XLISP > Internals

Xlisp Internals

92Jan14 jsp@glia.biostr.washington.edu (Jeff Prothero). Public Domain.

                 | xlisp 2.1 internals |

         "Trust the Source, Luke, trust the Source!"


  Who should read this?     What is an LVAL?  
  What is the obarray?     The Interpreter Stacks  
  What is a context?     What is an environment?  
  How are xlisp entities stored and identified?     How are vectors implemented?  
  How are strings implemented?     How are symbols implemented?  
  How are closures implemented?     How are objects implemented?  
  How are classes implemented?     How is the class hierarchy laid out?  
  How do we look up the value of a variable?     How are function calls implemented?  
  How are message-sends implemented?     How is garbage collection implemented?  
  How are the source files laid out?     How do I add a new primitive fn to xlisp?  
  Minor Observations.     Acknowledgements. 

Who should read this ?

Anyone poking through the C implementation of xlisp for the first time. This is intended to provide a rough roadmap of the global xlisp structures and algorithms. If you just want to write lisp code in xlisp, you don't need to read this file -- go read xlisp.doc (XLisp Appendix in the Nyquist manual), XlispOOP.doc (XLisp Objects Primer), and XlispRef.doc (XLisp Language Reference), in about that order. If you want to tinker with the xlisp implementation code, you should *still* read those three before reading this. The following isn't intended to be exhaustively precise -- that's what the source code is for! It is intended only to allow you a fighting change of understanding the code the first time through (instead of the third time).

At the bottom of the file you'll find an example of how to add new primitive functions to xlisp.

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What is an LVAL ?

An "LVAL" is the C type for a generic pointer to an xlisp garbage-collectable something. (Cons cell, object, string, closure, symbol, vector, whatever.) Virtually every variable in the interpreter is an LVAL. Cons cells contain two LVAL slots, symbols contains four LVAL slots, etc.

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What is the obarray ?

The obarray is the xlisp symbol table. More precisely, it is a hashtable mapping ascii strings (SYMBOL names) to SYMBOLs. (The name "obarray" is traditional but a bit of a misnomer, since it contains only xlisp SYMBOLs, and in particular contains no xlisp OBJECTs.) It is used when converting lisp expressions from text to internal form. Since it is a root for the garbage collector, it also serves to distinguish permanent global-variable SYMBOLs from other SYMBOLs -- you can permanently protect a SYMBOL from the garbage collector by entering it into the obarray. This is called "interning" the SYMBOL. The obarray is called "obarray" in C and "*OBARRAY*" in xlisp. It is physically implemented as a VECTOR-valued SYMBOL.

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The Interpreter Stacks

xlisp uses two stacks, an "evaluation stack" and an "argument stack". Both are roots for the garbage collector. The evaluation stack is largely private to the interpreter and protects internal values from garbage collection, while the argument stack holds the conventional user-visible stackframes.

The evaluation stack is an EDEPTH-long array of "LVAL" allocated by xldmem.c:xlminit(). It grows zeroward.

xlstkbase points to the zero-near end of the evaluation stack.

xlstktop points to the zero-far end of the evaluation stack; the occupied part of the stack lies between xlstack and xlstktop. NOTE that xlstktop is *NOT* the top of the stack in the conventional sense of indicating the most recent entry on the stack: xlstktop is a static bounds pointer which never changes once the stack is allocated.

xlstack starts at the zero-far end of the evaluation stack. *xlstack is the most recent LVAL on the stack. The garbage collector MARKs everything reachable from the evaluation stack (among other things), so we frequently push things on this stack while C code is manipulating them. (Via xlsave(), xlprotect(), xlsave1(), xlprot1().)

The argument stack is an ADEPTH-long array of "LVAL". It also grows zeroward. The evaluator pushes arguments on the argument stack at the start of a function call (form evaluation). Built-in functions usually eat them directly off the stack. For user-lisp functions xleval.c:evfun() pops them off the stack and binds them to the appropriate symbols before beginning execution of the function body proper.

xlargstkbase is the zero-near end of argument stack.

xlargstktop is the zero-far end of argument stack. Like xlstktop, xlargstktop is a static bounds pointer which never changes after the stack is allocated.

