The threading surface is not where FPC code looks for it

Type: compat (RTL parity) — Track B (lib/rtl)
Status: backlog
Opened: 2026-08-05
Found by: writing the tools/fpc_diff_probe.sh thread cases. Every one needed a {$IFDEF FPC} split in its uses line before it would build on both sides — which is the finding.

pxx has a real, working native TThread (lib/rtl/palthreadobj.pas, futex/PAL based, no libc). The gap is entirely in where the names live and what they look like, i.e. the mission's "compile real-world code as-is" line.

The four differences

TThread is in palthreadobj, not Classes. Every threaded FPC/Delphi program says uses Classes. Making Classes re-export TThread is the obvious fix and has a real cost to weigh: classes currently does not depend on the thread machinery, and pulling palthreadobj in makes every Classes-using program carry it. Worth a Track U call if the size/dependency trade-off is not obvious — do not just do it.
No cthreads unit. On Unix, FPC code must have cthreads first in the program's uses, so portable sources start with {$IFDEF UNIX}cthreads,{$ENDIF}. pxx has no thread manager to install, so an empty compat shim unit would cost nothing and let those sources through unedited.
WaitFor is a procedure. FPC's is function WaitFor: LongWord, returning the thread's ReturnValue, and r := t.WaitFor is the common idiom. Today pxx compiles that expression and yields garbage — but that is a separate and much worse bug, filed as [[bug-p-procedure-method-in-an-expression-yields-garbage]]. Fixing that one turns this into an honest compile error; fixing this one makes the idiom work.
BeginThread / EndThread / TThreadID / WaitForThreadTerminate do not exist. The low-level FPC thread API. palthread has the machinery under its own names.

Already done, separately

InterLockedIncrement and family were missing entirely and now exist in lib/rtl/palatomic.pas, verified identical to FPC across all eleven return-value cases. They still need an explicit uses palatomic where FPC has them in system — [[bug-a-interlocked-family-needs-a-uses-clause-unlike-fpc]].

Coverage

Six thread cases in tools/fpc_diff_probe.sh (thread-*). The ones this ticket blocks are tagged [known]; thread-tthread-waitfor already passes, so the core Create/Start/WaitFor/Finished path is verified against FPC today.

Gate

Track B: make lib-test / tools/gate.sh lib. If Classes gains the re-export, re-check binary size on a Classes-only demo before and after.