Admin/PLATFORMS

merged, with minor edits: Admin/PLATFORMS, CONTRIBUTORS;

Multi-platform support of Isabelle
==================================

Preamble
--------

The general programming model is that of a stylized ML + Scala + POSIX
environment, with a minimum of system-specific code in user-space
tools.

The Isabelle system infrastructure provides some facilities to make
this work, e.g. see the ML and Scala modules File and Path, or
functions like Isabelle_System.bash.  The settings environment also
provides some means for portability, e.g. the bash function
"platform_path" to keep the impression that Windows/Cygwin adheres to
Isabelle/POSIX standards, although Poly/ML and the JVM are native on
Windows.

When producing add-on tools, it is important to stay within this clean
room of Isabelle, and refrain from non-portable access to operating
system functions. The Isabelle environment uses peculiar scripts for
GNU bash and perl as system glue: this style should be observed as far
as possible.


Supported platforms
-------------------

A broad range of hardware and operating system platforms are supported
by building executables on base-line versions that are neither too old
nor too new. Common OS families work: Linux, Windows, macOS, but
exotic ones are unsupported: BSD, Solaris, NixOS.

Official (full support):

  x86_64-linux      Ubuntu 14.04 LTS

  x86_64-darwin     macOS 10.13 High Sierra (lapbroy68 MacBookPro11,2)
                    macOS 10.14 Mojave (mini2 Macmini8,1)
                    macOS 10.15 Catalina (laramac01 Macmini8,1)
                    macOS 11.1 Big Sur (mini1 Macmini8,1)

  x86_64-windows    Windows 10
  x86_64-cygwin     Cygwin 3.1.x https://isabelle.sketis.net/cygwin_2021 (x86_64/release)

New (experimental):

  arm64-linux       Raspberry Pi OS 64bit beta (Debian 10 / Buster)

  arm64-darwin      macOS 11.1 Big Sur


64 bit vs. 32 bit platform personality
--------------------------------------

Isabelle requires 64 bit hardware running a 64 bit operating
system. Only Windows still supports native x86 executables, but the
POSIX emulation on Windows via Cygwin64 works exclusively for x86_64.

The Isabelle settings environment provides variable
ISABELLE_PLATFORM64 to refer to the standard platform personality. On
Windows this is for Cygwin64, but the following native platform
identifiers are available as well:

  ISABELLE_WINDOWS_PLATFORM64
  ISABELLE_WINDOWS_PLATFORM32

These are always empty on Linux and macOS, and non-empty on
Windows. For example, this is how to refer to native Windows and
fall-back on Unix (always 64 bit):

  "${ISABELLE_WINDOWS_PLATFORM64:-$ISABELLE_PLATFORM64}"

And this is for old 32 bit executables on Windows, but still 64 bit on
Unix:

  "${ISABELLE_WINDOWS_PLATFORM32:-$ISABELLE_PLATFORM64}"


Dependable system tools
-----------------------

The following portable system tools can be taken for granted:

* Scala on top of Java.  Isabelle/Scala irons out many oddities and
  portability issues of the Java platform.

* GNU bash as uniform shell on all platforms. The POSIX "standard"
  shell /bin/sh does *not* work portably -- there are too many
  non-standard implementations of it. On Debian and Ubuntu /bin/sh is
  actually /bin/dash and introduces many oddities.

* Perl as largely portable system programming language, with its
  fairly robust support for processes, signals, sockets etc.


Known problems
--------------

* macOS: If MacPorts is installed there is some danger that
  accidental references to its shared libraries are created
  (e.g. libgmp).  Use otool -L to check if compiled binaries also work
  without MacPorts.

* macOS: If MacPorts is installed and its version of Perl takes
  precedence over /usr/bin/perl in the PATH, then the end-user needs
  to take care of installing extra modules, e.g. for HTTP support.
  Such add-ons are usually included in Apple's /usr/bin/perl by
  default.

* Common Unix tools like /bin/sh, /bin/kill, sed, ulimit are
  notoriously non-portable an should be avoided.

* The traditional "uname" Unix tool only tells about its own executable
  format, not the underlying platform!