## Unit lolevel

This unit provides a number of handy low-level operations. Use at your own risk.

This unit uses the `srfi-4` and `extras` units.

### Foreign pointers

The abstract class of pointer is divided into 2 categories:

pointer object
  is a foreign pointer object, a tagged foreign pointer object, or a SWIG-pointer.
pointer-like object
  is a closure, port, locative, or a pointer object.

SWIG-pointers are currently an issue due to "bitrot" in the SWIG CHICKEN translator. While they are considered a pointer object unexpected results are possible.

Note that Locatives, while technically pointers, are not considered a pointer object, but a pointer-like object. The distinction is artificial.

#### address->pointer

`(address->pointer ADDRESS)` procedure

  Creates a new foreign pointer object initialized to point to the address given in the integer `ADDRESS`.

#### allocate

`(allocate BYTES)` procedure

  Returns a foreign pointer object to a freshly allocated region of static memory.

  This procedure could be defined as follows:

      (define allocate (foreign-lambda c-pointer "malloc" integer))

#### free

`(free POINTER)` procedure

  Frees the memory pointed to by `POINTER`.

  This procedure could be defined as follows:

      (define free (foreign-lambda void "free" c-pointer))

#### object->pointer

`(object->pointer X)` procedure

  Returns a foreign pointer object pointing to the Scheme object X, which should be a non-immediate object. ("foreign" here is a bit of a misnomer.)

  Note that data in the garbage collected heap moves during garbage collection.

#### pointer->object

`(pointer->object POINTER)` procedure

  Returns the Scheme object pointed to by the pointer object `POINTER`.

  Whether the `POINTER` actually points to a Scheme object is not guaranteed. Use at your own risk.

#### pointer?

`(pointer? X)` procedure

  Returns `#t` if `X` is a pointer object, or `#f` otherwise.

#### pointer-like?

`(pointer-like? X)` procedure

  Returns `#t` if `X` is a pointer-like object, or `#f` otherwise.

#### pointer=?

`(pointer=? POINTER*1 POINTER*2)` procedure

  Returns `#t` if the pointer-like objects `POINTER*1` and `POINTER*2` point to the same address, or `#f` otherwise.

#### pointer->address

`(pointer->address POINTER*)` procedure

  Returns the address, to which the pointer-like object `POINTER*` points.

#### pointer+

`(pointer+ POINTER* N)` procedure

  Returns a new foreign pointer object representing the pointer-like object `POINTER*` address value increased by the byte-offset `N`.

  Use of anything other than a pointer object as an argument is questionable.

#### align-to-word

`(align-to-word POINTER*-OR-INT)` procedure

  Accepts either a pointer-like object or an integer as the argument and returns a new foreign pointer or integer aligned to the native word size of the host platform.

  Use of anything other than an integer or pointer object as an argument is questionable.

### SRFI-4 Foreign pointers

These procedures actually accept a pointer-like object as the `POINTER` argument. However, as usual, use of anything other than a pointer object is questionable.

#### pointer-u8-ref

`(pointer-u8-ref POINTER)` procedure

  Returns the unsigned byte at the address designated by `POINTER`.

#### pointer-s8-ref

`(pointer-s8-ref POINTER)` procedure

  Returns the signed byte at the address designated by `POINTER`.

#### pointer-u16-ref

`(pointer-u16-ref POINTER)` procedure

  Returns the unsigned 16-bit integer at the address designated by `POINTER`.

#### pointer-s16-ref

`(pointer-s16-ref POINTER)` procedure

  Returns the signed 16-bit integer at the address designated by `POINTER`.

#### pointer-u32-ref

`(pointer-u32-ref POINTER)` procedure

  Returns the unsigned 32-bit integer at the address designated by `POINTER`.

#### pointer-s32-ref

`(pointer-s32-ref POINTER)` procedure

  Returns the signed 32-bit integer at the address designated by `POINTER`.

#### pointer-f32-ref

`(pointer-f32-ref POINTER)` procedure

  Returns the 32-bit float at the address designated by `POINTER`.

#### pointer-f64-ref

`(pointer-f64-ref POINTER)` procedure

  Returns the 64-bit double at the address designated by `POINTER`.

#### pointer-u8-set!

`(pointer-u8-set! POINTER N)` procedure
`(set! (pointer-u8-ref POINTER) N)` procedure

  Stores the unsigned byte `N` at the address designated by `POINTER`.

#### pointer-s8-set!

`(pointer-s8-set! POINTER N)` procedure
`(set! (pointer-s8-ref POINTER) N)` procedure

  Stores the signed byte `N` at the address designated by `POINTER`.

#### pointer-u16-set!

`(pointer-u16-set! POINTER N)` procedure
`(set! (pointer-u16-ref POINTER) N)` procedure

  Stores the unsigned 16-bit integer `N` at the address designated by `POINTER`.

