Data representation in Guile
****************************

by Jim Blandy

[Due to the rather non-orthogonal and performance-oriented nature of the SCM interface, you need to understand SCM internals *before* you can use the SCM API.  That's why this chapter comes first.]

[NOTE: this is Jim Blandy's essay almost entirely unmodified.  It has to be adapted to fit this manual smoothly.]

In order to make sense of Guile's SCM_ functions, or read libguile's source code, it's essential to have a good grasp of how Guile actually represents Scheme values.  Otherwise, a lot of the code, and the conventions it follows, won't make very much sense.  This essay is meant to provide the background necessary to read and write C code that manipulates Scheme values in a way that is compatible with libguile.

We assume you know both C and Scheme, but we do not assume you are familiar with Guile's implementation.

Data Representation in Scheme
=============================

Scheme is a latently-typed language; this means that the system cannot, in general, determine the type of a given expression at compile time. Types only become apparent at run time.  Variables do not have fixed types; a variable may hold a pair at one point, an integer at the next, and a thousand-element vector later.  Instead, values have fixed types.

In order to implement standard Scheme functions like pair? and string? and provide garbage collection, the representation of every value must contain enough information to accurately determine its type at run time.  Often, Scheme systems also use this information to determine whether a program has attempted to apply an operation to an inappropriately typed value (such as taking the car of a string).

Because variables, pairs, and vectors may hold values of any type, Scheme implementations use a uniform representation for values --- a single type large enough to hold either a complete value or a pointer to a complete value, along with the necessary typing information.

The following sections will present a simple typing system, and then make some refinements to correct its major weaknesses.  However, this is not a description of the system Guile actually uses.  It is only an illustration of the issues Guile's system must address.  We provide all the information one needs to work with Guile's data in How Guile does it.

A Simple Representation
-----------------------

The simplest way to meet the above requirements in C would be to represent each value as a pointer to a structure containing a type indicator, followed by a union carrying the real value.  Assuming that SCM is the name of our universal type, we can write:

     enum type { integer, pair, string, vector, ... };

     typedef struct value *SCM;

     struct value {
       enum type type;
       union {
         int integer;
         struct { SCM car, cdr; } pair;
         struct { int length; char *elts; } string;
         struct { int length; SCM  *elts; } vector;
         ...
       } value;
     };

with the ellipses replaced with code for the remaining Scheme types.

This representation is sufficient to implement all of Scheme's semantics.  If X is an SCM value:

  * To test if X is an integer, we can write X->type == integer.

  * To find its value, we can write X->value.integer.

  * To test if X is a vector, we can write X->type == vector.

  * If we know X is a vector, we can write X->value.vector.elts[0] to refer to its first element.

  * If we know X is a pair, we can write X->value.pair.car to extract its car.

Faster Integers
---------------

Unfortunately, the above representation has a serious disadvantage.  In order to return an integer, an expression must allocate a struct value, initialize it to represent that integer, and return a pointer to it.  Furthermore, fetching an integer's value requires a memory reference, which is much slower than a register reference on most processors.  Since integers are extremely common, this representation is too costly, in both time and space.  Integers should be very cheap to create and manipulate.

One possible solution comes from the observation that, on many architectures, structures must be aligned on a four-byte boundary. (Whether or not the machine actually requires it, we can write our own allocator for struct value objects that assures this is true.) In this case, the lower two bits of the structure's address are known to be zero.

This gives us the room we need to provide an improved representation for integers.  We make the following rules:

  * If the lower two bits of an SCM value are zero, then the SCM value is a pointer to a struct value, and everything proceeds as before.

  * Otherwise, the SCM value represents an integer, whose value appears in its upper bits.

Here is C code implementing this convention:

     enum type { pair, string, vector, ... };

     typedef struct value *SCM;

     struct value {
       enum type type;
       union {
         struct { SCM car, cdr; } pair;
         struct { int length; char *elts; } string;
         struct { int length; SCM  *elts; } vector;
         ...
       } value;
     };

     #define POINTER_P(x) (((int) (x) & 3) == 0)
     #define INTEGER_P(x) (! POINTER_P (x))

     #define GET_INTEGER(x)  ((int) (x) >> 2)
     #define MAKE_INTEGER(x) ((SCM) (((x) << 2) | 1))

Notice that integer no longer appears as an element of enum type, and the union has lost its integer member.  Instead, we use the POINTER_P and INTEGER_P macros to make a coarse classification of values into integers and non-integers, and do further type testing as before.

Here's how we would answer the questions posed above (again, assume X is an SCM value):

  * To test if X is an integer, we can write INTEGER_P (X).

  * To find its value, we can write GET_INTEGER (X).

  * To test if X is a vector, we can write:

            POINTER_P (X) && X->type == vector

     Given the new representation, we must make sure X is truly a pointer before we dereference it to determine its complete type.

  * If we know X is a vector, we can write X->value.vector.elts[0] to refer to its first element, as before.

  * If we know X is a pair, we can write X->value.pair.car to extract its car, just as before.

This representation allows us to operate more efficiently on integers than the first.  For example, if X and Y are known to be integers, we can compute their sum as follows:

     MAKE_INTEGER (GET_INTEGER (X) + GET_INTEGER (Y))

Now, integer math requires no allocation or memory references.  Most real Scheme systems actually use an even more efficient representation, but this essay isn't about bit-twiddling.  (Hint: what if pointers had 01 in their least significant bits, and integers had 00?)

