Pascal - Memory Management

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This chapter explains dynamic memory management in Pascal. Pascal programming language provides several functions for memory allocation and management.

Allocating Memory Dynamically

While doing programming, if you are aware about the size of an array, then it is easy and you can define it as an array. For example, to store a name of any person, it can go max 100 characters so you can define something as follows:

var
name: array[1..100] of char;

But now, let us consider a situation, where you have no idea about the length of the text you need to store, for example, you want to store a detailed description about a topic. Here, we need to define a pointer to string without defining how much memory is required.

Pascal provides a procedure newto create pointer variables.

program exMemory;
var
name: array[1..100] of char;
description: ^string;
begin
   name:= 'Zara Ali';
   new(description);
      if not assigned(description) then
         writeln(' Error - unable to allocate required memory')
      else
         description^ := 'Zara ali a DPS student in class 10th';
   writeln('Name = ', name );
   writeln('Description: ', description^ );
end.

When the above code is compiled and executed, it produces the following result:

Name = Zara Ali
Description: Zara ali a DPS student in class 10th

Now, if you need to define a pointer with specific number of bytes to be referred by it later, you should use the getmem function or the getmem procedure, which has the following syntax:

procedure Getmem(
   out p: pointer;
   Size: PtrUInt
);

function GetMem(
   size: PtrUInt
):pointer;

In the previous example, we declared a pointer to a string. A string has a maximum value of 255 bytes. If you really don't need that much space, or a larger space, in terms of bytes, getmem subprogram allows specifying that. Let us rewrite the previous example, using getmem:

program exMemory;
var
name: array[1..100] of char;
description: ^string;
begin
   name:= 'Zara Ali';
   description := getmem(200);
      if not assigned(description) then
         writeln(' Error - unable to allocate required memory')
      else
         description^ := 'Zara ali a DPS student in class 10th';
   writeln('Name = ', name );
   writeln('Description: ', description^ );
   freemem(description);
end.

When the above code is compiled and executed, it produces the following result:

Name = Zara Ali
Description: Zara ali a DPS student in class 10th

So, you have complete control and you can pass any size value while allocating memory unlike arrays, where once you defined the size cannot be changed.

Resizing and Releasing Memory

When your program comes out, operating system automatically releases all the memory allocated by your program, but as a good practice when you are not in need of memory anymore, then you should release that memory.

Pascal provides the procedure dispose to free a dynamically created variable using the procedure new. If you have allocated memory using the getmem subprogram, then you need to use the subprogram freemem to free this memory. The freemem subprograms have the following syntax:

procedure Freemem(
   p: pointer;
  Size: PtrUInt
);

function Freemem(
   p: pointer
):PtrUInt;

Alternatively, you can increase or decrease the size of an allocated memory block by calling the function ReAllocMem. Let us check the above program once again and make use of ReAllocMem and freemem subprograms. Following is the syntax for ReAllocMem:

function ReAllocMem(
   var p: pointer;
   Size: PtrUInt
):pointer;   

Following is an example which makes use of ReAllocMem and freemem subprograms:

program exMemory;
var
name: array[1..100] of char;
description: ^string;
desp: string;
begin
   name:= 'Zara Ali';
   desp := 'Zara ali a DPS student.';
   description := getmem(30);
   if not assigned(description) then
      writeln('Error - unable to allocate required memory')
   else
      description^ := desp;

   (* Suppose you want to store bigger description *)
   description := reallocmem(description, 100);
   desp := desp + ' She is in class 10th.';
   description^:= desp; 
   writeln('Name = ', name );
   writeln('Description: ', description^ );
   freemem(description);
end.

When the above code is compiled and executed, it produces the following result:

Name = Zara Ali
Description: Zara ali a DPS student. She is in class 10th

Memory Management Functions

Pascal provides a hoard of memory management functions that is used in implementing various data structures and implementing low-level programming in Pascal. Many of these functions are implementation dependent. Free Pascal provides the following functions and procedures for memory management:

S.NFunction Name & Description
1function Addr(X: TAnytype):Pointer;
Returns address of variable
2function Assigned(P: Pointer):Boolean;
Checks if a pointer is valid
3function CompareByte(const buf1; const buf2; len: SizeInt):SizeInt;
Compares 2 memory buffers byte per byte
4function CompareChar(const buf1; const buf2; len: SizeInt):SizeInt;
Compares 2 memory buffers byte per byte
5function CompareDWord(const buf1; const buf2; len: SizeInt):SizeInt;
Compares 2 memory buffers byte per byte
6function CompareWord(const buf1; const buf2; len: SizeInt):SizeInt;
Compares 2 memory buffers byte per byte
7function Cseg: Word;
Returns code segment
8procedure Dispose(P: Pointer);
Frees dynamically allocated memory
9procedure Dispose(P: TypedPointer; Des: TProcedure);
Frees dynamically allocated memory
10function Dseg: Word;
Returns data segment
11procedure FillByte(var x; count: SizeInt; value: Byte);
Fills memory region with 8-bit pattern
12procedure FillChar( var x; count: SizeInt; Value: Byte|Boolean|Char);
Fills memory region with certain character
13procedure FillDWord( var x; count: SizeInt; value: DWord);
Fills memory region with 32-bit pattern
14procedure FillQWord( var x; count: SizeInt; value: QWord);
Fills memory region with 64-bit pattern
15procedure FillWord( var x; count: SizeInt; Value: Word);
Fills memory region with 16-bit pattern
16procedure Freemem( p: pointer; Size: PtrUInt);
Releases allocated memory
17procedure Freemem( p: pointer );
Releases allocated memory
18procedure Getmem( out p: pointer; Size: PtrUInt);
Allocates new memory
19procedure Getmem( out p: pointer);
Allocates new memory
20procedure GetMemoryManager( var MemMgr: TMemoryManager);
Returns current memory manager
21function High( Arg: TypeOrVariable):TOrdinal;
Returns highest index of open array or enumerated
22function IndexByte( const buf; len: SizeInt; b: Byte):SizeInt;
Finds byte-sized value in a memory range
23function IndexChar( const buf; len: SizeInt; b: Char):SizeInt;
Finds char-sized value in a memory range
24function IndexDWord( const buf; len: SizeInt; b: DWord):SizeInt;
Finds DWord-sized (32-bit) value in a memory range
25function IndexQWord( const buf; len: SizeInt; b: QWord):SizeInt;
Finds QWord-sized value in a memory range
26function Indexword( const buf; len: SizeInt; b: Word):SizeInt;
Finds word-sized value in a memory range
27function IsMemoryManagerSet: Boolean;
Is the memory manager set
28function Low( Arg: TypeOrVariable ):TOrdinal;
Returns lowest index of open array or enumerated
29procedure Move( const source; var dest; count: SizeInt );
Moves data from one location in memory to another
30procedure MoveChar0( const buf1; var buf2; len: SizeInt);
Moves data till first zero character
31procedure New( var P: Pointer);
Dynamically allocate memory for variable
32procedure New( var P: Pointer; Cons: TProcedure);
Dynamically allocates memory for variable
33function Ofs( var X ):LongInt;
Returns offset of variable
34function ptr( sel: LongInt; off: LongInt):farpointer;
Combines segment and offset to pointer
35function ReAllocMem( var p: pointer; Size: PtrUInt):pointer;
Resizes a memory block on the heap
36function Seg( var X):LongInt;
Returns segment
37procedure SetMemoryManager( const MemMgr: TMemoryManager );
Sets a memory manager
38function Sptr: Pointer;
Returns current stack pointer
39function Sseg: Word;
Returns stack segment register value


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