如何在Squeak FFI中对齐结构体字段？含实际结构体示例

To match the C struct's aligned offsets (0, 4, 8, 16) in Squeak's ExternalStructure, you have two reliable approaches to handle the padding between fields:

1. Explicitly Specify Field Offsets #

The simplest way is to directly define the byte offset of each field in the fields method. This tells Squeak exactly where each member starts, overriding default packing rules.

Here's the implementation:

Foo class>>fields 
  ^#(
    (x 'char' 0)       "Starts at byte 0"
    (y 'float' 4)      "Starts at byte 4 (3 bytes padding after x)"
    (z 'char' 8)       "Starts at byte 8"
    (t 'double' 16)    "Starts at byte 16 (7 bytes padding after z)"
  )

This approach is concise if you already know the exact offsets required by your target platform's alignment rules.

2. Add Explicit Padding Fields #

If you prefer to make the padding logic visible and self-documenting, you can add dummy padding fields between actual members to fill the gaps. Use the array syntax ((name type count)) to create padding of the required number of bytes:

Foo class>>fields 
  ^#(
    (x 'char')
    (paddingAfterX 'unsignedChar' 3)  "3 bytes to align y to 4-byte boundary"
    (y 'float')
    (z 'char')
    (paddingAfterZ 'unsignedChar' 7)  "7 bytes to align t to 8-byte boundary"
    (t 'double')
  )

Each unsignedChar occupies 1 byte, so specifying a count of 3 gives exactly the padding needed between x and y, while 7 bytes fill the gap between z and t to reach the 16-byte offset for t.

Key Notes #

• Both methods ensure your Squeak Foo struct matches the memory layout of the C struct foo with the specified offsets.
• Choose explicit offsets for brevity, or padding fields if you want to make the alignment logic obvious to anyone reading the code.

内容的提问来源于stack exchange，提问作者aka.nice