What and where are the stack and heap

Question

Programming language books explain that value types are created on the stack  and reference types are created on the heap  without explaining what these two things are  I haven t read a clear explanation of this   I understand what a stack is  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  What is their scope  What determines the size of each of them  What makes one faster

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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... up to Microsoft and its Intermediate Language (IL) that changed the paradigm to have a OO virtual machine assembly language. So we'll be able to have some CLI/CIL CPU in the future (one project of MS).

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 .NET

Stack vs Heap: Know the Difference

Static class memory allocation where it is stored C#

What and where are the stack and heap?

https://en.wikipedia.org/wiki/Memory_management

https://en.wikipedia.org/wiki/Stack_register

Assembly language resources:

Assembly Programming Tutorial

Intel® 64 and IA-32 Architectures Software Developer Manuals

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I have moved this answer from another question that was more or less a dupe of this one    The answer to your question is implementation specific and may vary across compilers and processor architectures  However  here is a simplified explanation    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 list of blocks on the heap  This meta information about the blocks on the heap is also stored on the heap often in a small area just in front of every block  As the heap grows new blocks are often allocated from lower addresses towards higher addresses  Thus you can think of the heap as a heap of memory blocks that grows in size as memory is allocated  If the heap is too small for an allocation the size can often be increased by acquiring more memory from the underlying operating system  Allocating and deallocating many small blocks may leave the heap in a state where there are a lot of small free blocks interspersed between the used blocks  A request to allocate a large block may fail because none of the free blocks are large enough to satisfy the allocation request even though the combined size of the free blocks may be large enough  This is called heap fragmentation  When a used block that is adjacent to a free block is deallocated the new free block may be merged with the adjacent free block to create a larger free block effectively reducing the fragmentation of the heap      The stack   The stack often works in close tandem with a special register on the CPU named the stack pointer  Initially the stack pointer points to the top of the stack  the highest address on the stack   The CPU has special instructions for pushing values onto the stack and popping them back from the stack  Each push stores the value at the current location of the stack pointer and decreases the stack pointer   A pop retrieves the value pointed to by the stack pointer and then increases the stack pointer  don t be confused by the fact that adding a value to the stack decreases the stack pointer and removing a value increases it  Remember that the stack grows to the bottom   The values stored and retrieved are the values of the CPU registers  When a function is called the CPU uses special instructions that push the current instruction pointer  i e  the address of the code executing on the stack  The CPU then jumps to the function by setting the  instruction pointer to the address of the function called  Later  when the function returns  the old instruction pointer is popped from the stack and execution resumes at the code just after the call to the function  When a function is entered  the stack pointer is decreased to allocate more space on the stack for local  automatic  variables  If the function has one local 32 bit variable four bytes are set aside on the stack  When the function returns  the stack pointer is moved back to free the allocated area  If a function has parameters  these are pushed onto the stack before the call to the function  The code in the function is then able to navigate up the stack from the current stack pointer to locate these values  Nesting function calls work like a charm  Each new call will allocate function parameters  the return address and space for local variables and these activation records can be stacked for nested calls and will unwind in the correct way when the functions return  As the stack is a limited block of memory  you can cause a stack overflow by calling too many nested functions and or allocating too much space for local variables  Often the memory area used for the stack is set up in such a way that writing below the bottom  the lowest address  of the stack will trigger a trap or exception in the CPU  This exceptional condition can then be caught by the runtime and converted into some kind of stack overflow exception         Can a function be allocated on the heap instead of a stack    No  activation records for functions  i e  local or automatic variables  are allocated on the stack that is used not only to store these variables  but also to keep track of nested function calls   How the heap is managed is really up to the runtime environment  C uses malloc and C   uses new  but many other languages have garbage collection   However  the stack is a more low-level feature closely tied to the processor architecture  Growing the heap when there is not enough space isn t too hard since it can be implemented in the library call that handles the heap  However  growing the stack is often impossible as the stack overflow only is discovered when it is too late  and shutting down the thread of execution is the only viable option

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Others have answered the broad strokes pretty well  so I ll throw in a few details    Stack and heap need not be singular  A common situation in which you have more than one stack is if you have more than one thread in a process   In this case each thread has its own stack  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  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  This memory won t survive your return statement  but it s useful for a scratch buffer  Making a huge temporary buffer on Windows that you don t use much of is not free  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  If you access memory more than one page off the end of the stack you will crash   Example    void myfunction        char big 10000000         Do something that only uses for first 1K of big 99  of the time

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I have something to share  although the major points are already covered   Stack     Very fast access  Stored in RAM  Function calls are loaded here along with the local variables and function parameters passed  Space is freed automatically when program goes out of a scope  Stored in sequential memory    Heap   Slow access comparatively to Stack  Stored in RAM  Dynamically created variables are stored here  which later requires freeing the allocated memory after use  Stored wherever memory allocation is done  accessed by pointer always    Interesting note    Should the function calls had been stored in heap  it would had resulted in 2 messy points      Due to sequential storage in stack  execution is faster  Storage in heap would have resulted in huge time consumption thus making the whole program execute slower  If functions were stored in heap  messy storage pointed by pointer   there would have been no way to return to the caller address back  which stack gives due to sequential storage in memory

