s9core.txt

	================================================================
	|                           S9CORE                             |
	|         A Toolkit for Implementing Dynamic Languages         |
	|                           Mk IIIc                            |
	================================================================

	                     Nils M Holm, 2007-2017
			      In the public domain

	----------------------------------------------------------------
	RATIONALE
	----------------------------------------------------------------

	Dynamic languages typically require some basic infrastructure
	that is common in their implementations, including

	- garbage collection
	- primitive functions
	- dynamic type checking
	- bignum arithmetics"
	- heap images

	S9core offers all of the above, and some more, in a single
	object file that can be linked against a dynamic language
	implementation. It takes care of all the nitty gritty stuff and
	allows the implementor to focus on the design of the language
	itself.

	----------------------------------------------------------------
	FEATURES
	----------------------------------------------------------------

	- Precise, constant-space, stop-the-world garbage collection
	  with vector pool compaction (defragmentation) and finalization
	  of I/O ports

	- Non-copying GC, all nodes stay in their original locations

	- Bignum (unbounded-precision) integer arithmetics

	- Decimal-based, platform-independent real number arithmetics

	- Persistent heap images

	- Type-checked primitive functions

	- Symbol identity

	- Memory allocation on heap exclusively (no malloc() until the
	  heap grows)

	- A basis for implementing interpreters, runtime libraries, etc

	- Statically or dynamically linked

	- Available on Unix, Plan 9, and in C89/POSIX environments

	----------------------------------------------------------------
	REFERENCE MANUAL
	----------------------------------------------------------------

	===== SETUP AND NAMESPACE ======================================

	A module that intends to use the S9core tool kit must include
	the S9core header using

	#include <s9core.h>

	As of Mk II, the tool kit has a separate name space which is
	implemented by beginning all symbol names with a

	S9_ or s9_

	prefix. However, many symbols can be "imported" by adding

	#include <s9import.h>

	Doing so will create aliases of most definitions with the prefix
	removed, so you can write, for instance:

	cons(a, cons(b, NIL))

	instead of

	s9_cons(a, s9_cons(b, S9_NIL))

	There are some symbol names that will not have aliases, mostly
	tuneable parameters of s9core.h. Those names will print with
	their prefixes in this text. All other names will have their
	prefixes removed.

	When a module wants to use S9core functions without importing
	them, the following rules apply:

	A lower-case function or macro name is prefixed with s9_, e.g.
	bignum_add() becomes s9_bignum_add().

	A capitalized function or macro name has its first letter
	converted to lower case and an S9_ prefix attached, e.g.:
	Real_exponent becomes S9_real_exponent.

	An upper-case symbol gets an S9_ prefix, e.g.: NIL becomes
	S9_NIL.

	----- S9_VERSION -----------------------------------------------

	The S9_VERSION macro expands to a string holding the S9core
	version in "YYYYMMDD" (year, month, day) format.

	===== C-LEVEL DATA TYPES =======================================

	At C level, there are only two data types in S9core. Dynamic
	typing is implemented by adding type tags to objects on the heap.

	----- cell -----------------------------------------------------

	A "cell" is a reference to an object on the heap. All objects
	are addressed using cells. A cell is wide enough to hold a
	pointer on the host platform (typically a ptrdiff_t.

	Example: cell x, y;

	----- PRIM (struct S9_primitive) -------------------------------

	A PRIM is a structure containing information about a primitive
	procedure:

	struct S9_primitive { 
	        char    *name;  
	        cell    (*handler)(cell expr);
	        int     min_args;
	        int     max_args;
	        int     arg_types[3];
	};

	The name field names the primitive procedure. The handler is
	a pointer to a function from cell to cell implementing the
	primitive function. Because a cell may reference a list or
	vector, functions may in fact have any number of arguments
	(and, for that matter, return values).

	The min_args, max_args, and arg_types[] fields define the type
	of the primitive function. min_args and max_args specify the
	expected argument count. When they are equal, the argument count
	is fixed. When max_args is less then zero, the function accepts
	any number of arguments that is greater than or equal to
	min_args.

	The arg_types[] array holds the type tags of the first three
	argument of the primitive. Functions with more than three
	arguments must check additional arguments internally. Unused
	argument slots can be set to T_ANY (any type accepted).

	Example:

	PRIM Prims[] = {
	  { "cons", p_cons, 2, 2, { T_ANY,  T_ANY, T_ANY } },
	  { "car",  p_car,  1, 1, { T_PAIR, T_ANY, T_ANY } },
	  { "cdr",  p_cdr.  1, 1, { T_PAIR, T_ANY, T_ANY } },
	  { NULL }
	};

	Where p_cons, p_car, and p_cdr are the functions implementing
	the corresponding primitives.

	===== CALLING CONVENTIONS ======================================

	All S9core functions protect their parameters from the garbage
	collector so it is safe, for example, to write

	make_real(1, 0, make_integer(x));

	or

	cell n = cons(c, NIL);
	n = cons(b, n);
	n = cons(a, n);

	In the first case, the integer created by make_integer() will
	be protected in the application of make_real(). In the second
	example, the object /c/ will be protected in the first call,
	and the list /n/ will be protected in all subsequent applications
	cons(). Note that the objects /b/ and /a/ are not protected
	during the first call and /a/ is not protected during the second
	call, though.

	Use save() and unsave()  to protect objects temporarily.

	===== INITIALIZATION AND SHUTDOWN ==============================

	----- void s9_init(cell **extroots); ---------------------------

	The s9_init() function initializes the memory pools, connects
	the first three I/O ports to stdin, stdout, and stderr, and sets
	up the internal S9core structures. It must be called before any
	other S9core functions can be used.

	The extroots parameter is a pointer to an array of addresses of
	cells that will be protected from the garbage collector (the
	so-called "GC roots"). The last array member must be NULL.
	Because cells can reference trees, lists, or vectors, larger
	structures may be protected from GC by including their handles
	in this array.

	Example:

	cell Environment;
	cell *GC_roots[] = { &Environment, NULL };
	...
	s9_init(GC_roots);

	----- void s9_fini(void); --------------------------------------

	The s9_fini() function shuts down S9core and releases all memory
	allocated by it. This function is normally never called, because
	clean-up is done by the operating system.

	The only reason to call it is to prepare for the
	re-initialization of the toolkit, for example to
	recover from a failed image load (see load_image()).

	----- MEMORY ALLOCATION ----------------------------------------

	----- S9_NODE_LIMIT --------------------------------------------
	----- S9_VECTOR_LIMIT ------------------------------------------

	The S9_NODE_LIMIT and S9_VECTOR_LIMIT constants specify the
	maximum sizes of the node pool and the vector pool, respectively.
	The "pools" are used to allocate objects at run time. Their
	sizes are measured in "nodes" for the node pool and cells for
	the vector pool. Both sizes default to 14013 times 1024
	(14,013K).

	The size of a cell is the size of a pointer on the host
	platform. The size of a node is two cells plus a char. So the
	total node memory limit using the default settings on a 64-bit
	host would be:

	14013 times 1024 times (2 times 8+1)  =  243,938,304 bytes

	The default vector pool limit would be:

	14013K cells  =  114,794,496 bytes

	At run time, the S9core toolbox will /never/ allocate more
	memory than the sum of the above (plus the small amount
	allocated to primitive functions at initialization time).

	When S9core runs out of memory, it will print a message and
	terminate program execution. However, a program can request to
	handle memory allocation failure itself by passing a handler to
	the mem_error_handler() function (further explanations can be
	found below).

	The amount allocated to S9core can be changed by the user.
	See the set_node_limit() and set_vector_limit() functions for
	details.

	----- void mem_error_handler(void (*h)(int src)); --------------

	When a function pointer is passed to mem_error_handler(), S9core
	will no longer terminate program execution when a node or vector
	allocation request fails. The request will /succeed/ and the
	function passed to mem_error_handler() will be called.