Cheaper Pairs
-------------

However, there is yet another issue to confront.  Most Scheme heaps contain more pairs than any other type of object; Jonathan Rees says that pairs occupy 45% of the heap in his Scheme implementation, Scheme 48.  However, our representation above spends three SCM-sized words per pair --- one for the type, and two for the CAR and CDR.  Is there any way to represent pairs using only two words?

Let us refine the convention we established earlier.  Let us assert that:

  * If the bottom two bits of an SCM value are #b00, then it is a pointer, as before.

  * If the bottom two bits are #b01, then the upper bits are an integer.  This is a bit more restrictive than before.

  * If the bottom two bits are #b10, then the value, with the bottom two bits masked out, is the address of a pair.

Here is the new C code:

     enum type { string, vector, ... };

     typedef struct value *SCM;

     struct value {
       enum type type;
       union {
         struct { int length; char *elts; } string;
         struct { int length; SCM  *elts; } vector;
         ...
       } value;
     };

     struct pair {
       SCM car, cdr;
     };

     #define POINTER_P(x) (((int) (x) & 3) == 0)

     #define INTEGER_P(x)  (((int) (x) & 3) == 1)
     #define GET_INTEGER(x)  ((int) (x) >> 2)
     #define MAKE_INTEGER(x) ((SCM) (((x) << 2) | 1))

     #define PAIR_P(x) (((int) (x) & 3) == 2)
     #define GET_PAIR(x) ((struct pair *) ((int) (x) & ~3))

Notice that enum type and struct value now only contain provisions for vectors and strings; both integers and pairs have become special cases.  The code above also assumes that an int is large enough to hold a pointer, which isn't generally true.

Our list of examples is now as follows:

  * To test if X is an integer, we can write INTEGER_P (X); this is as before.

  * To find its value, we can write GET_INTEGER (X), as before.

  * To test if X is a vector, we can write:

            POINTER_P (X) && X->type == vector

     We must still make sure that X is a pointer to a struct value before dereferencing it to find its type.

  * If we know X is a vector, we can write X->value.vector.elts[0] to refer to its first element, as before.

  * We can write PAIR_P (X) to determine if X is a pair, and then write GET_PAIR (X)->car to refer to its car.

This change in representation reduces our heap size by 15%.  It also makes it cheaper to decide if a value is a pair, because no memory references are necessary; it suffices to check the bottom two bits of the SCM value.  This may be significant when traversing lists, a common activity in a Scheme system.

Again, most real Scheme systems use a slighty different implementation; for example, if GET_PAIR subtracts off the low bits of x, instead of masking them off, the optimizer will often be able to combine that subtraction with the addition of the offset of the structure member we are referencing, making a modified pointer as fast to use as an unmodified pointer.

Guile Is Hairier
----------------

We originally started with a very simple typing system --- each object has a field that indicates its type.  Then, for the sake of efficiency in both time and space, we moved some of the typing information directly into the SCM value, and left the rest in the struct value. Guile itself employs a more complex hierarchy, storing finer and finer gradations of type information in different places, depending on the object's coarser type.

In the author's opinion, Guile could be simplified greatly without significant loss of efficiency, but the simplified system would still be more complex than what we've presented above.

How Guile does it
=================

Here we present the specifics of how Guile represents its data.  We don't go into complete detail; an exhaustive description of Guile's system would be boring, and we do not wish to encourage people to write code which depends on its details anyway.  We do, however, present everything one need know to use Guile's data.

General Rules
-------------

Any code which operates on Guile datatypes must #include the header file <libguile.h>.  This file contains a definition for the SCM typedef (Guile's universal type, as in the examples above), and definitions and declarations for a host of macros and functions that operate on SCM values.

All identifiers declared by <libguile.h> begin with scm_ or SCM_.

The functions described here generally check the types of their SCM arguments, and signal an error if their arguments are of an inappropriate type.  Macros generally do not, unless that is their specified purpose.  You must verify their argument types beforehand, as necessary.

Macros and functions that return a boolean value have names ending in P or _p (for ``predicate'').  Those that return a negated boolean value have names starting with SCM_N.  For example, SCM_IMP (X) is a predicate which returns non-zero iff X is an immediate value (an IM).  SCM_NCONSP (X) is a predicate which returns non-zero iff X is not a pair object (a CONS).

Garbage Collection
------------------

Aside from the latent typing, the major source of constraints on a Scheme implementation's data representation is the garbage collector. The collector must be able to traverse every live object in the heap, to determine which objects are not live.

There are many ways to implement this, but Guile uses an algorithm called mark and sweep.  The collector scans the system's global variables and the local variables on the stack to determine which objects are immediately accessible by the C code.  It then scans those objects to find the objects they point to, et cetera.  The collector sets a mark bit on each object it finds, so each object is traversed only once.  This process is called tracing.

When the collector can find no unmarked objects pointed to by marked objects, it assumes that any objects that are still unmarked will never be used by the program (since there is no path of dereferences from any global or local variable that reaches them) and deallocates them.

In the above paragraphs, we did not specify how the garbage collector finds the global and local variables; as usual, there are many different approaches.  Frequently, the programmer must maintain a list of pointers to all global variables that refer to the heap, and another list (adjusted upon entry to and exit from each function) of local variables, for the collector's benefit.

