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Modula-3 Frequently Asked Questions (FAQ)

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Archive-name: Modula-3-faq

See reader questions & answers on this topic! - Help others by sharing your knowledge
   Michel Dagenais Michel Dagenais, GNU General Public License, 1998-2001
    Ecole Polytechnique
    C.P. 6079, Succ. Centre-Ville
    Montreal, Quebec, H3C 3A7
                Modula-3 Frequently asked questions and answers
   Maintained by Michel Dagenais ([1],
   suggestions are most welcome. Last updated January 15 2002. The latest
   copy of this FAQ may be obtained from the [2]Polytechnique Montreal
   Modula-3 Home.

What is Modula-3?

   Modula-3 is a systems programming language that descends from Mesa,
   Modula-2, Cedar, and Modula-2+. It also resembles its cousins Object
   Pascal, Oberon, and Euclid.
   The goal of Modula-3 is to be as simple and safe as it can be while
   meeting the needs of modern systems programmers. Instead of exploring
   new features, we studied the features of the Modula family of
   languages that have proven themselves in practice and tried to
   simplify them into a harmonious language. We found that most of the
   successful features were aimed at one of two main goals: greater
   robustness, and a simpler, more systematic type system.
   Modula-3 retains one of Modula-2's most successful features, the
   provision for explicit interfaces between modules. It adds objects and
   classes, exception handling, garbage collection, lightweight processes
   (or threads), and the isolation of unsafe features.
Where is Modula-3 used? Is it used in industry?

   A number of programming teams selected Modula-3 for industrial and
   research projects, and for teaching. It encourages good programming
   practices and comes with excellent libraries for distributed
   programming and graphical user interfaces. A non exhaustive list is
   available at [3][Modula-3 at Work].
Is commercial support available?

   Critical Mass Corporation used to offer their own version of SRC
   Modula-3, CM3, an integrated development environment for Modula-3,
   [4]Reactor, as well as training and consulting services.
   Olaf Wagner from [5]Elego Software Solutions is now maintaining [6]CM3
   as an open source package and offers commercial support.
Where can I get documents and information on Modula-3?

   A [7]concise bibliography and a more complete [8]bibliography describe
   Modula-3 related books, technical reports, and papers. The definition
   of Modula-3 is contained in: [9]"System Programming with Modula-3"
   also known as SPwM3. Sam Harbison has written a more tutorial book
   titled [10]Modula3.
   Three main Web servers contain Modula-3 related information:
   [11], [12]DEC SRC Modula-3 home page, and [13]Ecole
   Polytechnique de Montré Modula-3 home page.
   The Usenet newsgroup comp.lang.modula3 is the official meeting place
   for Modula-3 related discussions.
Is Modula-3 a superset of Modula-2?

   No; valid Modula-2 programs are not valid Modula-3 programs. However,
   there is a tool to help convert Modula-2 programs to Modula-3.
Comparisons between Modula-3 and other languages?

   From: (Laszlo BOESZOERMENYI)
   A Comparison of Modula-3 and Oberon-2 by myself in Structured
   Programming 1993, 14:15-22
   Robert Henderson, Benjamin Zorn, A Comparison of Object-Oriented
   Programming in Four Modern Languages, Department of Computer Science,
   University of Colorado, Boulder, Colorado, [14]Technical Report
   The paper evaluates Oberon, Modula-3, Sather, and Self in the context
   of object-oriented programming. While each of these programming
   languages provide support for classes with inheritance, dynamic
   dispatch, code reuse, and information hiding, they do so in very
   different ways and with varying levels of efficiency and simplicity. A
   single application was coded in each language and the experience
   gained forms the foundation on which the subjective critique is based.
What implementations are available, how do they compare?

   All implementations are based on [15]DEC SRC Modula-3. [16]Critical
   Mass offered an improved version with commercial support. It features
   incremental garbage collection on NT, and a few additional packages
   like ODBC database access. This is now [17]open sourced and maintained
   by Olaf Wagner. Ecole Polytechnique de Montréal has been maintaining
   an [18]updated distribution. It features integrated documentation, and
   NT support through the gcc cygwin compiler
Can I contribute Modula-3 software?

   Contributions are most welcome. The primary contact to offer
   contributions is comp.lang.modula3.
   [19]The Ecole Polytechnique de Montréal Modula-3 distribution is the
   most regularly updated and may be a good place to submit your
Why use Modula-3?

   Here is what John Polstra, author of the popular CVSup, replied:
Subject: Re: SUP on
Date:Wed, 06 Nov 1996 12:31:26 -0800
From:John Polstra
> Erhm, why on earth did you chose Modula3 ??

   Modula-3 really is a different language, designed specifically for
   systems programming by some extremely competent and experienced people
   who knew what they were doing.
> Oh and yes I have seen apps written in modula3, all of which was
> horrible performers, and impossible to port to new platforms, so
> the management decide a complete rewrite in, guess what, C!

