Program code stack heap

It allocates memory for automatic variables on the stack and dynamic variables, created with the new operator, on the heap. When the program deallocates memory, the memory management system returns it to its origin, either the stack or the heap.

The vital concept that we can draw from Figure 1 is that a running program maintains its data in one of three logical regions of memory.

The OS allocates the memory for global and static variables when it loads the program into memory and doesn't deallocate it until the program terminates. Values stored in these variables do not change unless the program explicitly changes them barring changes made by external - outside of the program - systems. The other two regions, the stack and the heap, are more dynamic - the memory is allocated and deallocated by the program as needed. The differences between these two regions are the algorithms that manage the memory and, therefore, how they behave.

A stack is a simple last-in, first-out LIFO data structure. Imagining a stack of plates in a cafeteria is common to introduce stacks to new computer scientists. In the cafeteria example, plates are removed from or added to the stack only at the top. One of our first actions upon entering the cafeteria is to pop a plate off the top of the stack of plates before we go through the line. Similarly, the dishwashers push each clean plate on top of the stack one at a time.

In this way, the last clean plate pushed on the stack is the first plate that a customer pops off of the top. Learn to implement data structures like Heap, Stacks, Linked List and many more!

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They are not designed to be fast, they are designed to be useful. How the programmer utilizes them determines whether they are "fast" or "slow". A lot of answers are correct as concepts, but we must note that a stack is needed by the hardware i. OOP guys will call it methods. You can use the stack to pass parameters..

The stack is essentially an easy-to-access memory that simply manages its items as a - well - stack. Only items for which the size is known in advance can go onto the stack. This is the case for numbers, strings, booleans. Since objects and arrays can be mutated and change at runtime, they have to go into the heap. Source: Academind. CPU stack and heap are physically related to how CPU and registers works with memory, how machine-assembly language works, not high-level languages themselves, even if these languages can decide little things.

All modern CPUs work with the "same" microprocessor theory: they are all based on what's called "registers" and some are for "stack" to gain performance. All CPUs have stack registers since the beginning and they had been always here, way of talking, as I know. Assembly languages are the same since the beginning, despite variations CPUs have stack registers to speed up memories access, but they are limited compared to the use of others registers to get full access to all the available memory for the processus.

It why we talked about stack and heap allocations. In summary, and in general, the heap is hudge and slow and is for "global" instances and objects content, as the stack is little and fast and for "local" variables and references hidden pointers to forget to manage them. So when we use the new keyword in a method, the reference an int is created in the stack, but the object and all its content value-types as well as objects is created in the heap, if I remember.

But local elementary value-types and arrays are created in the stack. The difference in memory access is at the cells referencing level: addressing the heap, the overall memory of the process, requires more complexity in terms of handling CPU registers, than the stack which is "more" locally in terms of addressing because the CPU stack register is used as base address, if I remember.

It is why when we have very long or infinite recurse calls or loops, we got stack overflow quickly, without freezing the system on modern computers C Heap ing Vs Stack ing In. Stack vs Heap: Know the Difference.

Static class memory allocation where it is stored C. What and where are the stack and heap? Assembly Programming Tutorial. Thank you for a really good discussion but as a real noob I wonder where instructions are kept? Ultimately, we went with the von Neumann design and now everything is considered 'the same'. Everything above talks about DATA. My guess is that since an instruction is a defined thing with a specific memory footprint, it would go on the stack and so all 'those' registers discussed in assembly are on the stack.

Of course then came object oriented programming with instructions and data comingled into a structure that was dynamic so now instructions would be kept on the heap as well? When a process is created then after loading code and data OS setup heap start just after data ends and stack to top of address space based on architecture.

When more heap is required OS will allocate dynamically and heap chunk is always virtually contiguous. Please see brk , sbrk and alloca system call in linux. To read anything, you must have a book open on your desk, and you can only have as many books open as fit on your desk.

To get a book, you pull it from your bookshelf and open it on your desk. To return a book, you close the book on your desk and return it to its bookshelf. Stack and heap are names we give to two ways compilers store different kinds of data in the same place i. What is their scope? What determines the size of each of them? What makes one faster? It allocates a fixed amount of memory for these variables.

This size of this memory cannot grow. The memory is contiguous a single block , so access is sometimes faster than the heap. An object placed on the stack that grows in memory during runtime beyond the size of the stack causes a stack overflow error. The amount of memory is limited only by the amount of empty space available in RAM i.

The amount used can grow or shrink as needed at runtime. 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.

Programmers manually put items on the stack with the new keyword and MUST manually deallocate this memory when they are finished using it. Code that repeatedly allocates new memory without deallocating it when it is no longer needed to a memory leak. The stack and heap were not primarily introduced to improve speed; they were introduced to handle memory overflow. The first concern regarding use of the stack vs. If an object is intended to grow in size to an unknown amount like a linked list or an object whose members can hold an arbitrary amount of data , place it on the heap.