The list of global variables is usually not too difficult to maintain, since global variables are relatively rare.  However, an explicitly maintained list of local variables (in the author's personal experience) is a nightmare to maintain.  Thus, Guile uses a technique called conservative garbage collection, to make the local variable list unnecessary.

The trick to conservative collection is to treat the stack as an ordinary range of memory, and assume that every word on the stack is a pointer into the heap.  Thus, the collector marks all objects whose addresses appear anywhere in the stack, without knowing for sure how that word is meant to be interpreted.

Obviously, such a system will occasionally retain objects that are actually garbage, and should be freed.  In practice, this is not a problem.  The alternative, an explicitly maintained list of local variable addresses, is effectively much less reliable, due to programmer error.

To accomodate this technique, data must be represented so that the collector can accurately determine whether a given stack word is a pointer or not.  Guile does this as follows:

  * Every heap object has a two-word header, called a cell.  Some objects, like pairs, fit entirely in a cell's two words; others may store pointers to additional memory in either of the words.  For example, strings and vectors store their length in the first word, and a pointer to their elements in the second.

  * Guile allocates whole arrays of cells at a time, called heap segments.  These segments are always allocated so that the cells they contain fall on eight-byte boundaries, or whatever is appropriate for the machine's word size.  Guile keeps all cells in a heap segment initialized, whether or not they are currently in use.

  * Guile maintains a sorted table of heap segments.

Thus, given any random word W fetched from the stack, Guile's garbage collector can consult the table to see if W falls within a known heap segment, and check W's alignment.  If both tests pass, the collector knows that W is a valid pointer to a cell, intentional or not, and proceeds to trace the cell.

Note that heap segments do not contain all the data Guile uses; cells for objects like vectors and strings contain pointers to other memory areas.  However, since those pointers are internal, and not shared among many pieces of code, it is enough for the collector to find the cell, and then use the cell's type to find more pointers to trace.

Immediates vs. Non-immediates
-----------------------------

Guile classifies Scheme objects into two kinds: those that fit entirely within an SCM, and those that require heap storage.

The former class are called immediates.  The class of immediates includes small integers, characters, boolean values, the empty list, the mysterious end-of-file object, and some others.

The remaining types are called, not suprisingly, non-immediates. They include pairs, procedures, strings, vectors, and all other data types in Guile.

 -- Macro: int SCM_IMP (SCM X)
     Return non-zero iff X is an immediate object.

 -- Macro: int SCM_NIMP (SCM X)
     Return non-zero iff X is a non-immediate object.  This is the exact complement of SCM_IMP, above.

     You must use this macro before calling a finer-grained predicate to determine X's type.  For example, to see if X is a pair, you must write:

          SCM_NIMP (X) && SCM_CONSP (X)

     This is because Guile stores typing information for non-immediate values in their cells, rather than in the SCM value itself; thus, you must determine whether X refers to a cell before looking inside it.

     This is somewhat of a pity, because it means that the programmer needs to know which types Guile implements as immediates vs. non-immediates. There are (possibly better) representations in which SCM_CONSP can be self-sufficient.  The immediate type predicates do not suffer from this weakness.

Immediate Datatypes
-------------------

The following datatypes are immediate values; that is, they fit entirely within an SCM value.  The SCM_IMP and SCM_NIMP macros will distinguish these from non-immediates; see Immediates vs. Non-immediates for an explanation of the distinction.

Note that the type predicates for immediate values work correctly on any SCM value; you do not need to call SCM_IMP first, to establish that a value is immediate.  This differs from the non-immediate type predicates, which work correctly only on non-immediate values; you must be sure the value is SCM_NIMP before applying them.

Integers
........

Here are functions for operating on small integers, that fit within an SCM.  Such integers are called immediate numbers, or INUMs.  In general, INUMs occupy all but two bits of an SCM.

Bignums and floating-point numbers are non-immediate objects, and have their own, separate accessors.  The functions here will not work on them.  This is not as much of a problem as you might think, however, because the system never constructs bignums that could fit in an INUM, and never uses floating point values for exact integers.

 -- Macro: int SCM_INUMP (SCM X)
     Return non-zero iff X is a small integer value.

 -- Macro: int SCM_NINUMP (SCM X)
     The complement of SCM_INUMP.

 -- Macro: int SCM_INUM (SCM X)
     Return the value of X as an ordinary, C integer.  If X is not an INUM, the result is undefined.

 -- Macro: SCM SCM_MAKINUM (int I)
     Given a C integer I, return its representation as an SCM. This function does not check for overflow.

Characters
..........

Here are functions for operating on characters.

 -- Macro: int SCM_ICHRP (SCM X)
     Return non-zero iff X is a character value.

 -- Macro: unsigned int SCM_ICHR (SCM X)
     Return the value of x as a C character.  If X is not a Scheme character, the result is undefined.

 -- Macro: SCM SCM_MAKICHR (SCM C)
     Given a C character C, return its representation as a Scheme character value.

Booleans
........

Here are functions and macros for operating on booleans.

 -- Macro: SCM SCM_BOOL_T
 -- Macro: SCM SCM_BOOL_F
     The Scheme true and false values.

 -- Macro: int SCM_NFALSEP (X)
     Convert the Scheme boolean value to a C boolean.  Since every object in Scheme except #f is true, this amounts to comparing X to #f; hence the name.

