# C record layout

Records declared in a `DEFINITION MODULE FOR "C"` use the **C record
layout**:

- declaration-order fields;
- natural alignment for each field;
- trailing padding to the record's maximum alignment;
- `SIZE` is the C `sizeof` value;
- field offsets match C `offsetof`.

Example:

```modula2
DEFINITION MODULE FOR "C" CStructLayout;

FROM SYSTEM IMPORT ADDRESS;

TYPE
  Mixed = RECORD
    b : CBool;       (* 1-byte _Bool *)
    f : SHORTREAL;   (* 4-byte float *)
    d : REAL;        (* 8-byte double *)
    i : INTEGER;     (* 4-byte int *)
    p : ADDRESS;     (* 8-byte pointer *)
  END;
END CStructLayout.
```

The layout is:

```text
b  @ 0
f  @ 4
d  @ 8
i  @16
p  @24
sizeof(Mixed) = 32
```

This is covered by `compiler/tests/cstruct_layout.def` and
`CStructLayoutProg`, with the C side checked using `sizeof` and
`offsetof`.

## Initial boundary policy

Records are currently passed to C **by pointer** only:

```modula2
c_function(ADR(record))
```

By-value C struct parameters and results are not yet part of the ABI;
use a C shim for APIs that require them.

C structs containing inline C arrays cannot be represented by V3 array
fields, which are descriptors/pointers. Such records need a C shim or
an array-free view.

## Next

Callback/function-pointer ABI, with top-level trampolines and a
`user_data` + destroy convention such as the existing `GtkClosures`
design.