*xlsp ("sp"=="stack pointer") is the most recent item on the argument stack.

xlfp ("fp"=="frame pointer") is the base of the current stackframe.

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What is a context ?

An xlisp "context" is something like a checkpoint, recording a particular point buried in the execution history so that we can abort/return back to it. Contexts are used to implement call/return, catch/throw, signals, gotos, and breaks. xlcontext points to the chain of active contexts, the top one being the second-newest active context. (The newest -- that is, current -- active context is implemented by the variables xlstack xlenv xlfenv xldenv xlcontext xlargv xlargc xlfp xlsp.) Context records are written by xljump.c:xlbegin() and read by xljump.c:xljump(). Context records are C structures on the C program stack; They are not in the dynamic memory pool or on the lisp execution or argument stacks.

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What is an environment ?

An environment is basically a store of symbol-value pairs, used to resolve variable references by the lisp program. xlisp maintains three environments, in the global variables xlenv, xlfenv and xldenv.

Lisp supports two sorts of binding, "lexical binding" and "dynamic binding". Lexically bound variables are 'visible' only in code textually within their binding form. Dynamically bound variables are 'visible' in any code *called* from the binding form. (Either kind of binding may be shadowed by other declarations, of course.) Historically, lisp has been moving from dynamic binding (which is easy for interpreters to handle), to lexical binding (which is easy for humans and compilers to handle). Almost all xlisp binding forms are lexically scoped. The most important exception is progv.

xlenv and xlfenv track lexical bindings. xlenv and xlfenf are conceptually a single environment, although they are implemented separately. They are linked-list stacks which are pushed when we enter a function and popped when we exit it. We also switch xlenv+xlfenf environments entirely when we begin executing a new closure (user-fn written in lisp).

The xlenv environment is the most heavily used environment. It is used to resolve everyday data references to local variables. It consists of a list of frames (and objects). Each frame is a list of sym-val pairs. In the case of an object, we check all the instance and class variables of the object, then do the same for its superclass, until we run out of superclasses.

The xlfenv environment is maintained strictly parallel to xlenv, but is used to find function values instead of variable values. The separation may be partly for lookup speed and partly for historical reasons. Merging these two lists into a single list (while distinguishing function bindings from variable bindings, of course) would slightly decrease fn enter/exit overhead while increasing the overhead of looking up each variable or function binding.

When we send a message, we set xlenv to the value it had when the message CLOSURE was built, then push on (obj msg-class), where msg-class is the [super]class defining the method. (We also set xlfenv to the value xlfenv had when the method was built.) This makes the object instance variables part of the environment, and saves the information needed to correctly resolve references to class variables, and to implement SEND-SUPER.

The xldenv environment tracks dynamic bindings. It is a simple list of sym-val pairs, treated as a stack. Progv uses it to save the old values of symbols it binds, and it is also used to save old values of s_evalhook and s_applyhook (*EVALHOOK* and *APPLYHOOK*). (These latter mostly support the debug facilities.)

These environments are manipulated in C via the xlisp.h macros xlframe(e), xlbind(s,v), xlfbind(s,v), xlpbind(s,v,e), xldbind(s,v), xlunbind(e).

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How are xlisp entities stored and identified ?

Conceptually, xlisp manages memory as a single array of fixed-size objects. Keeping all objects the same size simplifies memory management enormously, since any object can be allocated anywhere, and complex compacting schemes aren't needed. Every LVAL pointer points somewhere in this array. Every xlisp object has the basic format (xldmem.h:typdef struct node)

  struct node {
      char n_type;
      char n_flags;
      LVAL car;
      LVAL cdr;

where n_type is one of:

  FREE      A node on the freelist.
  SUBR      A function implemented in C. (Needs evaluated arguments.)
  FSUBR     A "special form" implemented in C. (Needs unevaluated arguments).
  CONS      A regular lisp cons cell.
  SYMBOL    A symbol.
  FIXNUM    An integer.
  FLONUM    A floating-point number.
  STRING    A string.
  OBJECT    Any object, including class objects.
  STREAM    An input or output file.
  VECTOR    A variable-size array of LVALs.
  CLOSURE   Result of DEFUN or LAMBDA -- a function written in lisp.
  CHAR      An ascii character.
  USTREAM   An internal stream.
  STRUCT    A structure.

