Current limits

PXX is experimental. This page collects practical limits a user should know before treating a successful compile as a production-ready result.

General use

• Do not use PXX-built programs for security-sensitive, safety-sensitive, financial, legal, medical, infrastructure, or public network-facing workloads.
• The supported surface is what is tested in this repository. Uncovered FPC language or RTL behavior may compile incorrectly or not compile at all.
• Error messages are improving, but some unsupported constructs still fail with compiler-internal wording.
• The runtime reserves a 256 MiB heap arena. The first heap allocation in any PXX program maps 256 MiB in a single MAP_PRIVATE|MAP_ANONYMOUS request. Pages fault in lazily, so actual use stays small — a string-concatenating test program peaks at 392 KB resident and the compiler at about 15 MB compiling it — but the mapping is requested without MAP_NORESERVE, so a small VM, container, or memory-capped environment can refuse it, and the program then exits with pxx: out of memory (heap arena mmap failed) before doing any work. A program that allocates nothing (a writeln of a literal) makes no such request. Measured 2026-09-10 on x86-64 Linux.

Language and compatibility

• PXX does not implement the full Free Pascal language and RTL.
• {$mode objfpc} and -Mobjfpc are accepted as compatibility markers, not as a switch to a complete FPC semantic mode.
• Some FPC directives are accepted only as comments or compatibility markers.
• Range ({$R+}), overflow ({$Q+}), and IO ({$I+}) checking are implemented but opt-in per region — the lax default does not check. Many other compile-switch states are still accepted only as inert markers. See directives.
• The FPC package ecosystem is not bundled.

Targets

• Linux x86_64 is the primary path.
• Linux i386, aarch64, and arm32 are cross-output targets with growing test coverage.
• riscv32 and xtensa are embedded/ESP32-oriented targets. Treat them as active bring-up surfaces rather than stable general-purpose release targets.
• --shared (.so output) is x86-64 only, introduced for and validated with the .asm assembly-source frontend.

Libraries

• Library documentation describes the intended user-facing surface, but the RTL and PCL are still young.
• HTTPS requires a registered TLS backend. The OpenSSL backend is opt-in and depends on a system libssl.
• Some GUI examples require GTK/OpenGL development libraries and a display server.

Reporting gaps

If a documented example fails against the pinned compiler, file a progress ticket under devdocs/progress/backlog with the command, source, expected result, and actual result.

Next

• Command line
• FPC compatibility
• Targets