Memory management is a crucial aspect of programming, especially in languages like ALGOL, which provide low-level access to memory. This module will cover the fundamental concepts of memory management, including allocation, deallocation, and best practices to avoid common pitfalls such as memory leaks and fragmentation.
Key Concepts
- Memory Allocation: The process of reserving a portion of memory for use by a program.
- Memory Deallocation: The process of freeing up previously allocated memory.
- Static vs. Dynamic Memory: Understanding the difference between memory allocated at compile-time (static) and at runtime (dynamic).
- Heap and Stack: Two primary areas of memory used for different purposes.
- Garbage Collection: Automatic memory management to reclaim memory that is no longer in use.
Memory Allocation
Static Memory Allocation
Static memory allocation occurs at compile-time. The size and location of the memory are fixed and cannot be changed during runtime.
In this example, a and b are statically allocated. Their memory is reserved when the program starts and released when the program ends.
Dynamic Memory Allocation
Dynamic memory allocation occurs at runtime, allowing for more flexible memory usage. ALGOL uses procedures like new and dispose for dynamic memory management.
Here, array is dynamically allocated using new and deallocated using dispose.
Heap and Stack
Stack
The stack is used for static memory allocation, typically for function calls and local variables. It operates in a Last-In-First-Out (LIFO) manner.
Heap
The heap is used for dynamic memory allocation. It is more flexible but requires careful management to avoid fragmentation and leaks.
Garbage Collection
ALGOL does not have built-in garbage collection like some modern languages. Therefore, it is the programmer's responsibility to manage memory manually.
Practical Example
Let's create a dynamic array and manage its memory:
begin
integer array[1:10];
new(array);
for i := 1 step 1 until 10 do
array[i] := i * 2;
for i := 1 step 1 until 10 do
print(array[i]);
dispose(array);
end;Explanation
- Allocation:
new(array)dynamically allocates memory for the array. - Usage: The
forloops initialize and print the array elements. - Deallocation:
dispose(array)frees the allocated memory.
Common Mistakes and Tips
- Memory Leaks: Always ensure that dynamically allocated memory is deallocated using
dispose. - Double Free: Avoid deallocating memory that has already been freed.
- Null Pointers: Check for null pointers before accessing memory to prevent crashes.
Exercises
Exercise 1: Dynamic String Allocation
Write a program that dynamically allocates memory for a string, assigns a value, and then deallocates the memory.
Solution
Exercise 2: Dynamic Matrix Allocation
Write a program that dynamically allocates memory for a 2x2 matrix, assigns values, prints them, and then deallocates the memory.
begin
integer matrix[1:2, 1:2];
new(matrix);
matrix[1,1] := 1;
matrix[1,2] := 2;
matrix[2,1] := 3;
matrix[2,2] := 4;
for i := 1 step 1 until 2 do
for j := 1 step 1 until 2 do
print(matrix[i,j]);
dispose(matrix);
end;Solution
begin
integer matrix[1:2, 1:2];
new(matrix);
matrix[1,1] := 1;
matrix[1,2] := 2;
matrix[2,1] := 3;
matrix[2,2] := 4;
for i := 1 step 1 until 2 do
for j := 1 step 1 until 2 do
print(matrix[i,j]);
dispose(matrix);
end;Conclusion
In this module, we covered the basics of memory management in ALGOL, including static and dynamic memory allocation, heap and stack usage, and manual memory management. Understanding these concepts is crucial for writing efficient and error-free programs. In the next module, we will explore concurrency in ALGOL, which will build on the memory management concepts discussed here.