Messages may be sent only to nodes with n_type == OBJECT.

Obviously, several of the above types won't fit in a fixed-size two-slot node. The escape is to have them malloc() some memory and have one of the slots point to it -- VECTOR is the archetype. For example, see xldmem.c:newvector(). To some extent, this malloc() hack simply exports the memory- fragmentation problem to the C malloc()/free() routines. However, it helps keep xlisp simple, and it has the happy side-effect of unpinning the body of the vector, so that vectors can easily be expanded and contracted.

The garbage collector has special-case code for each of the above node types, so it can find all LVAL slots and recycle any malloc()ed ram when a node is garbage-collected.

Xlisp pre-allocates nodes for all ascii characters, and for small integers. These nodes are never garbage-collected.

As a practical matter, allocating all nodes in a single array is not very sensible. Instead, nodes are allocated as needed, in segments of one or two thousand nodes, and the segments linked by a pointer chain rooted at xldmem.c:segs.

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How are vectors implemented ?

An xlisp vector is a generic array of LVAL slots. Vectors are also the canonical illustration of xlisp's escape mechanism for node types which need more than two LVAL slots (the maximum possible in the fixed-size nodes in the dynamic memory pool). The node CAR/CDR slots for a vector hold a size field plus a pointer to a malloc()ed ram chunk, which is automatically free()ed when the vector is garbage-collected.

xldmem.h defines macros for reading and writing vector fields and slots: getsize(), getelement() and setelement(). It also defines macros for accessing each of the other types of xlisp nodes.

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How are strings implemented ?

Strings work much like vectors: The node has a pointer to a malloc()ed ram chunk which is automatically free()ed when the string gets garbage-collected.

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How are symbols implemented ?

A symbol is a generic user-visible lisp variable, with separate slots for print name, value, function, and property list. Any or all of these slots (including name) may be NIL.

You create an xlisp symbol from C by calling "xlmakesym(name)" or "xlenter(name)" (to make a symbol and enter it in the obarray).

You may create symbols from xlisp by explicitly calling GENSYM or MAKE-SYMBOL (uninterned symbols), or INTERN (interned symbol). However, the lisp reader will create symbols on sight, so most lisp symbols are created as side-effects of expressions like 'name.

(A symbol is 'interned' if it is listed in *OBARRAY*. Various parts of the system, like the lisp reader, treat *OBARRAY* essentially as the list of all known symbols. It is unusual but occasionally useful to create uninterned symbols. You can make READ create an uninterned symbol by preceding it with #:. In xlisp, a newly created symbol has no value unless its name begins with a ':', in which case it has itself for its value -- handy for keywords and message selectors.)

Most of the symbol-specific code in the interpreter is in xlsym.c.

Physically, a symbol is implemented like a four-slot vector.

Random musing: Abstractly, the LISP symbols plus cons cells (etc) constitute a single directed graph, and the symbols mark spots where normal recursive evaluation should stop. Normal lisp programming practice is to have a symbol in every cycle in the graph, so that recursive traversal can be done without MARK bits.

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How are closures implemented ?

A closure, the return value from a lambda, is a regular coded-in-lisp fn. Physically, it is implemented like an eleven-slot vector, with the node n_type field hacked to contain CLOSURE instead of VECTOR. The vector slots contain:

  name     symbol -- 1st arg of DEFUN.  NIL for LAMBDA closures.
  type     (s_lambda or s_macro). Must be s_lambda to be executable.
  args     List of "required" formal arguments (as symbols)
  oargs    List of "optional" args, each like: (name (default specified-p))
  rest     Name of "&rest" formal arg, else NIL.
  kargs    keyword args, each like: ((':foo 'bar default specified-p))
  aargs    &aux vars, each like: (('arg default))
  body     actual code (as lisp list) for fn. 
  env      value of xlenv when the closure was built.  NIL for macros.
  fenv     value of xlfend when the closure was built. NIL for macros.
  lambda   The original formal args list in the DEFUN or LAMBDA.

