Lua Academy · Lesson

Pointers, Structs, and Arrays in FFI

Work with C pointers, struct types, and array allocations in FFI.

Lesson 3 of 413 steps

Pointers, Structs, and Arrays in FFI is a free Lua Academy lesson on CoddyKit. This is lesson 3 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Lua Academy learning path, and your progress syncs across the web and the CoddyKit app. The Lua Academy course includes 4 lessons in total.

C Types in FFI

FFI supports all standard C types: int, float, double, char, pointers, arrays, and structs. Declare them in ffi.cdef.

Creating C Arrays

Allocate a C array with ffi.new("type[N]"). Access elements with 0-based indexing (C convention).

local ffi = require("ffi")
local arr = ffi.new("int[5]", {10, 20, 30, 40, 50})
print(arr[0], arr[1], arr[4])  -- 10  20  50

Pointers

Create a pointer to a value or allocate pointer types. Use ptr[0] to dereference.

local ffi = require("ffi")
local p = ffi.new("int[1]", 42)  -- int* pointing to 42
print(p[0])   -- 42
p[0] = 99
print(p[0])   -- 99

Struct Definition and Access

Declare a struct in cdef, then create instances with ffi.new and access fields with dot notation.

ffi.cdef[[
  typedef struct {
    float x, y, z;
  } Vec3;
]]
local v = ffi.new("Vec3", 1.0, 2.0, 3.0)
print(v.x, v.y, v.z)

Array of Structs

Create arrays of structs for bulk data processing — common in game engines and numerical computing.

local particles = ffi.new("Vec3[1000]")
for i = 0, 999 do
  particles[i].x = i * 0.1
  particles[i].y = 0
  particles[i].z = 0
end

Pointer Arithmetic

Advance a pointer with ptr + n. This returns a new pointer offset by n elements (not bytes).

local arr = ffi.new("int[5]", {1,2,3,4,5})
local p = arr + 2   -- pointer to arr[2]
print(p[0])         -- 3

ffi.cast: Type Casting

ffi.cast(ctype, val) reinterprets a value as another C type. Use carefully — wrong casts cause undefined behavior.

local ffi = require("ffi")
local i = ffi.cast("int32_t", -1)
print(i)  -- -1
local u = ffi.cast("uint32_t", i)
print(u)  -- 4294967295

ffi.sizeof and ffi.alignof

ffi.sizeof("Vec3") returns the byte size of a C type. ffi.alignof returns its alignment requirement.

ffi.gc: Attaching Finalizers

ffi.gc(ptr, cleanup) attaches a Lua function to a C pointer so it is called when the pointer is GC'd — enabling RAII for C memory.

local p = ffi.gc(ffi.C.malloc(1024), ffi.C.free)
-- p is automatically freed when GC collects it

Null Pointer Checks

Compare pointers to nil (which equals the C NULL pointer in FFI comparisons).

local ptr = lib.allocate()
if ptr == nil then error("allocation failed") end

Performance of C Arrays

C arrays accessed through FFI are significantly faster than Lua tables for numeric data — no boxing, no hash lookups, no type checks.

FFI Arrays Question

What index does FFI use for C arrays?

Answer it in the interactive lesson

Recap: Pointers, Structs, Arrays in FFI

Create C arrays with ffi.new("type[N]") (0-indexed), access struct fields with dot notation, use ffi.cast for type reinterpretation, and ffi.gc for automatic C memory cleanup.

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Frequently Asked Questions

Is the “Pointers, Structs, and Arrays in FFI” lesson free?

Yes — any 3 lessons of the Lua Academy learning path, including “Pointers, Structs, and Arrays in FFI”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Lua Academy course includes 4 lessons in total.

What will I learn in “Pointers, Structs, and Arrays in FFI”?

Work with C pointers, struct types, and array allocations in FFI. You practise Lua 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 Lua Academy?

No prior experience is required. Lua Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 3 of 4.

How long does the “Pointers, Structs, and Arrays in FFI” 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.

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