   Are you sure it was Modula-3? The SRC Modula-3 compiler supports about
   25 different platforms.
   Plenty of real world apps (*big* ones) have been written in Modula-3,
   and they perform pretty well. There's also the SPIN OS project
   [20][SPIN] at University of Washington, in which the kernel was
   written in Modula-3. It performs well, too.
   Now, you can always argue that a program would be somewhat faster and
   somewhat smaller if it had been written in C. Hey, guess what? I was
   around when Unix V6 came out, and the same stuff was written about it.
   Just substitute "C" for "Modula-3" and "assembly language" for "C".
   The answer is the same in both cases: Unix would not exist as we know
   it today if it had been written in assembly language. CVSup would not
   exist as we know it today if it had been written in C (or C++, for
   that matter).
   OK, so why on earth did I choose Modula-3? In no particular order:
    1. I needed application level threads, and threads are an integral
       part of the Modula-3 language. About the only reasonable
       alternative was to use pthreads with C or C++. But pthreads was
       not well supported under FreeBSD at that time.
    2. I needed a graphical display during development so that I could
       monitor the 3 client threads as they were running, debug them,
       appraise their relative performance, and find the bottlenecks.
       Modula-3 has a very nice toolkit for creating GUIs quickly and
       painlessly. (OK, so the scrollbars are as ugly as sin.)
    3. Modula-3 is a compiled language that is reasonably efficient.
    4. I needed to use some low level system functions, e.g., mapping
       files into memory. Modula-3 provides good access to such
       functions, and it is quite easy to add interfaces to foreign
       libraries such as "libz".
    5. Modula-3 has good support for networking.
    6. It is a mature and stable language that has been used in a number
       of serious, large projects. The language and compiler have been
       stable for about 5 years, which is more than you can say for C++.
    7. It has nice support for object oriented programming, including a
       good type system, a nice exception model, and a modern
       high-performance garbage collector. These traits, IMHO, contribute
       powerfully to producing well-structured, maintainable programs.
       Now before you label me an unstudly OO weenie, please consider
       this. I've been programming in C professionally for 19 years. I
       made my living for many years writing C compilers and related
       tools such as assemblers, linkers, and disassemblers. I still use
       C and C++ when I feel they are appropriate for a project, not to
       mention when I have to because that's what the client wants to
       use. I have experience programming in many many different
       languages. Different languages are good for different things. I
       still like programming in C (and C++ for some things), but I'm
       glad I didn't use it for CVSup.
    8. I had just come off a huge 3+ year C++ project. During that time,
       I learned just how much C++ sucks. I did not feel like doing it
       again right away "for fun."
    9. I have spent my entire professional career getting paid to use the
       wrong tools, because, e.g., the manager read that C++ was
       "popular." For once, just once, on a _hobby_ project, I decided I
       was going to use the tool I felt was the best for the job at hand.
       I thought about it long and hard, evaluated several options (C and
       C++ among them), and eventually chose Modula-3. I have never
       regretted that decision.
   Any questions? :-)
- --
   John Polstra                             
   John D. Polstra & Co., Inc.                Seattle, Washington USA
   "Self-knowledge is always bad news."                 -- John Barth


Why program receives a SEGV signal under the debugger?

   The garbage collector on some platforms uses the SEGV (segmentation
   violation) signal to detect modified portions of the dynamically
   allocated space. It is possible to disable this feature or to inform
   the debugger to let these signals propagate. See the [21]m3gdb
Problems with threads, fork and VTALARM?

   The threads are implemented using the virtual timer interrupt.
   Normally, the run time environment will catch the interrupt and
   determine if thread switching is appropriate. However, if a new
   process is created with fork, it will have the virtual timer activated
   and no interrupt handler to receive it, resulting in a core dump. If
   you use the standard procedure Process.Create to fork new processes,
   this will be handled automatically for you. If you insist on using
   fork, you need to disable the timer, fork and then reenable the timer.
X libraries not found?

   The position of X libraries is stored, for instance for pre-compiled
   PM3 LINUXELF binaries, in the template file m3config/src/LINUXELF as
   well as in X11/LINUXELF/.M3EXPORTS (m3build/templates/LINUXELF, and
   X11R4/LINUXELF/.M3EXPORTS for SRC-M3). Thus you may want to edit these
   files if your X libraries are located in an uncommon place.
What means Missing RTHooks or similar messages?

   The standard library, libm3, is not included by default. You need in
   your m3makefiles to import("libm3") or to import a library which
   imports libm3. Otherwise, messages about run time procedures such as
   RTHooks not being available are produced.
M3build versus Make or why m3 does not work?

   The Modula-3 compiler m3 does a much finer grained dependency analysis
   than possible with make. For this reason, a very flexible front end,
   m3build, reads the program description files, m3makefile, and
   generates the commands required to compile and link Modula-3 programs
   and libraries. The m3makefile content is documented in the m3build
   documentation. Calling the m3 compiler directly is difficult and thus
   not recommended, especially on PM3 where it is now merged with
Why are exceptions raised by X or Network Objects applications?

   Graphical applications (based on Trestle/X Windows) raise the
   TrestleComm.Failure exception when the DISPLAY environment variable is
   incorrect or the X server is refusing the connection. They raise
   MachineIDPosix.Failure if the network configuration files are
   incorrectly set up, especially on LINUX; /etc/hosts must contain at
   least a loopback address ( and the /etc/rc scripts an
   appropriate ifconfig command (/etc/ifconfig lo; /etc/route
   add Applications with Network Objects may also raise
   exceptions or consume all the CPU time available when the network
   configuration files are incorrect.
What is the story with Trestle and OpenWindows?

   Mark Manasse says:
   I think that the OpenWindows release should be enough (no need to get
   the MIT X release), although there are a few things in Trestle that
   trigger devastating bugs in OpenWindows. But the only library we
   depend on is Xlib, R4 or later.
   The main thing I know that crashes OW 2.0 is the code where we call
   GrabKey specifying AnyKey. You can either loop over all of the keys,
   or you can just comment out the call; programs won't run exactly the
   same, but you probably won't notice the difference.
Why so many problems installing on Solaris?

   These notes were contributed by ( while
   installing PM3-1.1.14 on a Sun Ultra 5 running Solaris 2.8. They
   describe various problems and their solution or workaround.
   The installation of PM3 on Solaris systems is particularly prone to
   problems as these systems tend to be an unpredictable (from the point
   of view of the PM3 people) mixture of Sun and Gnu software --- Sun do
   not bundle a C compiler with the operating system.
   My machine has gcc version 2.95.2 installed; it has Sun's versions of
   make, ld, as and ar installed within /usr/ccs/bin; Gnu's version of
   these tools are not installed.
   My installation was successful, after a bit of fiddling around with
   the configuration/template files and environment variables. Some of
   the fixes are trivial (if you know what you are doing), while others
   --- for me at least --- were not --- I am a Modula 3 novice and far
   from experienced with Solaris.
   The issues that arose are:
     * Paths needed to be set to find tools such as make.
     * LD_LIBRARY_PATH needed to be set to ensure libstdc++.a.2.10.0
       and/or were found.
     * A link needed to be set so that byacc points to yacc.
     * The configuration for linking needed to be changed since only the
       Sun version of ld was installed, not Gnu's.
     * The build of m3gdb failed to build.
     * The gnuemacs package failed to build.
Initial Problems