The lambda field is for printout purposes. The remaining fields store a predigested version of the formal args list. This is a limited form of compilation: by processing the args list at closure-creation time, we reduce the work needed during calls to the closure.

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How are objects implemented ?

An object is implemented like a vector, with the size determined by the number of instance variables. The first slot in the vector points to the class of the object; the remaining slots hold the instance variables for the object. An object needs enough slots to hold all the instance variables defined by its class, *plus* all the instance variables defined by all of its superclasses.

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How are classes implemented ?

A class is a specific kind of object, hence has a class pointer plus instance variables. All classes have the following instance variables:

  MESSAGES     A list of (interned-symbol method-closure) pairs.
  IVARS        Instance variable names: A list of interned symbols.
  CVARS        Class variable names:    A list of interned symbols.
  CVALS        Class variable values:   A vector of values.
  SUPERCLASS   A pointer to the superclass.
  IVARCNT      Number of class instance variables, as a fixnum.
  IVARTOTAL    Total number of instance variables, as a fixnum.

IVARCNT is the count of the number of instance variables defined by our class. IVARTOTAL is the total number of instance variables in an object of this class -- IVARCNT for this class plus the IVARCNTs from all of our superclasses.

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How is the class hierarchy laid out ?

The fundamental objects are the OBJECT and CLASS class objects. (Both are instances of class CLASS, and since CLASSes are a particular kind of OBJECT, both are also objects, with n_type==OBJECT. Bear with me!)

OBJECT is the root of the class hierarchy: everything you can send a message to has OBJECT as its class or super*class. (Vectors, chars, integers and so forth stand outside the object hierarchy -- you can't send messages to them. I'm not sure why Dave did it this way.) OBJECT defines the messages:

  :isnew -- Does nothing but return the object.
  :class -- Returns contents of class-pointer slot.
  :show  -- Prints names of obj, obj->class and instance vars.

Since a CLASS is a specialized type of OBJECT (with instance variables like MESSAGES which generic OBJECTs lack), class CLASS has class OBJECT as its superclass. The CLASS object defines the messages:

  :new    -- Create new object with self.IVARTOTAL LVAR slots, plus
             one for the class pointer. Point class slot to self.
             Set new.n_type char to OBJECT.
             IVARTOTAL, using parameters from :new call.  (The
             :isnew msg inherits the :new msg parameters because
             the  :isnew msg is generated automatically after
             each :new   msg, courtesy of a special hack in
  :answer -- Add a (msg closure) pair to self.MESSAGES.

Here's a figure to summarize the above, with a generic object thrown in for good measure. Note that all instances of CLASS will have a SUPERCLASS pointer, but no normal object will. Note also that the messages known to an object are those which can be reached by following exactly one Class Ptr and then zero or more Superclass Ptrs. For example, the generic object can respond to :ISNEW, :CLASS and :SHOW, but not to :NEW or :ANSWER. (The functions implementing the given messages are shown in parentheses.)

                                      |Superclass Ptr
  :isnew (xlobj.c:obisnew) <----|  class |Class Ptr
  :class (xlobj.c:obclass) <----| OBJECT |------------------------------>+
  :show  (xlobj.c:objshow) <----|        |                               |
                                +--------+                               |
                                  ^  ^  ^                                |
        +---------+               |  |  |                                |
        | generic |Class Ptr      |  |  +<---------------+               |
        | object  |-------------->+  |Superclass Ptr     ^Superclass Ptr |
        +---------+                  |                   |               |
                             Msg+--------+          +---------+          |
  :isnew (xlobj.c:clnew)   <----| class  |Class Ptr | generic |Class Ptr |
  :new   (xlobj.c:clisnew) <----| CLASS  |------->+ | CLASS   |------->+ |
  :answer(xlobj.c:clanswer)<----|        |        | |         |        | |
                                +--------+        | +---------+        | |
                                   ^  ^           |                    | |
                                   |  |           v                    v |
                                   |  +-----------+ <------------------+ v

Thus, class CLASS inherits the :CLASS and :SHOW messages from class OBJECT, overrides the default :ISNEW message, and provides new messages :NEW and :ANSWER.

