macOS Implementation Status

Phase 1: Build Pipeline (Complete)

The Nix + Protobuf + Rust build pipeline is fully operational. A demo binary compiles the protobuf schema, populates a sample BatchMessage with two socket records and a system summary, and serializes it to pretty-printed JSON.

What was built

Nix build system (flake.nix, nix/)

Flake inputs: nixpkgs-unstable, rust-overlay, flake-utils, advisory-db
Rust 1.93.0 pinned via rust-overlay with clippy, rustfmt, rust-src extensions
Custom rustPlatform via makeRustPlatform for reproducible builds
nix/constants.nix — centralized pname, version, rustVersion, systems, tool lists, crossTargets
nix/package.nix — buildRustPackage with protobuf, pkg-config, darwin framework support
nix/cross.nix — cross-compilation derivation using cargo-zigbuild + zig for macOS targets
nix/proto.nix — standalone proto validation (nix build .#proto)
nix/checks.nix — clippy, fmt, test as separate derivations for parallel CI
nix/shell.nix — dev shell with explicit deps, CARGO_HOME=.cargo to isolate from rustup

Protobuf schema (proto/tcp_stats.proto)

Full schema from design doc Section 19.2
4 enums: TcpState (12 values), Platform (3), IpVersion (3), DataSource (7)
6 messages: CollectionMetadata (11 fields), TcpSocketRecord (78 fields), StateBucket, SystemSummary (18 fields), BatchMessage
Proto3 optional on all data fields to distinguish absent vs zero

Rust code generation (build.rs, src/proto_gen.rs)

prost-build compiles proto and writes descriptor set
pbjson-build generates serde Serialize/Deserialize impls from the descriptor
btree_map(["."]) for deterministic serialization order
Generated code included via include! macro in proto_gen::tcpstats_reader module

Issues found and resolved

Issue	Root cause	Fix
`IpVersion::V4` not found	prost strips `IP_VERSION_` prefix, generates `IpVersion4`	Changed to `IpVersion::IpVersion4`
`cargo clippy` reported rustc 1.77.2	`buildRustPackage` hooks set `RUSTC_WRAPPER` to cargo-auditable	Replaced `inputsFrom` with explicit dependency list in shell.nix
`cargo clippy` still found old rustc	cargo checks `~/.cargo/bin` for subcommands, found rustup proxy	Set `CARGO_HOME=.cargo` in dev shell env
`cargoLock.lockFile` eval error	String interpolation `"${src}/..."` not valid for Nix path	Changed to path concatenation `src + "/Cargo.lock"`
`.cargo/` registry staged in git	`CARGO_HOME=.cargo` + `git add -A`	Added `/.cargo` to .gitignore
`sysctlbyname` newp arg type mismatch	macOS `libc` expects `mut c_void`, code used `std::ptr::null()` (`const`)	Changed to `std::ptr::null_mut()` — silent on native builds, caught by cross-compile
cargo-zigbuild HOME cache dir	Nix sandbox sets `$HOME=/homeless-shelter` (doesn't exist)	Set `export HOME=$(mktemp -d)` in buildPhase

Cross-compilation (Complete)

Cross-compile macOS binaries from Linux using cargo-zigbuild + zig (which bundles macOS SDK/linker stubs). No Xcode or macOS SDK installation required.

Nix files added/changed:

nix/constants.nix — added crossTargets mapping (cross-x86_64-darwin, cross-aarch64-darwin) to Rust target triples
nix/cross.nix (new) — stdenv.mkDerivation using cargo zigbuild --release --target <triple>:
- nativeBuildInputs: rustToolchainWithTargets, cargo-zigbuild, zig, protobuf, pkg-config, cargoSetupHook
- cargoDeps: rustPlatform.importCargoLock for offline builds
- doCheck = false (can't run Mach-O on Linux)
- meta.platforms = platforms.linux (only builds on Linux hosts)
flake.nix — added rustToolchainWithTargets (base toolchain + darwin targets), crossPackages conditional on stdenv.isLinux, merged into packages output

Key insight: No Apple frameworks are actually linked at build time. The SystemConfiguration entry in package.nix is precautionary — Cargo.lock shows zero crates that depend on it. The only native dependency is libc (for sysctlbyname), which zig's bundled libSystem.B.dylib stubs satisfy.