#### pointer-s16-set!

`(pointer-s16-set! POINTER N)` procedure
`(set! (pointer-s16-ref POINTER) N)` procedure

  Stores the signed 16-bit integer `N` at the address designated by `POINTER`.

#### pointer-u32-set!

`(pointer-u32-set! POINTER N)` procedure
`(set! (pointer-u32-ref POINTER) N)` procedure

  Stores the unsigned 32-bit integer `N` at the address designated by `POINTER`.

#### pointer-s32-set!

`(pointer-s32-set! POINTER N)` procedure
`(set! (pointer-s32-ref POINTER) N)` procedure

  Stores the 32-bit integer `N` at the address designated by `POINTER`.

#### pointer-f32-set!

`(pointer-f32-set! POINTER N)` procedure
`(set! (pointer-f32-ref POINTER) N)` procedure

  Stores the 32-bit floating-point number `N` at the address designated by `POINTER`.

#### pointer-f64-set!

`(pointer-f64-set! POINTER N)` procedure
`(set! (pointer-f64-ref POINTER) N)` procedure

  Stores the 64-bit floating-point number `N` at the address designated by `POINTER`.

### Tagged pointers

Tagged pointers are foreign pointer objects with an extra tag object.

#### tag-pointer

`(tag-pointer POINTER* TAG)` procedure

  Creates a new tagged foreign pointer object from the pointer-like object `POINTER*` with the tag `TAG`, which may an arbitrary Scheme object.

  Use of anything other than a pointer object is questionable.

#### tagged-pointer?

`(tagged-pointer? X [TAG])` procedure

  Returns `#t` if `X` is a tagged foreign pointer object, or `#f` otherwise.

  Further, returns `#t` when `X` has the optional tag `TAG` (using an `equal?` comparison), or `#f` otherwise.

#### pointer-tag

`(pointer-tag POINTER*)` procedure

  If `POINTER` is a tagged foreign pointer object, its tag is returned. If `POINTER*` is any other kind of pointer-like object `#f` is returned. Otherwise an error is signalled.

### Pointer vectors

Pointer vectors are specialized and space-efficient vectors for foreign pointer objects. All procedures defined below that accept a pointer object allow `#f` as an alternative representation of the `NULL`-pointer.

#### make-pointer-vector

`(make-pointer-vector LENGTH [INIT])` procedure

  Creates a pointer-vector of the given length and optionally initializes each element to `INIT`, which should be a pointer or `#f`, which represents the `NULL` pointer.

#### pointer-vector?

`(pointer-vector? X)` procedure

  Returns `#t` if `X` is a pointer-vector or `#f` otherwise.

#### pointer-vector

`(pointer-vector POINTER ...)` procedure

  Returns a pointer-vector from the given pointer arguments.

#### pointer-vector-length

`(pointer-vector-length POINTERVECTOR)` procedure

  Returns the length of the given pointer-vector.

#### pointer-vector-ref

`(pointer-vector-ref POINTERVECTOR INDEX)` procedure

  Returns the pointer at `INDEX` in the given pointer-vector or `#f` if the element is a `NULL`- pointer.

#### pointer-vector-set!

`(pointer-vector-set! POINTERVECTOR INDEX POINTER)` procedure

  Sets the element at the position `INDEX` in the given pointer-vector to `POINTER`. The alternative syntax

       (set! (pointer-vector-ref POINTERVECTOR INDEX) POINTER)

  is also allowed.

### Locatives

A locative is an object that points to an element of a containing object, much like a pointer in low-level, imperative programming languages like C. The element can be accessed and changed indirectly, by performing access or change operations on the locative. The container object can be computed by calling the `locative->object` procedure.

Locatives may be passed to foreign procedures that expect pointer arguments. The effect of creating locatives for evicted data (see `object-evict`) is undefined.

#### make-locative

`(make-locative OBJ [INDEX])` procedure

  Creates a locative that refers to the element of the non-immediate object `OBJ` at position `INDEX`. `OBJ` may be a vector, pair, string, blob, SRFI-4 number-vector, or record structure. `INDEX` should be a fixnum. `INDEX` defaults to 0.

#### make-weak-locative

`(make-weak-locative OBJ [INDEX])` procedure

  Creates a weak locative. Even though the locative refers to an element of a container object, the container object will still be reclaimed by garbage collection if no other references to it exist.

#### locative?

`(locative? X)` procedure

  Returns `#t` if `X` is a locative, or `#f` otherwise.

#### locative-ref

`(locative-ref LOC)` procedure

  Returns the element to which the locative `LOC` refers. If the containing object has been reclaimed by garbage collection, an error is signalled.