 -- Macro: SCM SCM_BOOL_NOT (X)
     Return the boolean inverse of X.  If X is not a Scheme boolean, the result is undefined.

Unique Values
.............

The immediate values that are neither small integers, characters, nor booleans are all unique values --- that is, datatypes with only one instance.

 -- Macro: SCM SCM_EOL
     The Scheme empty list object, or ``End Of List'' object, usually written in Scheme as '().

 -- Macro: SCM SCM_EOF_VAL
     The Scheme end-of-file value.  It has no standard written representation, for obvious reasons.

 -- Macro: SCM SCM_UNSPECIFIED
     The value returned by expressions which the Scheme standard says return an ``unspecified'' value.

     This is sort of a weirdly literal way to take things, but the standard read-eval-print loop prints nothing when the expression returns this value, so it's not a bad idea to return this when you can't think of anything else helpful.

 -- Macro: SCM SCM_UNDEFINED
     The ``undefined'' value.  Its most important property is that is not equal to any valid Scheme value.  This is put to various internal uses by C code interacting with Guile.

     For example, when you write a C function that is callable from Scheme and which takes optional arguments, the interpreter passes SCM_UNDEFINED for any arguments you did not receive.

     We also use this to mark unbound variables.

 -- Macro: int SCM_UNBNDP (SCM X)
     Return true if X is SCM_UNDEFINED.  Apply this to a symbol's value to see if it has a binding as a global variable.

Non-immediate Datatypes
-----------------------

A non-immediate datatype is one which lives in the heap, either because it cannot fit entirely within a SCM word, or because it denotes a specific storage location (in the nomenclature of the Revised^4 Report on Scheme).

The SCM_IMP and SCM_NIMP macros will distinguish these from immediates; see Immediates vs. Non-immediates.

Given a cell, Guile distinguishes between pairs and other non-immediate types by storing special tag values in a non-pair cell's car, that cannot appear in normal pairs.  A cell with a non-tag value in its car is an ordinary pair.  The type of a cell with a tag in its car depends on the tag; the non-immediate type predicates test this value.  If a tag value appears elsewhere (in a vector, for example), the heap may become corrupted.

Non-immediate Type Predicates
.............................

As mentioned in Garbage Collection, all non-immediate objects start with a cell, or a pair of words.  Furthermore, all type information that distinguishes one kind of non-immediate from another is stored in the cell.  The type information in the SCM value indicates only that the object is a non-immediate; all finer distinctions require one to examine the cell itself, usually with the appropriate type predicate macro.

The type predicates for non-immediate objects generally assume that their argument is a non-immediate value.  Thus, you must be sure that a value is SCM_NIMP first before passing it to a non-immediate type predicate.  Thus, the idiom for testing whether a value is a cell or not is:

     SCM_NIMP (X) && SCM_CONSP (X)

Pairs
.....

Pairs are the essential building block of list structure in Scheme.  A pair object has two fields, called the car and the cdr.

It is conventional for a pair's CAR to contain an element of a list, and the CDR to point to the next pair in the list, or to contain SCM_EOL, indicating the end of the list.  Thus, a set of pairs chained through their CDRs constitutes a singly-linked list. Scheme and libguile define many functions which operate on lists constructed in this fashion, so although lists chained through the CARs of pairs will work fine too, they may be less convenient to manipulate, and receive less support from the community.

Guile implements pairs by mapping the CAR and CDR of a pair directly into the two words of the cell.

 -- Macro: int SCM_CONSP (SCM X)
     Return non-zero iff X is a Scheme pair object. The results are undefined if X is an immediate value.

 -- Macro: int SCM_NCONSP (SCM X)
     The complement of SCM_CONSP.

 -- Macro: void SCM_NEWCELL (SCM INTO)
     Allocate a new cell, and set INTO to point to it.  This macro expands to a statement, not an expression, and INTO must be an lvalue of type SCM.

     This is the most primitive way to allocate a cell; it is quite fast.

     The CAR of the cell initially tags it as a ``free cell''.  If the caller intends to use it as an ordinary cons, she must store ordinary SCM values in its CAR and CDR.

     If the caller intends to use it as a header for some other type, she must store an appropriate magic value in the cell's CAR, to mark it as a member of that type, and store whatever value in the CDR that type expects.  You should generally not do this, unless you are implementing a new datatype, and thoroughly understand the code in <libguile/tags.h>.

 -- Function: SCM scm_cons (SCM CAR, SCM CDR)
     Allocate (``CONStruct'') a new pair, with CAR and CDR as its contents.

The macros below perform no typechecking.  The results are undefined if CELL is an immediate.  However, since all non-immediate Guile objects are constructed from cells, and these macros simply return the first element of a cell, they actually can be useful on datatypes other than pairs.  (Of course, it is not very modular to use them outside of the code which implements that datatype.)

 -- Macro: SCM SCM_CAR (SCM CELL)
     Return the CAR, or first field, of CELL.

 -- Macro: SCM SCM_CDR (SCM CELL)
     Return the CDR, or second field, of CELL.

 -- Macro: void SCM_SETCAR (SCM CELL, SCM X)
     Set the CAR of CELL to X.

 -- Macro: void SCM_SETCDR (SCM CELL, SCM X)
     Set the CDR of CELL to X.

