1. File I/O, Command Line Parameters, Endian-ness
Operating Systems
CS3430
Sarah Diesburg

2. Outline File I/O in C Passing arguments on the command line
Starter C code
More on serialization (hexedit, endian, safe functions)

3. File I/O in C Two resources:
A C language file I/O tutorial (link also found on class Resources page)
Take a look at endian_test.c
Linked on today session webpage

4. Starting our Utility $>./fat32_reader fat32.img /]
Fat32_reader is name of our utility
Should return a different prompt (like “/]”) to symbolize that user is inside utility

5. Alternatively…
$>java fat32_reader fat32.img /] $>python3 fat32_reader fat32.img $>python2 fat32_reader fat32.img

6. Passing in Command Line Parameters
Can be done with any language
Usually uses an argv[] and argc passed into main
Let’s look at a C example:
args.c

7. Main Structure of our Utility
Our utility will
Prompt the user for a command
Perform the command (or quit)
Repeat
Need a main loop and a way to compare input to commands
fat32_reader.c - C starter code

8. Additional Reserved Sectors (Optional)
Reserved Region
Reserved Region – Includes the boot sector, the extended boot sector, the file system information sector, and a few other reserved sectors
Reserved
Region
FAT
Data
Boot Sector
FS Information Sector
Additional Reserved Sectors (Optional)

9. Additional Reserved Sectors (Optional)
Beginning Project 3
Start by reading boot sector fat32 information
As you work, it might make sense to first take a look at the raw file system image
Hexedit to the rescue!
Reserved
Region
FAT
Data
Boot Sector
FS Information Sector
Additional Reserved Sectors (Optional)

10. Hexedit To install (run once): To run: $> hexedit [filename]
$> sudo apt-get update
$> sudo apt-get install hexedit
To run:
$> hexedit [filename]
View files in hexadecimal or ASCII
Why wouldn’t you want to view the file system image file in your regular editor?

11. Hexedit

12. Hexedit
Line numbers in hex

13. Hexedit
Content in hex

14. Content in printable ASCII
Hexedit
Content in printable ASCII

15. Hexadecimal Hints Hex is base 16 – one hexadecimal can represent 0-15
It takes 4 binary bits to represent values 0-15
0000 = 0
1111 = 15

16. Hexadecimal Hints
If it takes 4 bits to represent one hexadecimal number, it takes 8 bits to represent two hexadecimal numbers
8 bits = 1 byte
Two hex numbers together symbolize one byte
That’s why hex numbers are in groups of two

17. Endianness FAT32 is represented in little endian byte order
Reading left to right, you encounter least-significant byte first
What 32-bit number is this? 0x or 0x ?

18. Endianness
Why are characters in order (readable) if some numbers are not?

19. Endianness
You must account for little endianness across bytes when reading in numbers of size larger than one byte
Characters are only one byte, no re-ordering necessary
Use only safe functions when reading, to make sure to adjust for the endian-ness of your architecture
Ex. le8toh(num), le16toh(num), le32toh(num)

20. Important Boot Sector Information
Size of each region
BPB_BytesPerSec
BPB_SecPerClus
BPB_RsvdSecCnt
BPB_NumFATS
BPB_FATSz32
Root directory (first directory in tree)
BPB_RootClus
Warning: this list is not exhaustive!