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Version: Zig 0.12.0

Extern Structs

Normal structs in Zig do not have a defined layout; extern structs are required for when you want the layout of your struct to match the layout of your C ABI.

Let's create an extern struct. This test should be run with x86_64 with a gnu ABI, which can be done with -target x86_64-native-gnu.

const expect = @import("std").testing.expect;

const Data = extern struct { a: i32, b: u8, c: f32, d: bool, e: bool };

test "hmm" {
const x = Data{
.a = 10005,
.b = 42,
.c = -10.5,
.d = false,
.e = true,
const z = @as([*]const u8, @ptrCast(&x));

try expect(@as(*const i32, @ptrCast(@alignCast(z))).* == 10005);
try expect(@as(*const u8, @ptrCast(@alignCast(z + 4))).* == 42);
try expect(@as(*const f32, @ptrCast(@alignCast(z + 8))).* == -10.5);
try expect(@as(*const bool, @ptrCast(@alignCast(z + 12))).* == false);
try expect(@as(*const bool, @ptrCast(@alignCast(z + 13))).* == true);

This is what the memory inside our x value looks like.


Note how there are gaps in the middle and at the end - this is called "padding". The data in this padding is undefined memory, and won't always be zero.

As our x value is that of an extern struct, we could safely pass it into a C function expecting a Data, providing the C function was also compiled with the same gnu ABI and CPU arch.