 -- Macro: SCM SCM_CAAR (SCM CELL)
 -- Macro: SCM SCM_CADR (SCM CELL)
 -- Macro: SCM SCM_CDAR (SCM CELL) ...
 -- Macro: SCM SCM_CDDDDR (SCM CELL)
     Return the CAR of the CAR of CELL, the CAR of the CDR of CELL, et cetera.

Vectors, Strings, and Symbols
.............................

Vectors, strings, and symbols have some properties in common.  They all have a length, and they all have an array of elements.  In the case of a vector, the elements are SCM values; in the case of a string or symbol, the elements are characters.

All these types store their length (along with some tagging bits) in the CAR of their header cell, and store a pointer to the elements in their CDR.  Thus, the SCM_CAR and SCM_CDR macros are (somewhat) meaningful when applied to these datatypes.

 -- Macro: int SCM_VECTORP (SCM X)
     Return non-zero iff X is a vector. The results are undefined if X is an immediate value.

 -- Macro: int SCM_STRINGP (SCM X)
     Return non-zero iff X is a string. The results are undefined if X is an immediate value.

 -- Macro: int SCM_SYMBOLP (SCM X)
     Return non-zero iff X is a symbol. The results are undefined if X is an immediate value.

 -- Macro: int SCM_LENGTH (SCM X)
     Return the length of the object X. The results are undefined if X is not a vector, string, or symbol.

 -- Macro: SCM * SCM_VELTS (SCM X)
     Return a pointer to the array of elements of the vector X. The results are undefined if X is not a vector.

 -- Macro: char * SCM_CHARS (SCM X)
     Return a pointer to the characters of X. The results are undefined if X is not a symbol or a string.

There are also a few magic values stuffed into memory before a symbol's characters, but you don't want to know about those.  What cruft!

Procedures
..........

Guile provides two kinds of procedures: closures, which are the result of evaluating a lambda expression, and subrs, which are C functions packaged up as Scheme objects, to make them available to Scheme programmers.

(There are actually other sorts of procedures: compiled closures, and continuations; see the source code for details about them.)

 -- Function: SCM scm_procedure_p (SCM X)
     Return SCM_BOOL_T iff X is a Scheme procedure object, of any sort.  Otherwise, return SCM_BOOL_F.

Closures
........

[FIXME: this needs to be further subbed, but texinfo has no subsubsub]

A closure is a procedure object, generated as the value of a lambda expression in Scheme.  The representation of a closure is straightforward --- it contains a pointer to the code of the lambda expression from which it was created, and a pointer to the environment it closes over.

In Guile, each closure also has a property list, allowing the system to store information about the closure.  I'm not sure what this is used for at the moment --- the debugger, maybe?

 -- Macro: int SCM_CLOSUREP (SCM X)
     Return non-zero iff X is a closure.  The results are undefined if X is an immediate value.

 -- Macro: SCM SCM_PROCPROPS (SCM X)
     Return the property list of the closure X.  The results are undefined if X is not a closure.

 -- Macro: void SCM_SETPROCPROPS (SCM X, SCM P)
     Set the property list of the closure X to P.  The results are undefined if X is not a closure.

 -- Macro: SCM SCM_CODE (SCM X)
     Return the code of the closure X.  The results are undefined if X is not a closure.

     This function should probably only be used internally by the interpreter, since the representation of the code is intimately connected with the interpreter's implementation.

 -- Macro: SCM SCM_ENV (SCM X)
     Return the environment enclosed by X. The results are undefined if X is not a closure.

     This function should probably only be used internally by the interpreter, since the representation of the environment is intimately connected with the interpreter's implementation.

Subrs
.....

[FIXME: this needs to be further subbed, but texinfo has no subsubsub]

A subr is a pointer to a C function, packaged up as a Scheme object to make it callable by Scheme code.  In addition to the function pointer, the subr also contains a pointer to the name of the function, and information about the number of arguments accepted by the C fuction, for the sake of error checking.

There is no single type predicate macro that recognizes subrs, as distinct from other kinds of procedures.  The closest thing is scm_procedure_p; see Procedures.

 -- Macro: char * SCM_SNAME (X)
     Return the name of the subr X.  The results are undefined if X is not a subr.

 -- Function: SCM scm_make_gsubr (char *NAME, int REQ, int OPT, int REST, SCM (*FUNCTION)())
     Create a new subr object named NAME, based on the C function FUNCTION, make it visible to Scheme the value of as a global variable named NAME, and return the subr object.

     The subr object accepts REQ required arguments, OPT optional arguments, and a REST argument iff REST is non-zero.  The C function FUNCTION should accept REQ + OPT arguments, or REQ + OPT + 1 arguments if rest is non-zero.

     When a subr object is applied, it must be applied to at least REQ arguments, or else Guile signals an error.  FUNCTION receives the subr's first REQ arguments as its first REQ arguments.  If there are fewer than OPT arguments remaining, then FUNCTION receives the value SCM_UNDEFINED for any missing optional arguments.  If RST is non-zero, then any arguments after the first REQ + OPT are packaged up as a list as passed as FUNCTION's last argument.

     Note that subrs can actually only accept a predefined set of combinations of required, optional, and rest arguments.  For example, a subr can take one required argument, or one required and one optional argument, but a subr can't take one required and two optional arguments. It's bizarre, but that's the way the interpreter was written.  If the arguments to scm_make_gsubr do not fit one of the predefined patterns, then scm_make_gsubr will return a compiled closure object instead of a subr object.

