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What are ‘bitfields’ in C?

I’ve previously written that “Struct fields have a fixed byte offset”. This is not actually true, because of a feature called bitfields. They allow us to do bit packing, but without all the bitwise operators, with greater safety, and greater clarity. The cost is some language complexity.

I’ll take the previous example and rewrite it using bitfield feature:

#include <stdbool.h>
#include <stdio.h>
#include <stdint.h>

struct player {
  bool is_male : 1;
  bool is_cpu : 1;
  unsigned char num_lives : 2;
  unsigned short points : 10;

int main(void) {
  struct player p;
  p.is_male = true;
  p.is_cpu = false;
  p.num_lives = 2;
  p.points = 789;
  printf("p.is_male = %d, p.is_cpu = %d, p.num_lives = %d, p.points = %d\n", p.is_male, p.is_cpu, p.num_lives, p.points);
  // Prints:
  //   p.is_male = 1, p.is_cpu = 0, p.num_lives = 2, p.points = 789

  return 0;

So much shorter! But how does this language feature work? Find out in the next episode of jameshfisher

What can computers do? What are the limits of mathematics? And just how busy can a busy beaver be? This year, I’m writing Busy Beavers, a unique interactive book on computability theory. You and I will take a practical and modern approach to answering these questions — or at least learning why some questions are unanswerable!

It’s only $19, and you can get 50% off if you find the discount code ... Not quite. Hackers use the console!

After months of secret toil, I and Andrew Carr released Everyday Data Science, a unique interactive online course! You’ll make the perfect glass of lemonade using Thompson sampling. You’ll lose weight with differential equations. And you might just qualify for the Olympics with a bit of statistics!

It’s $29, but you can get 50% off if you find the discount code ... Not quite. Hackers use the console!

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