fpc-filexfer

Free Pascal library of FidoNet-era file-transfer engines. Sibling to fpc-binkp in the fpc-* library ecosystem: fpc-binkp handles the BinkP mailer protocol; fpc-filexfer handles everything that came before.

Currently at 0.2.0 (April 2026).

What's in it

Unit	Protocol	Spec
fx.xmodem	XModem / XModem-CRC / XModem-1K / YModem / YModem-G	Christensen/Forsberg
fx.sealink	SEAlink sliding-window	FSC-0019
fx.zmodem	ZModem / ZedZap / DirectZap	FTS-0018 / FTS-0020
fx.hydra	Hydra bidirectional	FTS-0057
fx.janus	Janus bidirectional	BinkleyTerm XE tradition
fx.nova	Nova bidirectional with SHA-256	Xenia Mailer (Ken Johnson)

Framework:

• fx.transport — IFXTransport interface (non-blocking byte stream). Shape matches fpc-binkp's IBPTransport with an added Flush for the modem-era byte-framing engines. A consumer can back both libraries onto one socket implementation via a thin adapter.
• fx.provider — IFXFileProvider interface (pluggable file I/O). Mirrors fpc-binkp's IBPFileProvider ownership contract: NextOutbound / AcknowledgeSent / AcknowledgeFailed for TX; OpenForReceive / FinalizeReceive / CleanupReceive for RX.
• fx.provider.fs — reference local-filesystem provider.
• fx.transport.tcp — reference UNIX TCP transport (non-UNIX consumers plug their own Watt-32 / WinSock / OS2 equivalent).
• fx.events — progress + session-end callback signatures.
• fx.crc — CRC-16 CCITT, CRC-16 SEAlink, CRC-16 reflected (Hydra), CRC-32 (ZModem-32 / Hydra / Nova).
• fx.sha — SHA-256 for Nova's content-addressed transfer verification.

TLogProc from fpc-log is used throughout; there is no per-library log shape.

Using it

Protocol engines are session classes; each has a Run, SendBatch, or ReceiveBatch entry point that takes a transport and a provider:

uses fx.types, fx.transport.tcp, fx.provider.fs, fx.sealink;

var
  Transport: IFXTransport;
  Provider:  IFXFileProvider;
  Session:   TFXSEAlinkSession;
  Result:    TFXResult;
begin
  Transport := TFXTcpTransport.CreateClient('peer.example', 24554);
  Provider  := TFXFsProvider.Create('/inbound', '/tmp/fx');

  Session := TFXSEAlinkSession.Create;
  try
    Result := Session.Run(Transport, Provider, @MyLogProc,
                          {IsOriginator=}True);
    if not Result.Success then
      Writeln('transfer failed: ', Result.ErrorMessage);
  finally
    Session.Free;
  end;
end.

See fx.zmodem.pas / fx.hydra.pas / fx.janus.pas / fx.nova.pas for the equivalent entry points on the other engines.

Building

./build.sh              # builds all supported targets
./build.sh x86_64-linux # single target

Supported targets: x86_64-linux, x86_64-freebsd, x86_64-win64, i386-linux, i386-freebsd, i386-go32v2, i386-os2, i386-win32 (8 total). The reference TCP transport (fx.transport.tcp) is UNIX-only; other targets get the library minus that unit.

Dependencies:

• FPC 3.2.2+
• fpc-log >= 0.1.0 (pulled via sibling directory ../fpc-log)
• paszlib (FPC RTL) for fx.nova's per-block ZLIB compression
• Nothing else. No external crypto library — SHA-256 is in-tree.

Testing

The in-tree tests/ harness ships a smoke test (test_crc) run via ./run_tests.sh. Cross-library parity against the Fimail sources is verified externally; downstream consumers should pin by tag and run their own integration harness.

The 0.2.0 release was a full re-extraction of all six protocol engines from the pre-extraction Fimail sources after the v0.1.x mechanical carve-out was discovered to have silently dropped 30–50 % of protocol code per file. See CHANGELOG.md for the per-protocol breakdown and source-of-truth references.

Ecosystem

fpc-filexfer is part of a family of Free Pascal libraries for FidoNet-related work:

• fpc-log — log interface (TLogProc)
• fpc-msgbase — message-base readers/writers
• fpc-ftn-transport — PKT / BSO / bundles / TIC
• fpc-binkp — BinkP mailer
• fpc-comet — Comet native mailer
• fpc-filexfer — this library — pre-BinkP transfer engines
• fpc-emsi — EMSI / WaZOO / FTS-0001 session handshakes

fpc-emsi sits above fpc-filexfer: once its dispatcher has identified which transfer engine the peer wants, the caller hands a matching fx.* session off to run the transfer.

Licence

Translations of protocol code (sealink, zmodem, hydra, janus) are line-for-line ports of public-domain or GPL implementations (Xenia Mailer, BinkleyTerm XE). Nova is original. See each unit's header for attribution.

This library is Free Pascal code. Use it.