Stack vs. Heap — What's the Difference?
By Maham Liaqat & Fiza Rafique — Updated on April 8, 2024
Stack is used for static memory allocation with LIFO access, ideal for local variables. Heap allows dynamic memory allocation, suitable for variables whose size may change.
Difference Between Stack and Heap
Table of Contents
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Key Differences
The stack is a region of memory where data is added or removed in a last-in, first-out (LIFO) manner. This makes it perfect for storing local variables and function calls. On the other hand, the heap is used for dynamic memory allocation, where blocks of memory are allocated and freed in an arbitrary order, making it suitable for variables that can grow or shrink in size, such as arrays or strings.
Stack memory allocation is managed by the system: memory is allocated and deallocated automatically when a function is called and returns. This results in faster memory access but limits flexibility. Whereas, memory allocation on the heap is manually controlled by the programmer via allocation (e.g., malloc in C) and deallocation (e.g., free in C), offering more control at the cost of complexity and the risk of memory leaks.
Variables stored on the stack are accessible only within the function they were declared in, providing a degree of security and preventing accidental access from other parts of the program. On the other hand, variables allocated on the heap are accessible from any part of the program, provided the pointer to the heap memory is available, facilitating global access but increasing the risk of unintended modifications.
The size of the stack is predetermined by the system and can lead to a stack overflow if the limit is exceeded, especially in the case of deep recursive functions. The heap, while theoretically only limited by the size of the virtual memory, can suffer from fragmentation over time as blocks of memory are allocated and freed.
The stack's LIFO structure ensures a well-organized memory allocation and deallocation process, leading to efficient memory use and minimal fragmentation. In contrast, the heap's flexibility in memory allocation order can lead to fragmentation, requiring additional algorithms to manage free memory efficiently.
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Comparison Chart
Memory Allocation
Static, automatic by system
Dynamic, manual by programmer
Access Pattern
Last-in, first-out (LIFO)
Arbitrary access
Allocation Speed
Fast
Slower than stack
Memory Management
Automatic
Manual
Suitable For
Local variables, function calls
Dynamically sized variables, such as arrays
Compare with Definitions
Stack
Uses LIFO access method.
The last function called is the first to be completed and removed from the stack.
Heap
A memory area for dynamic allocation.
Int* numbers = new int[10]; allocates memory on the heap.
Stack
A memory region for local variables.
Int number in a function is stored in the stack.
Heap
Global access.
Variables allocated on the heap can be accessed from anywhere in the program.
Stack
Fast allocation.
Stack allows for quick memory allocation for temporary variables.
Heap
Manually managed memory.
Programmers must explicitly allocate and deallocate heap memory.
Stack
Limited by size.
Excessive memory allocation on the stack can lead to stack overflow.
Heap
Suitable for large, dynamic data.
The heap is used for data whose size can change at runtime.
Stack
Automatically managed memory.
Memory for local variables is automatically allocated and deallocated.
Heap
Prone to fragmentation.
Frequent allocations and deallocations can fragment the heap, reducing efficiency.
Stack
An orderly pile, especially one arranged in layers
A stack of newspapers.
Heap
A group of things placed or thrown, one on top of the other
A heap of dirty rags lying in the corner.
Stack
A large, usually conical pile of straw or fodder arranged for outdoor storage.
Heap
Often heaps(Informal) A great deal; a lot
We have heaps of homework tonight.
Stack
(Computers) A section of memory and its associated registers used for temporary storage of information in which the item most recently stored is the first to be retrieved.
Heap
(Slang) An old or run-down car.
Stack
A group of three rifles supporting each other, butt downward and forming a cone.
Heap
To put or throw in a pile
Heaped the clothes on the bed.
Stack
A chimney or flue.
Heap
To fill completely or to overflowing
Heap a plate with vegetables.
Stack
A group of chimneys arranged together.
Heap
To bestow in abundance or lavishly
Heaped praise on the rescuers.
Stack
A vertical exhaust pipe, as on a ship or locomotive.
Heap
A crowd; a throng; a multitude or great number of people.
Stack
An extensive arrangement of bookshelves.
Heap
A pile or mass; a collection of things laid in a body, or thrown together so as to form an elevation.
A heap of earth; a heap of stones
Stack
The area of a library in which most of the books are shelved.
Heap
A great number or large quantity of things.
Stack
A stackup.
Heap
(computing) A data structure consisting of trees in which each node is greater than all its children.
Stack
An English measure of coal or cut wood, equal to 108 cubic feet (3.06 cubic meters).
Heap
(computing) Memory that is dynamically allocated.
You should move these structures from the stack to the heap to avoid a potential stack overflow.
Stack
(Informal) A large quantity
A stack of work to do.
