FunctionPackage: systemToCDocOverviewCGDocRelNotesFAQIndexPermutedIndex
Allegro CL version 10.1
Unrevised from 10.0 to 10.1.
10.0 version


Arguments: address offset pos type &optional coerce

This function and the related function memref calculate an address in memory and performs a read from that address. This function takes an address as an argument while memref takes a Lisp object.

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 is :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 address argument must be an integer and the offset and pos arguments must be fixnum values. The sum of the three arguments 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 is :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 (4-byte access, fixnum result [top bits lost on overflow])  
:lisp (4-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) 
  ;; 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 :signed-natural and :unsigned-natural, see 64 bit Allegro CL Implementations in implementation.htm.

Programming Notes

  1. The function memref-int is very similar to the memref function, but because the base address determination is significantly different the descriptions are different in several subtle but important ways.
  2. In place of using memref-int and memref, operators like single-float-to-shorts, double-float-to-shorts, shorts-to-single-float, shorts-to-double-float may be used for accessing/creating floats. A foreign type can be defined (see ftype.htm) to access arbitrary memory areas. See also write-vector.
  3. When optimization is declared at a sufficient level, the call to memref-int and to its setf form, is compiled in line and is thus very efficient. Note that when the optional coerce argument is supplied, then the call is not open-coded when compiled.
  4. 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 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)
  5. The offset and pos arguments are equivalent and interchangeable. They are both added as byte-offsets to the base address to determine the final memory address. Usually, the offset is some constant that adjusts for the start of the data in some object being accessed. 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.
  6. One application of memref-int is to access large blocks of memory (larger than array-total-size-limit).

    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:

    (dotimes (i size) (memref-int (+ addr i) 0 0 :byte))

    Note how the address arithmetic is performed before the call to memref-int. If the index were passed as the offset or the pos argument, it would be subject to the fixnum limitation.

    To avoid bignum boxing in this iteration, memref with a fixnum argument may be used to keep all numbers in the fixnum range. See the description of memref for a detailed analysis of this technique.

Copyright (c) 1998-2017, Franz Inc. Oakland, CA., USA. All rights reserved.
This page was not revised from the 10.0 page.
Created 2017.2.15.

Allegro CL version 10.1
Unrevised from 10.0 to 10.1.
10.0 version