packed struct for Exact Layout
Lay out bits with no padding.
packed struct for Exact Layout is a free Zig Academy lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Zig Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Layout Matters
Sometimes you need bytes laid out in an exact shape, like a hardware register or a network packet. That is when a packed struct shines.
Normal Structs Add Padding
A plain struct lets Zig reorder and pad fields for speed, so its in-memory size may be larger than the sum of its parts.
const Plain = struct {
flag: bool,
value: u32,
};Enter packed struct
Write packed struct and Zig stores fields back-to-back with no padding, in the exact order you declared them.
const Tight = packed struct {
flag: bool,
value: u7,
};It Is Bit-Packed
A packed struct is measured in bits, not bytes. The fields above use 1 + 7 bits, fitting neatly into a single byte.
A Known Backing Integer
Because the layout is exact, a packed struct has a guaranteed backing integer type that holds all its bits together.
const Flags = packed struct {
a: bool,
b: bool,
rest: u6,
};Convert to the Integer
Use @bitCast to turn a packed struct into its backing integer, so you can store or transmit it as a raw number.
const f = Flags{ .a = true, .b = false, .rest = 0 };
const bits: u8 = @bitCast(f);Convert Back Again
The same @bitCast reads an integer back into a packed struct, decoding raw bits into named fields you can use.
const raw: u8 = 0b0000_0001;
const f: Flags = @bitCast(raw);Check the Exact Size
Use @bitSizeOf to confirm a packed struct occupies precisely the bit count you expect, with zero hidden padding.
const n = @bitSizeOf(Flags); // 8Declaration Order Is the Layout
In a packed struct, the first field occupies the lowest bits. Order is the layout, so declare fields exactly as the format demands.
Great for Protocols
Packed structs map cleanly onto binary formats, letting you describe a hardware register or wire packet as readable named fields.
Trade-Off to Know
Bit packing can make field access a touch slower than a normal struct. Reach for packed only when exact layout actually matters. 🧩
Quick Check
What does a packed struct guarantee about its fields?
Recap
You learned that packed struct gives an exact, padding-free bit layout, has a backing integer, and converts with @bitCast. Perfect for binary formats. 🎉
Frequently asked questions
Is the “packed struct for Exact Layout” lesson free?
Yes — the full text of “packed struct for Exact Layout” is free to read here on the web, and the Zig Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Zig Academy course, upgrade to CoddyKit PRO.
What will I learn in “packed struct for Exact Layout”?
Lay out bits with no padding. You practise Zig Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Zig Academy?
No prior experience is required. Zig Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “packed struct for Exact Layout” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Zig Academy lesson?
Yes. Every Zig Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- packed struct for Exact Layout
- Arbitrary-Width Integers like u3
- Bit Fields and Flags
- extern struct and ABI Layout