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.
Now let's get to some details.
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.
The stack is much faster than the heap but also smaller and more expensive.
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.
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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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
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:
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 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
- 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.
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