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.

Field	a	a	a	a	b				c	c	c	c	d	e
Bytes	15	27	00	00	2A	00	00	00	00	00	28	C1	00	01	00	00

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.