The Real dd

• A highly configurable command-line utility used to convert and copy files
• Performs block level I/O, as opposed to filesystem-level I/O for increased performance

What is a Block?

• A sequence of bytes with a predefined size
• Hard drives / SSDs typically have a sector (block) size of 4 kB
• This means read/write operations to the disk can only address 4 kB portions at a time.
• If you write a 64 kB file to the disk, it will be broken down into 16 writes of 4 kB each

Real dd Usage

Some notable parameters:

• if=FILE: read from FILE instead of stdin
• of=FILE: write to FILE instead of stdout
• bs=BYTES: bs is the block size; read and write up to BYTES bytes at a time (default: 512);
• count=N: copy only N input blocks
• seek=N: skip N obs-sized blocks at start of output
• skip=N: skip N ibs-sized blocks at start of input

Applications of dd

• Data transfer, and in-place modification of files
• Wiping disks for security (e.g. prior to recycling hardware)
  • Contents of a file are not necessarily overwritten on regular deletion
• Generating files with random data (will be useful for testing in upcoming Networking MP)
• “Flashing” / installing custom ROMs (enhanced versions of vanilla OS) on Android devices

Advanced Applications

• Kernel profiling (measure the speed of dd if=/dev/zero of=/dev/null)
• Obtaining a finite data subsection of an infinite data stream
• Implementing conversion/analysis tools (e.g. hexdump)

Warnings

dd misuse can cause interesting errors:

• bs and count definition errors can lead to infinite size files
  • This can break password login on your machine!
• of definition errors can overwrite endpoints:
  • Worst case: it can corrupt your hard disk/SSD

So define each parameter carefully, especially if running as root!

Our Implementation of dd

You will use:

• File-level I/O (fread, fwrite, etc.) to copy data
• Interrupts (signal on SIGUSR1) to get status reports

Our dd’s parameters

• -i <file>: input file (defaults to stdin)
• -o <file>: output file (defaults to stdout)
• -b <size>: block size, the number of bytes copied at a time (defaults to 512)
• -c <count>: total number of blocks copied (defaults to the entire file)
• -p <count>: number of blocks to skip at the start of the input file (defaults to 0)
• -k <count>: number of blocks to skip at the start of the output file (defaults to 0)

Argument Parsing: getopt()

• getopt parses function arguments passed through the command line.
• Specify and handle all special case function parameters.

getopt(argc, argv, "i:o:b:c:p:k:")

Example Usage

./dd -i input_file -o output_file -b 256 -c 1

• Defines block size to be 256 bytes
• Copies 1 block from offset 0 of input_file to offset 0 of output_file

Questions?