The
stack is a place in the computer memory where all the variables that are declared and initialized
before runtime are stored. The
heap is the section of computer memory where all the variables created or initialized
at runtime are stored.
What are the memory segments?
The distinction between
stack and
heap relates to programming. When you look at your computer memory, it is organized into three segments:
- text (code) segment
- stack segment
- heap segment
The
text segment (often called
code segment)
is where the compiled code of the program itself resides. When you open
some EXE file in Notepad, you can see that it includes a lot of
"Gibberish" language, something that is not readable to human. It is the
machine code, the computer representation of the program instructions.
This includes all user defined as well as system functions.
Now let's get to some details.
What is stack?
The two sections other from the code segment in the memory are used for
data. The
stack is the section of memory that is allocated for
automatic variables within functions.
Data
is stored in stack using the Last In First Out (LIFO) method. This
means that storage in the memory is allocated and deallocated at only
one end of the memory called the
top of the stack. Stack
is a section of memory and its associated registers that is used for
temporary storage of information in which the most recently stored item
is the first to be retrieved.
What is heap?
On the other hand,
heap is an area of memory used for
dynamic memory allocation.
Blocks of memory are allocated and freed in this case in an arbitrary
order. The pattern of allocation and size of blocks is not known until
run time. Heap is usually being used by a program for many different
purposes.
The stack is much faster than the heap but also smaller and more expensive.
Heap and stack from programming perspective
Most object-oriented languages have some defined structure, and some come with so-called
main() function. When a program begins running, the system calls the function
main() which marks the entry point of the program. For example every C, C++, or C# program must have one function named
main(). No other function in the program can be called
main(). Before we start explaining, let's take a look at the following example:
int x; /* static stack storage */
void main() {
int y; /* dynamic stack storage */
char str; /* dynamic stack storage */
str = malloc(50); /* allocates 50 bytes of dynamic heap storage */
size = calcSize(10); /* dynamic heap storage */
When a program begins executing in the main() function,
all variables declared within main() will be stored on the stack.
If the
main() function calls another function in the program, for example
calcSize(), additional storage will be allocated for the variables in
calcSize(). This storage will be allocated in the
heap memory segment.
Notice that the parameters passed by
main() to
calcSize() are also stored on the stack. If the
calcSize() function calls to any additional functions, more space would be allocated at the heap again.
When the
calcSize()
function returns the value, the space for its local variables at heap
is then deallocated and heap clears to be available for other functions.
The memory allocated in the heap area is used and reused during program execution.
It should be noted that memory allocated in heap will contain garbage values left over from previous usage.
Memory space for
objects is always allocated in heap. Objects are placed on the heap.
Built-in datatypes like int, double, float and parameters to methods are allocated on the stack.
Even though objects are held on heap, references to them are also variables and they are placed on stack.
The stack
segment provides more stable storage of data for a program. The memory
allocated in the stack remains in existence for the duration of a
program. This is good for global and static variables. Therefore, global
variables and static variables are allocated on the stack.
Why is stack and heap important?
When
a program is loaded into memory, it takes some memory management to
organize the process. If memory management was not present in your
computer memory, programs would clash with each other leaving the
computer non-functional.
Heap and stack in Java
When you create an object using the
new operator, for example
myobj = new Object();, it allocates memory for the myobj object on the
heap. The stack memory space is used when you declare automatic variables.
Note, when you do a string initialization, for example
String myString;, it is a reference to an object so it will be created using
new and hence it will be placed on the heap.
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The stack is the memory set aside as scratch
space for a thread of execution. When a function is called, a block is
reserved on the top of the stack for local variables and some
bookkeeping data. When that function returns, the block becomes unused
and can be used the next time a function is called. 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.
The heap is memory set aside for dynamic allocation. Unlike the
stack, there's no enforced pattern to the allocation and deallocation of
blocks from the heap; 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; there are
many custom heap allocators available to tune heap performance for
different usage patterns.
Stack:
- Stored in computer RAM like the heap.
- Variables created on the stack will go out of scope and automatically deallocate.
- Much faster to allocate in comparison to variables on the heap.
- Implemented with an actual stack data structure.
- Stores local data, return addresses, used for parameter passing
- Can have a stack overflow when too much of the stack is used.
(mostly from inifinite (or too much) recursion, very large allocations)
- Data created on the stack can be used without pointers.
- You would use the stack if you know exactly how much data you need to allocate before compile time and it is not too big.
- Usually has a maximum size already determined when your program starts
Heap:
- Stored in computer RAM like the stack.
- Variables on the heap must be destroyed manually and never fall out of scope. The data is freed with delete, delete[] or free
- Slower to allocate in comparison to variables on the stack.
- Used on demand to allocate a block of data for use by the program.
- Can have fragmentation when there are a lot of allocations and deallocations
- In C++ data created on the heap will be pointed to by pointers and allocated with new or malloc
- Can have allocation failures if too big of a buffer is requested to be allocated.
- You would use the 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.
- Responsible for memory leaks
Each thread gets a stack, while there's typically only one heap for
the application (although it isn't uncommon to have multiple heaps for
different types of allocation).
- The OS allocates the stack for each system-level thread when the
thread is created. Typically the OS is called by the language runtime
to allocate the heap for the application.
- The stack is attached to a thread, so when the thread exits the
stack is reclaimed. The heap is typically allocated at application
startup by the runtime, and is reclaimed when the application
(technically process) exits.
- The size of the stack is set when a thread is created. The size
of the heap is set on application startup, but can grow as space is
needed (the allocator requests more memory from the operating system).
- The stack is faster because the access pattern makes it trivial
to allocate and deallocate memory from it (a pointer/integer is simply
incremented or decremented), while the heap has much more complex
bookkeeping involved in an allocation or free. Also, each byte in the
stack tends to be reused very frequently which means it tends to be
mapped to the processor's cache, making it very fast.
