Lean 4 FFI Constructor Memory Layout

Lean 4 FFI Constructor Memory Layout

Problem

When writing C FFI bindings for Lean 4 inductives or structures that contain both object fields (pointers to Lean objects) and scalar fields (small integers like UInt8, UInt16), the memory layout is non-obvious and incorrect assumptions cause assertion failures at runtime.

Context / Trigger Conditions

• Assertion: i < lean_ctor_num_objs(o) when calling lean_ctor_get(obj, i)
• Assertion: offset >= lean_ctor_num_objs(o) * sizeof(void*) when calling scalar getters
• lean_ctor_get_uint8, lean_ctor_get_uint16, etc. return garbage values
• Creating FFI for inductives like:

  inductive SockAddr where
    | ipv4 (addr : IPv4Addr) (port : UInt16)  -- 1 object field, 2 scalar bytes
  

Solution

Memory Layout Rules

Lean 4 constructors store fields in this order:

1. Object fields (pointers): indices 0, 1, 2, ... accessible via lean_ctor_get(obj, i)
2. Scalar fields (small integers): stored AFTER all object fields

Scalar Offset Calculation

The offset for scalar fields is relative to the START of the constructor's data, not the start of the scalar section. Therefore:

// For a constructor with N object fields:
scalar_offset = sizeof(void*) * num_object_fields + byte_offset_within_scalars

For the FIRST scalar field, the offset is sizeof(void*) * num_object_fields.

Which Types Are Scalars?

• UInt8, UInt16, UInt32, UInt64, Float, Bool → Scalar in inductives
• String, Array, other inductives/​structures → Object (pointer)
• A structure with ONLY scalar fields (like 4x UInt8) → Still uses constructor, but with 0 object fields

Example: Correct FFI for Mixed Constructor

-- Lean definition
inductive SockAddr where
  | ipv4 (addr : IPv4Addr) (port : UInt16)  -- IPv4Addr is object, UInt16 is scalar

// WRONG - treats UInt16 as object field
uint16_t port = lean_unbox(lean_ctor_get(addr, 1));  // CRASHES!

// CORRECT - UInt16 is scalar at offset after 1 object field
uint16_t port = lean_ctor_get_uint16(addr, sizeof(void*));  // Works!

Creating Constructors with Mixed Fields

// lean_alloc_ctor(tag, num_object_fields, num_scalar_bytes)

// For SockAddr.ipv4: 1 object field (IPv4Addr), 2 scalar bytes (UInt16)
lean_obj_res result = lean_alloc_ctor(0, 1, 2);
lean_ctor_set(result, 0, ipv4_obj);           // Object at index 0
lean_ctor_set_uint16(result, sizeof(void*), port);  // Scalar at offset 8 (64-bit)

Structure with Only Scalars

structure IPv4Addr where
  a : UInt8
  b : UInt8
  c : UInt8
  d : UInt8

// 0 object fields, 4 scalar bytes - scalar offset starts at 0
lean_obj_res ipv4 = lean_alloc_ctor(0, 0, 4);
lean_ctor_set_uint8(ipv4, 0, a);  // offset 0
lean_ctor_set_uint8(ipv4, 1, b);  // offset 1
lean_ctor_set_uint8(ipv4, 2, c);  // offset 2
lean_ctor_set_uint8(ipv4, 3, d);  // offset 3

Verification

1. Build passes without Lean assertion violations
2. Run tests that exercise the FFI functions
3. Values round-trip correctly (create in Lean, read in C, write in C, read in Lean)

Common Mistakes

Notes

• On 64-bit systems, sizeof(void*) is 8
• The scalar offset is in BYTES, not in field indices
• Multiple scalars are packed contiguously after the object fields
• Lean's runtime does NOT automatically box small integers in inductives—they're truly stored inline

References

• Lean 4 source: lean.h contains the constructor macros with assertions
• FFI patterns observed in Lean's own C runtime and mathlib FFI code