bit flags

bit flags are a clever way to store lots of information in one integer

If you have many options which are true or false, you can encode them all into an integer, with 1 bit for each option. 32 bits 32 options!

For example, some of the bit flags the open function in C uses: - nofollow - append - truncate - create - write only - read write

(this is on Linux)

where you’ll see bit flags

In libc, the open, socket, and mmap functions use bit flags to pass options.

The TCP and UDP protocol headers both have a flags field which has bit flags.

bit flags are used a lot in C code

Here’s some C code that opens a new file: fd = open("file.txt", O_RDWR | O_CREAT, 0666);

O_RDWR is: 00000010

O_CREAT is: 01000000

O_RDWR | O_CREAT is: 01000010

You can check if a bit flag is set in C like this:

if (flags & O_RDWR) { ... }

fun example: tic tac toe!

Here’s a way to encode the state of a tic tac toe game in 18 bits:

x positions:

100

010

010

O positions:

010

001

100