Build convenience targets:

Three ways to build cross targets, all producing per-target output directories:

Method	Command	Output
Makefile	`make cross-aarch64-darwin`	`result-cross-aarch64-darwin/bin/tcpstats-reader`
Makefile	`make cross-x86_64-darwin`	`result-cross-x86_64-darwin/bin/tcpstats-reader`
Makefile	`make cross-all`	Both targets
nix run	`nix run .#cross-aarch64-darwin`	`result-cross-aarch64-darwin/bin/tcpstats-reader`
nix run	`nix run .#cross-x86_64-darwin`	`result-cross-x86_64-darwin/bin/tcpstats-reader`
nix run	`nix run .#build-cross-all`	Both targets with separate output dirs
nix build	`nix build .#cross-all`	`result/bin/tcpstats-reader-{x86_64,aarch64}-apple-darwin`

The apps outputs wrap nix build with --out-link result-<target> so each target gets its own output directory automatically. The cross-all package collects all targets into a single output with binaries named by target triple.

Binary sizes (release, stripped):

Target	Size
`x86_64-apple-darwin`	~613 KB
`aarch64-apple-darwin`	~633 KB

Tested end-to-end: built on Linux via nix build .#cross-x86_64-darwin, scp'd to macOS 11.7.10 x86_64 host, confirmed working with live TCP socket data:

$ nix build .#cross-x86_64-darwin
$ file ./result/bin/tcpstats-reader
tcpstats-reader: Mach-O 64-bit x86_64 executable, flags:<NOUNDEFS|DYLDLINK|TWOLEVEL|NO_REEXPORTED_DYLIBS|PIE|HAS_TLV_DESCRIPTORS>

$ scp ./result/bin/tcpstats-reader 172.16.50.135:
tcpstats-reader                                          100%  613KB  17.3MB/s   00:00

$ ssh 172.16.50.135 './tcpstats-reader --count 1'
{"metadata":{"timestampNs":"1772408699298216000","hostname":"dass-MBP.localdomain",
"platform":"PLATFORM_MACOS","osVersion":"11.7.10","intervalMs":1000,
"dataSources":["DATA_SOURCE_MACOS_PCBLIST_N"],"collectionDurationNs":"50816",
"pcblistGeneration":"626","batchSequence":"1","toolVersion":"tcpstats-reader 0.1.0"},
"records":[...7 sockets...],"summary":{"timestampNs":"1772408699298229000",
"intervalMs":1000,"totalSockets":7,"stateCounts":[{"state":"TCP_STATE_CLOSED","count":7}]}}

Phase 2: Sysctl Reader (Complete)

src/sysctl.rs — Shared sysctl reader with platform cfg-gating.

read_sysctl(name) — two-call pattern (get size, allocate +25% headroom, read)
read_pcblist_validated(name, max_retries) — reads sysctl, parses xinpgen header/trailer, retries if xig_gen mismatch between header and trailer
read_clock_hz() — reads kern.clockrate struct, returns hz field for RTT tick conversion
read_os_version() — reads kern.osproductversion for metadata
All real implementations #[cfg(any(target_os = "macos", target_os = "freebsd"))]
Stubs return Err(SysctlError::UnsupportedPlatform) on Linux
SysctlError enum with thiserror derives: NameToMib, ReadFailed, GenerationMismatch, TooSmall, UnsupportedPlatform

Phase 3: macOS pcblist_n Parser (Complete)

src/platform/macos_layout.rs — Offset constants isolated for easy correction.

