So many answers and I don't think one of them got it right 1) Where and what are they (physically in a real computer's memory)? Function calls are loaded here along with the local variables and function parameters passed. When using fibers, green threads or coroutines, you usually have a separate stack per function. Wow! The stack is a portion of memory that can be manipulated via several key assembly language instructions, such as 'pop' (remove and return a value from the stack) and 'push' (push a value to the stack), but also call (call a subroutine - this pushes the address to return to the stack) and return (return from a subroutine - this pops the address off of the stack and jumps to it). @ZaeemSattar Think of the static function variable like a hidden global or like a private static member variable. What Is the Difference Between 'Man' And 'Son of Man' in Num 23:19? The heap is the area of memory dynamic memory allocations are made out of (explicit "new" or "allocate" calls). The machine follows instructions in the code section. The stack is faster because all free memory is always contiguous. Heap space is used for the dynamic memory allocation of Java objects and JRE classes at runtime. In other words stack memory is kind of private memory of Java Threads, while heap memory is shared . A stack is a pile of objects, typically one that is neatly arranged. Can a function be allocated on the heap instead of a stack? Modern systems have good heap managers, and modern dynamic languages use the heap extensively (without the programmer really worrying about it). The reference variable of the String emp_name argument will point to the actual string from the string pool into the heap memory. Stored in computer RAM just like the heap. The stack memory is organized and we already saw how the activation records are created and deleted. As mentioned, heap and stack are general terms, and can be implemented in many ways. Heap memory is divided into Young-Generation, Old-Generation etc, more details at Java Garbage Collection. You can reach in and remove items in any order because there is no clear 'top' item. Yes, heap memory is a type of memory that is stored in the RAM (Random Access Memory) of a computer. Heap memory is also not as threaded-safe as Stack-memory because data stored in Heap-memory are visible to all threads. This answer was the best in my opinion, because it helped me understand what a return statement really is and how it relates to this "return address" that I come across every now and then, what it means to push a function onto the stack, and why functions are pushed onto stacks. Probably you may also face this question in your next interview. Is hardware, and even push/pop are very efficient. From the perspective of Java, both are important memory areas but both are used for different purposes. It is managed by Java automatically. In modern processors and operating systems the exact way it works is very abstracted anyway, so you don't normally need to worry much about how it works deep down, except that (in languages where it lets you) you mustn't use memory that you haven't allocated yet or memory that you have freed. In Java, memory management is a vital process. I say sometimes slower/faster above because the speed of the program might not have anything to do with items being allocated on the stack or heap. 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. Go memory usage (Stack vs Heap) Now that we are clear about how memory is organized let's see how Go uses Stack and Heap when a program is executed. You just move a pointer. \>>> Profiler image. "MOVE", "JUMP", "ADD", etc.). why memory for primitive data types is not allocated? See [link]. Memory is allocated in a contiguous block. why people created them in the first place?) It is a special data structure that can keep track of blocks of memory of varying sizes and their allocation status. That's what the heap is meant to be. This is because the compiler will generate a stack probe loop that is called every time your function is entered to make sure the stack exists (because Windows uses a single guard page at the end of your stack to detect when it needs to grow the stack. Consider real-time processing as an example. Usually has a maximum size already determined when your program starts. 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. Stacks in computing architectures are regions of memory where data is added or removed in a last-in-first-out manner. In java, a heap is part of memory that comprises objects and reference variables. On the stack you save return addresses and call push / ret pop is managed directly in hardware. Which is faster the stack or the heap? Assembly languages are the same since the beginning, despite variations up to Microsoft and its Intermediate Language (IL) that changed the paradigm to have a OO virtual machine assembly language. It is termed a heap because it is a collection of memory space that programmers can allocate and deallocate. One typical memory block was BSS (a block of zero values) Both the stack and the heap are memory areas allocated from the underlying operating system (often virtual memory that is mapped to physical memory on demand). PS: Those are just general rules, you can always find edge cases and each language comes with its own implementation and resulting quirks, this is meant to be taken as a guidance to the concept and a rule of thumb. I am getting confused with memory allocation basics between Stack vs Heap. Why is there a voltage on my HDMI and coaxial cables? If a law is new but its interpretation is vague, can the courts directly ask the drafters the intent and official interpretation of their law? (It may help to set a breakpoint here as well.) The stack is essentially an easy-to-access memory that simply manages its items "Static" (AKA statically allocated) variables are not allocated on the stack. ? Yum! A program doesn't really have runtime control over it; it's determined by the programming language, OS and even the system architecture. The stack and heap are traditionally located at opposite ends of the process's virtual address space. Then we find the main() method in the next line which is stored in the stack along with all its primitive(or local) and the reference variable Emp of type Emp_detail will also be stored in the Stack and will point out to the corresponding object stored in Heap memory. The heap is a different space for storing data where JavaScript stores objects