Lecture 9 Memory Allocation in C++
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Questions and Answers

Which memory segment is also known as the 'free store'?

  • Data Segment
  • Heap Segment
  • Code Segment (correct)
  • BSS Segment

The Code Segment typically contains read-only data, including the program's executable instructions.

True (A)

What does LIFO stand for in the context of a call stack?

Last-In, First-Out

The BSS Segment contains all ______ global and static variables.

<p>uninitialized</p> Signup and view all the answers

Match the memory allocation types with their characteristics:

<p>Static memory allocation = Allocated once, persists throughout the program's life. Automatic memory allocation = Allocated for local variables and freed when the block is exited. Dynamic memory allocation = Requested when needed and must be explicitly freed.</p> Signup and view all the answers

In C++, which type of memory allocation requires the programmer to manually deallocate the memory?

<p>All types of memory allocation (C)</p> Signup and view all the answers

Automatic variables persist throughout the entire execution of a program.

<p>False (B)</p> Signup and view all the answers

What is a 'stack frame' in the context of the call stack?

<p>Data associated to one function call</p> Signup and view all the answers

A ______ register tracks the top of the stack, adjusted each time a value is pushed onto or popped from the stack.

<p>stack pointer</p> Signup and view all the answers

Match the memory areas with the type of variables typically stored in them:

<p>Stack = Local variables and function parameters BSS = Uninitialized global and static variables Data Segment = Initialized static variables Heap = Dynamically allocated variables</p> Signup and view all the answers

Which of the following is an advantage of using the stack for memory allocation?

<p>Size is relatively large. (B)</p> Signup and view all the answers

Heap memory is automatically deallocated when the program no longer needs it.

<p>False (B)</p> Signup and view all the answers

What is heap fragmentation?

<p>When a large allocation fails because none of the free blocks are of a large enough size</p> Signup and view all the answers

The size of variables/arrays allocated with static memory allocation must be known at ______.

<p>compile time</p> Signup and view all the answers

Match each function with its purpose:

<p>malloc = Allocates memory without initializing it. calloc = Allocates memory and initializes it to 0. realloc = Resizes previously allocated memory. free = Frees the allocated memory.</p> Signup and view all the answers

Which function copies memory from one block to another?

<p>memset (C)</p> Signup and view all the answers

It is acceptable to access memory after it has been freed, as long as you reallocate it later.

<p>False (B)</p> Signup and view all the answers

What value does malloc return if the memory allocation fails?

<p>NULL</p> Signup and view all the answers

It is good practice to initialize dynamically allocated memory to all ______, using memset() if the allocation succeeds.

<p>zeros</p> Signup and view all the answers

Match each operation with the appropriate memory management function:

<p>Allocate memory for a string and duplicate it = strdup Set all bytes of a memory region to a specific value = memset Dynamically allocate memory = malloc Allocate with initialization = calloc</p> Signup and view all the answers

What can happen if a program loses the pointer to dynamically allocated memory?

<p>A memory leak occurs. (C)</p> Signup and view all the answers

It is acceptable practice for some programs to never free memory.

<p>True (A)</p> Signup and view all the answers

What is the primary difference between malloc and calloc?

<p><code>calloc</code> initializes memory to zero</p> Signup and view all the answers

Dynamically sized arrays should not be ______ variables.

<p>local</p> Signup and view all the answers

Match the C memory areas with where the allocation occurs:

<p>Global variables = Data Segment or BSS Local variables = Stack Memory allocated with malloc = Heap</p> Signup and view all the answers

Which of the following is NOT a valid memory allocation function in C++?

<p>alloc (D)</p> Signup and view all the answers

The strdup function only duplicates the string, not including the null-terminating character.

<p>False (B)</p> Signup and view all the answers

In the following code snippet, indicate where the variable temp is allocated: void func() { if(true) { int temp = 0; } temp = 1; }

<p>Stack</p> Signup and view all the answers

A stack uses a ______ data structure.

<p>LIFO</p> Signup and view all the answers

Analyze the following C++ code snippet and select the statement that best describes its memory allocation behavior:

struct TestClass {};
TestClass* ptr_test() {
 int i; //automatic, local, stack
 TestClass objTC; //automatic, local, stack
 TestClass* ptrA = &objTC; //automatic, local, stack
 TestClass* ptrB = (TestClass*)malloc(sizeof(TestClass)); //ptr local, on heap
 return ptrB; //is this safe?
 return ptrA; //is this safe?
}

<p><code>ptrB</code> points to memory allocated on the stack, and returning it is unsafe. (C)</p> Signup and view all the answers

Memory allocated using calloc is guaranteed to be zero-initialized across all operating systems and hardware architectures.

