Feature flags

machbus separates hosted integration surfaces from embedded protocol surfaces. The default build is intentionally convenient for Linux/desktop development, while embedded users opt into a smaller no_std + alloc surface by disabling default features.

Current feature model

[features]
default = ["wirebit", "dep:pyo3", "dep:concord", "tracing/std"]
embedded = []
wirebit = ["dep:wirebit", "wirebit/socketcan"]
async = ["dep:futures-core"]

There are exactly four features: default, embedded, wirebit, and async. There is no separate std or alloc knob — the no_std switch is keyed off embedded, so a normal build is std (with an allocator) automatically, and an embedded build is no_std + alloc. An allocator is always available.

Unless you select embedded, the C ABI, the Python bindings (pyo3), and the rich geo conversions (concord) are always compiled — they are part of the default hosted build, not separate features.

The feature table

Hosted/default mode

With normal dependency syntax:

[dependencies]
machbus = { path = "../machbus" }

you get the hosted default surface. This is the right mode for examples that use the virtual bus, C ABI work, Python binding work, rich concord conversions, and ordinary Linux/desktop development.

Embedded no_std + alloc mode

Embedded users should disable defaults:

[dependencies]
machbus = { path = "../machbus", default-features = false, features = ["embedded"] }

This compiles the embedded public surface as no_std + alloc. The embedded loop owns:

• monotonic time (machbus::time::Instant values supplied by the caller);
• CAN receive/transmit through machbus::net::CanTransport;
• storage for NIU config text, IOP bytes, and VT stored-pool blobs;
• scheduling and task wakeups.

The embedded feature does not compile:

• the C ABI;
• the Python bindings;
• SocketCAN / wirebit;
• concord;
• host file load/save helpers;
• host-clock Driver::poll() convenience.

Use Driver::poll_at(now) or the embedded Session loop shape instead. The embedded Session enables IsoNet’s direct message-capture queue, so decoded messages are drained without a boxed callback listener at the session boundary. Hosted callback dispatch remains explicit and opt-in. Selected ISO 11783 application codecs are also embedded-available, including AUX, Functionalities, Group Function, guidance codecs, TIM, implement/tractor message codecs, and Sequence Control core state/recording/TAN helpers. machbus::isobus::tc also builds in embedded mode for heap-backed DDOP/object codecs, TC client/server pump state, TC-GEO, grids, task lifecycle/logging, rate limiting, outstanding-request tracking, ISOXML parsing, and task totals. machbus::isobus::fs builds in embedded mode for File Client / File Server pump state, ISO 11783-13 operation/type/property codecs, path validation, in-memory file storage, and volume helpers; real media persistence stays application-owned. machbus::isobus::vt builds in embedded mode for object pools, VT Client / VT Server pump state, update helpers, storage-agnostic stored-version blobs, and working-set state; VT render/GTUI remains hosted.

Fixed-capacity helpers (embedded)

The embedded profile also ships allocation-free fixed-capacity helper primitives for bounded-memory critical paths:

• machbus::fixed::FixedQueue<T, N>, FixedFrameQueue<N>, FixedSlots<T, N>, FixedBytes<N>, FixedMessage<N>;
• machbus::net::TpCmdtTx<'_>, TpRxFixed<N>, EtpCmdtTx<'_>, EtpRxFixed<N>;
• machbus::session::FixedEvent<N>;
• Session::poll_fixed_event::<N>() / Driver::poll_fixed_at::<N>();
• examples/embedded_fixed_queue.rs, which uses fixed RX/TX transport buffers.

In no_std builds IsoNet uses a fixed-capacity pending TP/ETP transmit queue for the deferred multi-frame send path; TP and ETP active-session tables use fixed-capacity slot storage; Fast Packet receive-session slots and reassembly payloads are fixed-capacity. FastPacketProtocol::process_frame_fixed::<N>() returns a bounded FixedMessage<N> without allocating, and FastPacketProtocol::send_fixed::<N>() avoids a growable frame vector. TP/CMDT and ETP/DPO pending transmit windows have fixed variants (get_pending_data_frames_fixed::<N>()), broadcast BAM transfers can use send_bam_fixed::<N>(), and the borrowing transmit/receive helpers (TpCmdtTx, TpRxFixed, EtpCmdtTx, EtpRxFixed) reassemble or emit fixed-capacity batches without copying into the heap-backed session stores. Oversized fixed events are reported explicitly instead of being truncated.

This is not yet a full no-alloc profile: the session core still uses the heap-backed embedded profile. The fixed-capacity helpers are the allocation-free critical-path building blocks introduced ahead of the larger internal queue/cache/reassembly migration.

Validation targets

Use the Makefile targets rather than ad-hoc Cargo commands:

make no-std-check
make no-std-target-check
make no-std-surface-check
make embedded-examples-check
make wirebit-examples-check

make no-std-target-check checks the embedded feature on the documented embedded target. make no-std-surface-check compiles the embedded public API imports and minimal loop shape in a dedicated test.

For dependency audits:

cargo tree --no-default-features --features embedded -e normal

The embedded graph should stay free of wirebit, pyo3, and concord.

Combining features

Hosted features combine freely when they make sense. wirebit implies the hosted transport path and is not part of the embedded profile. async is runtime-agnostic, but it does not make the session Send or thread-safe; it remains a local, explicitly pumped model.

What this proves / does not prove

Feature flags describe compile-time surface area and dependencies. They do not claim official ISO 11783, SAE J1939, NMEA, or AEF certification. A real deployment still needs official standards access, hardware evidence, and interoperability testing.

See also

• no_std on microcontrollers
• Crate map
• Validation gates
• Hardware evidence
• Bindings overview