          gcc is usually installed in /usr/local/bin; on a Solaris
          machine, ar, as, make and ld are all in /usr/ccs/bin, by
          default. Hence these must both be on root's path (assuming you
          are installing as root). Neither were; I have not changed any
          paths since installation of Solaris 2.8 on a new machine a few
          days ago.
          This is in addition to /usr/local/pm3/bin, as mentioned by the
          PM3 installation instructions.
          The build required byacc. yacc is installed in /usr/ccs/bin; a
          soft link:
     lrwxrwxrwx   1 root     other          4 Aug 11 15:45 byacc -> yacc

          solved this problem.
   Library Paths
          In addition to /usr/local/pm3/lib/m3 as mentioned by the PM3
          installation instructions LD_LIBRARY_PATH must include
          /usr/local/lib so that libstdc++ (part of the gcc distribution)
          can be found.
          In addition I found that the environment variable CC needed to
          be set to /usr/local/bin/gcc. This is of course mentioned in
          the PM3 installation instructions.
Miscellaneous Questions

Can I get Modula-3 other than by FTP or HTTP?

   Prime Time Freeware (PTF) includes Modula-3. PTF is a set of two
   ISO-9660 CDroms filled with 3GB of freeware, issued semi-annually. PTF
   is distributed via bookstores and mail. You can reach PTF using:
        Fax:    [1] (408) 738 2050
        Voice:  [1] (408) 738 4832
        Mail:   Prime Time Freeware
                415-112 N. Mary Ave., Suite 50
                Sunnyvale, CA 94086

   Many Linux CDroms include a copy of the FTP site which
   has Linux binaries for Modula-3.
How to call Modula-3 procedures from a C program?

   Calling Modula-3 from C is tricky because M3 has a more elaborate
   run-time environment. The simplest solution is to make the main
   program M3 and then call C via EXTERNAL routines. Calling back into M3
   is then relatively straightforward.
   Here's an example. It calls the C code to lodge the identity of the M3
   procedure to be called back which avoids having to know the actual
   name used by the linker.
   First a little M3 module to be called from C (M3code), then a C module
   called by the M3 main and calling the M3 module (Ccode), and finally
   the main program (Main):
(* M3code.i3 *)

IMPORT Ctypes;
PROCEDURE put (a: Ctypes.char_star);
END M3code.

(* M3code.m3 *)

IMPORT Ctypes, IO, M3toC;

PROCEDURE put (a: Ctypes.char_star) =
    IO.Put (M3toC.StoT (a) & "\n");
  END put;

END M3code.

(* Ccode.i3 *)

IMPORT Ctypes;
PROCEDURE set (p: PROCEDURE (a: Ctypes.char_star));
PROCEDURE act (a: Ctypes.char_star);
END Ccode.

/* Ccode.c */

typedef void (*PROC)();
static PROC action;

void set (p)
  PROC p;
    action = p; /* register the M3 procedure */

void act (a)
  char *a;
    action (a); /* call the M3 procedure */

(* Main.m3 *)


IMPORT Ccode, M3code, M3toC;

  Ccode.set (M3code.put);
  Ccode.act (M3toC.TtoS ("Hello world"));
END Main.

(* m3makefile *)


interface ("Ccode")
c_source ("Ccode")
module ("M3code")

Can Modula-3 code call C++ and vice-versa?

   There is no problem to call C++ functions declared as extern C. You
   must use a C++ aware linker (e.g. the C++ compiler). A complete
   example of M3 calling C++ objects, which in turn call M3 callbacks, is
   available in [22]the sgml library.
   On some platforms, a call to get the static variables constructors
   called may be required:
   From: gwyant@cloyd.East.Sun.COM (Geoffrey Wyant - Sun Microsystems
   Labs BOS)
   You must use your C++ compiler as the linker, rather than /bin/cc or
   You need to call the function '_main'. The easiest way to do this is
   to have the following set of interfaces and implementations:
        INTERFACE CXXMain;
          <*EXTERN "_main"*> CxxMain;
        END CXXMain;

        MODULE CXXMain;

   and then import CXXMain into your M3 main module. This will ensure
   that the C++ function _main gets called.
How to copy heap objects?

   Deep copies are easily performed using Pickles. An object graph is
   Pickled to a text writer into a TEXT. Then, a copy is created by
   unpickling a new object graph from a text reader created from the
   Shallow copies are less often needed but may be performed with the
   following procedure:
    tc     := TYPECODE (r);
    n_dims : INTEGER;
    res    : REFANY;
    shape  : RTHeapRep.ArrayShape;

    (* allocate a new object of the same type (and shape) as the old one *)
    RTHeapRep.UnsafeGetShape (r, n_dims, shape);
    IF (n_dims <= 0)
      THEN res := RTAllocator.NewTraced (tc);
      ELSE res := RTAllocator.NewTracedArray (tc, SUBARRAY(shape^, 0, n_dims));

    (* copy the old data into the new object *)
    RTMisc.Copy (RTHeap.GetDataAdr (r), RTHeap.GetDataAdr (res),
                 RTHeap.GetDataSize (r));

    RETURN res;
  END Duplicate;

How to get output messages to appear immediately (flushing writers)?

   Modula-3 Writers are buffered. Thus, you need to issue a Wr.Flush when
   the output should appear immediately, for instance to prompt the user
   for some input. Since this can become annoying, libraries in other
   languages sometimes offer the option of unbuffered writes. In
   Modula-3, an equivalent behavior is obtained with AutoFlushWr which
   gets a background thread to flush a writer at a specified interval.
How to read a single character as soon as typed?