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I think many other people have given you mostly correct answers on this matter   One detail that has been missed  however  is that the  heap  should in fact probably be called the  free store    The reason for this distinction is that the original free store was implemented with a data structure known as a  binomial heap    For that reason  allocating from early implementations of malloc   free   was allocation from a heap   However  in this modern day  most free stores are implemented with very elaborate data structures that are not binomial heaps

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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       The heap is a memory for items of which you can   t predetermine the   exact size and structure  Since objects and arrays can be mutated and   change at runtime  they have to go into the heap    Source  Academind

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The most important point is that heap and stack are generic terms for ways in which memory can be allocated   They can be implemented in many different ways  and the terms apply to the basic concepts    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      The simplicity of a stack is that you do not need to maintain a table containing a record of each section of allocated memory  the only state information you need is a single pointer to the end of the stack   To allocate and de-allocate  you just increment and decrement that single pointer   Note  a stack can sometimes be implemented to start at the top of a section of memory and extend downwards rather than growing upwards  In a heap  there is no particular order to the way items are placed   You can reach in and remove items in any order because there is no clear  top  item     Heap allocation requires maintaining a full record of what memory is allocated and what isn t  as well as some overhead maintenance to reduce fragmentation  find contiguous memory segments big enough to fit the requested size  and so on   Memory can be deallocated at any time leaving free space   Sometimes a memory allocator will perform maintenance tasks such as defragmenting memory by moving allocated memory around  or garbage collecting - identifying at runtime when memory is no longer in scope and deallocating it     These images should do a fairly good job of describing the two ways of allocating and freeing memory in a stack and a heap   Yum    To what extent are they controlled by the OS or language runtime   As mentioned  heap and stack are general terms  and can be implemented in many ways   Computer programs typically have a stack called a call stack which stores information relevant to the current function such as a pointer to whichever function it was called from  and any local variables   Because functions call other functions and then return  the stack grows and shrinks to hold information from the functions further down the call stack   A program doesn t really have runtime control over it  it s determined by the programming language  OS and even the system architecture   A heap is a general term used for any memory that is allocated dynamically and randomly  i e  out of order   The memory is typically allocated by the OS  with the application calling API functions to do this allocation   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  What is their scope   The call stack is such a low level concept that it doesn t relate to  scope  in the sense of programming   If you disassemble some code you ll see relative pointer style references to portions of the stack  but as far as a higher level language is concerned  the language imposes its own rules of scope   One important aspect of a stack  however  is that once a function returns  anything local to that function is immediately freed from the stack   That works the way you d expect it to work given how your programming languages work   In a heap  it s also difficult to define   The scope is whatever is exposed by the OS  but your programming language probably adds its rules about what a  scope  is in your application   The processor architecture and the OS use virtual addressing  which the processor translates to physical addresses and there are page faults  etc   They keep track of what pages belong to which applications   You never really need to worry about this  though  because you just use whatever method your programming language uses to allocate and free memory  and check for errors  if the allocation freeing fails for any reason   What determines the size of each of them   Again  it depends on the language  compiler  operating system and architecture   A stack is usually pre-allocated  because by definition it must be contiguous memory   The language compiler or the OS determine its size   You don t store huge chunks of data on the stack  so it ll be big enough that it should never be fully used  except in cases of unwanted endless recursion  hence   stack overflow   or other unusual programming decisions   A heap is a general term for anything that can be dynamically allocated   Depending on which way you look at it  it is constantly changing size   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  What makes one faster   The stack is faster because all free memory is always contiguous   No list needs to be maintained of all the segments of free memory  just a single pointer to the current top of the stack   Compilers usually store this pointer in a special  fast register for this purpose   What s more  subsequent operations on a stack are usually concentrated within very nearby areas of memory  which at a very low level is good for optimization by the processor on-die caches

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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   The Memory Management Glossary web page has a diagram of this memory layout   The stack and heap are traditionally located at opposite ends of the process s virtual address space  The stack grows automatically when accessed  up to a size set by the kernel  which can be adjusted with setrlimit RLIMIT STACK         The heap grows when the memory allocator invokes the brk   or sbrk   system call  mapping more pages of physical memory into the process s virtual address space    In systems without virtual memory  such as some embedded systems  the same basic layout often applies  except the stack and heap are fixed in size  However  in other embedded systems  such as those based on Microchip PIC microcontrollers   the program stack is a separate block of memory that is not addressable by data movement instructions  and can only be modified or read indirectly through program flow instructions  call  return  etc    Other architectures  such as Intel Itanium processors  have multiple stacks  In this sense  the stack is an element of the CPU architecture

