
See the functions htons, htonl, ntohs and ntohl.
The side effect of using these functions will be that your code will work on multiple platforms. (On machines that natively store integers in Big Endian format, like the Mac (not the Intel-based ones), the above functions don’t actually do anything but you should still use them!)
What to hand in: You need to hand in a .tar.gz file containing all your source code and a Makefile that produces the executables for parts I - V.
Please include a readme.txt file that explains your design and implementation.