Unix Compatibility

Unix Compatibility

XAIOS is a freestanding Unix-like operating system with its own syscall and userspace ABI. FreeBSD is the primary external behavioral reference for portable command, SSH/SFTP and network work.

That is a design and test policy, not a lineage claim. XAIOS is not derived from FreeBSD, does not implement the FreeBSD kernel ABI, and does not implement a Linux binary ABI either. Protocol interoperability is deliberately operating-system independent: a FreeBSD or Linux OpenSSH client passing a gate proves the tested wire behavior and nothing about binary compatibility with that client's OS.

What is and is not implemented

Surface	Status	Boundary
XAIOS kernel and userspace	Native freestanding ABI	C99 code uses XAIOS syscalls and services. It depends on neither glibc, Linux syscalls, FreeBSD libc, nor either host kernel.
Unix command behavior	Portable subset, QEMU-tested	Local and SSH sessions are stateful: independent working directory, prompt, command errors, recursive trees, interactive nano, alternate-screen less. Pipes and redirection are bounded shell features. XAIOS is not POSIX-certified and provides no general POSIX process environment.
FreeBSD client interoperability	QEMU-tested subset	An official FreeBSD 15.1 AArch64 VM uses base-system OpenSSH/SFTP and nc against XAIOS public-key login and rejection, xaiosctl, SFTP operations, interactive PTY ANSI xtop, and UDP echo.
Linux client interoperability	QEMU-tested subset	A disposable Debian 13 container is an independent OpenSSH/SFTP/network client for the broader administration, password, stateful shell, interactive nano, recursive filesystem, rekey, concurrency and malformed-traffic suite.
Linux binary ABI	Not implemented	XAIOS runs no Linux ELF programs and implements no Linux syscalls, procfs, namespaces, cgroups or distribution package semantics.
FreeBSD binary ABI	Not implemented	XAIOS runs no FreeBSD ELF programs and implements no FreeBSD syscalls, jails, rc.d, ports/pkg or kernel interfaces.
Hosted inference engine	macOS and Linux hosts	The portable engine builds as a native process on macOS and Linux. That host support does not define the XAIOS guest ABI.

Current evidence

Client	Current QEMU evidence
FreeBSD 15.1 AArch64	Official checksum-pinned VM; OpenSSH public-key acceptance and rejection, xaiosctl, SFTP write/stat/read/rename/remove, interactive PTY ANSI xtop, and UDP echo.
macOS	Native OpenSSH/SFTP administration and one-guest parallel load coverage.
Debian 13	Independent Linux/OpenSSH cross-client coverage for password policy, stateful shell prompts, interactive nano, recursive filesystem operations, command errors, rekey, administration, persistence, concurrency, interactive PTY xtop and malformed network traffic.

The command subset covers ls, cd, pwd, mkdir, rm, cp, mv, cat, alternate-screen less, grep, find, ps, df, du, tar, cpio, zip, unzip, outbound ssh, and recursive scp. The exact options each one accepts are on Applications; the session commands the kernel itself owns are on Commands. Those two pages are the command surface, and this page does not restate them.

What matters here is the shape of the subset rather than its contents. An option that is not implemented fails explicitly; nothing is silently accepted and ignored, because a flag that appears to work and does nothing is worse than one that is refused. -- ends option parsing where a Unix reader expects it. grep implements basic regular expressions — ., *, ^, $ and escapes — not extended or Perl syntax, and find matches * and ? in a filename rather than a full glob language. Outbound ssh supports one password-authenticated -J hop; multi-hop -J, -J with agent authentication, ProxyCommand, and the complete OpenSSH algorithm matrix are not implemented.

Archives, and the limits underneath them

POSIX ustar and PAX paths, GNU long-name records, validated single-member gzip/DEFLATE, and stored or Deflate ZIP entries from Unix, Windows and macOS tools are all read. Creation is ustar, newc cpio, and stored ZIP with CRC32 and Unix attributes. Multi-member gzip, gzip creation, encryption, ZIP64, symlinks and device nodes are rejected rather than partially handled. Extraction also rejects absolute paths, .., Windows drive prefixes, backslashes, checksum failures, integer overflow and unsupported entry types.

The container limits people meet in practice are XAIOS storage limits, not archive-format limits, and it is worth keeping the two apart when a transfer fails. xaibootFS bounds paths to 256 bytes and open handles to 256, and the file API caps a write at 256 KiB — below what the on-disk format itself allows — so an archive has to fit in a 256 KiB file whatever tar would otherwise accept. Filesystem and Storage carries the current figures.

Interoperability evidence includes exact-content round trips for XAIOS-created ustar and ZIP through macOS and Debian 13 readers, macOS PAX tar and Debian GNU long-name tar into XAIOS, Windows-origin ZIP metadata with Deflate data into XAIOS, and recursive XAIOS-to-Debian and Debian-to-XAIOS scp over OpenSSH SFTP. The Windows case validates the standard container contract only; no physical Windows OpenSSH client was available for that gate.

The FreeBSD reference gate

make qemu-freebsd-network-suite
make qemu-freebsd-bidirectional-suite

The first target downloads the official FreeBSD 15.1 AArch64 BASIC-CLOUDINIT-ufs.qcow2.xz into ~/.cache/xaios/freebsd, verifies the pinned SHA-256 of both the compressed archive and the decompressed image before use, creates a disposable copy-on-write overlay and a cidata disk, and boots that beside one XAIOS guest. Set XAIOS_FREEBSD_IMAGE to an already-verified uncompressed QCOW2 to skip the download.

The seed disables FreeBSD's two first-boot updater services so the run is deterministic and bounded; it does not modify the cached vendor image. XAIOS stays on TCG for correctness, while the FreeBSD client defaults to HVF on macOS and TCG elsewhere. The run leaves build/qemu-freebsd-client.log, build/qemu-freebsd-xaios.log and build/qemu-freebsd-network-suite.json.

The bidirectional gate puts the same FreeBSD VM inside a Dockerized QEMU harness, starts base-system sshd, and drives XAIOS's own outbound SSH and recursive SFTP-backed SCP client in both transfer directions. Docker supplies process isolation and repeatable QEMU dependencies there; it does not substitute a Linux kernel for FreeBSD. See Networking and SSH for the complete local and Intel VPS contract.

The rule for new code

Guest code should use XAIOS interfaces, or a Unix semantic chosen and documented deliberately. Do not import a Linux-only API because Linux happens to be a CI host, and do not copy FreeBSD internals because FreeBSD happens to be the behavioral reference. A future POSIX, FreeBSD or Linux compatibility layer has to be explicit, separately versioned and separately tested — never inferred from matching command names or from protocol interoperability that already works.