Record kind tags: XSO_SOCKET=0x001, XSO_RCVBUF=0x002, XSO_SNDBUF=0x004, XSO_STATS=0x008, XSO_INPCB=0x010, XSO_TCPCB=0x020
Named offset constants for each struct (e.g. XSOCKET_N_SO_LAST_PID_OFFSET, XTCPCB_N_T_SRTT_OFFSET)
roundup64() for XNU 8-byte record alignment
TCP_RTT_SHIFT=3, TCP_RTTVAR_SHIFT=2, INP_IPV4=0x1, INP_IPV6=0x2

src/platform/macos.rs — Pure parsing functions (testable on all platforms).

parse_pcblist_n(buf, hz) — walks tagged records, returns Vec<RawSocketRecord>
ConnectionAccumulator — collects fields from tagged records for one connection:
- parse_xsocket_n() — socket_id (so_pcb), uid, pid, effective_pid
- parse_rcvbuf() / parse_sndbuf() — buffer cc + hiwat
- parse_xinpcb_n() — IP addrs (v4/v6 based on inp_vflag), ports (network byte order), inp_gencnt
- parse_xtcpcb_n() — state, flags, cwnd, ssthresh, maxseg, windows, RTT (raw ticks), seq nums, window scale, dupacks, rxtshift, starttime
- build() — tags data source, returns RawSocketRecord
RTT conversion: ((t_srtt >> TCP_RTT_SHIFT) * 1_000_000) / hz for microseconds
Byte-reading helpers: read_u8_at, read_u16_be_at, read_i32_at, read_u32_at, read_u64_at
New XSO_SOCKET record = new connection group; emit previous when complete
Unknown kinds skipped gracefully (forward compat)

src/platform/mod.rs — Error types and platform dispatch.

CollectError enum: Sysctl, Parse, Truncated, UnknownKind, UnsupportedPlatform
CollectionResult { records, generation, collection_duration_ns }
collect_tcp_sockets() — cfg-dispatches to macos::collect() or stub::collect()

src/platform/stub.rs — Linux CI stub returning Err(CollectError::UnsupportedPlatform).

Design decisions

Cursor-based parsing, not #[repr(C)] structs — offsets in macos_layout.rs are more robust across XNU versions and testable with synthetic byte blobs on Linux CI.
macos.rs always compiled — only collect() (which calls sysctl) is cfg-gated. The pure parse_pcblist_n() function and all tests run on all platforms.

Phase 4: Record Types + Proto Conversion (Complete)

src/record.rs — Internal intermediate types.

enum IpAddr { V4([u8; 4]), V6([u8; 16]) } — raw byte representation
struct RawSocketRecord — ~35 fields with Option<T>, all in native Rust types with normalized units (RTT in microseconds)
Bridge between platform parser and proto conversion

src/convert.rs — Proto conversion functions.

kernel_state_to_proto(i32) -> i32 — macOS TCPS_* (0-10) maps to proto enum (1-11), offset by +1
ip_version_to_proto(u8) -> i32, ip_addr_to_bytes(&IpAddr) -> Vec<u8>
raw_to_proto(&RawSocketRecord) -> TcpSocketRecord — maps all fields, sets sources = [MacosPcblistN]
build_metadata(generation, duration, count, seq, interval_ms) -> CollectionMetadata — timestamp, hostname, platform, os_version
build_summary_from_records(&[TcpSocketRecord], interval_ms) -> SystemSummary — counts states using BTreeMap
build_batch() — assembles a full BatchMessage from raw records

Phase 5: JSON Output (Complete)

src/output/mod.rs — Output abstraction.

OutputError enum: Serialization, Io
trait OutputSink { emit(&mut self, &BatchMessage), flush(&mut self), format_name() }

src/output/json.rs — JSON Lines sink.

JsonSink<W: Write> wrapping BufWriter<W>
emit() uses serde_json::to_writer (or to_writer_pretty with --pretty)
One JSON object per line (JSON Lines format)

Phase 6: CLI + Collection Loop (Complete)

src/config.rs — Minimal hand-rolled CLI config.

Config { interval: Duration, count: u64, pretty: bool }
Config::from_args() — parses --interval SECS, --count N, --pretty, --help
No clap dependency

src/main.rs — Synchronous collection loop.

Parse config, create JsonSink on stdout
Loop: collect_tcp_sockets() -> build_batch() -> sink.emit() -> sleep
Stop after --count passes (0 = infinite)
Uses anyhow::Result for top-level error handling
No tokio — single loop with std::thread::sleep

Dependencies added

libc = "0.2" — sysctl FFI calls
thiserror = "2" — typed error enums in library code
anyhow = "1" — error handling in main.rs
hostname = "0.4" — hostname detection for metadata

Not added (intentionally): tokio, clap, tracing, byteorder, nix crate.