New classes are created by (send CLASS :NEW ...) messages. Their Class Ptr will point to CLASS. By default, they will have OBJECT as their superclass, but this can be overridden by the second optional argument to :NEW.

The above basic structure is set up by xlobj.c:xloinit().

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How do we look up the value of a variable ?

When we're cruising along evaluating an expression and encounter a symbol, the symbol might refer to a global variable, an instance variable, or a class variable in any of our superclasses. Figuring out which means digging through the environment. The canonical place this happens is in xleval.c:xleval(), which simply passes the buck to xlsym.c:xlgetvalue(), which in turn passes the buck to xlxsym.c:xlxgetvalue(), where the fun of scanning down xlenv begins. The xlenv environment looks something like

           Backbone    Environment frame contents
           --------    --------------------------
  xlenv --> frame      ((sym val) (sym val) (sym val) ... )
            frame      ...
            object     (obj msg-class)
            frame      ...
            object     ...
            frame      ...

The "frame" lines are due to everyday nested constructs like LET expressions, while the "object" lines represent an object environment entered via a message send. xlxgetvalue scans the enviroment left to right, and then top to bottom. It scans down the regular environment frames itself, and calls xlobj.c:xlobjgetvalue() to search the object environment frames.

xlobjgetvalue() first searches for the symbol in the msg-class, then in all the successive superclasses of msg-class. In each class, it first checks the list of instance-variable names in the IVARS slot, then the list of class-variables name in the CVARS slot.

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How are function calls implemented ?

xleval.c contains the central expression-evaluation code. xleval.c:xleval() is the standard top-level entrypoint. The two central functions are xleval.c:xlevform() and xleval.c:evfun(). xlevform() can evaluate four kinds of expression nodes:

SUBR: A normal primitive fn coded in C. We call evpushargs() to evaluate and push the arguments, then call the primitive.

FSUBR: A special primitive fn coded in C, which (like IF) wants its arguments unevaluated. We call pushargs() (instead of evpushargs()) and then the C fn.

CLOSURE: A preprocessed written-in-lisp fn from a DEFUN or LAMBDA. We call evpushargs() and then evfun().

CONS: We issue an error if CONS.car isn't a LAMBDA, otherwise we call xleval.c:xlclose() to build a CLOSURE from the LAMBDA, and fall into the CLOSURE code.

The common thread in all the above cases is that we call evpushargs() or pushargs() to push all the arguments on the evaluation stack, leaving the number and location of the arguments in the global variables xlargc and xlargv. The primitive C functions consume their arguments directly from the argument stack.

xleval.c:evfun() evaluates a CLOSURE by:

(1) Switching xlenv and xlfenv to the values they had when the CLOSURE was built. (These values are recorded in the CLOSURE.)

(2) Binding the arguments to the environment. This involves scanning through the section of the argument stack indicated by xlargc/xlargv, using information from the CLOSURE to resolve keyword arguments correctly and assign appropriate default values to optional arguments, among other things.

(3) Evaluating the body of the function via xleval.c:xleval().

(4) Cleaning up and restoring the original environment.

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How are message-sends implemented ?

We scan the MESSAGES list in the CLASS object of the recipient, looking for a (message-symbol method) pair that matches our message symbol. If necessary, we scan the MESSAGES lists of the recipient's superclasses too. (xlobj.c:sendmsg().) Once we find it, we basically do a normal function evaluation. (xlobjl.c:evmethod().)

Two differences between message-send and normal function invocation:

1) We need to replace the message-symbol by the recipient on the argument stack to make "self" available in the method closure.

2) We need to push an 'object' stack entry on the xlenv environment to record which class is handling the message (so that, for example, SEND-SUPER can find our superclass).

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How is garbage collection implemented ?

The dynamic memory pool managed by xlisp consists of a chain of memory segments (xldmem.h:struct segment) rooted at global C variable "segs". Each segment contains an array of "struct node"s plus a pointer to the next segment. Each node contains a n_type field and a MARK bit, which is zero except during garbage collection.

