packed struct (#181)

* code recognize * nothing * remove useless file * source file * 增加 packed 重新排序说明 * 格式化
zigcc · Sep 4, 2024 · 6e1a764 · 6e1a764
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diff --git a/bun.lockb b/bun.lockb
diff --git a/course/basic/advanced_type/struct.md b/course/basic/advanced_type/struct.md
@@ -197,24 +197,43 @@ zig 在使用结构体的时候还支持省略结构体类型，只要能让 zig
 
 3. 还可以对 `packed` 的结构体的指针设置内存对齐来访问对应的字段：
 
-> 这里说明可能有些不清楚，请见谅！
-
-```zig
-const std = @import("std");
-const expect = std.testing.expect;
-
-const S = packed struct {
-    a: u32,
-    b: u32,
-};
-test "overaligned pointer to packed struct" {
-    var foo: S align(4) = .{ .a = 1, .b = 2 };
-    const ptr: *align(4) S = &foo;
-    const ptr_to_b: *u32 = &ptr.b;
-    try expect(ptr_to_b.* == 2);
-}
+:::details 示例
+
+<<<@/code/release/struct.zig#aligned_struct
+
+:::
+
+4. 默认情况下 zig 还会对字段进行重新排序，但是在 packed 下并不会重新排序。
+
+:::details 示例
+
+<<<@/code/release/struct.zig#reorder_struct
+
+输出为：
+
+```sh
+16
+12
+0
+4
 ```
 
+在 64 位系统上，Foo 的内存布局是：
+
+```sh
+|   4(i32)  |  8(pointer)    |   4(padding)  |
+```
+
+如果去掉 packed，则是
+
+```sh
+|   8(pointer  |  4(int32)    |   4(padding)  |
+```
+
+可以看到， Zig 会对字段进行重新排序。
+
+:::
+
 ### 命名规则
 
 由于在 zig 中很多结构是匿名的（例如可以把一个源文件看作是一个匿名的结构体），所以 zig 基于一套规则来进行命名：

diff --git a/course/code/12/build_system/test/src/root.zig b/course/code/12/build_system/test/src/root.zig
diff --git a/course/code/12/struct.zig b/course/code/12/struct.zig
@@ -363,3 +363,40 @@ const PackedCast = struct {
     }
     // #endregion packed_cast
 };
+
+const aligned_struct = struct {
+    // #region aligned_struct
+    const std = @import("std");
+    const expect = std.testing.expect;
+
+    const S = packed struct {
+        a: u32,
+        b: u32,
+    };
+    test "overaligned pointer to packed struct" {
+        var foo: S align(4) = .{ .a = 1, .b = 2 };
+        const ptr: *align(4) S = &foo;
+        const ptr_to_b: *u32 = &ptr.b;
+        try expect(ptr_to_b.* == 2);
+    }
+    // #endregion aligned_struct
+};
+
+const reorder_struct = struct {
+    // #region reorder_struct
+    const std = @import("std");
+
+    const Foo = packed struct {
+        x: i32,
+        y: [*]i32, // 一个多项指针
+    };
+
+    pub fn main() !void {
+        std.debug.print("{any}\n", .{@sizeOf(Foo)});
+        std.debug.print("{any}\n", .{@bitSizeOf(Foo) / 8});
+
+        std.debug.print("{any}\n", .{@bitOffsetOf(Foo, "x") / 8});
+        std.debug.print("{any}\n", .{@bitOffsetOf(Foo, "y") / 8});
+    }
+    // #endregion reorder_struct
+};
diff --git a/course/code/14/struct.zig b/course/code/14/struct.zig
@@ -363,3 +363,40 @@ const PackedCast = struct {
     }
     // #endregion packed_cast
 };
+
+const aligned_struct = struct {
+    // #region aligned_struct
+    const std = @import("std");
+    const expect = std.testing.expect;
+
+    const S = packed struct {
+        a: u32,
+        b: u32,
+    };
+    test "overaligned pointer to packed struct" {
+        var foo: S align(4) = .{ .a = 1, .b = 2 };
+        const ptr: *align(4) S = &foo;
+        const ptr_to_b: *u32 = &ptr.b;
+        try expect(ptr_to_b.* == 2);
+    }
+    // #endregion aligned_struct
+};
+
+const reorder_struct = struct {
+    // #region reorder_struct
+    const std = @import("std");
+
+    const Foo = packed struct {
+        x: i32,
+        y: [*]i32, // 一个多项指针
+    };
+
+    pub fn main() !void {
+        std.debug.print("{any}\n", .{@sizeOf(Foo)});
+        std.debug.print("{any}\n", .{@bitSizeOf(Foo) / 8});
+
+        std.debug.print("{any}\n", .{@bitOffsetOf(Foo, "x") / 8});
+        std.debug.print("{any}\n", .{@bitOffsetOf(Foo, "y") / 8});
+    }
+    // #endregion reorder_struct
+};