Memory

Question

what is difference between Stack and Heap?

Narasimha Reddy Chennupalli · Answer

Stack is used for static memory allocation and Heap for dynamic memory allocation both stored in the computer's RAM . 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. 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 . 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.

Blackbarbie Bb · Answer

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 (last in first out) 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. 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). To answer your questions directly: To what extent are they controlled by the OS or language runtime? 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. What is their scope? 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. What determines the size of each of them? 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). What makes one faster? 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. Another performance hit for the heap is that the heap being mostly a global resource typically has to be multi-threading safe i.e. each allocation and deallocation needs to be - typically - synchronized with all other heap accesses in the program.