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The Stack When you call a function the arguments to that function plus some other overhead is put on the stack  Some info  such as where to go on return  is also stored there  When you declare a variable inside your function  that variable is also allocated on the stack    Deallocating the stack is pretty simple because you always deallocate in the reverse order in which you allocate  Stack stuff is added as you enter functions  the corresponding data is removed as you exit them  This means that you tend to stay within a small region of the stack unless you call lots of functions that call lots of other functions  or create a recursive solution    The Heap The heap is a generic name for where you put the data that you create on the fly  If you don t know how many spaceships your program is going to create  you are likely to use the new  or malloc or equivalent  operator to create each spaceship  This allocation is going to stick around for a while  so it is likely we will free things in a different order than we created them    Thus  the heap is far more complex  because there end up being regions of memory that are unused interleaved with chunks that are - memory gets fragmented  Finding free memory of the size you need is a difficult problem  This is why the heap should be avoided  though it is still often used    Implementation Implementation of both the stack and heap is usually down to the runtime   OS  Often games and other applications that are performance critical create their own memory solutions that grab a large chunk of memory from the heap and then dish it out internally to avoid relying on the OS for memory    This is only practical if your memory usage is quite different from the norm - i e for games where you load a level in one huge operation and can chuck the whole lot away in another huge operation   Physical location in memory This is less relevant than you think because of a technology called Virtual Memory which makes your program think that you have access to a certain address where the physical data is somewhere else  even on the hard disc    The addresses you get for the stack are in increasing order as your call tree gets deeper  The addresses for the heap are un-predictable  i e implimentation specific  and frankly not important

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The Stack When you call a function the arguments to that function plus some other overhead is put on the stack  Some info  such as where to go on return  is also stored there  When you declare a variable inside your function  that variable is also allocated on the stack    Deallocating the stack is pretty simple because you always deallocate in the reverse order in which you allocate  Stack stuff is added as you enter functions  the corresponding data is removed as you exit them  This means that you tend to stay within a small region of the stack unless you call lots of functions that call lots of other functions  or create a recursive solution    The Heap The heap is a generic name for where you put the data that you create on the fly  If you don t know how many spaceships your program is going to create  you are likely to use the new  or malloc or equivalent  operator to create each spaceship  This allocation is going to stick around for a while  so it is likely we will free things in a different order than we created them    Thus  the heap is far more complex  because there end up being regions of memory that are unused interleaved with chunks that are - memory gets fragmented  Finding free memory of the size you need is a difficult problem  This is why the heap should be avoided  though it is still often used    Implementation Implementation of both the stack and heap is usually down to the runtime   OS  Often games and other applications that are performance critical create their own memory solutions that grab a large chunk of memory from the heap and then dish it out internally to avoid relying on the OS for memory    This is only practical if your memory usage is quite different from the norm - i e for games where you load a level in one huge operation and can chuck the whole lot away in another huge operation   Physical location in memory This is less relevant than you think because of a technology called Virtual Memory which makes your program think that you have access to a certain address where the physical data is somewhere else  even on the hard disc    The addresses you get for the stack are in increasing order as your call tree gets deeper  The addresses for the heap are un-predictable  i e implimentation specific  and frankly not important

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A couple of cents  I think  it will be good to draw memory graphical and more simple      Arrows - show where grow stack and heap  process stack size have limit  defined in OS  thread stack size limits by parameters in thread create API usually  Heap usually limiting by process maximum virtual memory size  for 32 bit 2-4 nbsp GB for example   So simple way  process heap is general for process and all threads inside  using for memory allocation in common case with something like malloc     Stack is quick memory for store in common case function return pointers and variables  processed as parameters in function call  local function variables