Ports
.....

Haven't written this yet, 'cos I don't understand ports yet.

Signalling Type Errors
----------------------

Every function visible at the Scheme level should aggressively check the types of its arguments, to avoid misinterpreting a value, and perhaps causing a segmentation fault.  Guile provides some macros to make this easier.

 -- Macro: void SCM_ASSERT (int TEST, SCM OBJ, int POSITION, char *SUBR)
     If TEST is zero, signal an error, attributed to the subroutine named SUBR, operating on the value OBJ.  The POSITION value determines exactly what sort of error to signal.

     If POSITION is a string, SCM_ASSERT raises a ``miscellaneous'' error whose message is that string.

     Otherwise, POSITION should be one of the values defined below.

 -- Macro: int SCM_ARG1
 -- Macro: int SCM_ARG2
 -- Macro: int SCM_ARG3
 -- Macro: int SCM_ARG4
 -- Macro: int SCM_ARG5
     Signal a ``wrong type argument'' error.  When used as the POSITION argument of SCM_ASSERT, SCM_ARGN claims that OBJ has the wrong type for the N'th argument of SUBR.

     The only way to complain about the type of an argument after the fifth is to use SCM_ARGn, defined below, which doesn't specify which argument is wrong.  You could pass your own error message to SCM_ASSERT as the POSITION, but then the error signalled is a ``miscellaneous'' error, not a ``wrong type argument'' error.  This seems kludgy to me.

 -- Macro: int SCM_ARGn
     As above, but does not specify which argument's type is incorrect.

 -- Macro: int SCM_WNA
     Signal an error complaining that the function received the wrong number of arguments.

     Interestingly, the message is attributed to the function named by OBJ, not SUBR, so OBJ must be a Scheme string object naming the function.  Usually, Guile catches these errors before ever invoking the subr, so we don't run into these problems.

 -- Macro: int SCM_OUTOFRANGE
     Signal an error complaining that OBJ is ``out of range'' for SUBR.

Defining New Types (Smobs)
==========================

Smobs are Guile's mechanism for adding new non-immediate types to the system. (The term ``smob'' was coined by Aubrey Jaffer, who says it comes from ``small object'', referring to the fact that only the CDR and part of the CAR of a smob's cell are available for use.)  To define a new smob type, the programmer provides Guile with some essential information about the type --- how to print it, how to garbage collect it, et cetera --- and Guile returns a fresh type tag for use in the CAR of new cells.  The programmer can then use scm_make_gsubr to publish a set of C functions to the Scheme world that create and operate on these objects.

Describing a New Type
---------------------

To define a new type, the programmer must fill in an scm_smobfuns structure with functions to manage instances of the type.  Here is the definition of the structure:

     typedef struct scm_smobfuns
     {
       SCM       (*mark) (SCM OBJ);
       scm_sizet (*free) (SCM OBJ);
       int       (*print) (SCM OBJ,
                           SCM PORT,
                           scm_print_state *PSTATE);
       SCM       (*equalp) (SCM A, SCM B);
     } scm_smobfuns;

mark
     Guile will apply this function to each instance of the new type it encounters during garbage collection.  This function is responsible for telling the collector about any other non-immediate objects the object refers to.  See Garbage Collecting Smobs, for more details.

free
     Guile will apply this function to each instance of the new type it could not find any live pointers to.  The function should release all resources held by the object and return. See Garbage Collecting Smobs, for more details.

print
     Guile will apply this function to each instance of the new type to print the value, as for display or write.  The function should write a printed representation of EXP on PORT, in accordance with the parameters in PSTATE.  (For more information on print states, see Ports.)

equalp
     If Scheme code asks the equal? function to compare two instances of the same smob type, Guile calls this function.  It should return SCM_BOOL_T if A and B should be considered equal?, or SCM_BOOL_F otherwise.  If equalp is zero, equal? will assume that two instances of this type are never equal? unless they are eq?.

Once you have built a scm_smobfuns structure, you can call the scm_newsmob function to add the type to the system.

 -- Function: long scm_newsmob (scm_smobfuns *FUNS)
     This function adds the type described by FUNS to the system.  The return value is a tag, used in creating instances of the type.

For example, here is how one might declare and register a new type representing eight-bit grayscale images:

     #include <libguile.h>

     scm_smobfuns image_funs = {
       mark_image, free_image, print_image, 0
     };

     long image_tag;

     void
     init_image_type ()
     {
       image_tag = scm_newsmob (&image_funs);
     }

Creating Instances
------------------

Like other non-immediate types, smobs start with a cell whose CAR contains typing information, and whose cdr is free for any use. To create an instance of a smob type, you must allocate a fresh cell, by calling SCM_NEWCELL, and store the tag returned by scm_smobfuns in its car.

Guile provides the following functions for managing memory, which are often helpful when implementing smobs:

 -- Function: char * scm_must_malloc (long LEN, char *WHAT)
     Allocate LEN bytes of memory, using malloc, and return a pointer to them.

     If there is not enough memory available, invoke the garbage collector, and try once more.  If there is still not enough, signal an error, reporting that we could not allocate WHAT.

     This function also helps maintain statistics about the size of the heap.