and functions. In no language does static allocation mean "not dynamic". The pointer pBuffer and the value of b are located on the stack, and are mostly likely allocated at the entrance to the function. Stack Vs Heap Java. I quote "Static items go on the stack". i. Memory Management in JavaScript. When you call a function the arguments to that function plus some other overhead is put on the stack. A sample assembly program showing stack pointers/registers being used vis a vis function calls would be more illustrative. In this sense, the stack is an element of the CPU architecture. Once a stack variable is freed, that region of memory becomes available for other stack variables. In C you can get the benefit of variable length allocation through the use of alloca, which allocates on the stack, as opposed to alloc, which allocates on the heap. Here is a list of the key differences between Stack and Heap Memory in C#. This kind of memory allocation is also known as Temporary memory allocation because as soon as the method finishes its execution all the data belonging to that method flushes out from the stack automatically. The heap is typically allocated at application startup by the runtime, and is reclaimed when the application (technically process) exits. Stack vs Heap Memory - Java Memory Management (Pointers and dynamic memory) Naveen AutomationLabs 315K subscribers Join Subscribe Share 69K views 2 years ago Whiteboard Learning - By. The difference is the cost of allocating heap memory, which is expensive, where as allocating stack memory is basically a nop. Since some answers went nitpicking, I'm going to contribute my mite. That doesn't work with modern multi-threaded OSes though. No list needs to be maintained of all the segments of free memory, just a single pointer to the current top of the stack. B. Stack 1. The PC and register data gets and put back where it was as it is popped, so your program can go on its merry way. Heap memory is accessible or exists as long as the whole application (or java program) runs. While the objects stored on the stack are gone when the containing stack frame is popped, memory used by objects stored on the heap needs to be freed up by the garbage collector. @JatinShashoo Java runtime, as bytecode interpreter, adds one more level of virtualization, so what you referred to is just Java application point of view. The net result is a percentage of the heap space that is not usable for further memory allocations. What is the difference between memory, buffer and stack? These images should do a fairly good job of describing the two ways of allocating and freeing memory in a stack and a heap. In a stack of items, items sit one on top of the other in the order they were placed there, and you can only remove the top one (without toppling the whole thing over). Compiler vs Interpreter. Stack vs heap allocation of structs in Go, and how they relate to garbage collection. Most importantly, CPU registers.) Stack and a Heap ? Stack Memory vs. Heap Memory. This all happens using some predefined routines in the compiler. When a function is entered, the stack pointer is decreased to allocate more space on the stack for local (automatic) variables. There is a fair bit of overhead required in managing dynamically allocated memory, which is usually handled by the runtime code of the programming language or environment used. Also, stack vs. heap is not only a performance consideration; it also tells you a lot about the expected lifetime of objects. out of order. How to deallocate memory without using free() in C? This is incorrect. I'm really confused by the diagram at the end. Its only disadvantage is the shortage of memory, since it is fixed in size. Use the allocated memory. When the heap is used. To follow a pointer through memory: Example: Others have directly answered your question, but when trying to understand the stack and the heap, I think it is helpful to consider the memory layout of a traditional UNIX process (without threads and mmap()-based allocators). However, the stack is a more low-level feature closely tied to the processor architecture. But the allocation is local to a function call, and is limited in size. Since items are allocated on the heap by finding empty space wherever it exists in RAM, data is not always in a contiguous section, which sometimes makes access slower than the stack. In any case, the purpose of both fibers, green threads and coroutines is having multiple functions executing concurrently, but not in parallel (see this SO question for the distinction) within a single OS-level thread, transferring control back and forth from one another in an organized fashion. Objects (which vary in size as we update them) go on the heap because we don't know at creation time how long they are going to last. @SnowCrash one question about your picture - how do I access, I would refer to a static variable declared within a function as having only local, @supercat That all makes sense. In interviews, difference between heap memory and stack memory in java is a commonly asked question. The heap is a generic name for where you put the data that you create on the fly. Note that the name heap has nothing to do with the heap data structure. Some info (such as where to go on return) is also stored there. Variables allocated on the stack are stored directly to the memory and access to this memory is very fast, and its allocation is dealt with when the program is compiled. Memory on the heap is allocated, deallocated, and resized regularly during program execution, and this can lead to a problem called fragmentation. What does "relationship" and "order" mean in this context? Nevertheless, the global var1 has static allocation. 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 code in the function is then able to navigate up the stack from the current stack pointer to locate these values. When you construct an object, it is always in Heap-space, and the referencing information for these objects is always saved in Stack-memory. Stack vs Heap Know the differences. You can 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. You can also have more than one heap, for example some DLL configurations can result in different DLLs allocating from different heaps, which is why it's generally a bad idea to release memory allocated by a different library. Moreover stack and heap are two commonly used terms in perspective of java.. . 40 RVALUE. 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.