   Characters typed on the keyboard are usually buffered. They become
   visible to the reading program only when the buffer is full or after,
   for example, a carriage return is received. This is not specific to
   Modula-3. To access the characters as they are typed, single character
   commands in a full screen editor for example, the input reader must be
   configured properly.
   From: [23] (Richard Watts)
   The POSIX way of doing it is to use tcsetattr(), and here is some code
   that does it under Solaris 2.x (this was written for serial ports, but
   the same thing applies) :
PROCEDURE Open(port : CHAR; timeout : INTEGER := 30) : T RAISES {Error} =
    term : TcPosix.termios;
    file : TEXT;
    fd : T;
    rc : INTEGER;
    (* Figure out which device we want to open : *)

    CASE port OF
      'A' => file := "/dev/ttya";
    | 'B' => file := "/dev/ttyb";
    ELSE RAISE Error("Invalid port " & Fmt.Char(port) & " specified.\n");

    (* Open it. 700 is a good default mode for serial ports. *)
    fd :=,  Unix.O_RDWR
                                            , 8_700);
    IF fd = -1 THEN
      RAISE Error("Open() on " & file & " failed.\n");

    (* Get the termios structure for it *)
    rc := TcPosix.tcgetattr(fd, ADR(term));
    IF rc # 0 THEN
      EVAL Unix.close(fd);
      RAISE Error("Couldn't get terminal attributes for " & file & ".\n");

    (* Modify the termios structure *)

    (* The default baud rate is right, but we'd better set it anyway
       in case someone left it set up wrong : *)
    rc := TcPosix.cfsetospeed(ADR(term), TcPosix.B9600);

    IF rc # 0 THEN
      EVAL Unix.close(fd);
      RAISE Error("Couldn't set output speed for " & file & "\n");

    rc := TcPosix.cfsetispeed(ADR(term), TcPosix.B9600);

    IF rc # 0 THEN
      EVAL Unix.close(fd);
      RAISE Error("Couldn't set input speed for " & file & "\n");

    (* Modify the line discipline - reset ECHO and ICANON *)
    term.c_lflag := Word.And( term.c_lflag,
    term.c_cc[TcPosix.VMIN] := 0;
    term.c_cc[TcPosix.VTIME] := 0; (* Set up timing right *)

    (* Now reset the terminal attributes *)
    rc := TcPosix.tcsetattr(fd, TcPosix.TCSANOW, ADR(term));

    IF rc # 0 THEN
      EVAL Unix.close(fd);
      RAISE Error("Can't set attributes for " & file & "\n");
    RETURN fd;
  END Open;

   (TcPosix.i3 is one of my interfaces, not libm3's, and I'll supply it
   if you like, but it's just a wrapper to tcgetattr and friends. The
   baud rate stuff shouldn't be necessary for terminals (or serial
   ports..) ). You should be able to somehow get an Rd.T out of this, I
   think, but it may involve a bit of hacking. The University of
   Cambridge can't have these opinions even if it wants them.
Why is Hello World larger in Modula-3 than in C?

   Modula-3 programs are slightly larger than C programs because the
   generated code includes runtime type information, and runtime checks
   for out-of-bound array references and NIL pointers. Many of these
   checks could be removed by a more sophisticated compiler.
   The fixed runtime is substantially larger (there is no runtime support
   in C). It contains a garbage collector, a thread runtime, and
   exception support. It is typically placed in a dynamically linked
   library, shared on disk and in memory between all the Modula-3
What is SRC Modula-3?

   [24]SRC-Modula-3 was built by the DEC Systems Research Center and is
   freely available and redistributable, with source code. In Europe it
   is also available from in
   pub/Modula-3. The most recent version is release 3.6
   The DEC SRC Modula-3 contains the following:
     * A native code compiler: uses the GCC backend; on
       machines/operating systems that have self-describing stacks, an
       optimized exception handling mechanism is provided, on other
       architectures, setjmp/longjmp is used. A very fast integrated
       backend is available on some platforms (currently NT386 and Linux
       The compilation system provides for minimal recompilation. Only
       those units that depend on the modified interface item will be
     * m3build: tool that performs dependency analysis and builds the
       Modula-3 programs and libraries.
     * m3gdb: a Modula-3 aware version of GDB.
     * Several tools for performance and coverage analysis.
     * A large standard library (libm3) providing
          + A multithread, incremental, generational, conservative
            garbage collector
          + Text manipulation.
          + Generic Containers: Lists, Sequences, Tables, SortedLists,
          + Atoms and Symbolic expressions (Lisp like lists)
          + An extensible stream IO system
          + Typesafe binary object transcription (persistent objects)
          + Operating system interfaces
          + Portable interfaces to the language runtime
       All standard libraries are thread-friendly. Modula-3 can readily
       link with existing C libraries; many libraries including X11R4 and
       various UNIX libraries are available as part of libm3.
     * Several other libraries for designing graphical user interfaces
       and distributed applications.
Why are there strange pragmas for Locking levels and other properties?

   The Trestle (ui library) interfaces contain Locking level pragmas. The
   base interfaces (libm3 library) contain SPEC pragmas. These are not
   processed by the compiler. Instead the Extended Static Checker,
   currently under development at DEC SRC, will report on problems
   detected based on the program content and the information specified in
   these pragmas [25][ESC]. The Extended Static Checker is not yet
   available, it may be some time in the future.
Design Issues

Why objects and interfaces?

   Allan Heydon on comp.lang.modula3, May 4th 1993:
   Modula-3 provides two separate mechanisms for data-hiding: one for
   hiding details about how interfaces are implemented, and the other for
   hiding details about how objects are implemented.
   The first data-hiding mechanism is realized by the distinction between
   interfaces and modules. Clients can only import interfaces, so the
   names declared in the modules implementing those interfaces are hidden
   from clients. Note that this mechanism has only two levels; a name is
   either declared in an interface, or it isn't. If a name is only
   declared in a module, it can't be used by a client.
   The second data-hiding mechanism is realized by opaque types and
   revelations. A Modula-3 interface may declare an object type to be
   opaque, in which case only a subset of the fields and methods of that
   object are revealed to clients importing the interface. Furthermore,
   the Modula-3 revelation mechanism allows a designer to reveal
   successively more fields and methods of an object in a series of
   interfaces. The fields and methods visible to a client then depends on
   which interfaces the client imports.
   The latter mechanism is quite flexible. As opposed to the
   interface/module data-hiding mechanism, opaque types allow you to
   define an arbitrary number of levels at which more and more
   information about the implementation of your object is revealed.
   See Sections 2.2.10, 2.4.6, and 2.4.7 of "Systems Programming with
   Modula-3" for more information about opaque types and about partial
   and complete revelations.
What is the purpose of BRANDED and REVEAL?

