start with Header

README.md

@@ -156,7 +156,7 @@ OFFSET     TYPE     NAME                          DESCRIPTION
    0      [4]u8     magic                         always 0xf09fa4b7
    4         u8     version                       now `0`
    5        u16     bom                           0x1234
-   7         u6     num_shells                    max value: 63
+             u8     num_shells                    max value: 63. Padding is strange on little endian.
    8        u32     num_users                     number of passwd entries
   12        u32     num_groups                    number of group entries
   16        u32     offset_cmph_uid2user
@@ -186,6 +186,11 @@ header field. For example, `cmph_gid2group` comes immediately after the header,
 and `idx_groupname2group` comes after `idx_gid2group`, whose offset is
 `offset_idx`, and size can be calculated.
 
+`num_shells` would fit to u6; however, we would need 2 bits of padding (all
+other fields are byte-aligned). If we instead do `u2` followed by `u6`, the
+byte would look very unusual on a little-endian architecture. Therefore we will
+just refuse loading the file if the number of shells exceeds 63.
+
 Primitive types
 ---------------
 

src/header.zig

@@ -0,0 +1,63 @@
+const std = @import("std");
+
+const HeaderSize = @bitSizeOf(Header) / 8;
+const Magic = [4]u8{ 0xf0, 0x9f, 0xa4, 0xb7 };
+const Version = 0;
+const Bom = 0x1234;
+
+const Header = packed struct {
+    magic: [4]u8,
+    version: u8,
+    bom: u16,
+    num_shells: u8,
+    num_users: u32,
+    num_groups: u32,
+    offset_cmph_uid2user: u32,
+    offset_cmph_groupname2group: u32,
+    offset_cmph_username2user: u32,
+    offset_idx: u32,
+    offset_groups: u32,
+    offset_users: u32,
+    offset_groupmembers: u32,
+    offset_additional_gids: u32,
+
+    // TODO(motiejus) should we be returning *Header instead? That way all
+    // stacks could be smaller by 48 bytes, and we would be referring to
+    // it's fields in the mmap'ed region.
+    pub fn init(blob: [HeaderSize]u8) Header {
+        return @bitCast(Header, blob);
+    }
+
+    pub fn asArray(self: *const Header) [HeaderSize]u8 {
+        return @bitCast([HeaderSize]u8, self.*);
+    }
+};
+
+const testing = std.testing;
+
+test "header size is byte-aligned" {
+    try testing.expectEqual(HeaderSize * 8, @bitSizeOf(Header));
+}
+
+test "header pack and unpack" {
+    const header = Header{
+        .magic = Magic,
+        .version = Version,
+        .bom = Bom,
+        .num_shells = 0,
+        .num_users = 0,
+        .num_groups = 0,
+        .offset_cmph_uid2user = 0,
+        .offset_cmph_groupname2group = 0,
+        .offset_cmph_username2user = 0,
+        .offset_idx = 0,
+        .offset_groups = 0,
+        .offset_users = 0,
+        .offset_groupmembers = 0,
+        .offset_additional_gids = 0,
+    };
+
+    const blob = header.asArray();
+    const header2 = Header.init(blob);
+    try testing.expectEqual(header, header2);
+}

src/shell.zig

@@ -4,7 +4,6 @@ const PriorityDequeue = std.PriorityDequeue;
 const StringArrayHashMap = std.StringArrayHashMap;
 const StringHashMap = std.StringHashMap;
 const BoundedArray = std.BoundedArray;
-const testing = std.testing;
 const StringContext = std.hash_map.StringContext;
 
 // MaxShells is the maximum number of "popular" shells.
@@ -182,6 +181,8 @@ inline fn roundUp4Padding(n: u12) u12 {
     return ((n + 3) & ~@intCast(u12, 3)) - n;
 }
 
+const testing = std.testing;
+
 test "basic shellpopcon" {
     var popcon = ShellWriter.init(testing.allocator);
     defer popcon.deinit();

src/test_main.zig

@@ -1,4 +1,5 @@
 test "turbonss test suite" {
     _ = @import("main.zig");
     _ = @import("shell.zig");
+    _ = @import("header.zig");
 }