After getting your code to run, if you find it is running unacceptably slow, then go back and refactor your code and see if it can be programmed more efficiently. It may turn out the problem has nothing to do with the stack or heap directly at all e. CPU-bound tasks, perhaps add multithreading or multiprocessing.

The heap size varies during runtime. The heap works with the OS during runtime to allocate memory. Each computer has a unique instruction set architecture ISA , which are its hardware commands e.

The ISA of the OS is called the bare machine and the remaining commands are called the extended machine. The kernel is the first layer of the extended machine. It controls things like. The machine code gets passed to the kernel when executed, which determines when it should run and take control, but the machine code itself contains ISA commands for requesting files, requesting memory, etc. So the code issues ISA commands, but everything has to pass by the kernel. Stack Overflow for Teams — Collaborate and share knowledge with a private group.

Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Ask Question. Asked 13 years, 4 months ago. Active 1 month ago. Viewed 1. But, Where and what are they physically in a real computer's memory? To what extent are they controlled by the OS or language run-time? Improve this question.

Also really good: codeproject. Related, see Stack Clash. Also see Red Hat Issue — jww. In other words, the stack and heap can be fully defined even if value and reference types never existed.

Further, when understanding value and reference types, the stack is just an implementation detail. Show 2 more comments. Active Oldest Votes. To answer your questions directly: To what extent are they controlled by the OS or language runtime? Improve this answer. Jeff Hill Jeff Hill Good answer - but I think you should add that while the stack is allocated by the OS when the process starts assuming the existence of an OS , it is maintained inline by the program.

This is another reason the stack is faster, as well - push and pop operations are typically one machine instruction, and modern machines can do at least 3 of them in one cycle, whereas allocating or freeing heap involves calling into OS code.

I'm really confused by the diagram at the end. I thought I got it until I saw that image. Anarelle the processor runs instructions with or without an os. Allocating on a stack is addition and subtraction on these systems and that is fine for variables destroyed when they are popped by returning from the function that created them, but constrast that to, say, a constructor, of which the result can't just be thrown away.

For that we need the heap, which is not tied to call and return. But where is it actually "set aside" in terms of Java memory structure?? JatinShashoo Java runtime, as bytecode interpreter, adds one more level of virtualization, so what you referred to is just Java application point of view. From operating system point of view all that is just a heap, where Java runtime process allocates some of its space as "non-heap" memory for processed bytecode.

Rest of that OS-level heap is used as application-level heap, where object's data are stored. Show 9 more comments. Stack: Stored in computer RAM just like the heap. Variables created on the stack will go out of scope and are automatically deallocated. 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 infinite or too deep 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 just like the stack. 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. 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 run time or if you need to allocate a lot of data. Responsible for memory leaks. Brian R.

Bondy Brian R. Bondy k gold badges silver badges bronze badges. The pointer pBuffer and the value of b are located on the stack, and are mostly likely allocated at the entrance to the function.

Depending on the compiler, buffer may be allocated at the function entrance, as well. It is a common misconception that the C language, as defined by the C99 language standard available at open-std. In fact, the word 'stack' does not even appear in the standard. See knosof. Brian You should explain why buffer[] and the pBuffer pointer are created on the stack and why pBuffer's data is created on the heap.

I think some ppl might be confused by your answer as they might think the program is specifically instructing that memory be allocated on the stack vs heap but this is not the case. Is it because Buffer is a value type whereas pBuffer is a reference type?

Also the comments about scope and allocation are wrong - Scope is not connected to the stack or the heap at all. Variables on the heap must be destroyed manually and never fall out of scope. It's up to you or the garbage collector to free them. Show 8 more comments. To what extent are they controlled by the OS or language runtime? David I don't agree that that is a good image or that "push-down stack" is a good term to illustrate the concept. When you add something to a stack, the other contents of the stack aren't pushed down, they remain where they are.

This answer includes a big mistake. Static variables are not allocated on the stack. See my answer [link] stackoverflow. Specifically, you say "statically allocated local variables" are allocated on the stack. Actually they are allocated in the data segment. Only automatically allocated variables which includes most but not all local variables and also things like function parameters passed in by value rather than by reference are allocated on the stack.

I've just realised you're right - in C, static allocation is its own separate thing rather than a term for anything that's not dynamic. I've edited my answer, thanks. It's not just C. Java, Pascal, Python and many others all have the notions of static versus automatic versus dynamic allocation. Saying "static allocation" means the same thing just about everywhere. In no language does static allocation mean "not dynamic". You want the term "automatic" allocation for what you are describing i.

I have moved this answer from another question that was more or less a dupe of this one. 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. In a multi-threaded environment each thread will have its own completely independent stack but they will share the heap.

Concurrent access has to be controlled on the heap and is not possible on the stack. The heap The heap contains a linked list of used and free blocks. New allocations on the heap by new or malloc are satisfied by creating a suitable block from one of the free blocks. This requires updating the list of blocks on the heap.