      (locative-ref (make-locative "abc" 1)) ==> #\b

#### locative-set!

`(locative-set! LOC X)` procedure
`(set! (locative-ref LOC) X)` procedure

  Changes the element to which the locative `LOC` refers to `X`. If the containing object has been reclaimed by garbage collection, an error is signalled.

#### locative->object

`(locative->object LOC)` procedure

  Returns the object that contains the element referred to by `LOC` or `#f` if the container has been reclaimed by garbage collection.

      (locative->object (make-locative "abc" 1)) ==> "abc"

### Extending procedures with data

#### extend-procedure

`(extend-procedure PROCEDURE X)` procedure

  Returns a copy of the procedure `PROCEDURE` which contains an additional data slot initialized to `X`. If `PROCEDURE` is already an extended procedure, then its data slot is changed to contain `X` and the same procedure is returned. Signals an error when `PROCEDURE` is not a procedure.

#### extended-procedure?

`(extended-procedure? PROCEDURE)` procedure

  Returns `#t` if `PROCEDURE` is an extended procedure, or `#f` otherwise.

#### procedure-data

`(procedure-data PROCEDURE)` procedure

  Returns the data object contained in the extended procedure `PROCEDURE`, or `#f` if it is not an extended procedure.

#### set-procedure-data!

`(set-procedure-data! PROCEDURE X)` procedure

  Changes the data object contained in the extended procedure `PROCEDURE` to `X`. Signals an error when `PROCEDURE` is not an extended procedure.

      (define foo
        (letrec ((f (lambda () (procedure-data x)))
                 (x #f) )
          (set! x (extend-procedure f 123))
          x) )
      (foo)                                         ==> 123
      (set-procedure-data! foo 'hello)
      (foo)                                         ==> hello

### Low-level data access

These procedures operate with what are known as `vector-like objects`. A `vector-like object` is a vector, record structure, pair, symbol or keyword.

Note that strings and blobs are not considered vector-like.

#### vector-like?

`(vector-like? X)` procedure

  Returns `#t` when `X` is a vector-like object, returns `#f` otherwise.

#### block-ref

`(block-ref VECTOR* INDEX)` procedure

  Returns the contents of the `INDEX`th slot of the vector-like object `VECTOR*`.

#### block-set!

`(block-set! VECTOR* INDEX X)` procedure
`(set! (block-ref VECTOR* INDEX) X)` procedure

  Sets the contents of the `INDEX`th slot of the vector-like object `VECTOR*` to the value of `X`.

#### number-of-slots

`(number-of-slots VECTOR*)` procedure

  Returns the number of slots that the vector-like object `VECTOR*` contains.

#### number-of-bytes

`(number-of-bytes BLOCK)` procedure

  Returns the number of bytes that the object `BLOCK` contains. `BLOCK` may be any non-immediate value.

#### object-copy

`(object-copy X)` procedure

  Copies `X` recursively and returns the fresh copy. Objects allocated in static memory are copied back into garbage collected storage.

#### move-memory!

`(move-memory! FROM TO [BYTES [FROM-OFFSET [TO-OFFSET]]])` procedure

  Copies `BYTES` bytes of memory from `FROM` to `TO`. `FROM` and `TO` may be strings, blobs, SRFI-4 number-vectors, memory mapped files, foreign pointers (as obtained from a call to `foreign-lambda`, for example), tagged-pointers or locatives. if `BYTES` is not given and the size of the source or destination operand is known then the maximal number of bytes will be copied. Moving memory to the storage returned by locatives will cause havoc, if the locative refers to containers of non-immediate data, like vectors or pairs.

  The additional fourth and fifth argument specify starting offsets (in bytes) for the source and destination arguments.

  Signals an error if any of the above constraints is violated.

### Data in unmanaged memory

#### object-evict

`(object-evict X [ALLOCATOR])` procedure

  Copies the object `X` recursively into the memory pointed to by the foreign pointer object returned by `ALLOCATOR`. The freshly copied object is returned. `ALLOCATOR` should be a procedure of a single argument (the number of bytes to allocate), and defaults to `allocate`.

  This facility allows moving arbitrary objects into static memory, but care should be taken when mutating evicted data: setting slots in evicted vector-like objects to non-evicted data is not allowed. It is possible to set characters/bytes in evicted strings or byte-vectors, though. It is advisable not to evict ports, because they might be mutated by certain file-operations. `object-evict` is able to handle circular and shared structures.