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To clarify  this answer has incorrect information  thomas fixed his answer after comments  cool       Other answers just avoid explaining what static allocation means  So I will explain the three main forms of allocation and how they usually relate to the heap  stack  and data segment below  I also will show some examples in both C C   and Python to help people understand    Static   AKA statically allocated  variables are not allocated on the stack  Do not assume so - many people do only because  static  sounds a lot like  stack   They actually exist in neither the stack nor the heap  The are part of what s called the data segment   However  it is generally better to consider  scope  and  lifetime  rather than  stack  and  heap    Scope refers to what parts of the code can access a variable  Generally we think of local scope  can only be accessed by the current function  versus global scope  can be accessed anywhere  although scope can get much more complex   Lifetime refers to when a variable is allocated and deallocated during program execution  Usually we think of static allocation  variable will persist through the entire duration of the program  making it useful for storing the same information across several function calls  versus automatic allocation  variable only persists during a single call to a function  making it useful for storing information that is only used during your function and can be discarded once you are done  versus dynamic allocation  variables whose duration is defined at runtime  instead of compile time like static or automatic    Although most compilers and interpreters implement this behavior similarly in terms of using stacks  heaps  etc  a compiler may sometimes break these conventions if it wants as long as behavior is correct  For instance  due to optimization a local variable may only exist in a register or be removed entirely  even though most local variables exist in the stack  As has been pointed out in a few comments  you are free to implement a compiler that doesn t even use a stack or a heap  but instead some other storage mechanisms  rarely done  since stacks and heaps are great for this    I will provide some simple annotated C code to illustrate all of this  The best way to learn is to run a program under a debugger and watch the behavior  If you prefer to read python  skip to the end of the answer        Statically allocated in the data segment when the program DLL is first loaded    Deallocated when the program DLL exits    scope - can be accessed from anywhere in the code int someGlobalVariable      Statically allocated in the data segment when the program is first loaded    Deallocated when the program DLL exits    scope - can be accessed from anywhere in this particular code file static int someStaticVariable       someArgument  is allocated on the stack each time MyFunction is called     someArgument  is deallocated when MyFunction returns    scope - can be accessed only within MyFunction   void MyFunction int someArgument            Statically allocated in the data segment when the program is first loaded        Deallocated when the program DLL exits        scope - can be accessed only within MyFunction       static int someLocalStaticVariable          Allocated on the stack each time MyFunction is called        Deallocated when MyFunction returns        scope - can be accessed only within MyFunction       int someLocalVariable          A  pointer  is allocated on the stack each time MyFunction is called        This pointer is deallocated when MyFunction returns        scope - the pointer can be accessed only within MyFunction       int  someDynamicVariable          This line causes space for an integer to be allocated in the heap        when this line is executed  Note this is not at the beginning of        the call to MyFunction    like the automatic variables        scope - only code within MyFunction   can access this space         through this particular variable          However  if you pass the address somewhere else  that code        can access it too     someDynamicVariable   new int           This line deallocates the space for the integer in the heap         If we did not write it  the memory would be  leaked          Note a fundamental difference between the stack and heap        the heap must be managed  The stack is managed for us      delete someDynamicVariable          In other cases  instead of deallocating this heap space you        might store the address somewhere more permanent to use later         Some languages even take care of deallocation for you    but        always it needs to be taken care of at runtime by some mechanism          When the function returns  someArgument  someLocalVariable        and the pointer someDynamicVariable are deallocated         The space pointed to by someDynamicVariable was already        deallocated prior to returning      return        Note that someGlobalVariable  someStaticVariable and    someLocalStaticVariable continue to exist  and are not    deallocated until the program exits    A particularly poignant example of why it s important to distinguish between lifetime and scope is that a variable can have local scope but static lifetime - for instance   someLocalStaticVariable  in the code sample above  Such variables can make our common but informal naming habits very confusing  For instance when we say  local  we usually mean  locally scoped automatically allocated variable  and when we say global we usually mean  globally scoped statically allocated variable   Unfortunately when it comes to things like  file scoped statically allocated variables  many people just say     huh       Some of the syntax choices in C C   exacerbate this problem - for instance many people think global variables are not  static  because of the syntax shown below   int var1     Has global scope and static allocation static int var2     Has file scope and static allocation  int main    return 0     Note that putting the keyword  static  in the declaration above prevents var2 from having global scope  Nevertheless  the global var1 has static allocation  This is not intuitive  For this reason  I try to never use the word  static  when describing scope  and instead say something like  file  or  file limited  scope  However many people use the phrase  static  or  static scope  to describe a variable that can only be accessed from one code file  In the context of lifetime   static  always means the variable is allocated at program start and deallocated when program exits   Some people think of these concepts as C C   specific  They are not  For instance  the Python sample below illustrates all three types of allocation  there are some subtle differences possible in interpreted languages that I won t get into here    from datetime import datetime  class Animal       FavoriteFood    Undefined     FavoriteFood is statically allocated      def PetAnimal self           curTime   datetime time datetime now      curTime is automatically allocatedion         print  Thank you for petting me  But it s     str curTime       you should feed me  My favorite food is     self  FavoriteFood   class Cat Animal        FavoriteFood    tuna    Note since we override  Cat class has its own statically allocated  FavoriteFood variable  different from Animal s  class Dog Animal        FavoriteFood    steak    Likewise  the Dog class gets its own static variable  Important to note - this one static variable is shared among all instances of Dog  hence it is not dynamic    if   name         main         whiskers   Cat     Dynamically allocated     fido   Dog     Dynamically allocated     rinTinTin   Dog     Dynamically allocated      whiskers PetAnimal       fido PetAnimal       rinTinTin PetAnimal        Dog  FavoriteFood    milkbones      whiskers PetAnimal       fido PetAnimal       rinTinTin PetAnimal      Output is    Thank you for petting me  But it s 13 05 02 255000  you should feed me  My favorite food is tuna   Thank you for petting me  But it s 13 05 02 255000  you should feed me  My favorite food is steak   Thank you for petting me  But it s 13 05 02 255000  you should feed me  My favorite food is steak   Thank you for petting me  But it s 13 05 02 255000  you should feed me  My favorite food is tuna   Thank you for petting me  But it s 13 05 02 255000  you should feed me  My favorite food is milkbones   Thank you for petting me  But it s 13 05 02 256000  you should feed me  My favorite food is milkbones