   Allan Heydon writes:
   These two keywords are necessary because of two quite different
   features of the language. REVEAL is necessary because Modula-3 has
   opaque types and partial revelations. BRANDED is necessary because the
   Modula-3 type system uses structural equivalence instead of name
   In Modula-3, the concrete structure of a type can be hidden from
   clients in an interface. A common idiom is:

    T <: TPublic;
    TPublic = OBJECT
      (* fields *)
      (* methods *)

  END I.

   The line "T <: TPublic" introduces the type "I.T" as an opaque subtype
   of the type "I.TPublic". It does not reveal any of the other details
   of the concrete structure of "I.T" to clients. Hence, "I.T" is said to
   be an opaque type. Put another way, the structure of "I.T" is only
   partially revealed to clients.
   In addition, it is possible to reveal more of "I.T"'s structure in
   other interfaces, like this:


    TPrivate = I.TPublic OBJECT
      (* more fields *)
      (* more methods *)

    I.T <: TPrivate;

  END IRep.

   This interface declares a type "IRep.TPrivate" that is a subtype of
   "I.TPublic". It also asserts that "I.T" is also a subtype of
   "IRep.TPrivate". A client that imports only the interface "I" has
   access only to the fields and methods in "I.TPublic" when accessing an
   object of type "I.T", but a client that imports both "I" and "IRep"
   also has access to the fields and methods in "IRep.TPrivate" when
   accessing an object of type "I.T".
   The "REVEAL" statement in this module simply asserts a subtype
   relation. Unlike type declarations, revelations introduce no new
   names. Hence, we could not have used the "TYPE" keyword in this case
   because the type "I.T" has already been declared once (albeit
   opaquely) in interface "I".
   Every opaque type must have a complete revelation. A complete
   revelation has the form:
    T = TConcrete;

   The revelation specifies that "TConcrete" is the concrete type for the
   opaque type "T".
   The Modula-3 type system uses structural equivalence instead of name
   equivalence. This means that two types are equal iff they have the
   same structure. One consequence of this rule is that two types you
   might intend to be distinct may actually be equal. This can have
   unintended effects on the run-time behavior of your program. For
   example, if both types that you expect to be distinct are actually
   structurally equivalent and the two types guard two arms of a TYPECASE
   statement, the arm for the second type will never be taken.
   If you want to avoid accidental equalities between two types, you can
   brand one (or both) of them with the BRANDED keyword. A branded type
   is equivalent to no other type, even if it is structurally equivalent
   to some other type. In essence, the BRANDED keyword adds a bit of
   virtual structure to the type that guarantees it will be distinct from
   every other type.
   The Modula-3 syntax allows you to supply a text constant as a name for
   the brand. If you don't supply an explicit brand, the compiler will
   make one up; however, the implicit brand invented by the compiler is
   not guaranteed to be chosen deterministically. Hence, explicit brands
   are useful if you are communicating types from one process to another
   and if you want to be sure that the branded type written by one
   process matches the branded type read in by the other.
   Any two opaque types in a program must be distinct. Otherwise, it
   would be too easy for clients to accidentally trip over type
   collisions like the TYPECASE example mentioned above. To enforce the
   restriction that all opaque types are distinct, the language requires
   that the type "TConcrete" in the complete revelation above must be a
   branded type.
Can a program recover from running out of virtual memory?

   No, this turns out to be quite a thorny problem. I think the best
   thing I can do is by attaching to this message the dialog that went on
   during the "beta test" of the new library interfaces (SRC Research
   Report 113, "Some Useful Modula-3 Interfaces). The parties are Xerox
   PARC's David Goldberg, Hans Boehm, Alan Demers, and David Nichols, and
   SRC's John DeTreville, who designed and implemented the garbage
   collector in SRC Modula-3. The dialog covers many of the issues, and
   apparently ends when the participants run out of steam.
   Paul McJones (editor of SRC 113)
RTAllocator should allow handling out of memory

   David Goldberg: ... there is one system problem that is not currently
   handled, namely running out of memory. I would very, very much like to
   see this handled in RTAllocator. One approach was suggested by Roy
   Levin a while back: Have a RegisterNoMemory(proc) routine that causes
   proc() to be called when memory is gone (or very low). Example of use:
   in the 'Solitaire' program, the 'Hint' button generates a tree of
   possible moves. If this tree gets very big and consumes all memory,
   the RegisterNoMemory proc could abandon the search down the current
   branch, NIL-out that branch, and ask for a garbage collection.
   Currently what happens is that Solitaire crashes if you bug 'Hint' and
   memory is low.
   Interface Police: Ok, make a concrete proposal and we'll talk. How low
   should memory be before the runtime complains? Before or after a
   collection? Is it ok to call your procedure from inside a runtime
   critical section (after all, you're probably in the middle of a NEW)?
   Are multiple notification procedures allowed to be registered?
   Shouldn't a routine that consumes arbitrary amounts of memory be coded
   to poll the allocator to ask how much memory is available?
   Hans Boehm/Alan Demers/David Goldberg/David Nichols: We believe that
   programs wishing to handle allocation failures will be able to do so
   with high (but not perfect) reliability if the interface provides two
   features: versions of the RTAllocator.New routines that report if the
   allocation is not possible (either by returning NIL or raising an
   exception), and a way to register a callback when memory is low. Both
   features are necessary. Here are two typical scenarios:
     * The Solitaire program. Before starting, Solitaire allocates a
       'safety net' block of memory, and registers a callback. When
       memory is exhausted, the callback frees the safety net, sets a
       flag, and returns. In the Solitaire program proper, the inner loop
       of the move generator checks the flag immediately after allocating
       a new node. If the flag is set, it abandons the search. It would
       not work for Solitiare to allocate new tree nodes with
       RTAllocator.New() and check for an error: as memory gets low, a
       library routine in some other package could cause an allocation
       Unfortunately, there is a race condition since another thread
       could run and do an allocation between the time the faulting NEW
       returns and all references to the search tree are NIL'ed. This can
       be mimimized by adding some slop to the safety net.
     * An editor that allocates O(n) bytes of memory when opening an
       n-byte file. If the users tries to open a huge file, you don't
       want to crash, but rather tell the user that the file can't be
       opened (in UNIX, the user can then kill some processes to regain
       swap space and try again, or in an emacs-style editor he can
       delete some buffers and try again). A callback won't work for
       this, because when attempting to open a huge file, the allocation
       must be aborted: there just isn't enough memory to go around.
       Instead an RTAllocator.New() routine should be used for this
       However, the editor will also want to register a callback proc to
       guard against NEW()s in other parts of the program that can't be
       satisfied. If the callback is passed the size of the memory
       allocation that can't be satisfied, the callback will be able to
       pick between two strategies. If there is a 'safety net' which is
       larger than the block to be allocated, the callback can free it
       and set a "low on memory" flag, with the editor cleaning up
       properly later. If the safety net is not big enough, the callback
       itself can attempt an emergency save before crashing.
   Here's a specific proposal that embodies these ideas. We're not wedded
   to the details. Note that RTCollector.LimitRelative is not essential:
   it just lifts some of the functionality currently in RTHeapPolicy.
     * Add the following to RTCollector.i3:
      PROCEDURE LimitAbsolute(n: CARDINAL);