  Evicted symbols are no longer unique: a fresh copy of the symbol is created, so

      (define x 'foo)
      (define y (object-evict 'foo))
      y                              ==> foo
      (eq? x y)                      ==> #f
      (define z (object-evict '(bar bar)))
      (eq? (car z) (cadr z))         ==> #t

  This loss of uniqueness also implies that an evicted structure -- such as one created with `define-record` -- cannot be operated on with the existing predicate or accessors, as internally a symbol is used to denote the type:

      (define-record point x y)
      (point? (make-point x y))                  ; => #t
      (point? (object-evict (make-point x y)))   ; => #f

#### object-evict-to-location

`(object-evict-to-location X POINTER* [LIMIT])` procedure

  As `object-evict` but moves the object at the address pointed to by the pointer-like object `POINTER*`. If the number of copied bytes exceeds the optional `LIMIT` then an error is signalled (specifically a composite condition of types `exn` and `evict`. The latter provides a `limit` property which holds the exceeded limit. Two values are returned: the evicted object and a new pointer pointing to the first free address after the evicted object.

  Use of anything other than a pointer object as the `POINTER*` argument is questionable.

#### object-evicted?

`(object-evicted? X)` procedure

  Returns `#t` if `X` is a non-immediate evicted data object, or `#f` otherwise.

#### object-release

`(object-release X [RELEASER])` procedure

  Frees memory occupied by the evicted object `X` recursively. `RELEASER` should be a procedure of a single argument (a foreign pointer object to the static memory to be freed) and defaults to `free`.

#### object-unevict

`(object-unevict X [FULL])` procedure

  Copies the object `X` and nested objects back into the normal Scheme heap. Symbols are re-interned into the symbol table. Strings and byte-vectors are not copied, unless `FULL` is given and not `#f`.

#### object-size

`(object-size X)` procedure

  Returns the number of bytes that would be needed to evict the data object `X`. If `X` is an immediate object, zero is returned.

### Record instance

#### make-record-instance

`(make-record-instance SYMBOL ARG1 ...)` procedure

  Returns a new instance of the record type `SYMBOL`, with its slots initialized to `ARG1 ...`. To illustrate:

      (define-record-type point (make-point x y) point?
        (x point-x point-x-set!)
        (y point-y point-y-set!))

  expands into something quite similar to:

      (begin
        (define (make-point x y)
          (make-record-instance 'point x y) )
        (define (point? x)
          (and (record-instance? x)
               (eq? 'point (block-ref x 0)) ) )
        (define (point-x p) (block-ref p 1))
        (define (point-x-set! p x) (block-set! p 1 x))
        (define (point-y p) (block-ref p 2))
        (define (point-y-set! p y) (block-set! p 1 y)) )

#### record-instance?

`(record-instance? X [SYMBOL])` procedure

  Returns `#t` if `X` is a record structure, or `#f` otherwise.

  Further, returns `#t` if `X` is of type `SYMBOL`, or `#f` otherwise.

#### record-instance-type

`(record-instance-type RECORD)` procedure

  Returns type symbol of the record structure `RECORD`. Signals an error if `RECORD` is not a record structure.

#### record-instance-length

`(record-instance-length RECORD)` procedure

  Returns number of slots for the record structure `RECORD`. The record-instance type is not counted. Signals an error if `RECORD` is not a record structure.

#### record-instance-slot

`(record-instance-slot RECORD INDEX)` procedure

  Returns the contents of the `INDEX`th slot of the record structure `RECORD`. The slot index range is the open interval `[0 record-instance-length)`. Signals an error if `RECORD` is not a record structure.

#### record-instance-slot-set!

`(record-instance-slot-set! RECORD INDEX X)` procedure
`(set! (record-instance-slot RECORD INDEX) X)` procedure

  Sets the `INDEX`th slot of the record structure `RECORD` to `X`. The slot index range is the open interval `[0 record-instance-length)`. Signals an error if `RECORD` is not a record structure.

#### record->vector

`(record->vector RECORD)` procedure

  Returns a new vector with the type and the elements of the record structure `RECORD`. Signals an error if `RECORD` is not a record structure.

### Magic

#### object-become!

`(object-become! ALIST)` procedure

  Changes the identity of the value of the car of each pair in `ALIST` to the value of the cdr. Both values may not be immediate (i.e. exact integers, characters, booleans or the empty list).

      (define x "i used to be a string")
      (define y '#(and now i am a vector))
      (object-become! (list (cons x y)))
      x                                    ==> #(and now i am a vector)
      y                                    ==> #(and now i am a vector)
      (eq? x y)                            ==> #t

  Note: this operation invokes a major garbage collection.

  The effect of using `object-become!` on evicted data (see `object-evict`) is undefined.

#### mutate-procedure!

`(mutate-procedure! OLD PROC)` procedure

  Replaces the procedure `OLD` with the result of calling the one-argument procedure `PROC`. `PROC` will receive a copy of `OLD` that will be identical in behaviour to the result of `OLD`:

       ;;; Replace arbitrary procedure with tracing one:

       (mutate-procedure! my-proc
         (lambda (new)
           (lambda args
             (printf "~s called with arguments: ~s~%" new args)
             (apply new args) ) ) )

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