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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  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 deallocation   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   A clear demonstration   Image source  vikashazrati wordpress com

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Others have answered the broad strokes pretty well  so I ll throw in a few details    Stack and heap need not be singular  A common situation in which you have more than one stack is if you have more than one thread in a process   In this case each thread has its own stack  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  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  This memory won t survive your return statement  but it s useful for a scratch buffer  Making a huge temporary buffer on Windows that you don t use much of is not free  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  If you access memory more than one page off the end of the stack you will crash   Example    void myfunction        char big 10000000         Do something that only uses for first 1K of big 99  of the time

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stack  heap and data of each process in virtual memory

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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  In C    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   or C  data created on the heap will be pointed to by pointers and allocated with new or malloc respectively  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    Example   int foo       char  pBuffer     lt --nothing allocated yet  excluding the pointer itself  which is allocated here on the stack     bool b   true     Allocated on the stack    if b            Create 500 bytes on the stack     char buffer 500          Create 500 bytes on the heap     pBuffer   new char 500           lt -- buffer is deallocated here  pBuffer is not     lt --- oops there s a memory leak  I should have called delete   pBuffer

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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  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 deallocation   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   A clear demonstration   Image source  vikashazrati wordpress com

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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   The Memory Management Glossary web page has a diagram of this memory layout   The stack and heap are traditionally located at opposite ends of the process s virtual address space  The stack grows automatically when accessed  up to a size set by the kernel  which can be adjusted with setrlimit RLIMIT STACK         The heap grows when the memory allocator invokes the brk   or sbrk   system call  mapping more pages of physical memory into the process s virtual address space    In systems without virtual memory  such as some embedded systems  the same basic layout often applies  except the stack and heap are fixed in size  However  in other embedded systems  such as those based on Microchip PIC microcontrollers   the program stack is a separate block of memory that is not addressable by data movement instructions  and can only be modified or read indirectly through program flow instructions  call  return  etc    Other architectures  such as Intel Itanium processors  have multiple stacks  In this sense  the stack is an element of the CPU architecture

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Thank you for a really good discussion but as a real noob I wonder where instructions are kept  In the BEGINNING scientists were deciding between two architectures  von NEUMANN where everything is considered DATA and HARVARD where an area of memory was reserved for instructions and another for data   Ultimately  we went with the von Neumann design and now everything is considered  the same   This made it hard for me when I was learning assembly  https   www cs virginia edu  evans cs216 guides x86 html because they talk about registers and stack pointers     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

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Wow  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    The stack is memory that begins as the highest memory address allocated to your program image  and it then decrease in value from there  It is reserved for called function parameters and for all temporary variables used in functions   There are two heaps  public and private   The private heap begins on a 16-byte boundary  for 64-bit programs  or a 8-byte boundary  for 32-bit programs  after the last byte of code in your program  and then increases in value from there  It is also called the default heap   If the private heap gets too large it will overlap the stack area  as will the stack overlap the heap if it gets too big  Because the stack starts at a higher address and works its way down to lower address  with proper hacking you can get make the stack so large that it will overrun the private heap area and overlap the code area  The trick then is to overlap enough of the code area that you can hook into the code  It s a little tricky to do and you risk a program crash  but it s easy and very effective   The public heap resides in it s own memory space outside of your program image space  It is this memory that will be siphoned off onto the hard disk if memory resources get scarce   2  To what extent are they controlled by the OS or language runtime   The stack is controlled by the programmer  the private heap is managed by the OS  and the public heap is not controlled by anyone because it is an OS service -- you make requests and either they are granted or denied   2b  What is their scope   They are all global to the program  but their contents can be private  public  or global   2c  What determines the size of each of them   The size of the stack and the private heap are determined by your compiler runtime options  The public heap is initialized at runtime using a size parameter   2d  What makes one faster   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   REF   https   norasandler com 2019 02 18 Write-a-Compiler-10 html  https   docs microsoft com en-us windows desktop api heapapi nf-heapapi-getprocessheap  https   docs microsoft com en-us windows desktop api heapapi nf-heapapi-heapcreate

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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    It s the region of memory below the stack pointer register  which can be set as needed   The stack is also used for passing arguments to subroutines  and also for preserving the values in registers before calling subroutines   The heap is a portion of memory that is given to an application by the operating system  typically through a syscall like malloc   On modern OSes this memory is a set of pages that only the calling process has access to   The size of the stack is determined at runtime  and generally does not grow after the program launches   In a C program  the stack needs to be large enough to hold every variable declared within each function   The heap will grow dynamically as needed  but the OS is ultimately making the call  it will often grow the heap by more than the value requested by malloc  so that at least some future mallocs won t need to go back to the kernel to get more memory   This behavior is often customizable   Because you ve allocated the stack before launching the program  you never need to malloc before you can use the stack  so that s a slight advantage there   In practice  it s very hard to predict what will be fast and what will be slow in modern operating systems that have virtual memory subsystems  because how the pages are implemented and where they are stored is an implementation detail