      (* Don't let the heap grow beyond n bytes.  The collector/allocator
         should observe this in all heap growth decisions.  *)

      [Comment from Hans: I don't think there is a way to write
      programs that are reasonable citizens on a shared system without
      some such facility.]

      PROCEDURE LimitRelative(x: REAL);

      (* Advisory.  Try to keep the heap size at roughly x times the
         amount of live data. (For ref counting it affects only the
         backup collector for cycles.)  *)

      [Comments from Hans: The performance of all collectors with
      which I am familiar depends crucially on such a parameter.  Thus
      it might as well be exposed in some portable interface.  (The
      allocator should of course use less memory if it gains no time
      advantage from using more.)  The "amount of live data" is, of
      course, implementation defined, as are the minimum values of x
      that have any chance of being observed.]
     * In RTAllocator.i3, add OutOfMemory to RAISES clauses of all the
       New routines, and add the following:
      EXCEPTION OutOfMemory;

         CallBackObj = OBJECT notify(bytes: CARDINAL) END;

      PROCEDURE RegisterHeapFullCallback(obj: CallBackObj);

      (* Add obj.notify to the list of procs to be called if an
         allocation request is about to fail, either because of lack
         of memory, or due to violation of an
         RTCollector.LimitAbsolute imposed limit.  The notify method
         will be called with an argument specifying the size in bytes
         of the allocate call that triggered the callback.  The notify
         method may not allocate or acquire locks, or call any
         procedures that do.  It may be invoked synchronously with any
         combination of locks held.  (Should there be a way to delete
         a registered callback?).  If a garbage collection after this
         callback fails to reclaim enough memory to allocate the
         requested object, an exception will be raised if the
         allocation was through RTAllocator.  Otherwise a checked
         runtime error will result.  The notify proc is not called
         when memory fails from an RTAllocator.New call (these
         failures can be caught by the user).

         Typical actions by notify would include one of the following:

