Stack Vs Heap Memory

  1. Stack is used for static memory allocation and Heap for dynamic memoryallocation. It means “You can use the stack if you know exactly how much data you need to allocate before compile time and it is not too big. And You can use heap if you don’t know exactly how much data you will need at runtime or if you need to allocate a lot of data.

2) Both are stored in the computer’s RAM .

                                        Random-access memory  is a form of computer data storage which stores frequently used program instructions to increase the general speed of a system. A random-access memory device allows data items to be read or written in almost the same amount of time irrespective of the physical location of data inside the memory.

In contrast, with other direct-access data storage media such as hard disks, CD-RWs, DVD-RWs and the older drum memory, the time required to read and write data items varies significantly depending on their physical locations on the recording medium, due to mechanical limitations such as media rotation speeds and arm movement.

3) STACK -> Variables allocated on the stack are stored directly to the memory and access to this memory is very fast, and it’s allocation is dealt with when the program is compiled.

When a function or a method calls another function which in turns calls another function etc., the execution of all those functions remains suspended until the very last function returns its value. The stack is always reserved in a LIFO order, the most recently reserved block is always the next block to be freed. This makes it really simple to keep track of the stack, freeing a block from the stack is nothing more than adjusting one pointer.

HEAP -> Variables allocated on the heap have their memory allocated at run time and accessing this memory is a bit slower, but the heap size is only limited by the size of virtual memory (Memory that appears to exist as main storage.It maps memory addresses used by a program, called virtual addresses, into physical addresses in computer memory).

Element of the heap have no dependencies with each other and can always be accessed randomly at any time. You can allocate a block at any time and free it at any time. This makes it much more complex to keep track of which parts of the heap are allocated or free at any given time.

4) In a multi-threaded situation each thread will have its own completely independent stack but they will share the heap.

Stack is thread specific and Heap is application specific. The stack is important to consider in exception handling and thread executions.

How long does memory on the stack last versus memory on the heap?

Once a function call runs to completion, any data on the stack created specifically for that function call will automatically be deleted. Any data on the heap will remain there until it’s manually deleted by the programmer.

Can the stack grow in size? Can the heap grow in size?

The stack is set to a fixed size, and can not grow past it’s fixed size (although some languages have extensions that do allow this). So, if there is not enough room on the stack to handle the memory being assigned to it, a stack overflow occurs. This often happens when a lot of nested functions are being called, or if there is an infinite recursive call.

If the current size of the heap is too small to accommodate new memory, then more memory can be added to the heap by the operating system. This is one of the big differences between the heap and the stack.

Which is faster – the stack or the heap? And why?

The stack is much faster than the heap. This is because of the way that memory is allocated on the stack.

How is memory deallocated on the stack and heap?

Data on the stack is automatically deallocated when variables go out of scope. However, in languages like C and C++,  data stored on the heap has to be deleted manually by the programmer using one of the built in keywords like free, delete, or delete[ ]. Other languages like Java and .NET use garbage collection to automatically delete memory from the heap, without the programmer having to do anything.

What can go wrong with the stack and the heap?

If the stack runs out of memory, then this is called a stack overflow – and could cause the program to crash.

The heap could have the problem of fragmentation, which occurs when the available memory on the heap is being stored as noncontiguous (or disconnected) blocks – because used blocks of memory are in between the unused memory blocks. When excessive fragmentation occurs, allocating new memory may be impossible because of the fact that even though there is enough memory for the desired allocation, there may not be enough memory in one big block for the desired amount of memory.

Which one should I use – the stack or the heap?

For people new to programming, it’s probably a good idea to use the stack since it’s easier.
Because the stack is small, you would want to use it when you know exactly how much memory you will need for your data, or if you know the size of your data is very small.

It’s better to use the heap when you know that you will need a lot of memory for your data, or you just are not sure how much memory you will need (like with a dynamic array).

Stack vs Heap Pros and Cons


  • very fast access
  • don’t have to explicitly de-allocate variables
  • space is managed efficiently by CPU, memory will not become fragmented
  • local variables only
  • limit on stack size (OS-dependent)
  • variables cannot be resized


  • variables can be accessed globally
  • no limit on memory size
  • (relatively) slower access
  • no guaranteed efficient use of space, memory may become fragmented over time as blocks of memory are allocated, then freed
  • you must manage memory (you’re in charge of allocating and freeing variables)
  • variables can be resized using realloc()

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