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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    It s the region of memory below the stack pointer register  which can be set as needed   The stack is also used for passing arguments to subroutines  and also for preserving the values in registers before calling subroutines   The heap is a portion of memory that is given to an application by the operating system  typically through a syscall like malloc   On modern OSes this memory is a set of pages that only the calling process has access to   The size of the stack is determined at runtime  and generally does not grow after the program launches   In a C program  the stack needs to be large enough to hold every variable declared within each function   The heap will grow dynamically as needed  but the OS is ultimately making the call  it will often grow the heap by more than the value requested by malloc  so that at least some future mallocs won t need to go back to the kernel to get more memory   This behavior is often customizable   Because you ve allocated the stack before launching the program  you never need to malloc before you can use the stack  so that s a slight advantage there   In practice  it s very hard to predict what will be fast and what will be slow in modern operating systems that have virtual memory subsystems  because how the pages are implemented and where they are stored is an implementation detail

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Stack   Very fast access Don t have to explicitly de-allocate variables Space is managed efficiently by CPU  memory will not become fragmented Local variables only Limit on stack size  OS-dependent  Variables cannot be resized   Heap   Variables can be accessed globally No limit on memory size  Relatively  slower access No guaranteed efficient use of space  memory may become fragmented over time as blocks of memory are allocated  then freed You must manage memory  you re in charge of allocating and freeing variables  Variables can be resized using realloc

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A couple of cents  I think  it will be good to draw memory graphical and more simple      Arrows - show where grow stack and heap  process stack size have limit  defined in OS  thread stack size limits by parameters in thread create API usually  Heap usually limiting by process maximum virtual memory size  for 32 bit 2-4 nbsp GB for example   So simple way  process heap is general for process and all threads inside  using for memory allocation in common case with something like malloc     Stack is quick memory for store in common case function return pointers and variables  processed as parameters in function call  local function variables

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Wow  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    The stack is memory that begins as the highest memory address allocated to your program image  and it then decrease in value from there  It is reserved for called function parameters and for all temporary variables used in functions   There are two heaps  public and private   The private heap begins on a 16-byte boundary  for 64-bit programs  or a 8-byte boundary  for 32-bit programs  after the last byte of code in your program  and then increases in value from there  It is also called the default heap   If the private heap gets too large it will overlap the stack area  as will the stack overlap the heap if it gets too big  Because the stack starts at a higher address and works its way down to lower address  with proper hacking you can get make the stack so large that it will overrun the private heap area and overlap the code area  The trick then is to overlap enough of the code area that you can hook into the code  It s a little tricky to do and you risk a program crash  but it s easy and very effective   The public heap resides in it s own memory space outside of your program image space  It is this memory that will be siphoned off onto the hard disk if memory resources get scarce   2  To what extent are they controlled by the OS or language runtime   The stack is controlled by the programmer  the private heap is managed by the OS  and the public heap is not controlled by anyone because it is an OS service -- you make requests and either they are granted or denied   2b  What is their scope   They are all global to the program  but their contents can be private  public  or global   2c  What determines the size of each of them   The size of the stack and the private heap are determined by your compiler runtime options  The public heap is initialized at runtime using a size parameter   2d  What makes one faster   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   REF   https   norasandler com 2019 02 18 Write-a-Compiler-10 html  https   docs microsoft com en-us windows desktop api heapapi nf-heapapi-getprocessheap  https   docs microsoft com en-us windows desktop api heapapi nf-heapapi-heapcreate

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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  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 deallocation   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   A clear demonstration   Image source  vikashazrati wordpress com

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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  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 deallocation   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   A clear demonstration   Image source  vikashazrati wordpress com

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A lot of answers are correct as concepts  but we must note that a stack is needed by the hardware  i e  microprocessor  to allow calling subroutines  CALL in assembly language      OOP guys will call it methods   On the stack you save return addresses and call   push   ret   pop is managed directly in hardware   You can use the stack to pass parameters   even if it is slower than using registers  would a microprocessor guru say or a good 1980s BIOS book       Without stack no microprocessor can work   we can t imagine a program  even in assembly language  without subroutines functions  Without the heap it can   An assembly language program can work without  as the heap is a OS concept  as malloc  that is a OS Lib call    Stack usage is faster as    Is hardware  and even push pop are very efficient  malloc requires entering kernel mode  use lock semaphore  or other synchronization primitives  executing some code and manage some structures needed to keep track of allocation