Verified operations

Command	Result
`nix develop -c cargo build`	Compiles on Linux (stubs) and macOS
`nix develop -c cargo clippy --all-targets -- -D warnings`	Zero warnings
`nix develop -c cargo fmt --check`	Clean
`nix develop -c cargo test`	8 tests pass (state mapping, conversion, parser with synthetic byte buffers)
On macOS: `cargo run -- --count 1`	Prints one JSON BatchMessage with live socket data
On macOS: `cargo run -- --count 3 --interval 2`	Prints 3 batches, 2 seconds apart
`nix build .#cross-x86_64-darwin`	Cross-compiles Mach-O x86_64 binary (~613 KB) on Linux
`nix build .#cross-aarch64-darwin`	Cross-compiles Mach-O arm64 binary (~633 KB) on Linux
`nix build .#cross-all`	Both targets in `result/bin/` named by triple
`nix run .#cross-aarch64-darwin`	Builds to `result-cross-aarch64-darwin/` automatically
`nix run .#build-cross-all`	Builds both targets with separate output dirs
`make cross-all`	Builds both targets via Makefile
Cross-compiled x86_64 binary on macOS 11.7.10	Runs successfully, collects live TCP socket data

File inventory

Makefile                         # Build targets: build, test, clippy, fmt, cross-*
src/
  lib.rs                       # Library root — 7 module declarations
  main.rs                      # Collection loop with anyhow error handling
  proto_gen.rs                 # Include generated prost + pbjson code
  config.rs                    # CLI arg parser (--interval, --count, --pretty)
  record.rs                    # RawSocketRecord intermediate type
  sysctl.rs                    # Sysctl reader with cfg-gated BSD/Linux
  convert.rs                   # RawSocketRecord -> proto TcpSocketRecord
  platform/
    mod.rs                     # CollectError, CollectionResult, dispatch
    macos.rs                   # pcblist_n parser (always compiled, collect() cfg-gated)
    macos_layout.rs            # XNU struct offset constants
    stub.rs                    # Linux stub
  output/
    mod.rs                     # OutputSink trait, OutputError
    json.rs                    # JSON Lines sink

Known risks and next steps

Struct offsets: The macos_layout.rs offsets are derived from XNU headers. They must be validated on a real macOS host by comparing parsed output to netstat -an. First run may need offset corrections — the isolated layout file makes this a single-file fix.
IPv4-mapped IPv6: Some sockets use ::ffff:a.b.c.d with vflag=0x2. Handle in convert.rs later if needed.
LISTEN sockets: No remote addr. Parser handles all-zeros gracefully.

Remaining phases

Module	Phase	Purpose
`delta.rs`	7	Per-connection delta tracking (retransmit rate, byte deltas)
`platform/macos.rs` (enrich)	8	TCP_CONNECTION_INFO getsockopt for richer per-socket data
`output/binary.rs`	9	Length-delimited binary protobuf output
System summary enrichment	10	tcp.stats sysctl for system-wide counters in SystemSummary
FreeBSD platform	11-15	pcblist parser, tcpstats kernel module, kern.file join

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macOS Implementation Status

Phase 1: Build Pipeline (Complete)

What was built

Issues found and resolved

Cross-compilation (Complete)

Phase 2: Sysctl Reader (Complete)

Phase 3: macOS pcblist_n Parser (Complete)

Design decisions

Phase 4: Record Types + Proto Conversion (Complete)

Phase 5: JSON Output (Complete)

Phase 6: CLI + Collection Loop (Complete)

Dependencies added

Verified operations

File inventory

Known risks and next steps

Remaining phases

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macos.md

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macos.md

File metadata and controls

macOS Implementation Status

Phase 1: Build Pipeline (Complete)

What was built

Issues found and resolved

Cross-compilation (Complete)

Phase 2: Sysctl Reader (Complete)

Phase 3: macOS pcblist_n Parser (Complete)

Design decisions

Phase 4: Record Types + Proto Conversion (Complete)

Phase 5: JSON Output (Complete)

Phase 6: CLI + Collection Loop (Complete)

Dependencies added

Verified operations

File inventory

Known risks and next steps

Remaining phases