	****************************************************************
	The function is then required to handle the error as soon as
	possible, for example by interrupting program execution and
	returning to the REPL, or by signalling an exception.
	****************************************************************

	The integer argument passed to a memory error handler will
	identify the source of the error: 1 denotes the node allocator
	and 2 indicates the vector allocator.

	Allocation requests can still succeed in case of a low memory
	condition, because S9core never allocates more than 50% of each
	pool. (This is done, because using more than half of a pool will
	result in GC thrashing, which would reduce performance
	dramatically.)

	As soon as a memory error handler has been invoked, thrashing
	/will/ start immediately. Program execution will slow down to a
	crawl and eventually the allocator will fail to recover from a
	low-memory condition and kill the process, even with memory
	error handling enabled.

	The default handler (which just terminates program execution)
	can be reinstalled by passing NULL to mem_error_handler().

	----- void set_node_limit(int k); ------------------------------

	These functions modify the node pool and vector pool memory
	limits. The value passed to the function will become the new
	limit for the respective pool.  The limits must be set up
	immediately after initialization and may not be altered once
	set. Limits are specified in kilo nodes, i.e. they will be
	multiplied by 1024 internally.

	Setting either value to zero will disable the corresponding
	memory limit, i.e. S9core will grow the memory pools
	indefinitely until physical memory allocation fails. This may
	cause massive swapping in memory-heavy applications.

	S9core memory pools both start with a size of 32768 units
	(S9_INITIAL_SEGMENT_SIZE constant) and grow exponentially to
	a base of 3/2. With the default settings, the limit will be
	reached after growing either pool for 15 times.

	Note that actual memory limits all have the form 32768 * 1.5^n,
	so a limit that is not constructed using the above formula will
	probably be smaller than expected. Reasonable memory limits
	(using the default segment size) are listed in figure 1.

	As can be seen in the table, the minimal memory footprint of
	S9core is 416K bytes on 32-bit and 800K bytes on 64-bit systems.
	In order to obtain a smaller initial memory footprint, the
	S9_INITIAL_SEGMENT_SIZE constant has to be reduced and the table
	in figure 1 has to be recalculated.

	    Limit  64-bit memory  32-bit memory
	---------  -------------  -------------
	       32           800K           416K
	       48          1200K           625K
	       72          1800K           937K
	      108          2700K          1405K
	      162          4050K          2107K
	      243          6075K          3160K
	      364          9100K          4733K
	      546            14M          7089K
	      820            21M            11M
	    1,230            31M            16M
	    1,846            46M            24M
	    2,768            69M            36M
	    4,152           104M            54M
	    6,228           156M            81M
	    9,342           234M           121M
	---------------------------------------
	   14,013           350M           182M
	---------------------------------------
	   21,019           525M           273M
	   31,529           788M           410M
	   47,293          1182M           615M
	   70,939          1773M           922M
	  106,409          2660M          1383M
	  159,613          3990M          2075M
	  239,419          5985M          3112M
	  359,128          8978M          4669M
	  538,692            13G          7003M
	  808,038            20G            10G
	1,212,057            30G            16G
	1,818,085            45G            24G
	2,727,127            68G            35G
	4,090,690           102G            53G
	6,136,034           153G            80G
	---------------------------------------
	Fig 1. Memory Limits

	----- ARITHMETICS ----------------------------------------------

	----- S9_DIGITS_PER_CELL ---------------------------------------
	----- S9_INT_SEG_LIMIT -----------------------------------------

	S9_DIGITS_PER_CELL is the number of decimal digits that can be
	represented by a cell and S9_INT_SEG_LIMIT is the smallest
	integer that can /not/ be represented by an "integer segment"
	(which has the size of one cell). The integer segment limit is
	equal to 10^S9_DITIGS_PER_CELL.

	A cell is called an integer segment in S9core arithmetics,
	because numbers are represented by chains of cells (segments).

	The practical use of the S9_INT_SEG_LIMIT constant is that
	bignums that are smaller than this limit can be converted to
	(long) integers just be extracting their first segment.

	These values are /not/ tunable. S9_DIGITS_PER_CELL is 18 on
	64-bit machines, 9 on 32- bit machines, and (theoretically) 4 on
	16-bit machines.

	----- S9_MANTISSA_SEGMENTS -------------------------------------
	----- S9_MANTISSA_SIZE -----------------------------------------

	S9_MANTISSA_SEGMENTS his is the number of integer segments (see
	above) in the mantissae of real numbers. The default is one
	segment (18 digits of precision) on 64-bit hosts and two segments
	(also 18 digits) on 32-bit platforms. Each additional mantissa
	segment increases precision by S9_DIGITS_PER_CELL (see above),
	but also slows down real number computations.

	This is a compile-time option and cannot be tweaked at run time.

	S9_MANTISSA_SIZE is the number of decimal digits in a mantissa.
	It is used in the computation of various values, such as Epsilon.

	===== S9CORE TYPES =============================================

	S9core data types are pretty LISP- or Scheme-centric, but most
	of them can be used in a variety of languages.

	Each type may be associated with a predicate testing for the
	type, an allocator creating an object of the given type, and one
	or more accessors that extract values from the type. Predicates
	always return 0 (false) or 1 (true). Type predicates succeed
	(return 1) if the object passed to them is of the given type.

	----- SPECIAL VALUES -------------------------------------------

	Special values are constant, unique, can be compared with ==,
	and have no allocators.
	................................................................

	     Type: NIL
	Predicate: x == NIL

	NIL ("Not In List") denotes the end of a list, an empty list,
	or an empty return value. For example, to create a list of the
	objects /a/, /b/, and /c/, the following S9core code would be
	used:

	cell list = cons(c, NIL);
	list = cons(b, list);
	list = cons(a, list);

	See also: T_LIST
	................................................................

	     Type: END_OF_FILE
	Predicate: eof_p(x)
	           x == END_OF_FIL

	END_OF_FILE is an object that is reserved for indicating the end
	of file when reading from an input source. The eof_p() predicate
	returns truth only for the END_OF_FILE object.
	................................................................

	     Type: UNDEFINED
	Predicate: undefined_p(x)
	           x == UNDEFINED

	The UNDEFINED value is returned by a function to indicate that
	its value for the given arguments is undefined. For example,

	bignum_divide(One, Zero)

	would return UNDEFINED.
	................................................................

	Type: UNSPECIFIC
	Predicate: unspecific_p(x)
	           x == UNSPECIFIC

	The UNSPECIFIC value can be returned by functions to
	indicate that their return value is of no importance
	and should be ignored.
	................................................................

	     Type: USER_SPECIALS
	Predicate: special_p()

	When more special values are needed, they should be assigned
	/decreasing/ values starting at the value of the USER_SPECIALS
	constant. The predicate special_p() will return truth for all
	special values, including user-defined ones.

	Examples:

	#define TOP      (USER_SPECIALS-0)
	#define BOTTOM   (USER_SPECIALS-1)
	................................................................

	Type: VOID
	Predicate: x == VOID

	VOID denotes the absence of a value. While UNSPECIFIC is
	typically /returned/ by a function to indicate that its
	value is uninteresting, VOID may be /passed/ to a function
	to indicate that the corresponding argument may be ignored.

	----- TAGGED TYPES ---------------------------------------------

	A "tagged" object is a compound data object (pair, tree) with a
	type tag in its first slot. Tagged objects typically carry some
	payload, such as an integer value, an I/O port, or a symbol name.
	The internal structure of a tagged object does not matter; it is
	created using an allocator function and its payload is accessed
	using one or multiple accessor functions.