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In Short  A stack is used for static memory allocation and a heap for dynamic memory allocation  both stored in the computer s RAM     In Detail  The Stack  The stack is a  LIFO   last in  first out  data structure  that is managed and optimized by the CPU quite closely  Every time a function declares a new variable  it is  pushed  onto the stack  Then every time a function exits  all of the variables pushed onto the stack by that function  are freed  that is to say  they are deleted   Once a stack variable is freed  that region of memory becomes available for other stack variables   The advantage of using the stack to store variables  is that memory is managed for you  You don t have to allocate memory by hand  or free it once you don t need it any more  What s more  because the CPU organizes stack memory so efficiently  reading from and writing to stack variables is very fast   More can be found here     The Heap  The heap is a region of your computer s memory that is not managed automatically for you  and is not as tightly managed by the CPU  It is a more free-floating region of memory  and is larger   To allocate memory on the heap  you must use malloc   or calloc    which are built-in C functions  Once you have allocated memory on the heap  you are responsible for using free   to deallocate that memory once you don t need it any more   If you fail to do this  your program will have what is known as a memory leak  That is  memory on the heap will still be set aside  and won t be available to other processes   As we will see in the debugging section  there is a tool called Valgrind that can help you detect memory leaks   Unlike the stack  the heap does not have size restrictions on variable size  apart from the obvious physical limitations of your computer   Heap memory is slightly slower to be read from and written to  because one has to use pointers to access memory on the heap  We will talk about pointers shortly   Unlike the stack  variables created on the heap are accessible by any function  anywhere in your program  Heap variables are essentially global in scope   More can be found here     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  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  Elements 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     You can use the stack if you know exactly how much data you need to allocate before compile time  and it is not too big  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   In a multi-threaded situation each thread will have its own completely independent stack  but they will share the heap  The stack is thread specific and the heap is application specific  The stack is important to consider in exception handling and thread executions   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      At run-time  if the application needs more heap  it can allocate memory from free memory and if the stack needs memory  it can allocate memory from free memory allocated memory for the application   Even  more detail is given here and here     Now come to your question s answers      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   More can be found here      What is their scope    Already given in top       You can use the stack if you know exactly how much data you need to allocate before compile time  and it is not too big  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     More can be found in here      What determines the size of each of them    The size of the stack is set by OS when a thread is created  The size of the heap is set on application startup  but it can grow as space is needed  the allocator requests more memory from the operating system       What makes one faster    Stack allocation is much faster since all it really does is move the stack pointer  Using memory pools  you can get comparable performance out of heap allocation  but that comes with a slight added complexity and its own headaches   Also  stack vs  heap is not only a performance consideration  it also tells you a lot about the expected lifetime of objects   Details can be found from here

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Simply  the stack is where local variables get created  Also  every time you call a subroutine the program counter  pointer to the next machine instruction  and any important registers  and sometimes the parameters get pushed on the stack  Then any local variables inside the subroutine are pushed onto the stack  and used from there   When the subroutine finishes  that stuff all gets popped back off the stack  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   The heap is the area of memory dynamic memory allocations are made out of  explicit  new  or  allocate  calls   It is a special data structure that can keep track of blocks of memory of varying sizes and their allocation status   In  classic  systems RAM was laid out such that the stack pointer started out at the bottom of memory  the heap pointer started out at the top  and they grew towards each other  If they overlap  you are out of RAM  That doesn t work with modern multi-threaded OSes though  Every thread has to have its own stack  and those can get created dynamicly

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In the following C  code  public void Method1         int i   4      int y   2      class1 cls1   new class1        Here s how the memory is managed    Local Variables that only need to last as long as the function invocation go in the stack  The heap is used for variables whose lifetime we don t really know up front but we expect them to last a while  In most languages it s critical that we know at compile time how large a variable is if we want to store it on the stack    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  In many languages the heap is garbage collected to find objects  such as the cls1 object  that no longer have any references    In Java  most objects go directly into the heap  In languages like C   C    structs and classes can often remain on the stack when you re not dealing with pointers   More information can be found here   The difference between stack and heap memory allocation  laquo   timmurphy org  and here    Creating Objects on the Stack and Heap  This article is the source of picture above  Six important  NET concepts  Stack  heap  value types  reference types  boxing  and unboxing - CodeProject  but be aware it may contain some inaccuracies

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Thank you for a really good discussion but as a real noob I wonder where instructions are kept  In the BEGINNING scientists were deciding between two architectures  von NEUMANN where everything is considered DATA and HARVARD where an area of memory was reserved for instructions and another for data   Ultimately  we went with the von Neumann design and now everything is considered  the same   This made it hard for me when I was learning assembly  https   www cs virginia edu  evans cs216 guides x86 html because they talk about registers and stack pointers     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

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Stack   Very fast access Don t have to explicitly de-allocate variables Space is managed efficiently by CPU  memory will not become fragmented Local variables only Limit on stack size  OS-dependent  Variables cannot be resized   Heap   Variables can be accessed globally No limit on memory size  Relatively  slower access No guaranteed efficient use of space  memory may become fragmented over time as blocks of memory are allocated  then freed You must manage memory  you re in charge of allocating and freeing variables  Variables can be resized using realloc

