# Packet layout (/docs/reference/shared-memory)

Internal frame packets, packed structures and C++ layout checks.

`ShmPacket` is the internal snapshot produced by `GameReader` and consumed by the frame loop. The name remains from an earlier shared-memory bridge. The current reader uses `process_vm_readv` and copies snapshots under a mutex. It does not open POSIX shared memory or named semaphores.

The former `CHUDCS2_SHM_NAME` and `CHUDCS2_SEM_NAME` variables have no effect. This page keeps its original URL for existing links.

## Packed layout [#packed-layout]

Packet definitions live in `src/overlay/game_reader.hpp`. Packed structures use `#pragma pack(push, 1)` and compile-time size assertions. Preserve field order, scalar widths, array lengths and nested layouts when changing them.

The packet includes view and local-player state, weapon spread and recoil values, hitboxes, projectiles, infernos, doors, dropped items, smokes, spectators and player records. Player records contain model names, bone transforms, status flags, weapon data, velocity and shot state.

## Change checklist [#change-checklist]

1. Update the definitions and size assertions in `src/overlay/game_reader.hpp` together.
2. Update affected assertions and bounds checks in `tests/cpp/test_main.cpp`.
3. Update the producer in `src/overlay/game_reader.cpp` and affected consumers.
4. Build with `CHUDCS2_BUILD_TESTS=ON` and run CTest as described in [Testing](/docs/guides/testing).

The repository does not contain the former Python ctypes mirror or simulation fixtures. A test name such as `test_shm_reader_bounds` refers to the retained packet types and bounds logic.