Heap
(colloquial) A dilapidated place or vehicle.
My first car was an old heap.
Stack
To arrange in a stack; pile.
Heap
(colloquial) A lot, a large amount
Thanks a heap!
Stack
To load or cover with stacks or piles
Stacked the dishwasher.
Heap
(transitive) To pile in a heap.
He heaped the laundry upon the bed and began folding.
Stack
(Games) To prearrange the order of (a deck of cards) so as to increase the chance of winning.
Heap
(transitive) To form or round into a heap, as in measuring.
Stack
To prearrange or fix unfairly so as to favor a particular outcome
Tried to stack the jury.
Heap
(transitive) To supply in great quantity.
They heaped praise upon their newest hero.
Stack
To direct (aircraft) to circle at different altitudes while waiting to land.
Heap
Very; representing broken English stereotypically or comically attributed to Native Americans
Stack
To form a stack
Make sure the boxes stack neatly against the wall.
Heap
A crowd; a throng; a multitude or great number of persons.
The wisdom of a heap of learned men.
A heap of vassals and slaves.
He had heaps of friends.
Stack
(heading) A pile.
Heap
A great number or large quantity of things not placed in a pile; as, a heap of trouble.
A vast heap, both of places of scripture and quotations.
I have noticed a heap of things in my life.
Stack
A large pile of hay, grain, straw, or the like, larger at the bottom than the top, sometimes covered with thatch.
Heap
A pile or mass; a collection of things laid in a body, or thrown together so as to form an elevation; as, a heap of earth or stones.
Huge heaps of slain around the body rise.
Stack
A pile of similar objects, each directly on top of the last.
Please bring me a chair from that stack in the corner.
Heap
To collect in great quantity; to amass; to lay up; to accumulate; - usually with up; as, to heap up treasures.
Though he heap up silver as the dust.
Stack
(UK) A pile of poles or wood, indefinite in quantity.
Heap
To throw or lay in a heap; to make a heap of; to pile; as, to heap stones; - often with up; as, to heap up earth; or with on; as, to heap on wood or coal.
Stack
A pile of wood containing 108 cubic feet. (~3 m³)
Heap
To form or round into a heap, as in measuring; to fill (a measure) more than even full.
Stack
An extensive collection
Heap
A collection of objects laid on top of each other
Stack
A smokestack.
Heap
(often followed by `of') a large number or amount or extent;
A batch of letters
A deal of trouble
A lot of money
He made a mint on the stock market
It must have cost plenty
Stack
(heading) In computing.
Heap
A car that is old and unreliable;
The fenders had fallen off that old bus
Stack
(programming) A linear data structure in which items inserted are removed in reverse order (the last item inserted is the first one to be removed).
History stack
Heap
Bestow in large quantities;
He heaped him with work
She heaped scorn upon him
Stack
A stack data structure stored in main memory that is manipulated during machine language procedure call related instructions.
Heap
Arrange in stacks;
Heap firewood around the fireplace
Stack your books up on the shelves
Stack
An implementation of a protocol suite (set of protocols forming a layered architecture).
A TCP/IP stack is a library or set of libraries or of OS drivers that take care of networking.
Heap
Fill to overflow;
Heap the platter with potatoes
Stack
A combination of interdependent, yet individually replaceable, software components or technologies used together on a system.
Stack
(math) A generalization of schemes in algebraic geometry and of sheaves.
Stack
(geology) A coastal landform, consisting of a large vertical column of rock in the sea.
Stack
(library) Compactly spaced bookshelves used to house large collections of books.
Stack
(figuratively) A large amount of an object.
They paid him a stack of money to keep quiet.
Stack
(military) A pile of rifles or muskets in a cone shape.
Stack
(poker) The amount of money a player has on the table.
Stack
(heading) In architecture.
Stack
A number of flues embodied in one structure, rising above the roof.
Stack
A vertical drainpipe.
Stack
A fall or crash, a prang.
Stack
(bodybuilding) A blend of various dietary supplements or anabolic steroids with supposed synergistic benefits.
Stack
(aviation) A holding pattern, with aircraft circling one above the other as they wait to land.
Stack
(video games) The quantity of a given item which fills up an inventory slot or bag.
I've got 107 Golden Branches, but the stack size is 20 so they're taking up 6 spaces in my inventory.
Stack
(transitive) To arrange in a stack, or to add to an existing stack.
Please stack those chairs in the corner.
Stack
To arrange the cards in a deck in a particular manner.
This is the third hand in a row where you've drawn four of a kind. Someone is stacking the deck!
Stack
To take all the money another player currently has on the table.
I won Jill's last $100 this hand; I stacked her!
Stack
(transitive) To deliberately distort the composition of (an assembly, committee, etc.).