	----- type_tag(x) ----------------------------------------------

	The type_tag() accessor extracts the type tag, like T_BOOLEAN or
	T_INTEGER, from the given object. When the object does not have
	a type tag, it returns a special value, T_NONE.
	................................................................

	Type: T_ANY

	When used in a PRIM structure, this type tag matches any other
	type (i.e. the described primitive procedure will accept any
	type in its place).
	................................................................

	     Type: T_BOOLEAN
	Allocator: TRUE, FALSE
	Predicate: boolean_p(x)

	The TRUE and FALSE objects denote logical truth and falsity.
	................................................................

	     Type: T_CHAR
	Allocator: make_char(int c)
	Predicate: char_p(x)
	 Accessor: int char_value(x)

	T_CHAR objects store single characters. The make_char() function
	expects the character to store, and char_value() retrieves the
	character.

	Example:

	make_char('x')
	................................................................

	     Type: T_INPUT_PORT
	Allocator: make_port(int portno, T_INPUT_PORT)
	Predicate: input_port_p(x)
	 Accessor: int port_no(x)

	The make_port() allocator boxes a port handle. The port handle
	must be obtained by one of the I/O routines  before passing it
	to this function. port_no() returns the port handle stored in
	an T_INPUT_PORT (or T_OUTPUT_PORT) object.

	Example:

	cell p = open_input_port(path);
	if (p >= 0) return make_port(p, T_INPUT_PORT);
	................................................................

	     Type: T_INTEGER
	Allocator: make_integer(cell segment)
	Predicate: integer_p(x)
	 Accessor: cell bignum_to_int(x)

	The make_integer() function creates a single-segment bignum
	integer in the range from

	-10^S9_DITIGS_PER_CELL + 1  to  10^S9_DITIGS_PER_CELL - 1

	To create larger bignum integers, the string_to_bignum()
	function  has to be used.

	The bignum_to_int() accessor returns the value of a
	single-segment bignum integer or UNDEFINED, if the bignum
	has more than a single segment. There is no way to convert
	multi-segment bignums to a native C type.

	Example:

	cell x = make_integer(-12345);
	int  i = bignum_to_int(x);
	................................................................

	     Type: T_LIST
	           T_PAIR
	Allocator: cons(cell car_val, cell cdr_val)
	Predicate: pair_p(x)
	 Accessor: cell car(x)
	           cell cdr(x)

	The difference between the T_PAIR and T_LIST type tags is that
	T_LIST also includes NIL, which T_PAIR does not. Both type tags
	are used for primitive procedure type checking exclusively.

	The cons() allocator returns an ordered pair of any two values.
	It is in fact an incarnation of the LISP function of the same
	name. The accessors car() and cdr() retrieve the first and
	second value from a pair, respectively.

	pair_p() succeeds for pairs created by cons().
	T_LIST corresponds to

	pair_p(x) || x == NIL

	Further accessors, like caar() and friends, are also available
	and will be explained later in this text.

	Example:

	cons(One, NIL);          /* list */
	cell x = cons(One, Two); /* pair */
	car(x);                  /* One  */
	cdr(x);                  /* Two  */
	................................................................

	     Type: T_OUTPUT_PORT
	Allocator: make_port(int portno, T_OUTPUT_PORT)
	Predicate: output_port_p(x)
	 Accessor: int port_no(x)

	See T_INPUT_PORT, above, for details.

	Example:

	make_port(port_no, T_OUTPUT_PORT);
	................................................................

	     Type: T_PRIMITIVE
	Allocator: make_primitive(PRIM *p)
	Predicate: primitive_p(x)
	 Accessor: int prim_slot(x)
	           int prim_info(x)

	The make_primitive() function allocates a slot in an internal
	primitive function table, fills in the information in the given
	PRIM structure, and returns a primitive function object
	referencing that table entry. The prim_info() function retrieves
	the stored information (as a PRIM *).

	The prim_slot() accessor returns the slot number allocated for a
	given primitive function object in the internal table. Table
	offsets can be used to identify individual primitive functions.

	See the the discussion of the PRIM structure for an example of
	how to set up a primitive function. Given the table shown there,
	the following code would create the corresponding T_PRIMITIVE
	objects:

	for (i=0; p[i].name; i++) {
	        prim = make_primitive(&p[i]);
	        ...
	}
	................................................................

	     Type: T_FUNCTION
	Predicate: function_p(x)

	Function objects are deliberately underspecified. The user
	is required to define their own function object structure
	and accessors.

	For example, a LISP function allocator might look like this:

	cell make_function(cell args, cell body, cell env) {
	        /* args and body should be GC-protected! */
	        cell fun = cons(env, NIL);
	        fun = cons(body, fun);
	        fun = cons(args, fun);
	        return new_atom(T_FUNCTION, fun);
	}

	Given the structure of this function object, the corresponding
	accessors would look like this:

	#define fun_args(x) (cadr(x))
	#define fun_body(x) (caddr(x))
	#define fun_env(x)  (cadddr(x))
	................................................................

	     Type: T_REAL
	Allocator: make_real(int s, cell e, cell m)
	           Make_real(int f, cell e, cell m)
	Predicate: real_p(x)
	 Accessor: cell real_mantissa(x)
	           cell real_exponent(x)
	           Real_flags(x)

	A real number consists of three parts, a "mantissa" (the digits
	of the number), an exponent (the position of the decimal point),
	and a "flags" field, currently just containing the sign of the
	number.

	The value of a real number is

	sign * mantissa * 10^exponent

	The real_mantissa() and real_exponent() functions extract the
	mantissa and exponent, respectively. When applied to a bignum
	integer, the mantissa will be the number itself and the exponent
	will always be 0.

	Note that real_mantissa returns a bignum integer, but
	real_exponent returns an unboxed, cell-sized integer.

	The Real_flags() accessor can only be applied to real numbers.
	It extracts the flags field.

	The make_real() function is the principal real number allocator.
	It expects a sign /s/ (-1 or 1), an exponent as single cell, and
	a mantissa in the form of a bignum integer. When the mantissa is
	too large, the function will return UNDEFINED.

	Make_real() is a "quick and dirty" allocator. It expects a
	flags field in the place of a sign, a chain of integer segments
	instead of a bignum, and it does not perform any overflow
	checking.

	****************************************************************
	Caution: This function can create an invalid real number!
	****************************************************************

	Examples:

	cell m = make_integer(123);
	cell r = make_real( 1,  0, m); /* 123        */
	cell r = make_real( 1, 10, m); /*   1.23e+12 */
	cell r = make_real(-1, -5, m); /*  -0.00123  */
	................................................................

	     Type: T_STRING
	Allocator: make_string(char *s, int k)
	Predicate: string_p(x)
	 Accessor: char *string(x)
	           int string_len(x)

	The make_string() function creates a string of the length /k/
	and initializes it with the content of /s/. When the length /n/
	of /s/ is less than /k/, the last /k-n/ characters of the
	resulting string object will be undefined.

	Strings are counted /and/ NUL-terminated. The counted length of
	a given string is returned by the string_len() function, the C
	string length of /x/ is "strlen(string(x))" .

	................................................................
	The string() accessor returns a pointer to the char array
	holding the string.

	****************************************************************
	Note: no string obtained by string() or symbol_name() may be
	passed to make_string() as an initialization string, because
	vector objects (including strings and symbols) may move during
	heap compaction.
	****************************************************************

	The proper way to copy a string is

	int k = string_len(source);
	cell dest = make_string("", k-1);
	memcpy(string(dest), string(source), k);

	Alternatively, the copy_string() function may be used.
	................................................................

	     Type: T_SYMBOL
	Allocator: make_symbol(char *s, int k)
	           symbol_ref(char *s)
	Predicate: symbol_p(x)\fP"
	 Accessor: char *symbol_name(x)
	           int symbol_len(x)

	Typically, the symbol_ref() function is used to create or
	reference a symbol. A symbol is a unique string with an identity
	operation defined on it. I.e. referencing the same string twice
	using symbol_ref() will return /the same symbol/. Hence symbols
	can be compared using the == operator.