Xlisp uses a simple, classical mark-and-sweep garbage collector. When it runs out of memory (fnodes==NIL), it does a recursive traversal setting the MARK flag on all nodes reachable from the obarray, the three environments xlenv/xlfenv/xldenv, and the evaluation and argument stacks. (A "switch" on the n_type field tells us how to find all the LVAL slots in the node (plus associated storage), and a pointer-reversal trick lets us avoid using too much stack space during the traversal.) sweep() then adds all un-MARKed LVALs to fnodes, and clears the MARK bit on the remaining nodes. If this fails to produce enough free nodes, a new segment is malloc()ed.

The code to do this stuff is mostly in xldmem.c.

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How are the source files laid out ?

To really understand the source, of course, you simply have to sit down and read it. There's no royal road to hacking! So this is a map of the source, not a picture of it.

The core portable xlisp files have the prefix 'xl' (for 'xlisp').

The best place to start reading the code is in the two main header files, xlisp.h and xldmem.h.

The xldmem.h file ('dmem' for 'dynamic memory') defines the actual structure in memory of all the primitive xlisp data types. This is the file you will most likely refer to most often.

The xlisp.h file defines essentially all global constants and macros which don't need to know about the structures in xldmem.h, mainly manifest constants sizing various arrays for different machines, and macros to test for the type of a list object and to help parse xlisp argument lists.

The central code files to understand are xlmain.c, xleval.c, xlbfun.c, and xlsym.c.

xlmain.c contains both main() and the central read-eval-print loop, so it is the place from which to begin tracing flow of control.

xleval.c (with some support from xlbfun.) contains the heart of xlisp, the code to evaluate functions and macros.

xlsym.c contains the code to find the value of a symbol.

Once you understand the above three, you know where xlisp decides to evaluate an s-expression, how it evaluates it, and how it finds the values needed by the expression.

A good file to tackle next is xlobj.c, which handles much of the object-oriented support, and has much of the flavor of xleval.c.

Most of the other files are just libraries of functions to handle particular types of processing, and are easily understood once the central code is grokked.

One of the charms of xlisp *is* that it is small enough to easily read and comprehend. I hope it stays that way!

Here's a very brief file-by-file overview of the source. Your files will probably be laid out just a little differently. In particular, if you're not running on unix, instead of 'unixstuff.c' you'll have something like 'dosstuff.c'.

   Size Name        Contains
  ----- --------    --------
   2054 osdefs.h    System specific declarations.  Empty file in my case.
   2050 osptrs.h    System specific pointers.      Empty file in my case.
  14172 unixstuff.c Isolates I/O fns, which tend to be OS-specific. Unix version.
  19049 xlbfun.c    'Basic FUNctions': highlevel eval stuff + random support.
  30229 xlcont.c    'CONTrol': case, do, while, dotimes, other special forms.
   6380 xldbug.c    'DeBUG': break, abort, error, baktrace...
  18006 xldmem.c    'Dynamic MEMory': ram allocation, garbage collector.
   9431 xldmem.h    Declaration of LVAL, scads of useful macros.
  21220 xleval.c    'EVALuation': eval, apply, macroexpand and support for them.
  11935 xlfio.c     'File I/O': OPEN, READ-CHAR, WRITE-CHAR ...
  18481 xlftab.c    'Function TABle': Array of all xlisp primitives.
   4866 xlglob.c    'GLOBal variables':  Boring list of declarations.
  11048 xlimage.c   'memory IMAGE': Code to save/restore contents of xlisp.
  10579 xlinit.c    'INITialization': Start-of-the-world setup code.
   6016 xlio.c      'Input/Output': misc I/O stuff, some called by xlfio.c.
  12664 xlisp.h     consp() ..., xlgetarg() ..., misc types.
   5853 xljump.c    catch, throw, return ...
  20723 xllist.c    car, cdr, append, map ... basic list manipulation.
  11975 xlmath.c    Arithmetic functions -- addition, multiplication ...
  16425 xlobj.c     Object support -- create objects & classes, send msgs...
   4134 xlpp.c      A little prettyprinter.
   9487 xlprin.c    Print an arbitrary lisp value in ascii.
  19535 xlread.c    Read in an arbitrary ascii lisp expression.
  15062 xlstr.c     Manipulation of characters and strings.
  12889 xlstruct.c  Lisp structures -- defstruct and kin.
   5957 xlsubr.c    eq, equal, some internal utility fns.
   7019 xlsym.c     Symbols and obarrays ... maksym, getvalue, getprop...
   5566 xlsys.c     Misc stuff -- read file, print backtrace, peek/poke...
   3926 xmain.c     main(), with top read-eval-print loop.
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How do I add a new primitive fn to xlisp ?