         1) Clearing pointers to reduce the amount of accessible memory.
         2) Calling RTCollector.LimitAbsolute with a larger limit.
     * Variations on this proposal:
       Might want to consider adding:
      PROCEDURE GetLimitAbsolute(): CARDINAL;
      (* Return the current absolute heap limit *)
       The usefulness of RTCollector.LimitAbsolute in the callback would
       be increased if there was a way to tell if this actually freed up
       any more memory. One approach would be to change CallBackObj to
         CallBackObj = OBJECT
                          notify(bytes: CARDINAL; retry: BOOLEAN): BOOLEAN
       and change the action of RegisterHeapFullCallback to:
      (* If a garbage collection after all callbacks have been
         executed fails to reclaim enough memory to allocate the
         requested object, then any notify() procs that returned TRUE
         will be called again with retry := TRUE.  Otherwise an
         exception will be raised if the allocation was through
         RTAllocator, or else a checked runtime error will result. *)
       Thus, if you wanted to first try and get more memory in the
       callback by calling RTCollector.LimitAbsolute, you could return
       TRUE and wait for a callback with retry = TRUE. If this second
       callback occurs, you will need to clear some pointers to free up
       memory. Or another variation: add
      PROCEDURE GetTotalBytesAllocated(): CARDINAL;
      (* Returns the total number of bytes allocated since the program
         begin.  A CARDINAL may not be big enough, perhaps this should
         be a LONGREAL? *)
       Then the retry argument to the notify method can be eliminated,
       since a call is a retry only if GetTotalBytesAllocated() shows no
       additional allocations since the last callback.
   John DeTreville: When I read your March proposal for handling running
   out of memory on the traced heap, I didn't quite see how to implement
   the details you gave. I've been iterating to create mechanisms that
   are simpler and more implementable, and I've now arrived at quite a
   simple interface.
   In particular, I now believe that (almost) all the functionality you
   ask for is already provided by the current interface. I say "almost"
   because there's a few status calls to be added, and because some of
   the current mechanisms are clunky, but I believe I can tell a
   convincing story. Note that these mechanisms are or would be in
   SRC-specific interfaces (currently called RTAllocatorSRC,
   RTCollectorSRC, and RTHeapRep); I don't think we understand them well
   enough to put them into the public IP interfaces.
   Let's first distinguish VM limits from application-imposed limits. The
   amount of VM available to the application is a hard limit, although
   not one that can easily be predicted. In the current SRC M3
   implementation, both the allocator and the collector allocate VM from
   the kernel when necessary. If the collector tries to allocate VM and
   fails, the program must crash: there is no way to reestablish the
   necessary invariants to let it continue.
   I propose treating VM exhaustion as a checked runtime error, in the
   allocator and in the collector. The goal is then to establish and
   maintain an application-imposed limit that is uniformly stricter than
   the VM limit, whatever that may be.
   You propose a mechanism to allow calls to the New* procedures to fail
   if they would exceed the application-imposed limit. Of course, only a
   small part of the code would take advantage of this facility. This
   code could equally well query the heap to determine the current size,
   and compare it against the limit; if the program can also predict the
   size of the object to be allocated, it can decide whether or not to
   This approach requires some collector-dependent code in the
   application, but I doubt that it would be very much. It also allows
   possible race conditions, but I believe they're not much worse in
   practice than in the original proposal.
   You also propose a mechanism to notify the program whenever the limit
   is about to be exceeded. It's quite complicated to get such immediate
   notification. First, the procedures notified can't acquire locks or
   call most procedures in other modules. Second, it requires a new
   collection to run synchronously after the procedures to see if enough
   space has been freed and whether some of the procedures must be called
   again; this causes an interruption of service.
   Here's a different proposal, which might not allow space bounds as
   tight as in the original proposal, but which seems simpler. We would
   add a mechanism for an application thread to wait for the next
   collection to finish. This mechanism could replace the current
   mechanisms for registering and unregistering synchronous monitors,
   which have numerous complex and poorly documented constraints on what
   actions they can perform.
   Each time through, the thread could compare the amount of space still
   reachable to the application-imposed limit, and either free some data
   before the next collection (the ability to hold locks would be handy
   here) or increase "gcRatio" to make the collector work harder and keep
   the total heap size under control, or both.
   There is still the danger that the application could allocate so
   rapidly that this asynchronous thread might not be able to keep up,
   but otherwise asynchronous actions seem a lot more reasonable than
   This is one approach, and there are others. What's nice about this
   design is that it requires almost no changes to the interface, only
   better status reporting and a replacement of the mechanisms for
   registering and unregistering synchronous collection monitors. Maybe
   you could even work around the current lack of these facilities. Let
   me know what you think.
   Hans: One quick comment, without having thought much about the rest:
   "This code could equally well query the heap to determine the current
   size, and compare it against the limit; if the program can also
   predict the size of the object to be allocated, it can decide whether
   or not to proceed."
   Is this really true? Since the collector can't move some objects,
   there are presumably fragmentation issues. Am I guaranteed to be able
   to allocate 1 MB if the current heap size is 1 MB below the limit?
   This is certainly false in PCR, and I'm not sure how you could
   guarantee it without remapping pages.
   John: Hans Boehm notes that I was wrong about the client of New* being
   able to predict whether an allocation would succeed or fail, because
   of likely page-level fragmentation. This needs to be fixed in my
   To expand on my earlier message, let me outline a completely different
   approach for handling heap overflow, that perhaps has more in common
   with the original PARC proposal, but which seems far too complex and
   unwieldy to me. This complexity is why I tried to work out a simpler
   approach, even at the cost of providing fewer guarantees.
   We start by imagining that we want to be able to continue to run even
   if we exhaust VM. First, this means that we can never allocate VM from
   inside the collector. The implication is that whenever we allocate VM
   in the allocator, we allocate enough extra to tide us over through the
   next collection, no matter how much of the heap it retains. This
   suggests that we will significantly overallocate VM. For example, with
   a stop-and-copy collector and gcRatio set to 1, a program with a
   stably-sized reachable set currently requires 3 times as much space as
   the reachable set, but the "failsafe gc" would require 4 times.
   (Doing even this well depends crucially upon the SRC implementation
   detail that the current collector never copies objects bigger than one
   page, but leaves them in place. Otherwise, the possibility of
   fragmentation would make it much more difficult to determine how much
   memory to leave free for the collector, and in what sizes of runs of
   pages. It will also take some work to avoid off-by-one errors in
   predicting how much memory a collection could take.)
   Of course, if the client decreases gcRatio, or switches from
   sort-and-copy collection to concurrent collection, that would require
   allocating more VM, to ensure that the collector cannot run out of VM.
   That means that these operations can also fail, just like allocator
   Only some programs will want to be able to back off when they reach VM
   limits. Others won't mind getting a checked runtime error; in return,
   they will require less VM. Therefore, we need procedures to switch
   back and forth between these two modes. Again, attempting to switch to
   failsafe mode can fail.
   The collector currently allocates its own space on the traced heap
   during collections, which will have to be moved to the untraced heap
   if we are to predict how much traced heap a collection can use. Note
   that in general, once VM is exhausted, allocations on the untraced
   heap may start to fail, and so programs will probably die very quickly
   once VM is exhausted. But let's move on.
   In addition to the VM limit, we also want an application-imposed limit
   on heap size. The allocator and collector will guarantee that the heap
   size will never exceed this limit. Again, we will overallocate VM in
   the allocator to avoid exceeding the limit in the collector. Again,
   setting the limit may fail.
   So what happens when a NEW fails, or a New*, or switching to
   concurrent collection, or setting the application-imposed limit, or
   whatever? This happens whenever performing the operation would exceed
   the application-imposed limit, or when attempts to allocate enough
   extra VM fail.
   Some of these can signal an error, and the client can chose to do
   something else instead. In some cases, such as setting gcRatio, it
   might make sense for the failing operation to tell the client how
   close to the impossible value would be possible.
   NEW, though, should not signal an error; this would require massive
   changes in all existing modules that would not be add value to most
   clients. In this case, I can't think of anything much better than the
   original proposal. Before attempting to allocate the object, the
   collector will try to free up some storage. First, it can perform a
   collection, or finish the current one. If that doesn't do it, it can
   call one or more procedures registered by the application to drop
   links to some storage, or to change collector parameters. If that
   doesn't do it, we can perform another collection. And so on and so on,
   until the procedures say to give up.
   Note that these collections must be synchronous, since no allocations
   may be possible until this mechanism completes, and the collections
   will therefore cause interruptions of service. Note also that the
   procedures cannot acquire locks, cannot allocate storage, cannot call
   thread primitives, and so on, and therefore cannot call into most
   libraries; they are essentially restricted to reading and writing data
   structures whose implementations they know, and changing collector
   parameters. This seems excessively restrictive, but also unavoidable
   in this approach.
   In short, this seems like a lot of extra mechanism to add to the
   allocator and collector, that doesn't seem to do quite what you want;
   it gives you strict limits, but at a cost. My proposal of this morning
   is at least much simpler, although it can give looser limits.
   John, continuing: Thinking a little more about the problem of running
   out of storage in the untraced heap, it seems that the only reasonable
   thing to do is to merge the implementation of the untraced heap with
   the traced heap. This was, untraced NEWs that fail can be handled
   exactly the same way as traced NEWs, with a synchronous cleanup
   routine that frees enough VM to proceed, or resets parameters.
   This means that the allocator and collector cannot use the untraced
   heap, but must either use a static amount of storage which they could
   overflow, or must allocate enough extra in response to client
   applications that they cannot possibly run out of space. The potential
   space overhead for maximum-sized stacks, for example, is huge.
   The more this proposal is fleshed out, it more it seems that doing a
   good job of recovering from heap overflows is quite tricky, which is
   why I suggest a lower-tech approach for now.
   Hans: I just went over the last few messages in this thread again. I
   think the bottom line is, as you say:
   It's hard to implement an out-of-memory call-back on top of the
   current collector. Given the current collector, a collector call-back
   that allows polling is probably the best you can reasonably do, and
   should certainly be provided.
   The remaining question, which also seems to be motivated by other
   concerns here, is: To what extent are you tied to this collector
   design? The problem here seems to be mainly caused by copying old
   generation objects, since you could perhaps bound the size of the
   young generation? My suspicion, based unfortunately only on anecdotes,
   is that this is not a good idea anyway, since it uses too much space,
   and is also fairly expensive in copying cost. (PARCPlace seems to have
   arrived at the same conclusion, so there's probably at least one other
   supporter of this position. Some recent complaints here about space
   usage of Modula-3 programs also point a bit in this direction.)
   Do you agree? If so, should the interface be designed ignoring current
   constraints, and should we initially accept a partial implementation?
   John: It's been a while, so let me recap where we are, or at least
   where I am.
   We've been discussing mechanisms for Modula-3 programs to manage their
   memory better. In particular, we have proposed ways that programs
   could bound the heap size and recover from heap overflow.
   I think this topic is complicated enough, and new enough, that we
   shouldn't try to get it into the current set of portable interfaces.
   The Interface Police concur.
   We've floated two broad (families of) proposals for attacking this
     * Allow strong guarantees on the heap size; these guarantees would
       never be broken.
     * Allow the program to monitor its memory usage, discarding excess
       data as necessary.
   I think that the first is achievable. Adapting the current SRC
   Modula-3 (allocator and) collector to allow such guarantees would take
   a month or so. The principal problem is that the current collector
   tries to maximize space/time performance, and giving such guarantees
   will probably require extra memory to be set aside that will never be
   used. The collector would have two modes: with or without guarantees.
   Most programs would run without guarantees.
   I also think that a usable version of the second is possible with
   almost no change to the current collector. The programmer would have
   to do more work, and wouldn't get any strong guarantees, but this
   approach should work for many programs.
   We've also been discussing a third family of proposals, that seem to
   combine the worst features of the first and second: they require
   significant changes to the current collector, but son't give very
   strong guarantees. These seem much less interesting to me.
   Here's two pieces of opinion.
   First, I propose that we work out the details of #2, and you use it
   for a couple of programs. Get some experience with it. This could help
   inform a heavier-weight solution.
   Second, I wonder whether any of these solutions is a good candidate
   for a portable interface. It's one thing not to be strictly
   incompatible with a given collector strategy, but quite another to be
   easy to plug into an existing collector. Modula-3 currently doesn't
   require very much from its collector; making these proposals standard
   would significantly increase the requirements on a Modula-3
Why uppercase keywords?

