Incorrect byte offset and struct size for packed structs #2627
Description
The byte offsets of fields within a packed struct are sometimes incorrect, see the below example.
const std = @import("std");
const PackedStruct = packed struct {
/// bitoffset = 0, byteoffset = 0
bool_a: bool,
/// bitoffset = 1, byteoffset = 0
bool_b: bool,
/// bitoffset = 2, byteoffset = 0
bool_c: bool,
/// bitoffset = 3, byteoffset = 0
bool_d: bool,
/// bitoffset = 4, byteoffset = 0
bool_e: bool,
/// bitoffset = 5, byteoffset = 0
bool_f: bool,
/// bitoffset = 6, byteoffset = 0
u1_a: u1,
/// bitoffset = 7, byteoffset = 0
bool_g: bool,
/// bitoffset = 8, byteoffset = 1
u1_b: u1,
/// bitoffset = 9, byteoffset = 1
u3_a: u3,
/// bitoffset = 12, byteoffset = 1
u10_a: u10,
/// bitoffset = 22, byteoffset = 2
u10_b: u10,
};
pub fn main() void {
inline for (@typeInfo(PackedStruct).Struct.fields) |field| {
std.debug.warn("field={}, byteoffset={} bitoffset={}\n",
field.name,
usize(@byteOffsetOf(PackedStruct, field.name)),
usize(@bitOffsetOf(PackedStruct, field.name))
);
}
std.debug.warn("totalsize {} bytes\n", usize(@sizeOf(PackedStruct)));
}
The comments show what the bit offsets and byte offsets should be judging by the documentation[1], but below is the result:
field=bool_a, byteoffset=0 bitoffset=0
field=bool_b, byteoffset=0 bitoffset=1
field=bool_c, byteoffset=0 bitoffset=2
field=bool_d, byteoffset=0 bitoffset=3
field=bool_e, byteoffset=0 bitoffset=4
field=bool_f, byteoffset=0 bitoffset=5
field=u1_a, byteoffset=0 bitoffset=6
field=bool_g, byteoffset=0 bitoffset=7
field=u1_b, byteoffset=1 bitoffset=8
field=u3_a, byteoffset=1 bitoffset=9
field=u10_a, byteoffset=1 bitoffset=12
field=u10_b, byteoffset=1 bitoffset=22
totalsize 5 bytes
The size is also incorrect, since it should be 4 bytes (the sum of the fields) but is instead 5.
1: "bool fields use exactly 1 bit" and "Zig supports arbitrary width Integers and although normally, integers with fewer than 8 bits will still use 1 byte of memory, in packed structs, they use exactly their bit width" at https://ziglang.org/documentation/0.4.0/#packed-struct