A preliminary comment: You should have a copy of Guy L Steele's "Common Lisp: The Language" (2nd Edition), and make your new primitives compatible with the standard whenever practical.

Add a line to the end of xlftab.c:funtab[]. This table contains a list of triples:

The first element of each triple is the function name as it will appear to the programmer. Make it all upper case.

The second element is S (for SUBR) if (like most fns) your function wants its arguments pre-evaluated, else F (for FSUBR).

The third element is the name of the C function to call.

Remember that your arguments arrive on the xlisp argument stack rather than via the usual C parameter mechanism.

CAUTION: Try to keep your files separate from generic xlisp files, and to minimize the number of changes you make in the generic xlisp files. This way, you'll have an easier time re-installing your changes when new versions of xlisp come out. For example, if you are going to add many primitive functions to your xlisp, use an #include file rather than putting them all in xlftab.c. It's a good idea to put a marker (like a comment with your initials) on each line you change or insert in the generic xlisp fileset.

CAUTION: Remember that you usually need to protect the LVAL variables in your function from the garbage-collector. It never hurts to do this, and often produces obscure bugs if you do not. You protect uninitialized local variables with xlsave1() and initialized local variables with xlprot1().

BE CAREFUL NOT TO PROTECT UNINITIALIZED LOCAL VARIABLES WITH XLPROT1() OR XLPROTECT()! This will appear to work fine until garbage collection happens at an inconvenient moment, at which point the garbage collector will wind up following your uninitialized pointer off to never-never land.

Note: If you have several pointers to protect, you can save a little runtime and codespace by using xlstkcheck(number-of-variables-to-protect) followed by xlsave()s and xlprotect()s instead of the more expensive xlsave1()s and xlprot1()s.

Generic code for a new primitive fn:

  /* xlsamplefun - do useless stuff.        */
  /* Called like (samplefun '(a c b) 1 2.0) */
  LVAL xlsamplefun()
      /* Variables to hold the arguments: */
      LVAL    list_arg, integer_arg, float_arg;
      /* Get the arguments, with appropriate errors */
      /* if any are of the wrong type.  Look in     */
      /* xlisp.h for macros to read other types of  */
      /* arguments.  Look in xlmath.c for examples  */
      /* of functions which can handle an argument  */
      /* which may be either int or float:          */
      list_arg    = xlgalist()  ;  /* "XLisp Get A LIST"   */
      integer_arg = xlgafixnum();  /* "XLisp Get A FIXNUM" */
      float_arg   = xlgaflonum();  /* "XLisp Get A FLONUM" */
      /* Issue an error message if there are any extra arguments: */
      /* Call a separate C function to do the actual  */
      /* work.  This way, the main function can       */
      /* be called from both xlisp code and C code.   */
      /* By convention, the name of the xlisp wrapper */
      /* starts with "xl", and the native C function  */
      /* has the same name minus the "xl" prefix:     */
      return samplefun( list_arg, integer_arg, float_arg );
  LVAL samplefun( list_arg, integer_arg, float_arg )
  LVAL            list_arg, integer_arg, float_arg;
      FIXTYPE val_of_integer_arg;
      FLOTYPE val_of_float_arg;
      /* Variables which will point to LISP objects: */
      LVAL result;
      LVAL list_ptr;
      LVAL float_ptr;
      LVAL int_ptr;
      /* Protect our internal pointers by */
      /* pushing them on the evaluation   */
      /* stack so the garbage collector   */
      /* can't recycle them in the middle */
      /* of the routine:                  */
      xlstkcheck(4);    /* Make sure following xlsave */
                        /* calls won't overrun stack. */
      xlsave(list_ptr); /* Use xlsave1() if you don't */
      xlsave(float_ptr);/* do an xlstkcheck().        */
      /* Semantic check, illustrating use of xlfail(): */
      if (list_ptr == NIL) {
          xlfail("null list");
          /* Won't return. */
      /* Create an internal list structure, protected */
      /* against garbage collection until we exit fn: */
      list_ptr = cons(list_arg,list_arg);
      /* Get the actual values of our fixnum and flonum: */
      val_of_integer_arg = getfixnum( integer_arg );
      val_of_float_arg   = getflonum( float_arg   );
      /* Semantic check, illustrating use of xlerror(): */
      if (val_of_integer_arg < -2) {
          xlerror("bad integer",cvfixnum(val_of_integer_arg));
          /* Won't return. */
      /* You can have any amount of intermediate */
      /* computations at this point in the fn... */
      /* Make new numeric values to return: */
      integer_ptr = cvfixnum( val_of_integer_arg * 3   );
      float_ptr   = cvflonum( val_of_float_arg   * 3.0 );
      /* Cons it all together to produce a return value: */
      result = cons( float_ptr,   NIL    );
      result = cons( integer_ptr, result );
      result = cons( list_ptr,    result );
      /* Restore the stack, canceling the xlsave()s: */
      xlpopn(4); /* Use xlpop() for a single argument.*/
      return result;

