packed struct hidden padding issue

[b0bleet]

Zig Version

0.11.0-dev.1812+26196be34

Steps to Reproduce and Observed Behavior

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

pub fn main() anyerror!void {
    const e820map = packed struct {
        addr: u64,
        size: u64,
        type: u32,
    };
    try expect(@sizeOf(e820map) == 20);
}

Output:

$ zig run check.zig
error: TestUnexpectedResult
/opt/zig/lib/std/testing.zig:509:14: 0x20d807 in expect (test)
    if (!ok) return error.TestUnexpectedResult;
             ^
/tmp/check.zig:30:5: 0x20b695 in main (test)
    try expect(@sizeOf(e820map) == 20);
    ^

Expected Behavior

packed struct size should be 20 bytes.

[nektro]

is it 24 ? packed structs create an integer that is in bits the sum of all the fields and primarily used for bit-packing non-power-of-two values into one. here your struct is lowered as a u160 and likely padded to become either a u192 or u256 during codegen. also note you can still assert 20 with @bitSizeOf(e820map)/8.

if what you are trying to do is guarantee layout of fields of "normal" size you are likely looking for extern struct instead.

https://github.com/ziglang/zig/blob/26196be34/src/type.zig#L3577

[b0bleet]

I expected 20 bytes so padding bytes should be ignored, I tried it with 0.9.0 version it works as expected:
https://godbolt.org/z/rrb4WGxeK

[Vexu]

Packed structs were changed in 0.10.0, see #5049