	The make_symbol() function creates an uninterned symbol, i.e. a
	symbol with no identity (which cannot be compared or referenced).
	In a typical implementation, this function will not be used.

	See the T_STRING description for further details and caveats.

	Example:

	cell sym = symbol_ref("foo");
	................................................................

	     Type: T_SYNTAX
	Predicate: syntax_p(x)

	Like function objects, syntactic abstractions ("macros") are
	deliberately underspecified. Typically, the value of a T_SYNTAX
	object would be a T_FUNCTION object.
	................................................................

	     Type: T_VECTOR
	Allocator: make_vector(int k)
	Predicate: vector_p(x)
	 Accessor: cell *vector(x)
	           int vector_len(x)

	The make_vector() function returns a vector of /k/ elements
	(slots) with all slots set to UNDEFINED.

	vector() returns a pointer to the slots of the given vector,
	vector_len() returns the number of slots.

	Example:

	cell v = make_vector(100);
	save(v);
	for (i=0; i<100; i++) {
		x = make_integer(i);
	        vector(v)[i] = x;
	}
	unsave(1);

	****************************************************************
	Note: the result of vector() may not be used on the left side of
	an assignment where the right side allocates any objects. When
	in doubt, first assign the value to a temporary variable and
	then the variable to the vector. For an explanation see T_STRING.
	****************************************************************
	................................................................

	     Type: T_CONTINUATION
	Predicate: continuation_p(x)

	A "continuation" object is used to store the value of a captured
	continuation (as in Scheme's call/cc).  Its implementation is
	left to the user.

	----- ADDITIONAL ALLOCATORS ------------------------------------

	----- cell cons3(cell a, cell d, int t); -----------------------

	The cons3() function is the principal node allocator of S9core.
	It is like cons(), but has an additional parameter for the "tag"
	field. The tag field of a node assigns specific properties to a
	node. For example, it can turn a node into an "atom", a vector
	reference, or an I/O port reference. In fact, cons() is a
	wrapper around cons3() that supplies an empty (zero) tag field.

	The most interesting user-level application of cons3() is maybe
	the option to mix in a CONST_TAG in order to create an immutable
	node. Note though, that immutability is not enforced by S9core
	itself, because it never alters any nodes. However,
	implementations using S9core can use the constant_p() predicate
	to check for immutability.

	Also note that "atoms" are typically created by the new_atom()
	allocator, explained below.

	----- cell copy_string(cell x); --------------------------------

	This function creates an new string object with the same content
	as the given string object.

	----- new_atom(x, d) -------------------------------------------
	----- atom_p(x) ------------------------------------------------

	An "atom" is a node with its atom flag set. Unlike a "cons" node,
	as delivered by cons(), an atom has no reference to another node
	in its car field. Instead of a reference, it can carry any value
	in the car field, for example: the character of a character
	object, a bignum integer segment, or a type tag. The new_atom()
	function expects any value in the /x/ parameter and a node
	reference in the /d/ parameter.

	Tagged S9core objects are composed of multiple atoms. For
	example, the following program would create a "character"
	object containing the character 'x' :

	cell n = new_atom('x', NIL);
	n = new_atom(T_CHAR, n);

	(Don't do this, though; use make_char() instead!)

	The atom_p() function checks whether the given node is an atom.
	S9core atoms encompass all the special values (like NIL, TRUE,
	END_OF_FILE, etc), all nodes with the atom flag set (including
	all tagged types), and all vector objects (see below). In fact,
	only "conses" (as delivered by cons()) are considered to be
	non-atomic).

	----- cell new_port(void); -------------------------------------

	The new_port() function returns a handle to a port, but does not
	assign any FILE to it. A file can be assigned by using the
	return value of new_port() as an index to the Ports[] array. A
	negative return value indicates failure (out of free ports).

	Example:

	int p = new_port();
	if (p >= 0) {
	        Ports[p] = fopen(file, "r");
	}

	----- cell new_vec(cell type, int size); -----------------------

	This function allocates a new vector. A vector object has a type
	tag in its car field and a reference into the vector pool in its
	cdr field, that is, neither of its fields reference any other
	node. The /type/ parameter is the type tag to be installed in
	the new vector atom and /size/ is the number /bytes/ to allocate
	in the vector pool. The newly allocated segment of the vector
	pool will be left uninitialized except when /type/ is T_VECTOR.
	Slots of T_VECTOR objects will be initialized with NIL.

	Example:

	new_vec(T_STRING, 100);
	new_vec(T_VECTOR, 100 * sizeof(cell));

	----- save(n) --------------------------------------------------
	----- cell unsave(int k); --------------------------------------

	save() saves an object on the internal S9core stack and unsave(n)
	removes /n/ elements from the stack and returns the one last
	removed (i.e. the previously /n^th/ element on the stack).

	The S9core stack is mostly used to protect objects from being
	recycled by the GC.

	Removing an element from an empty stack will cause a fatal error
	and terminate program execution.

	Example:

	cell a = cons(One, NIL);
	save(a)		;
	cell b = cons(Two, NIL); /* a is protected */
	b = cons(b, NIL);        /* still protected */
	a = unsave(1);
	a = cons(a, b);

	----- ADDITIONAL PREDICATES ------------------------------------

	----- constant_p(x) --------------------------------------------

	This predicate succeeds, if the object passed to it has its
	CONST_TAG set, i.e. if it should be considered to be immutable.

	Example:

	if (constant_p(x))
	        /* error: x is constant */

	----- number_p(x) ----------------------------------------------

	The number_p() predicate succeeds, if its argument is either a
	bignum integer or a real number.

	----- ADDITIONAL ACCESSORS -------------------------------------

	----- caar(x)\fP ... \f[CB]cddddr(x) ---------------------------

	These are the usual LISP accessors for nested lists and trees.
	For instance,

	cadr(x)

	denotes the "car of the cdr of x". All names can be decoded by
	reading their "a"s and "d"s from the right to the left, where
	each "a" denotes a car accessor, and each "d" a cdr, e.g.

	  cadadr of ((1 2) (8 9))
	= cadar  of ((8 9))
	= cadr   of (8 9)
	= car    of (9)
	=           9

	----- tag(x) ---------------------------------------------------

	The tag() accessor extracts the "tag" field of a node. It is
	mostly used in the implementation of type predicates, to find
	out whether a node has its S9_ATOM_TAG set. For instance:

	#define T_DICTIONARY (USER_SPECIALS-1)
	#define dictionary_p(n)            \
	        (!special_p(n) &&          \
	         (tag(n) & S9_ATOM_TAG) && \
	         car(n) == T_DICTIONARY)

	===== PRIMITIVE PROCEDURES =====================================

	A S9core primitive function consists of a PRIM entry describing
	the primitive, and a "handler" implementing it. Here is a PRIM
	structure describing the Scheme procedure list-tail which, given
	a list and an integer /n/, returns the tail starting at the
	/n^th/ element of the list.

	{ "list-tail", p_list_tail, 2, 2,
	  { T_LIST,  T_INTEGER, T_ANY } },

	The corresponding handler, p_list_tail(), looks like this:

	cell pp_list_tail(cell x) {     
	    cell    p, n;
	
	    n = bignum_to_int(cadr(x));
	    if (n == UNDEFINED)
	        return error("int argument too big");
	    for (p = car(x); p != NIL; p = cdr(p), n--) {
	        if (!pair_p(p))
	            return error("not a proper list");
	        if (n <= 0)
	            break;
	    }
	    if (n != 0)
	        return error("int argument too big");
	    return p;
	}

	Like all primitive handlers, p_list_tail() is a function from
	cell to cell, but the argument it receives is actually a T_LIST
	of arguments, so car accesses the first argument and cadr the
	second one.