   Some people prefer uppercase keywords others hate them. Another
   possibility is to accept both forms for keywords. This topic has been
   discussed at length and there is no solution that will completely
   satisfy everyone's tastes. Fortunately this is a very minor issue and
   you can easily have lowercase keywords automatically converted for you
   using an emacs macro package like [26]m3su .
Why CONST Comments in Variables Declarations?

   John Kominek ( wrote: Sprinkled throughout
   SRC m3 you'll find "constant" variables exported in interfaces. For


   where Grain is assigned during module initialization. Instead, did the
   modula-3 designers consider doing this.

   Here the keyword permits only exporting modules to modify the Grain
   variable. Is there a problem with this proposal? The READONLY keyword
   is successfully used at procedure boundaries, so why not also at
   interface boundaries?
   Bill Kalsow replies:
   A problem with this proposal is that any module can claim to export
   the interface containing the variable, hence any module could modify
   the variable. Note that CONST says more than just READONLY. CONST
   implies that the variable should not be modified by clients and that
   once it is initialized, it won't be changed later by the
   implementation. READONLY would only mean that clients should not
   modify the variable. IMO, the "right" solution would have been to
    CONST x: T;

    MODULE Foo;
    CONST x: T := <value>;

   In the same way it checks revelations for opaque types, the compiler
   could check that only one module provided a value for the constant.
   But, this proposal doesn't quite hang together either. Consider this
    VAR   v: [0..x];

   The language definition says that "v"s definition is illegal if "x <
   0" because its type is "empty". The system could refuse to run the
   program by delaying the check until it had seem the corresponding
   implementation module. But, I think you'll agree that it could quickly
   turn into a mess. The most flexible handling of opacity I've seen is
   in Christian Collberg's PhD Thesis, "Flexible Encapsulation". It was
   published Dec 5, 1992 by the CS Dept at Lund University, Lund Sweden.
   If I remember correctly, his system was capable of deferring all
   checks and decisions imposed by opaque declarations until link time.


   7. file://localhost/home/m3/tmp/m3/pm3/intro/src/concise-bib.html
   8. file://localhost/home/m3/tmp/m3/pm3/intro/src/bib.html
   9. file://localhost/home/m3/tmp/m3/pm3/intro/src/bib.html#SPwM3
  10. file://localhost/home/m3/tmp/m3/pm3/intro/src/bib.html#m3-Har92
  21. file://localhost/home/m3/tmp/m3/pm3/language/modula3/m3tools/m3gdb/src
  22. file://localhost/home/m3/tmp/m3/pm3/text/sgmltools/sgml/src/nsgmls
  24. file://

Prof. Michel Dagenais     
Département de génie informatique
Ecole Polytechnique de Montréal     tel: (514) 340-4711 ext.4029

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