writeStructEndian writes padding bytes of packed structs due to alignment

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1

Am I using writeStructEndian wrong or is this a bug?

test "header write" {
    const EthernetHeader = packed struct(u112) {
        dest_mac: u48,
        src_mac: u48,
        ether_type: u16,
    };

    const header: EthernetHeader = .{
        .dest_mac = 0xAABB_CCDD_EEFF,
        .src_mac = 0x1122_3344_5566,
        .ether_type = 0x88a4,
    };

    var buf: [64]u8 = std.mem.zeroes([64]u8);
    var fbs = std.io.fixedBufferStream(&buf);
    var writer = fbs.writer();
    try writer.writeStructEndian(header, std.builtin.Endian.big);
    std.log.warn("wrote: {x}", .{fbs.getWritten()});
    // prints: { aa, bb, cc, dd, ee, ff, 11, 22, 33, 44, 55, 66, 88, a4, 0, 0 }
    // the 0, 0 at the end should not be there!                          ^  ^ bad!!
    try std.testing.expect(fbs.getWritten().len == 112 / 8);
}
2

The problem here is that the compiler adds alignment to integer types, which in this case adds padding zeroes. For example u112 is aligned to 16 bytes which makes
@sizeOf(u112) == 16 == @sizeOf(u128)

I think you can do the byteSwap manually and then use .writeInt(u112, @bitCast(byteSwappedStruct), .little);

3

yeah… this assertion fails, so i think you are right about the padding zeros

const EthernetHeader = packed struct(u112) {
        dest_mac: u48,
        src_mac: u48,
        ether_type: u16,

        comptime {
            std.debug.assert(@sizeOf(@This()) == 112 / 8);
        }
    };
4

this makes me really question if write struct endian should even exist, it already required packed structs:

pub fn writeStruct(self: Self, value: anytype) anyerror!void {
    // Only extern and packed structs have defined in-memory layout.
    comptime assert(@typeInfo(@TypeOf(value)).Struct.layout != .auto);
    return self.writeAll(mem.asBytes(&value));
}
5

or perhaps writeStructEndian should be changed to not add the padding of packed structs?