	The function first extracts the value of the integer argument
	and checks for overflow (multi-segment bignum). It then
	traverses the list argument, decrementing /n/ until n=0 or the
	end of the list is reached. After some final error checking, it
	returns the tail of the given list.

	Primitive handlers usually do not have to type-check their
	arguments, because there is a function that can do that /before/
	dispatching the handler. See below.

	----- char *typecheck(cell f, cell a); -------------------------

	The typecheck() function expects a primitive function object /f/
	and an argument list /a/. It checks the types of the arguments
	in /a/ against the type tags in the PRIM structure of /f/. When
	all arguments match, it returns NULL.

	When a type mismatch is found, the function returns a string
	explaining the nature of the type error in plain English. For
	example, passing a T_LIST and a T_CHAR to list-tail would return
	the message

	list-tail: expected integer in argument #2

	The program could then add a visual representation of the actual
	arguments that were about to be passed to the handler.

	----- cell apply_prim(cell f, cell a); -------------------------

	The apply_prim() function extracts the handler from the
	primitive function object /f/, calls it with the parameter /a/,
	and delivers the return value of the handler.

	apply_prim() itself does /not/ protect its arguments. Doing so
	is in the responsibility of the implementation.

	===== SYMBOL MANAGEMENT ========================================

	----- cell find_symbol(char *s); -------------------------------

	This function searches the internal symbol list for the given
	symbol. When the symbol is in the list ("interned"; see also
	intern_symbol(), below), then it returns a reference to it.
	Otherwise, it returns NIL.

	----- cell intern_symbol(cell y); ------------------------------

	This function adds the given symbol to an internal list of
	symbols. Symbols contained in that list are called "interned"
	symbols, and only those symbols can be checked for identity
	(i.e. compared with C's == operator).

	The intern_symbol() function should only be used to intern
	"uninterned" symbols, i.e. symbols created by make_symbol().
	Symbols created by symbol_ref() are automatically interned.

	Note: while uninterned symbols have their uses, almost all
	common use cases rely on interned symbols.

	----- cell symbol_to_string(cell x); ---------------------------
	----- cell string_to_symbol(cell x); ---------------------------

	symbol_to_string() returns a string object containing the name
	of the given symbol. string_to_symbol() is the inverse operation;
	it returns a symbol with the name given as its string argument.
	It also makes sure that the new symbol is interned.

	===== BIGNUM ARITHMETICS =======================================

	Bignum arithmetics can never overflow, but performance will
	degrade linearly as numbers grow bigger.

	----- Zero, One, Two, Ten --------------------------------------

	These are constants for common values, so you do not have to
	allocate them using make_integer().

	----- cell bignum_abs(cell a); ---------------------------------

	This function returns the absolute value (magnitude) of its
	argument, i.e.  the original value with a positive sign.

	----- cell bignum_add(cell a, cell b); -------------------------

	bignum_add() adds two integers and returns their result.

	----- cell bignum_divide(cell a, cell b); ----------------------

	bignum_divide() divides /a/ by /b/ and returns both the
	truncated integer quotient trunc(a/b) and the truncated
	division remainder a-trunc(a/b)*b (where trunc removes any
	non-zero fractional digits from its argument).

	The result is delivered as a cons node with the quotient in
	the car and the remainder in the cdr field. For example, given

	cell a = make_integer(-23),
	     b = make_integer(7);
	cell r = bignum_divide(a, b);

	the result would be equal to

	car(r) = make_integer(-3); /*       trunc(-23/7)   */
	cdr(r) = make_integer(-2); /* -23 - trunc(-23/7)*7 */

	----- int bignum_equal_p(cell a, cell b); ----------------------

	This predicate returns 1, if its arguments are equal.

	----- int bignum_even_p(cell a); -------------------------------

	This predicate returns 1, if its argument is divisible by 2 with
	a remainder of 0. See bignum_divide().

	----- int bignum_less_p(cell a, cell b); -----------------------

	This predicate returns 1, if its argument /a/ has a smaller value
	than its argument /b/.

	----- cell bignum_multiply(cell a, cell b); --------------------

	bignum_multiply() multiplies two integers and returns their
	product.

	----- cell bignum_negate(cell a); ------------------------------

	This function returns its argument with its sign reversed.

	----- cell bignum_shift_left(cell a, int fill); ----------------

	The bignum_shift_left() function shifts its argument /a/ to the
	left by one decimal digit and then replaces the rightmost digit
	with /fill/. Note that 0<=fill<=9 must hold!

	Example:

	cell n = make_integer(1234);
	bignum_shift_left(x, 5); /* 12345 */

	----- cell bignum_shift_right(cell a); -------------------------

	bignum_shift_right() shifts its argument to the right by one
	decimal digit. It returns a node with the shifted argument in
	the car part. The cdr part will contain the digit that "fell out"
	on the right side.

	Example:

	cell n = make_integer(12345);
	cell r = bignum_shift_right(n);

	The result would be equal to the following:

	car(r) = make_integer(1234);
	cdr(r) = make_integer(5);

	----- cell bignum_subtract(cell a, cell b); --------------------

	This function returns the difference /a-b/.

	----- cell bignum_to_real(cell a); -----------------------------

	The bignum_to_real() function converts a bignum integer to a
	real number. Note that for big integers, this will lead to a
	loss of precision. E.g., converting the integer

	340282366920938463463374607431768211456

	to real on a machine with a mantissa size of 18 digits will
	yield:

	3.40282366920938463e+38"

	Converting it back to bignum integer will give:

	340282366920938463000000000000000000000

	----- cell bignum_to_string(cell x); ---------------------------

	bignum_to_string() will return a string object containing the
	decimal representation of the given bignum integer. The string
	will be allocated in the vector pool, so it is safe to convert
	/really/ big integers.

	===== REAL NUMBER ARITHMETICS ==================================

	All real number operations except those with a Real_ or S9_
	prefix (capital first letter) accept bignum operands and
	convert them to real numbers silently. Of course, this may
	cause a loss of precision when large bignums are involved
	in a computation.

	When /both/ operands of a real number operation are bignums,
	the function will perform a precise bignum computation instead
	(except for real_divide(), which will always perform a real
	number division).

	Note that S9core real numbers are base-10 (ten), so 1/2, 1/4,
	1/5, 1/8 have exact results, but 1/3, 1/6, 1/7, and 1/9 do not.

	----- Epsilon --------------------------------------------------

	Epsilon (e) is a very small number: 10^-(S9_MANTISSA_SIZE+1).
	By all practical means, two numbers /a/ and /b/ should be
	considered to be equal, if their difference is not greater
	than /e/, i.e. |a-b|<=e.

	Of course, much smaller numbers can be expressed /and ordered/
	by S9core (using real_less_p()), but the difference between two
	very small numbers becomes insignificant as it approaches /e/.

	This is particularly important when computing converging series.
	Here the precision cannot increase any further when the
	difference between the current guess x_i and previous guess
	x_i-1 drops below /e/. So the computation has reached a fixed
	point when |x_i - x_i-1| <= e.

	Technically, the value of Epsilon is chosen in such a way that
	its number of fractional digits is one more than the mantissa
	size, so it cannot represent an /exact/ difference between
	/any/ two exact real numbers. For example (given a mantissa
	size of 9 digits,

	0.999999999 + 0.000000001 = 1.0

	but

	0.999999999 + 0.0000000001 = 0.999999999

	In this example, the smaller value in the second equation would
	be equal to Epsilon.

