File I/O, Command Line
Parameters, Endian-ness
Operating Systems
CS3430
Sarah Diesburg
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Outline




File I/O in C
Passing arguments on the command line
Starter C code
More on serialization (hexedit, endian, safe
functions)
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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
3
Starting our Utility
$>./fat32_reader fat32.img
/]
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Fat32_reader is name of our utility
Should return a different prompt (like “/]”) to
symbolize that user is inside utility
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Alternatively…
$>java fat32_reader fat32.img
/]
$>python3 fat32_reader fat32.img
/]
$>python2 fat32_reader fat32.img
/]
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Passing in Command Line
Parameters


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Can be done with any language
Usually uses an argv[] and argc passed into
main
Let’s look at a C example:

args.c
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Main Structure of our Utility

Our utility will
1.
2.
3.
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
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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
Boot Sector
FS Information
Sector
FAT
Region
Additional
Reserved Sectors
(Optional)
Data
Region
Beginning Project 3

Start by reading boot sector fat32 information
Reserved
Region
Boot Sector


FS Information
Sector
FAT
Region
Data
Region
Additional
Reserved Sectors
(Optional)
As you work, it might make sense to first take
a look at the raw file system image
Hexedit to the rescue!
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Hexedit
To install (run once):


$> 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?
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Hexedit
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Hexedit
Line
numbers in
hex
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Hexedit
Content in
hex
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Hexedit
Content in
printable
ASCII
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Hexadecimal Hints
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
Hex is base 16 – one hexadecimal can
represent 0-15
It takes 4 binary bits to represent values 0-15

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0000 = 0
1111 = 15
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Hexadecimal Hints

If it takes 4 bits to represent one
hexadecimal number, it takes 8 bits to
represent two hexadecimal numbers

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8 bits = 1 byte
Two hex numbers together symbolize one
byte
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That’s why hex numbers are in groups of two
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Endianness

FAT32 is represented in little endian byte
order

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Reading left to right, you encounter leastsignificant byte first
What 32-bit number is this? 0x0000040 or
0x40000000?
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Endianness

Why are characters in order (readable) if
some numbers are not?
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Endianness

You must account for little endianness
across bytes when reading in numbers of
size larger than one byte

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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)
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Important Boot Sector Information

Size of each region
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Root directory (first directory in tree)

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BPB_BytesPerSec
BPB_SecPerClus
BPB_RsvdSecCnt
BPB_NumFATS
BPB_FATSz32
BPB_RootClus
Warning: this list is not exhaustive!
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