sdl phase 2

specs.md

@@ -286,8 +286,26 @@ val := mp[2];             // 30
 
 **Implicit conversions**:
 - `*T` → `[*]T` (pointer to element → many-pointer)
+- `*[N]T` → `[*]T` (pointer to array → many-pointer)
+- `[N]T` → `[*]T` at call sites (array decays to many-pointer)
+- `[]T` → `[*]T` (slice decays to many-pointer, extracts `.ptr`)
+- `T` → `*T` at call sites (implicit address-of)
 - `null` (`*void`) → any `*T`
 
+**Fat pointer layout**: `[:0]u8`, `string`, and `[]T` are `{ptr, i64}` structs. The raw pointer is always the first field at offset 0. This means `*[:0]u8` works as C's `char**` — a C function dereferences through the outer pointer and reads the raw `char*` from offset 0.
+
+### C Interop Type Mapping
+
+| C type | sx type | Notes |
+|--------|---------|-------|
+| `const char*` (input) | `[:0]u8` | compiler extracts `.ptr` at call site |
+| `char*` (output buffer) | `[*]u8` | raw buffer, no length |
+| `const char**` | `*[:0]u8` | address of `[:0]u8` — `.ptr` at offset 0 |
+| `int*` (single out) | `*s32` | |
+| `unsigned*` (single out) | `*u32` | |
+| `float*` (buffer) | `[*]f32` | |
+| `void*` (generic) | `*void` | only for truly opaque/generic data |
+
 ### Vector Types (SIMD)
 LLVM SIMD vectors, parameterized by length and element type.
 ```sx