	----- Real_flags(x) --------------------------------------------
	----- Real_exponent(x) -----------------------------------------
	----- Real_mantissa(x) -----------------------------------------
	----- Real_negative_flag(x) ------------------------------------

	The Real_mantissa() and Real_exponent() macros are just more
	efficient versions of the real_mantissa() and real_exponent()
	functions. Unlike their function counterparts, they accept real
	number operands exclusively. Real_flags() is described in the
	section on tagged types. Real_negative_flag() extracts the
	"negative sign" flag from the flags field of the given real
	number.

	****************************************************************
	Note: Real_mantissa() returns a chain of integer segments
	without a type tag!
	****************************************************************

	----- Real_zero_p(x) -------------------------------------------
	----- Real_negative_p(x) ---------------------------------------
	----- Real_positive_p(x) ---------------------------------------

	These predicate macros test whether the given real number is
	zero, negative, or positive, respectively.

	----- Real_negate(a) -------------------------------------------

	This macro negates the given real number (returning a new real
	number object).

	****************************************************************
	Real_negate() does not protect its argument!
	****************************************************************

	----- cell real_abs(cell a); -----------------------------------

	The real_abs() function returns the magnitude (absolute value)
	of its argument (the original value with a positive sign).

	----- cell real_add(cell a, cell b); ---------------------------

	This function returns the sum of its arguments.

	****************************************************************
	When the arguments /a/ and /b/ differ by /n/ orders of magnitude,
	where n>=S9_MANTISSA_SIZE, then the sum will be equal to the
	larger of the two arguments. E.g. (given a mantissa size of 9):

	1000000000.0 + 9.0 = 1000000000.0"

	because the result (1000000009) would not fit in a mantissa.
	Even with values that overlap only partially, the result will
	be truncated, resulting in loss of precision.
	****************************************************************

	This is not a bug, but an inherent property of floating point
	arithmetics.

	----- cell real_divide(cell x, cell a, cell b); ----------------

	This function returns the quotient /a/b/. Loss of precision may
	occur, e.g.:

	1.0 / 3 * 3 = 0.999999999"

	(given a mantissa size of 9).

	The function /always/ performs a real number division, even if
	both arguments are integers.

	----- int real_equal_p(cell a, cell b); ------------------------

	The real_equal_p() predicate succeeds, if its arguments are
	equal. In S9core, two real numbers are equal, if they look
	equal when printed with print_real().

	However, the result of a real number operation may not be equal
	to a specific real number, even if expected. For instance,

	1.0 / 3 * 3  =/=  1.0

	Generally, equality of real numbers implemented using a floating
	point representation should be considered with care. This
	applies not only to the S9core operations, but even to common
	hardware implementations of real numbers. See also: Epsilon

	----- cell real_floor(cell x); ---------------------------------
	----- cell real_trunc(cell x); ---------------------------------
	----- cell real_ceil(cell x); ----------------------------------

	These functions round the given real number as shown in figure 2.

	           round
	function   toward sample rounded
	---------- ------ ------ -------
	real_floor   -inf    1.5     1.0
	                    -1.5    -2.0
	--------------------------------
	real_trunc      0    1.5     1.0
	                    -1.5    -1.0
	--------------------------------
	real_ceil    +inf    1.5     2.0
	                    -1.5    -1.0
	--------------------------------
	Fig 2. Rounding

	----- cell real_integer_p(cell x); -----------------------------

	This predicate succeeds, if the given number is an integer, i.e.
	has a fractional part of 0. This is trivially true for bignum
	integers.

	----- int real_less_p(cell a, cell b); -------------------------

	This predicate succeeds, if a<b.

	----- cell real_multiply(cell a, cell b); ----------------------

	This function returns the product of its arguments.

	----- cell real_negate(cell a); --------------------------------

	This function returns its argument with its sign reversed.

	----- cell real_negative_p(cell a); ----------------------------
	----- cell real_positive_p(cell a); ----------------------------
	----- cell real_zero_p(cell a); --------------------------------

	These predicates test whether the given number is zero, negative,
	or positive, respectively.

	----- cell real_power(cell a, cell b); -------------------------

	This function returns a^b. Both /a/ and /b/ may be real numbers,
	but when /b/ has a fractional part, /a/ must be positive (i.e.
	the result of real_power() may not be a complex number).

	----- cell real_subtract(cell a, cell b); ----------------------

	The real_subtract() function returns the difference a-b. The
	caveats regarding real number addition (see real_add()) also
	apply to subtraction.

	----- cell real_to_bignum(cell r); -----------------------------

	This function converts integers in real number format to bignum
	integers. Real numbers with a non-zero fractional part cannot be
	converted and will yield a result of UNDEFINED.

	Note that converting large real number will result in bignum
	integers with lots of zeros. Converting very large numbers may
	terminate the S9core process or, in case the memory limit has
	been removed, result in allocation of huge amounts of memory.
	For example, converting the number 1e+1000000 would create a
	string of 1 million zeros (and one one) and allocate about 25M
	bytes of memory in the process (on a 64-bit system). Also, the
	process would take a very long time.

	This function is most useful for real numbers with a magnitude
	not larger than the mantissa size.

	===== INPUT / OUTPUT ===========================================

	S9core input and output is based on "ports". A port is a handle
	to a garbage-collected object. On the C level, a port is a small
	integer (an index to the Ports[] array). On the S9core level, a
	T_INPUT_PORT or T_OUTPUT_PORT type tag is attached to the handle
	to make it distinguishable to the type checker.

	There are input ports and output ports, but no bidirectional
	ports for both input and output.

	When the garbage collector can prove that a port is inaccessible,
	it will finalize and recycle it. Of course, this works only for
	S9core ports. At C level, a port has to be "locked" (see
	lock_port()) to protect it from being recycled.

	Input ports are finalized by closing them, output ports by
	flushing and closing them.

	All I/O operations are performed on two implicit ports called
	the "current input port and "current output port". There are
	procedures for selecting these ports (e.g.  set_input_port()).

	The standard I/O files stdin, stdout, and stderr are assigned
	to the port handles 0, 1, and 2 when S9core is initialized.
	These ports are locked from the beginning.

	----- int blockread(char *s, int k); ---------------------------

	This function reads up to /k/ character from the current input
	port and stores them in /s/. It returns the number of characters
	read. When an I/O error occurs, it updates the internal I/O
	status (see io_status()).

	----- int readc(void) ------------------------------------------
	----- void rejectc(int c) --------------------------------------

	readc() reads a single character from the current input port and
	returns it. A return value of -1 indicates the EOF or an error.

	The rejectc() function inserts a character into the input port,
	so the next call to readc() (or blockread() ) will return it. In
	combination with readc(), it can be used to look ahead in the
	input stream.

	Example:

	cell peek = readc();
	rejectc(peek);

	At most two characters may be rejected subsequently, i.e. the
	reject buffer has a length of two characters.

	----- void blockwrite(char *s, int k); -------------------------

	This function writes /k/ characters from /s/ to the current
	output port. It returns the number of characters written. When
	an I/O error occurs, it updates the internal I/O status (see
	io_status()).

	----- int port_eof(int p); -------------------------------------

	This function returns a non-zero value, if reading beyond the
	EOF has been attempted on the given port. Otherwise it returns 0.

	----- void prints(char *s); ------------------------------------

	prints() writes the C string /s/ to the current output port.

	----- void print_bignum(cell n); -------------------------------

	The print_bignum() function writes the decimal representation of
	the bignum integer /n/ to the current output port.

	----- void print_expanded_real(cell x); ------------------------
	----- void print_real(cell x); ---------------------------------
	----- void print_sci_real(cell x); -----------------------------

	These functions all write representations of the real number /x/
	to the current output port. print_expanded_real() prints all
	digits of the real number, both the integer and fractional parts.
	print_sci_real() prints numbers in "scientific notation" with a
	normalized mantissa and an exponent. E.g., 123.45 will print as
	1.2345e+2, meaning 1.2345 times 10^2. The exponent character may
	vary; see the exponent_chars() function for details.