 -- Function: char * scm_must_realloc (char *ADDR, long OLEN, long LEN, char *WHAT)
     Resize (and possibly relocate) the block of memory at ADDR, to have a size of LEN bytes, by calling realloc.  Return a pointer to the new block.

     If there is not enough memory available, invoke the garbage collector, and try once more.  If there is still not enough, signal an error, reporting that we could not allocate WHAT.

     The value OLEN should be the old size of the block of memory at ADDR; it is only used for keeping statistics on the size of the heap.

 -- Function: void scm_must_free (char *ADDR)
     Free the block of memory at ADDR, using free.  If ADDR is zero, signal an error, complaining of an attempt to free something that is already free.

     This does no record-keeping; instead, the smob's free function must take care of that.

     This function isn't usually sufficiently different from the usual free function to be worth using.

Continuing the above example, if the global variable image_tag contains a tag returned by scm_newsmob, here is how we could construct a smob whose CDR contains a pointer to a freshly allocated struct image:

     struct image {
       int width, height;
       char *pixels;

       /* The name of this image */
       SCM name;

       /* A function to call when this image is
          modified, e.g., to update the screen,
          or SCM_BOOL_F if no action necessary */
       SCM update_func;
     };

     SCM
     create_image (SCM name, int width, int height)
     {
       struct image *image;
       SCM image_smob;

       image = (struct image *) scm_must_malloc (sizeof (*image), "image");
       image->width = width;
       image->height = height;
       image->pixels = scm_must_malloc (width * height, "image pixels");
       image->name = name;
       image->update_func = SCM_BOOL_F;

       SCM_NEWCELL (image_smob);
       SCM_SETCAR (image_smob, image_tag);
       SCM_SETCDR (image_smob, image);

       return image_smob;
     }

Typechecking
------------

Functions that operate on smobs should aggressively check the types of their arguments, to avoid misinterpreting some other datatype as a smob, and perhaps causing a segmentation fault.  Fortunately, this is pretty simple to do.  The function need only verify that its argument is a non-immediate, whose CAR is the type tag returned by scm_newsmob.

For example, here is a simple function that operates on an image smob, and checks the type of its argument.  We also present an expanded version of the init_image_type function, to make clear_image available to the Scheme level.

     SCM
     clear_image (SCM image_smob)
     {
       int area;
       struct image *image;

       SCM_ASSERT ((SCM_NIMP (image_smob)
                    && SCM_CAR (image_smob) == image_tag),
                   image_smob, SCM_ARG1, "clear-image");

       image = (struct image *) SCM_CDR (image_smob);
       area = image->width * image->height;
       memset (image->pixels, 0, area);

       /* Invoke the image's update function.  */
       if (image->update_func != SCM_BOOL_F)
         scm_apply (image->update_func, SCM_EOL, SCM_EOL);

       return SCM_UNSPECIFIED;
     }

     void
     init_image_type ()
     {
       image_tag = scm_newsmob (&image_funs);
       scm_make_gsubr ("clear-image", 1, 0, 0, clear_image);
     }

Note that checking types is a little more complicated during garbage collection; see the description of SCM_GCTYP16 in Garbage Collecting Smobs.

Garbage Collecting Smobs
------------------------

Once a smob has been released to the tender mercies of the Scheme system, it must be prepared to survive garbage collection.  Guile calls the mark and free functions of the scm_smobfuns structure to manage this.

As described before (see Garbage Collection), every object in the Scheme system has a mark bit, which the garbage collector uses to tell live objects from dead ones.  When collection starts, every object's mark bit is clear.  The collector traces pointers through the heap, starting from objects known to be live, and sets the mark bit on each object it encounters.  When it can find no more unmarked objects, the collector walks all objects, live and dead, frees those whose mark bits are still clear, and clears the mark bit on the others.

The two main portions of the collection are called the mark phase, during which the collector marks live objects, and the sweep phase, during which the collector frees all unmarked objects.

The mark bit of a smob lives in its CAR, along with the smob's type tag.  When the collector encounters a smob, it sets the smob's mark bit, and uses the smob's type tag to find the appropriate mark function for that smob: the one listed in that smob's scm_smobfuns structure.  It then calls the mark function, passing it the smob as its only argument.

The mark function's is responsible for marking any other Scheme objects the smob refers to.  If it does not do so, the objects' mark bits will still be clear when the collector begins to sweep, and the collector will free them.  If this occurs, it will probably break, or at least confuse, any code operating on the smob; the smob's SCM values will have become dangling references.

To mark an arbitrary Scheme object, the mark function may call this function:

 -- Function: void scm_gc_mark (SCM X)
     Mark the object X, and recurse on any objects X refers to. If X's mark bit is already set, return immediately.

Thus, here is how we might write the mark function for the image smob type discussed above:

     SCM
     mark_image (SCM image_smob)
     {
       /* Mark the image's name and update function.  */
       struct image *image = (struct image *) SCM_CDR (image_smob);

       scm_gc_mark (image->name);
       scm_gc_mark (image->update_func);

       return SCM_BOOL_F;
     }

Note that, even though the image's update_func could be an arbitrarily complex structure (representing a procedure and any values enclosed in its environment), scm_gc_mark will recurse as necessary to mark all its components.  Because scm_gc_mark sets an object's mark bit before it recurses, it is not confused by circular structures.

