Storage.md

Storage

Heap memory allocation and deallocation. This module provides the ALLOCATE and DEALLOCATE procedures that are called implicitly by the built-in NEW and DISPOSE operations. You rarely call these directly -- import Storage to enable NEW/DISPOSE, and the compiler handles the rest.

Available in PIM4 mode (the default). No special flags needed.

Procedures

PROCEDURE ALLOCATE(VAR p: ADDRESS; size: CARDINAL);

Allocate size bytes of heap memory and store the pointer in p. This is called automatically by NEW(p) -- the compiler inserts the call with the correct size based on the pointed-to type. You generally do not need to call this directly.

PROCEDURE DEALLOCATE(VAR p: ADDRESS; size: CARDINAL);

Free the heap block at p. Called automatically by DISPOSE(p). After deallocation, p is set to NIL.

How NEW and DISPOSE work

When you write NEW(p) where p is a POINTER TO SomeRecord, the compiler generates a call to ALLOCATE(p, SIZE(SomeRecord)). Similarly, DISPOSE(p) generates DEALLOCATE(p, SIZE(SomeRecord)). For this to work, ALLOCATE and DEALLOCATE must be visible in the current scope -- which means you need to import them from Storage.

Example

MODULE HeapDemo;
FROM InOut IMPORT WriteString, WriteInt, WriteLn;
FROM Storage IMPORT ALLOCATE, DEALLOCATE;

TYPE
  NodePtr = POINTER TO Node;
  Node = RECORD
    val: INTEGER;
    next: NodePtr
  END;

VAR p, q: NodePtr;

BEGIN
  (* NEW calls ALLOCATE internally *)
  NEW(p);
  p^.val := 42;
  p^.next := NIL;

  NEW(q);
  q^.val := 99;
  q^.next := p;

  (* Walk the list *)
  WriteString("List: ");
  WriteInt(q^.val, 1);
  WriteString(" -> ");
  WriteInt(q^.next^.val, 1);
  WriteLn;

  (* DISPOSE calls DEALLOCATE internally *)
  DISPOSE(q);
  DISPOSE(p)
END HeapDemo.