	The print_real() function will print numbers in expanded
	notation when there is an exact representation for that number,
	and otherwise it will print it in scientific notation.

	----- nl() -----------------------------------------------------

	nl() is short for prints("\n");.

	----- void flush(void); ----------------------------------------

	flush() commits all pending write operations on the current
	output port.

	----- int io_status(void); -------------------------------------
	----- void io_reset(void); -------------------------------------

	The io_status() function returns the internal I/O state. When it
	returns 0, no I/O error has occurred since the call of io_reset()
	(or the initialization of S9core). When it returns -1, an I/O
	error has occurred in between.

	io_reset() resets the I/O status to zero.

	These two functions can be used to perform multiple I/O
	operations in a row without having to check each return value.
	Once the I/O state was changed to -1, it will stay that way
	until explicitly reset using io_reset() .

	----- int open_input_port(char *path); -------------------------
	----- int open_output_port(char *path, int append); ------------
	----- void close_port(int port); -------------------------------

	open_input_port() opens a file for reading and returns a port
	handle for accessing that file. open_output_port() opens the
	given file for output and returns a handle. When the /append/
	flag is zero, it creates the file. It will truncate any
	preexisting file to zero length. When the /append/ flag is one,
	it will append data to an existing file. It still creates the
	file, if it does not exist.

	The port opening functions return a negative value in case of
	an error.

	The close_port() function closes the file associated with the
	given port handle and frees the handle. It can be used to close
	locked ports (see below), thereby unlocking them in the process.

	----- char *open_input_string(char *s); ------------------------
	----- void close_input_string(void); ---------------------------

	open_input_string() opens a string as input source and
	immediately redirects the current input port to that string.
	readc() and rejectc() work as expected on string input, but
	blockread() does not. The function returns the previous input
	string, if any, and NULL otherwise.

	close_input_string() ends input from a string and reestablishes
	the input port that was in effect before opening the string (it
	does not reestablish a previous input string, though!).

	----- int lock_port(int port); ---------------------------------
	----- int unlock_port(int port); -------------------------------

	These functions lock and unlock a port, respectively. Locking
	a port protects it from being finalized and recycled by the
	garbage collector. For example, a function opening a file and
	packaging the resulting port in a T_INPUT_PORT object, would
	need to lock the port:

	int port = open_input_port("some-file");
	lock_port(port);
	cell n = make_port(port, T_INPUT_PORT);
	unlock_port(port);

	Without locking the port, the make_port() function might close
	the freshly opened port when it triggers a GC. After unlocking
	the port, the T_INPUT_PORT object protects the port, /iff/
	it is accessible through a GC root (on the stack, bound to a
	symbol, etc).

	----- int input_port(void); ------------------------------------
	----- int output_port(void); -----------------------------------

	These functions return the current input port and current
	output port, respectively. input_port() returns -1 when
	input is currently being read from a string.

	----- cell set_input_port(cell port); --------------------------
	----- cell set_output_port(cell port); -------------------------
	----- void reset_std_ports(void); ------------------------------

	The set_input_port() function redirect all input to the given
	port. All read operations (readc(), blockread()) will use the
	given port after calling this function. The given port will
	become the new "current input port".

	set_output_port() changes the current output port, affecting
	blockwrite(), prints(), etc.

	The reset_std_ports() function sets the current input stream
	(handle 0) to stdin, the current output stream (handle 1) to
	stdout, and port handle 2 to stderr. It also clears the error
	and EOF flags of all standard ports.

	----- void set_printer_limit(int k); ---------------------------
	----- int printer_limit(void); ---------------------------------

	When set_printer_limit() is used to specify a non-zero
	"printer limit" /k/, then the output functions (like prints(),
	blockwrite(), etc) will write /k/ characters at most and
	discard any excess output. The printer_limit() function
	returns a non-zero value, if the printer limit has been
	reached (so that no further characters will be written).

	Specifying a printer limit of zero will remove any existing
	limit.

	Printer limits are useful for printing partial data, for
	instance in error messages. This is especially useful when
	outputting cyclic structures, which would otherwise print
	indefinitely.

	===== HEAP IMAGES ==============================================

	----- char *dump_image(char *path, char *magic); ---------------

	The dump_image() function writes a heap image to the given path.
	The /magic/ parameter must be a string of up to 16 characters
	that will be used for a magic ID when loading images.

	Heap images work only, if  /all/ state of the language
	implementation using S9core is kept on the heap. Internal
	variables referring to the heap must be included as image
	variables. See the image_vars() function, below.

	dump_image() will return NULL on success or an error message
	in case of failure.

	----- void image_vars(cell **v); -------------------------------
	----- void add_image_vars(cell **v); ---------------------------

	The parameter of image_vars() is a list of addresses of cells
	that need to be saved to a heap image. This basically includes
	all non-temporary cell variables that reference the node pool
	when an image is dumped, for example: a symbol table, an
	interpreter stack, etc.

	add_image_vars() is similar to image_vars(), but adds image
	variables to an existing list. Calling image_vars() will clear
	any previously existing list.

	All variables that are GC roots [see s9_init()] and all global
	symbols [see symbol_ref()] also have to be included in the
	image.

	Internal S9core variables are included automatically and do
	not have to be specified here.

	----- char *load_image(char *path, char *magic); ---------------

	The load_image() function loads a heap image file from the given
	path. It expects the heap image to contain the given magic ID
	(or the load will fail). See dump_image() for details.

	When an image could be successfully loaded, the function will
	return NULL.  In case of failure, it will deliver an explanatory
	error message in plain English.

	****************************************************************
	If load_image() fails, it leaves the heap in an undefined state.
	****************************************************************

	In this case, the following options exist:

	- Load a different image
	- Restart S9core by calling s9_fini() and then s9_init()
	- Terminate the S9core process by calling fatal()

	===== MEMORY MANAGEMENT ========================================

	----- int gc(void); --------------------------------------------
	----- int gcv(void); -------------------------------------------

	The gc() function starts a node pool garbage collection and
	returns the number of nodes reclaimed. gcv() starts a vector
	pool garbage collection and compaction and returns the number
	of free cells in the vector pool.

	GC is normally triggered by the allocator functions, but
	sometimes you might want to start from some known state (e.g.
	when benchmarking).

	----- void gc_verbosity(int n); --------------------------------

	When the parameter /n/ of gc_verbosity() is set to 1, S9core
	will print information about pool growth to stdout.  When n=2,
	it will also print the number of nodes/cells reclaimed in each
	GC. n=0 disables informational messages.

	===== STRING / NUMBER CONVERSION ===============================

	----- void exponent_chars(char *s); ----------------------------

	This function specifies the characters that will be interpreted
	as exponent signs in real numbers by string_numeric_p() and
	string_to_real().

	The first character of the string passed to this function will
	be used to denote exponents in the output of print_sci_real().

	The default exponent characters are "eE".

	----- int integer_string_p(char *s); ---------------------------
	----- int string_numeric_p(char *s); ---------------------------
	
	string_numeric_p() checks whether the given string represents a
	number. A number consists of the following parts:

	- an optional + or - sign
	- a non-empty sequence of decimal digits with an optional
	  decimal point at any position
	- an optional exponent character followed by another optional
	  sign and another non-empty sequence of decimal digits

	Subsequently, valid numbers would include, for instance:

	0  +123  -1  .1  +1.23e+5  1e6  .5e-2

	integer_string_p() checks whether a string represents an integer,
	i.e. a non-empty sequence of digits with an optional leading +/-
	sign. Each integer is trivially a number by the above rules.

	----- cell string_to_bignum(char *s); --------------------------

	The string_to_bignum() function converts a numeric string (see
	integer_string_p()) to a bignum integer and returns it. The
	result of this function is undefined, if its argument does not
	represent an integer.