As an optimization, the collector will mark whatever value is returned by the mark function; this helps limit depth of recursion during the mark phase.  Thus, the code above could also be written as:

     SCM
     mark_image (SCM image_smob)
     {
       /* Mark the image's name and update function.  */
       struct image *image = (struct image *) SCM_CDR (image_smob);

       scm_gc_mark (image->name);
       return image->update_func;
     }

Finally, when the collector encounters an unmarked smob during the sweep phase, it uses the smob's tag to find the appropriate free function for the smob.  It then calls the function, passing it the smob as its only argument.

The free function must release any resources used by the smob. However, it need not free objects managed by the collector; the collector will take care of them.  The return type of the free function should be scm_sizet, an unsigned integral type; the free function should return the number of bytes released, to help the collector maintain statistics on the size of the heap.

Here is how we might write the free function for the image smob type:

     scm_sizet
     free_image (SCM image_smob)
     {
       struct image *image = (struct image *) SCM_CDR (image_smob);
       scm_sizet size = image->width * image->height + sizeof (*image);

       free (image->pixels);
       free (image);

       return size;
     }

During the sweep phase, the garbage collector will clear the mark bits on all live objects.  The code which implements a smob need not do this itself.

There is no way for smob code to be notified when collection is complete.

Note that, since a smob's mark bit lives in its CAR, along with the smob's type tag, the technique for checking the type of a smob described in Typechecking will not necessarily work during GC.  If you need to find out whether a given object is a particular smob type during GC, use the following macro:

 -- Macro: void SCM_GCTYP16 (SCM X)
     Return the type bits of the smob X, with the mark bit clear.

     Use this macro instead of SCM_CAR to check the type of a smob during GC.  Usually, only code called by the smob's mark function need worry about this.

It is usually a good idea to minimize the amount of processing done during garbage collection; keep mark and free functions very simple.  Since collections occur at unpredictable times, it is easy for any unusual activity to interfere with normal code.

Garbage Collecting Simple Smobs
-------------------------------

It is often useful to define very simple smob types --- smobs which have no data to mark, other than the cell itself, or smobs whose CDR is simply an ordinary Scheme object, to be marked recursively.  Guile provides some functions to handle these common cases.

If the smob refers to no other Scheme objects, then no action is necessary; the garbage collector has already marked the smob cell itself.  In that case, you can use zero as your mark function.

 -- Function: SCM scm_markcdr (SCM X)
     Mark the references in the smob X, assuming that X's CDR contains an ordinary Scheme object, and X refers to no other objects.  This function simply returns X's CDR.

 -- Function: scm_sizet scm_free0 (SCM X)
     Do nothing; return zero.  This function is appropriate for smobs that use either zero or scm_markcdr as their marking functions, and refer to no heap storage, including memory managed by malloc, other than the smob's header cell.

A Complete Example
------------------

Here is the complete text of the implementation of the image datatype, as presented in the sections above.  We also provide a definition for the smob's print function.

     #include <stdlib.h>
     #include <libguile.h>

     long image_tag;

     struct image {
       int width, height;
       char *pixels;

       /* The name of this image */
       SCM name;

       /* A function to call when this image is
          modified, e.g., to update the screen,
          or SCM_BOOL_F if no action necessary */
       SCM update_func;
     };

     SCM
     create_image (SCM name, int width, int height)
     {
       struct image *image;
       SCM image_smob;

       image = (struct image *) scm_must_malloc (sizeof (*image), "image");
       image->width = width;
       image->height = height;
       image->pixels = scm_must_malloc (width * height, "image pixels");
       image->name = name;
       image->update_func = SCM_BOOL_F;

       SCM_NEWCELL (image_smob);
       SCM_SETCDR (image_smob, image);
       SCM_SETCAR (image_smob, image_tag);

       return image_smob;
     }

     SCM
     clear_image (SCM image_smob)
     {
       int area;
       struct image *image;

       SCM_ASSERT ((SCM_NIMP (image_smob)
                    && SCM_CAR (image_smob) == image_tag),
                   image_smob, SCM_ARG1, "clear-image");

       image = (struct image *) SCM_CDR (image_smob);
       area = image->width * image->height;
       memset (image->pixels, 0, area);

       /* Invoke the image's update function.  */
       if (image->update_func != SCM_BOOL_F)
         scm_apply (image->update_func, SCM_EOL, SCM_EOL);

       return SCM_UNSPECIFIED;
     }

     SCM
     mark_image (SCM image_smob)
     {
       struct image *image = (struct image *) SCM_CDR (image_smob);

       scm_gc_mark (image->name);
       return image->update_func;
     }

     scm_sizet
     free_image (SCM image_smob)
     {
       struct image *image = (struct image *) SCM_CDR (image_smob);
       scm_sizet size = image->width * image->height + sizeof (*image);

       free (image->pixels);
       free (image);

       return size;
     }

     int
     print_image (SCM obj, SCM port, scm_print_state *pstate)
     {
       struct image *image = (struct image *) SCM_CDR (image_smob);

       scm_gen_puts (scm_regular_string, "#<image ", port);
       scm_display (image->name, port);
       scm_gen_puts (scm_regular_string, ">", port);

       /* non-zero means success */
       return 1;
     }

     scm_smobfuns image_funs = {
       mark_image, free_image, print_image, 0
     };

     void
     init_image_type ()
     {
       image_tag = scm_newsmob (&image_funs);
       scm_make_gsubr ("clear-image", 1, 0, 0, clear_image);
     }