FixedBufferAllocator with No Heap
Allocate from a stack buffer.
FixedBufferAllocator with No Heap is a free Zig Academy lesson on CoddyKit — lesson 3 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.
Allocate Without the Heap
A FixedBufferAllocator hands out memory from a buffer you already own. No heap, no operating system call, just your own bytes. 🧱
You Supply the Buffer
You start with a plain array, often on the stack. That fixed region is the entire pool the allocator is allowed to give out.
var buffer: [1024]u8 = undefined;Wrap the Buffer
Pass your buffer to init to create the allocator. From now on, every allocation comes out of those exact bytes.
var fba = std.heap.FixedBufferAllocator.init(&buffer);Get Its Allocator
Call allocator() as usual to get the interface. Code using it cannot tell it is backed by a fixed buffer rather than the heap.
const a = fba.allocator();It Bumps a Pointer
Allocation just moves an offset forward through the buffer. That makes it extremely fast and completely predictable in cost.
It Can Run Out
The buffer has a fixed size, so a request can fail with OutOfMemory when there is no room left. You handle that like any allocation error.
Reset to Reuse
Call reset() to move the offset back to the start. The same buffer is now empty again, ready for a fresh round of allocations.
fba.reset();Great for Embedded
With no heap involved, this allocator suits embedded systems, kernels, and any place where dynamic memory is unavailable or forbidden.
Mind the Lifetime
If the buffer lives on the stack, its memory vanishes when the function returns. Never let allocations from it outlive that buffer.
Pair It With an Arena
A common trick is to wrap a fixed buffer in an arena, giving you bulk free semantics over a region that never touches the heap at all.
A Typical Setup
The recipe is short: declare a buffer, init the allocator over it, then allocate until full and optionally reset to start over.
var buf: [256]u8 = undefined;
var fba = std.heap.FixedBufferAllocator.init(&buf);
const a = fba.allocator();Quick Check
Think about where a FixedBufferAllocator gets its memory.
Recap
You met the FixedBufferAllocator: it serves memory from a buffer you own, never touches the heap, and resets in one cheap step. 🎯
Frequently asked questions
Is the “FixedBufferAllocator with No Heap” lesson free?
Yes — the full text of “FixedBufferAllocator with No Heap” 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 “FixedBufferAllocator with No Heap”?
Allocate from a stack buffer. 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 3 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “FixedBufferAllocator with No Heap” 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
- GeneralPurposeAllocator for Debug Safety
- Arena Allocators for Bulk Free
- FixedBufferAllocator with No Heap
- Choosing an Allocator per Workload