|Allegro CL version 9.0|
Unrevised from 8.2 to 9.0.
Arguments: object offset pos type &optional coerce
This function and the related function memref-int calculate an address in memory and performs a read from that address. This function takes an object as an argument while memref-int takes an address.
The width and style of the read depends upon the type. The possible values for the type argument are shown in a table below.
Except when the type argument
:lisp, the result is always a number, either an
integer or float. The type and possible range of that number
depends on the type argument.
This function is setf'able. The setf function writes
to the addressed memory instead of reading from it.
The object argument may be any Lisp value. If the Lisp value represents an object in the heap, the address of the object (including its tag) is used as the base address in the memory reference. If the Lisp value is an immediate value (a fixnum, a character, ...), then the actual bit representation of the Lisp value (including its tag) is used as the base address in the memory reference.
The offset and pos arguments must be fixnum values. The sum of thes two arguments is added to the base address value and the result is treated as an address in memory from which data is fetched, or to which data is stored.
When the coerce argument is non-
nil some checking is done in the setf form. The
value to be stored is coerced to the required type.
This function provides low-level memory access in Allegro CL. However, even though the name is exported and documented, we recommend against using this function in user code. Instead, we recommend using higher-level functions, such as those linked to below. However, the function is useful in the development cycle for debugging purposes.
WARNING about setf: except when the type
:lisp, the setf function does not do any
setf-protection. If it is used to store non-Lisp values into a memory
location that was intended to hold a Lisp value, then a
non-recoverable error is very likely to occur during the next GC.
The access-types (possible values of type) that are available are as follows:
:unsigned-byte, :signed-byte (1-byte access, 1-byte integer result) :unsigned-word, :signed-word (2-byte access, 2-byte integer result) :unsigned-long, :signed-long ;; 4-byte access, 4-byte integer result in 32-bit lisps ;; undetermined (4-byte or 8-byte) in 64-bit lisps :unsigned-long32 (4-byte access, 4-byte integer result) :unsigned-long64 (8-byte access, 8-byte integer result on 64-bit Lisps but 4-byte access, 4-byte integer result on all others) :fixnum (in 32-bit lisp, 4-byte access, fixnum result [top bits lost on overflow] in 64-bit lisp, 8-byte access, fixnum result [top bits lost on overflow]) :lisp (in 32-bit lisp, 4-byte access, lisp result in 64-bit lisp, 8-byte access, lisp result [careful, could confuse gc]) :single-float (or single-float) (4-byte access, 4-byte single-float result) :double-float (or double-float) (8-byte access, 8-byte double-float result) :signed-natural :unsigned-natural ;; For these last two, in 32-bit lisps, these will ;; construct 32-bit values, and in 64-bit lisps, it will construct ;; 64-bit values :nybble (4-bit -- half-byte -- value)
Be aware that though the access types might look similar to C types, they are not actually identical.
For more information on
:unsigned-natural, see 64 bit Allegro CL
Implementations in implementation.htm.
nilsome checking is done in the setf form. The value to be stored is coerced to the required type. This is especially useful when the argument is a float value, but of the wrong type, as in
(setf (sys:memref-int array-address 0 0 :double-float :coerce) 1.0s0)
An offset is necessary to access Lisp objects because a Lisp object pointer contains tag bits in the low-order positions. When the pointer is treated as a machine address, the numeric value is not the address of the Lisp data object in memory. The address is normally lower than the address of the object, by a fixed amount determined by the tag bits in the pointer.
Some of the offsets for Lisp objects are described in [Allegro directory]/misc/lisp.h in the Allegro CL distribution. In an earlier version of this documentation, it was stated that the offsets are available in Lisp itself by calling certain otherwise undocumented functions. That statement was incorrect. The offsets are not reliably available with Lisp.
For example, if the variable
addr holds the foreign
address of a large block of memory with a length stored in the
size variable, the following loop could be used to
extract bytes from the block:
((let ((addr/4 (ash addr -2))) (dotimes (i (size) (memref addr/4 i 0 :byte)))
If the index i can rise above the fixnum limit, the address arithmetic must be performed before the call to memref:
(let ((addr/4 (ash addr -2)) (size/4 (ash size -2))) (dotimes (i size/4) (dotimes (j 4) (memref (+ addr/4 i) j 0 :byte))))
The second inner loop is needed since only the offset and pos arguments allow byte addresses to be reached.
If the addresses in the memory block cross over into the negative fixnum range we need to break up the ranges of iteration:
(let ((addr/4 (ash addr -2)) (size/4 (ash size -2))) (if* (and (< addr/4 most-positive-fixnum) (<= size/4 (- most-positive-fixnum addr/4))) then ;; this is the simple case, all addresses in the positive ;; fixnum range (dotimes (i size/4) (dotimes (j 4) (memref (+ addr/4 i) j 0 :byte)))) elseif (< addr/4 most-positive-fixnum) then ;; the starting address is in the positive range ;; but the increment will cross over eventually ;; do the positive range first (dotimes (i (- most-positive-fixnum addr/4)) (dotimes (j 4) (memref (+ addr/4 i) j 0 :byte))) ;; then the negative range (dotimes (i (- size/4 (- most-positive-fixnum addr/4))) (dotimes (j 4) (memref (+ most-negative-fixnum i) j 0 :byte))) else ;; starting address is in the negative fixnum range ;; we create a negative fixnum that represents the word index (setq addr/4 (+ most-negative-fixnum (- addr/4 most-positive-fixnum))) (dotimes (i size/4) (dotimes (j 4) (memref (+ addr/4 i) j 0 :byte)))
Documentation for Allegro CL version 9.0. This page was not revised from the 8.2 page.
|Allegro CL version 9.0|
Unrevised from 8.2 to 9.0.