<p>True (A)</p> Signup and view all the answers

Explain how the stack pointer is modified when data is pushed onto the stack and subsequently popped off the stack. Assume the stack grows downwards (towards lower memory addresses).

<p>Pushed: decreased; popped: increased</p> Signup and view all the answers

When a function is called, a ______ is created on the stack to store parameters, local variables, and the return address.

<p>stack frame</p> Signup and view all the answers

Match each of the function code, with the likely memory segment it will exist in once produced as an executable:

<p><code>void my_function() { ... }</code> = Code Segment <code>int global_variable;</code> (declared outside any function) = BSS Segment (if uninitialized) or Data Segment (if initialized) <code>static int local_static_variable;</code> (declared inside a function) = Data Segment (if initialized) or BSS Segment (if uninitialized) <code>int* ptr = (int*)malloc(sizeof(int) * 10);</code> (inside a function) = Heap</p> Signup and view all the answers

What is the potential consequence of failing to check the return value of malloc?

<p>The memory allocation will automatically be retried. (B)</p> Signup and view all the answers

Using global variables is always recommended for efficient memory management.

<p>False (B)</p> Signup and view all the answers

Explain why allocating large arrays on the stack (as opposed to the heap) can be problematic.

<p>Stack overflow</p> Signup and view all the answers

The memset() function fills a block of memory with a specific ______.

<p>value</p> Signup and view all the answers

Match the term to the description:

<p>Code Segment = Contains a program's executable instructions. Data Segment = Contains all initialized static variables. Heap = Memory that must explicitly be allocated and freed.</p> Signup and view all the answers

Which memory area is primarily managed using a LIFO structure?

<p>Data Segment (C)</p> Signup and view all the answers

Static variables, even if uninitialized, are allocated on the stack.

<p>False (B)</p> Signup and view all the answers

In the context of the closest points problem, why is memory allocation a learning outcome?

<p>Because coordinates must have a certain amount of memory allocated, which directly impacts how the program is meant to be written and how the input is handled.</p> Signup and view all the answers

The primary outcome of calloc over malloc is that calloc also ______ the new memory.

<p>initializes</p> Signup and view all the answers

Flashcards

Code Segment

A memory area containing the program's executable instructions, typically read-only.

BSS Segment

A memory area for uninitialized global and static variables, initialized to 0 by default.

Data Segment

A memory area that is usually adjacent to the BSS Segment, it contains initialized static variables.

Heap Segment

Also known as the free store, this memory area stores dynamically allocated variables.

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Call Stack

A stack is a LIFO (Last In, First Out) structure.

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Static memory allocation

Memory allocation that occurs for static and global variables, persists throughout program life.

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Automatic memory allocation

Memory allocation for local variables and function parameters, freed when the block is exited.

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Dynamic memory allocation

Memory allocation when needed, requires manual freeing when no longer needed.

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Call stack

A container data structure holding multiple variables, keeps track of all active functions.

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Stack Frame

When a program encounters a function call, a region of memory can be added to the stack

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Call stack advantages

Stack operations are fast in memory

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Call stack disadvantages

The size on the call stack is limited to a fixed size

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Heap

A memory pool outside of the stack for dynamic memory allocation and deallocation, managed by the OS

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malloc

Allocates memory without initializing it.

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calloc

Allocates memory and initializes it to 0.

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realloc

Resizes previously allocated memory.

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memset

Fills memory with a specific value.

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memcpy

Copies memory from one block to another.

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free

Frees all allocated memory.

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free() function

Returns memory to the heap.

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malloc() function

Takes a positive integer and returns a pointer to allocated memory

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calloc()

Use to allocate and initialize in one step

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Memory allocation

When using dynamic memory allocation, always remember to return the memory

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Avoid header declarations!

Never put function or variable definitions in header files.

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Use header files!

Every code file, except the one containing main() should have a corresponding header file.

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Initialize variables

All variables should be initialized!

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Study Notes

  • Memory allocation involves managing how a computer program uses memory.

