Struct Layout as a Contract

Layout is determined, not arbitrary

Every member sits at its alignment boundary, and the struct's total size is a multiple of its strictest alignment (module 1's forensics, now as a contract). Two compilers claiming the same ABI must compute the same layout for the same declaration — otherwise binary modules cannot exchange structs.

The one-membership change that breaks everything

Adding a member in the middle shifts every later member's offset. Old binaries writing the old layout silently produce garbage for the new reader: no error, wrong data. This is why public C APIs append to the end of structs, or hide them behind opaque handles (module 3) entirely.

Versioning without breaking

• append-only: new members go last; old code ignores the tail (the size it knew still worked for the old prefix).
• reserved fields: pad the struct to a stable size from day one.
• opaque handles: layout becomes private; you can change anything.

Choose deliberately: every public struct in your career will carry one of these three decisions.