	----- cell string_to_real(char *s); ----------------------------

	The string_to_real() function converts a numeric string (as
	recognized by string_numeric_p()) to a real number and returns
	it. The result of this function is undefined, if its argument
	does not represent a real number. It returns UNDEFINED, if the
	given exponent is too large.

	Converting the string to real will lead to loss of precision,
	if the mantissa does not fit in the internal representation,
	e.g.

	string_to_real("3.1415926535897932384626")

	will result in 3.14159265 when the internal format uses a
	9-digit mantissa. In this case, the result will be truncated
	(rounded towards zero).

	----- cell string_to_number(char *s); --------------------------

	This function converts integer representations to bignums and
	real number representations (containing decimal points or
	exponent characters) to real numbers. Its result is undefined
	for non-numeric strings. See also: string_to_bignum(),
	string_to_real(), integer_string_p().

	===== COUNTERS =================================================

	----- counter --------------------------------------------------

	A "counter" is a structure for counting events. It can be reset,
	incremented, and read. See the following functions for details.

	----- void reset_counter(counter *c); --------------------------

	This function resets the given counter to zero.

	----- void count(counter *c); ----------------------------------

	This function increments the given counter by one. Counters
	overflow at one quadrillion (10^15). There is no overflow
	checking.

	----- cell read_counter(counter *c); ---------------------------

	This function converts the value of the given counter into a
	list of numbers in the range 0..999, where the first number
	represents the trillions, the second one the billions, etc.
	The last number contains the "ones" of the counter. E.g.
	reading a counter with a value of 12,345,678 would return

	(0 0 12 345 678)

	----- INTERNAL COUNTERS ----------------------------------------

	----- void run_stats(int run); ---------------------------------

	When run_stats() is called with a non-zero arguments, it resets
	all internal S9core counters and starts counting. When passed a
	zero argument, it stops counting and leaves the counters
	untouched. The counter values can be extracted using the
	get_counters() function.

	----- void cons_stats(int on); ---------------------------------

	Passing a non-zero value to cons_stats() activates the the
	internal cons counter of S9core. Passing zero to the function
	deactivates the counter (but does not reset it).

	Cons counting is usually activated before dispatching a
	primitive function and immediately deactivated thereafter.
	It counts allocation requests made by a program being
	interpreted rather than requests made by the interpreter.

	----- void get_counters(counter **n, counter **c, counter **g);

	This function retrieves the values of the three internal S9core
	counters that start when run_stats() is called with a non-zero
	argument. These counters count

	- the number of nodes allocated in total (/n/)
	- the number of nodes allocated by a program (/c/)
	- the number of garbage collections performed (/g/)

	The /n/, /c/, and /g/ varialbles can be passed to read_counter
	to convert them to a (machine-)readable form.

	===== UTILITY FUNCTIONS ========================================

	----- cell argv_to_list(char **argv); --------------------------

	The argv_to_list() function converts a C-style argv argument
	vector to a LISP-style list of strings, containing one command
	line argument per string. It returns the list.

	----- long asctol(char *s); ------------------------------------

	The asctol() function is like atol(), but does not interpret a
	leading 0 as a base-8 prefix, like Plan 9's atol() does.

	----- void fatal(char *msg); -----------------------------------

	This function prints the given message and then aborts program
	execution.

	----- cell flat_copy(cell n, cell *lastp); ---------------------

	flat_copy() copies the "spine" of the list /n/, i.e. the cons
	nodes connecting the elements of the list, giving a "shallow"
	or "flat" copy of the list, i.e. new spine, but identical
	elements.

	When /lastp/ is not NULL, it will be filled with the last cons
	of the fresh list, allowing, for instance, an O(1) destructive
	append. /lastp/ will be ignored, if /n/ is NULL.

	----- int length(cell n); --------------------------------------

	This function returns the number of elements in the list /n/.

	----------------------------------------------------------------
	CAVEATS
	----------------------------------------------------------------

	All caveats outlined here are due to garbage collection.
	This means that code exhibiting any of these issues /may/
	run properly most of the time and then fail unexpectedly.

	===== TEMPORARY VALUES =========================================

	A "temporary" value is a cell that is not part of any
	GC-protected structure, like the symbol table, the stack,
	or any other GC root. Temporary values are not protected in
	S9core and subject to recycling by the garcage collector.
	E.g. the value /n/ in

	cell n = cons(One, NIL);
	cell m = cons(Two, NIL); /* n is unprotected */

	is /not/ protected during the allocation of /m/ and may
	therefore be recycled.

	Most S9core functions allocate nodes, so a conservative
	premise would be that calling /any/ S9core function (with
	the obvious exception of accessors, like car(), string(),
	or port_no()), will destroy temporary values.

	There are several ways to protect temporary values. The
	most obvious one is to push the value on the stack during
	a critical phase:

	cell m, n = cons(One, NIL);
	save(n);
	m = cons(Two, NIL);
	unsave(1);

	A less versatile, but more lightweight approach would be to
	create a temporary protection object (Tmp) and add that to
	the GC root as specified in s9_init(). Using such an object,
	you could write:

	cell m, n = cons(One, NIL);
	Tmp = n;
	m = cons(Two, NIL);
	Tmp = NIL;

	Finally, all symbols created by symbol_ref() or interned by
	intern_symbol() are automatically protected, because they are
	stored in the internal S9core symbol table. So the following
	code is safe:

	cell n = symbol_ref("foo");
	cell m = cons(Two, NIL);

	Note that uninterned symbols (created by make_symbol()) are
	/not/ protected!

	===== LOCATIONS OF VECTOR PAYLOADS =============================

	Nodes never move once allocated, e.g., the location of /N/ will
	never change after executing

	N = make_vector(10);

	given that /N/ is protected from GC.

	However, "vector payloads" (the content of vectors, strings, and
	symbols) / will/ be moved during garbage collection by the
	vector pool compactor. Therefore, no S9core function may be
	called between retrieving the payload address of a vector and
	accessing it. For example, the following code WILL NOT WORK:

	cell S = make_string("foo", 3);
	save(S);
	char *s = string(S);
	cell n = make_string("", 10); /* string(S) may move */
	printf("%s\n", s);
	unsave(1);

	Because make_string() may trigger a vector pool garbage
	collection and compaction, the location of /s/ may change
	before it is printed by printf().

	To solve this issue, either make sure that no allocations
	take place after setting s = string(S) or always use the
	accessor string(S) in the place of /s/.

	Things are more complicated in statements like

	make_string(string(S), strlen(string(S)));

	As explained earlier [see T_STRING], this statement will /not/
	create a copy of the string /S/, because the location delivered
	by string(S) may become invalid before make_string() has a
	chance to copy it. See T_STRING for the proper procedure for
	copying strings.

	The same applies to locations delivered by the vector() and
	symbol_name() accessors.

	===== MIXING ASSIGNMENTS AND ALLOCATORS ========================

	Assignments to accessors must /never/ have an allocator in their
	rvalues. The statement

	car(n) = cons(One, Two); /* pool may move! */

	/will/ fail at some point, because the pool containing /n/ may
	move due to node pool reallocation.

	The cell /n/ is an index to an internal pool and car accesses a
	slot in that pool. When the cons in the above statement causes
	the node pool to grow, the pool will be realloc()'ed, so the
	original address of the pool may become invalid /before/ car
	can access the pool.

	The above works with some C compilers and does not with others,
	but either way, it is not covered by any C standard and should
	be avoided. The proper way to write the above would be:

	tmp = cons(One, Two);
	car(n) = tmp;

	For similar reasons, statements like

	return cdr(bignum_divide(a, b));

	will fail. Even here, storing the result in a temporary variable
	before taking the cdr would be the proper way.

	------------------------- THE END ------------------------------

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