The Government was accused of stacking the parliamentary committee.
Stack
To crash; to fall.
Jim couldn′t make it today as he stacked his car on the weekend.
Stack
(gaming) To operate cumulatively.
A magical widget will double your mojo. And yes, they do stack: if you manage to get two magical widgets, your mojo will be quadrupled. With three, it will be octupled, and so forth.
Stack
To place (aircraft) into a holding pattern.
Stack
To collect precious metal in the form of various small objects such as coins and bars.
Stack
(printing) To have excessive ink transfer.
Stack
A large and to some degree orderly pile of hay, grain, straw, or the like, usually of a nearly conical form, but sometimes rectangular or oblong, contracted at the top to a point or ridge, and sometimes covered with thatch.
But corn was housed, and beans were in the stack.
Stack
An orderly pile of any type of object, indefinite in quantity; - used especially of piles of wood. A stack is usually more orderly than a pile
Against every pillar was a stack of billets above a man's height.
Stack
A pile of wood containing 108 cubic feet.
Stack
A large quantity; as, a stack of cash.
Stack
A number of flues embodied in one structure, rising above the roof.
Stack
A section of memory in a computer used for temporary storage of data, in which the last datum stored is the first retrieved.
Stack
The section of a library containing shelves which hold books less frequently requested.
Stack
To lay in a conical or other pile; to make into a large pile; as, to stack hay, cornstalks, or grain; to stack or place wood.
Stack
To place in a vertical arrangement so that each item in a pile is resting on top of another item in the pile, except for the bottom item; as, to stack the papers neatly on the desk; to stack the bricks.
Stack
To select or arrange dishonestly so as to achieve an unfair advantage; as, to stack a deck of cards; to stack a jury with persons prejudiced against the defendant.
Stack
An orderly pile
Stack
(often followed by `of') a large number or amount or extent;
A batch of letters
A deal of trouble
A lot of money
He made a mint on the stock market
It must have cost plenty
Stack
A list in which the next item to be removed is the item most recently stored (LIFO)
Stack
A large tall chimney through which combustion gases and smoke can be evacuated
Stack
A storage device that handles data so that the next item to be retrieved is the item most recently stored (LIFO)
Stack
Load or cover with stacks;
Stack a truck with boxes
Stack
Arrange in stacks;
Heap firewood around the fireplace
Stack your books up on the shelves
Stack
Arrange the order of so as to increase one's winning chances;
Stack the deck of cards
Common Curiosities
Why would a programmer choose heap over stack?
For flexibility and to create large or dynamically sized data structures not suitable for the stack's size constraints.
What is stack memory?
Stack memory is a section of memory that stores local variables and function calls, using a last-in, first-out access pattern.
Can stack and heap memory run out?
Yes, both can run out: stack memory is limited in size and can cause stack overflow, while heap memory is limited by the system's available memory and can be fragmented.
How is memory managed on the stack and heap?
Stack memory management is automatic, while heap memory must be manually managed by the programmer.
Can variables on the heap be accessed globally?
Yes, heap-allocated variables can be accessed from anywhere in the program if the pointer is available.
What is heap memory?
Heap memory is used for dynamic memory allocation, allowing for the creation of variables that can change in size at runtime.
How do stack and heap differ in access patterns?
Stack uses a LIFO access pattern, while heap allows for arbitrary access to memory.
Can memory allocation on the heap be optimized?
Yes, through algorithms and techniques to manage free memory and reduce fragmentation.
Is heap memory slower than stack memory?
Yes, because dynamic memory allocation and deallocation on the heap are more complex and slower than the stack's automatic management.
What is a stack overflow?
A stack overflow occurs when there is no more space in the stack memory, often due to deep recursion or excessive local variable storage.
How does heap fragmentation occur?
Fragmentation happens due to the constant allocation and deallocation of memory blocks in an arbitrary order, leaving unusable gaps.
What are the risks of manual memory management?
Manual management, like on the heap, increases the risk of memory leaks and fragmentation.
What is automatic memory management?
It refers to the system's process of automatically allocating and deallocating memory, as seen with stack memory.
Are there size limitations for stack and heap?
The stack has a fixed size set by the system, while the heap is limited by the system's available memory or virtual memory space.
What happens when the heap runs out of memory?
Running out of heap memory can result in allocation failures, potentially causing the program to crash or behave unpredictably.
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Written by
Maham Liaqat
Co-written by
Fiza RafiqueFiza Rafique is a skilled content writer at AskDifference.com, where she meticulously refines and enhances written pieces. Drawing from her vast editorial expertise, Fiza ensures clarity, accuracy, and precision in every article. Passionate about language, she continually seeks to elevate the quality of content for readers worldwide.
