LECTURE No. 7. Dynamic memory

1. Reference data type. dynamic memory. Dynamic variables

A static variable (statically allocated) is a variable declared explicitly in the program, it is referred to by name. The place in memory for placing static variables is determined when the program is compiled. Unlike such static variables, Pascal programs can create dynamic variables. The main property of dynamic variables is that they are created and memory is allocated for them during program execution.

Dynamic variables are placed in a dynamic memory area (heap-area). A dynamic variable is not specified explicitly in variable declarations and cannot be referred to by name. Such variables are accessed using pointers and references.

A reference type (pointer) defines a set of values ​​that point to dynamic variables of a specific type, called a base type. A reference type variable contains the address of a dynamic variable in memory. If the base type is an undeclared identifier, then it must be declared in the same part of the type declaration as the pointer type.

The reserved word nil denotes a constant with a pointer value that does not point to anything.

Let us give an example of the description of dynamic variables.

var p1, p2 : ^real;

...

2. Working with dynamic memory. Untyped Pointers

Dynamic Memory Procedures and Functions

1. Procedure New(var p: Pointer).

Allocates space in the dynamic memory area to accommodate the dynamic variable p^Л, and assigns its address to the pointer p.

2. Procedure Dispose(varp: Pointer).

Frees the memory allocated for dynamic variable allocation by the New procedure, and the value of the pointer p becomes undefined.

3. Procedure GetMem(varp: Pointer; size: Word).

Allocates a memory section in the heap-area, assigns the address of its beginning to the p pointer, the size of the section in bytes is specified by the size parameter.

4. Procedure FreeMem(var p: Pointer; size: Word).

Frees the memory area, the beginning address of which is specified by the p pointer, and the size is specified by the size parameter. The value of the pointer p becomes undefined.

5. Procedure Mark(var p: Pointer)

Writes to the pointer p the address of the beginning of a section of free dynamic memory at the time of its call.

6. Procedure Release(var p: Pointer)

Releases a section of dynamic memory, starting from the address written to the pointer p by the Mark procedure, i.e. clears the dynamic memory that was occupied after the call to the Mark procedure.

7. MaxAvaikLongint Function

Returns the length, in bytes, of the longest free heap.

8. MemAvaikLongint Function

Returns the total amount of free dynamic memory in bytes.

9. Helper function SizeOf(X):Word

Returns the amount of bytes occupied by X, where X can be either a variable name of any type or a type name.

The built-in type Pointer denotes an untyped pointer, that is, a pointer that does not point to any particular type. Variables of type Pointer can be dereferenced: specifying the ^ character after such a variable causes an error.

Like the value denoted by nil, Pointer values ​​are compatible with all other pointer types.

Author: Tsvetkova A.V.

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