Example of what NOT to do:

Here's a function I wrote which does *NOT* correctly prevent the garbage collector from stealing its dynamically allocated cells:

  LVAL incorrect_Point_To_List( p )/*DON'T USE THIS CODE! */
  geo_point*                    p;
      Convert point to (x y z) list.
      LVAL result;
      result = cons(              /* THIS CODE IS BROKEN! */
          cvflonum(        p->x), /* THIS CODE IS BROKEN! */
          cons(                   /* THIS CODE IS BROKEN! */
              cvflonum(    p->y), /* THIS CODE IS BROKEN! */
              cons(               /* THIS CODE IS BROKEN! */
                  cvflonum(p->z), /* THIS CODE IS BROKEN! */
                  NIL             /* THIS CODE IS BROKEN! */
              )                   /* THIS CODE IS BROKEN! */
          )                       /* THIS CODE IS BROKEN! */
      );                          /* THIS CODE IS BROKEN! */
      return result;

The problem with the above function is that the "z" cell will be allocated first, and is not protected during the allocation of the "y" flonum (or vice versa, depending on the order the compiler chooses to evaluate these arguments). Similarly, the "y" cell is not protected during allocation of the "x" flonum. Here is a correct version, in which "result" always protects the list-to-date:

  LVAL correct_Point_To_List( p )
  geo_point*                  p;
      Convert point to (x y z) list.
      LVAL result;
      result = cons( cvflonum(p->z), NIL          );
      result = cons( cvflonum(p->y), result       );
      result = cons( cvflonum(p->x), result       );
      return result;
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Minor Observations:

xlapply, xlevform and sendmsg will issue an error if they encounter a s_macro CLOSURE. You are not allowed to use APPLY or FUNCALL with macros in Common Lisp. There is no way provided to declare macro methods, nor do they make much sense...

Neither xlapply nor sendmsg will handle FSUBRs. Common Lisp does not allow APPLYing a special form (FSUBR), and since SEND is a SUBR, all of its arguments are already evaluated, so it is not possible to have FSUBR methods.

Since xlisp tracks the three most recent input expressions (in variables +, ++ and +++) and three most recent results (in variables *, ** and ***), things may occasionally not get garbage-collected as soon as you expect!

Both xlobj.c:xloinit() and xlobj.c:obsymvols() initialize the "object" and "class" variables. This is neither redundant nor a bug:

xloinit creates the classes class and object, as well as the symbols, but sets the C variables class and object to point to the class and object.

obsymbols just sets the C variables by looking up the symbols. It is needed because when you restore a workspace you don't create new objects but still need to know where the existing objects are (they might be in a different location in the saved workspace). Notice that obsymbols is called by xlsymbols which is called both when initializing a new workspace and when restoring an old workspace.

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This document is considerably improved thanks to careful reading and thoughtful suggestions from Ken Whedbee (kcw@beach.cis.ufl.edu) and (especially) Tom Almy (toma@sail.labs.tek.com).

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