Program Memory Segments

  • Code Segment: Stores executable instructions and is typically read-only. Also known as the text segment.
  • BSS Segment: Stores uninitialized global and static variables, or those initialized to zero.
    • Example: static int i;
  • Data Segment: Stores initialized static variables, including global and static local variables.
    • The size is determined at compile time by the size of the data types.
    • Example: static int i = 3;
  • Heap Segment: Also called the free store, which stores dynamically allocated variables.
  • Call Stack: A LIFO (Last-In-First-Out) structure that stores function parameters, local variables, and other function-related information.
    • A stack pointer register tracks the top, adjusted as values are pushed or popped.

Memory Allocation Types in C++

  • Static Memory Allocation: Used for static and global variables.
    • Allocation occurs once when the program runs and lasts for its entire life.
  • Automatic Memory Allocation: Used for local variables and function parameters.
    • Allocation happens when the relevant code block, such as a function, is entered, and the memory is freed when the block exits.
  • Dynamic Memory Allocation: Used when memory is requested during runtime.
    • Requires explicit deallocation when the memory is no longer needed.

Static vs Automatic Allocation

  • Both static and automatic allocation require the size of the variable/array to be known at compile time.
  • Memory allocation and deallocation are automatic.
  • Static variables are allocated in the BSS (if uninitialized) or Data Segment (if initialized), not on the stack.
  • Automatic variables are allocated on the stack and persist only during a single call to a function.

Call Stack Details

  • Tracks all active functions and handles allocation of function parameters and local variables.
  • Implemented as a container data structure that can peek, top, pop, and push data.
  • Stores data associated with a function call in a stack frame.
  • A stack pointer tracks the top of the call stack.

Call Stack Operation

  1. Program Encounters a Function Call:
    • A stack frame is constructed and pushed onto the stack.
    • The stack frame contains the return address, function arguments, local variables, and register values to be restored.
    • CPU execution then moves to the function's start point.
  2. Upon Function Termination:
    • Registers are restored from the stack.
    • The stack frame is removed, freeing the memory allocated to local variables and arguments.
    • CPU execution resumes at the return address.

Call Stack: Pros and Cons

  • Advantages:
    • Fast stack operations.
    • Memory size is known at compile time, allowing direct access through a variable.
    • Automatic memory allocation and deallocation.
    • Contiguous, non-fragmented memory.
  • Disadvantages:
    • Relatively small size, typically 1 MB on Windows and up to 8 MB on Linux.
    • Potential for wasted memory if allocated space is unused such as char description[50];.
    • Variables cannot be resized.
    • LIFO (Last-In-First-Out) access only.

Heap Memory

  • A larger memory pool is managed by the OS.
  • Heap memory is dynamically allocated to variables as needed and can be returned when it's no longer used.
  • The program handles allocation and deallocation.
  • New memory is created by allocating a block of appropriate size from available memory blocks.
  • Requesting a large block may fail if no free blocks are large enough, leading to heap fragmentation.

Memory Allocation Functions

  • malloc: Allocates memory without initializing it.
  • Takes a positive integer specifying the amount of memory.
  • Should check if memory allocation was successful and returns NULL if it failed.
  • calloc: Allocates memory and initializes it to 0.
  • realloc: Resizes previously allocated memory.
  • memset: Fills memory with a specific value.
  • memcpy: Copies memory from one block to another.
  • strdup: Duplicates a string, including the null-terminating character.
  • free: Frees allocated memory.

Deallocating Memory

  • void free(void *ptr) takes a pointer to allocated memory and returns it to the heap.
  • Memory should not be accessed after it has been freed unless it is reallocated.
  • Results are undefined if the pointer doesn't point to a allocated memory.
  • All memory that a program has allocated is freed when it terminates.
  • Freeing memory once it's no longer needed is good practice.

Allocating Memory

  • void *malloc(int size) takes a positive integer and returns a pointer to the allocated memory, or NULL if the allocation fails.
  • It is good practice to initialize memory to all zeros using memset(), if the allocation succeeds, or using calloc to allocate and initialize in one step.
  • Memory will not be freed until the program terminates if the pointer to the memory is lost

C Programming Rules

  • Put function or variable definitions never in header files, to avoid multiple definitions of the same symbol.

Dynamically Sized Arrays Caution

  • Avoid allocating large or dynamically sized arrays as local variables to prevent stack overflow.

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Description

Explore the concepts of memory management using C++. Key areas include the code, BSS, data, heap, and stack segments. Learn about static vs dynamic allocation in C++.

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