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Since some answers went nitpicking  I m going to contribute my mite   Surprisingly  no one has mentioned that multiple  i e  not related to the number of running OS-level threads  call stacks are to be found not only in exotic languages  PostScript  or platforms  Intel Itanium   but also in fibers  green threads and some implementations of coroutines   Fibers  green threads and coroutines are in many ways similar  which leads to much confusion   The difference between fibers and green threads is that the former use cooperative multitasking  while the latter may feature either cooperative or preemptive one  or even both   For the distinction between fibers and coroutines  see here   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   When using fibers  green threads or coroutines  you usually have a separate stack per function   Technically  not just a stack but a whole context of execution is per function  Most importantly  CPU registers   For every thread there re as many stacks as there re concurrently running functions  and the thread is switching between executing each function according to the logic of your program  When a function runs to its end  its stack is destroyed  So  the number and lifetimes of stacks are dynamic and are not determined by the number of OS-level threads   Note that I said  usually have a separate stack per function   There re both stackful and stackless implementations of couroutines  Most notable stackful C   implementations are Boost Coroutine and Microsoft PPL s async await   However  C   s resumable functions  a k a   async and await    which were proposed to C  17  are likely to use stackless coroutines    Fibers proposal to the C   standard library is forthcoming  Also  there re some third-party libraries  Green threads are extremely popular in languages like Python and Ruby

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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  In C    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   or C  data created on the heap will be pointed to by pointers and allocated with new or malloc respectively  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    Example   int foo       char  pBuffer     lt --nothing allocated yet  excluding the pointer itself  which is allocated here on the stack     bool b   true     Allocated on the stack    if b            Create 500 bytes on the stack     char buffer 500          Create 500 bytes on the heap     pBuffer   new char 500           lt -- buffer is deallocated here  pBuffer is not     lt --- oops there s a memory leak  I should have called delete   pBuffer

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I think many other people have given you mostly correct answers on this matter   One detail that has been missed  however  is that the  heap  should in fact probably be called the  free store    The reason for this distinction is that the original free store was implemented with a data structure known as a  binomial heap    For that reason  allocating from early implementations of malloc   free   was allocation from a heap   However  in this modern day  most free stores are implemented with very elaborate data structures that are not binomial heaps

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OK  simply and in short words  they mean ordered and not ordered      Stack  In stack items  things get on the top of each-other  means gonna be faster and more efficient to be processed       So there is always an index to point the specific item  also processing gonna be faster  there is relationship between the items as well      Heap  No order  processing gonna be slower and values are messed up together with no specific order or index    there are random and there is no relationship between them    so execution and usage time could be vary     I also create the image below to show how they may look like

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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  In C    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   or C  data created on the heap will be pointed to by pointers and allocated with new or malloc respectively  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    Example   int foo       char  pBuffer     lt --nothing allocated yet  excluding the pointer itself  which is allocated here on the stack     bool b   true     Allocated on the stack    if b            Create 500 bytes on the stack     char buffer 500          Create 500 bytes on the heap     pBuffer   new char 500           lt -- buffer is deallocated here  pBuffer is not     lt --- oops there s a memory leak  I should have called delete   pBuffer

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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    It s the region of memory below the stack pointer register  which can be set as needed   The stack is also used for passing arguments to subroutines  and also for preserving the values in registers before calling subroutines   The heap is a portion of memory that is given to an application by the operating system  typically through a syscall like malloc   On modern OSes this memory is a set of pages that only the calling process has access to   The size of the stack is determined at runtime  and generally does not grow after the program launches   In a C program  the stack needs to be large enough to hold every variable declared within each function   The heap will grow dynamically as needed  but the OS is ultimately making the call  it will often grow the heap by more than the value requested by malloc  so that at least some future mallocs won t need to go back to the kernel to get more memory   This behavior is often customizable   Because you ve allocated the stack before launching the program  you never need to malloc before you can use the stack  so that s a slight advantage there   In practice  it s very hard to predict what will be fast and what will be slow in modern operating systems that have virtual memory subsystems  because how the pages are implemented and where they are stored is an implementation detail

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What is a stack  A stack is a pile of objects  typically one that is neatly arranged     Stacks in computing architectures are regions of memory where data is added or removed in a last-in-first-out manner   In a multi-threaded application  each thread will have its own stack    What is a heap  A heap is an untidy collection of things piled up haphazardly     In computing architectures the heap is an area of dynamically-allocated memory that is managed automatically by the operating system or the memory manager library   Memory on the heap is allocated  deallocated  and resized regularly during program execution  and this can lead to a problem called fragmentation   Fragmentation occurs when memory objects are allocated with small spaces in between that are too small to hold additional memory objects   The net result is a percentage of the heap space that is not usable for further memory allocations    Both together   In a multi-threaded application  each thread will have its own stack  But  all the different threads will share the heap   Because the different threads share the heap in a multi-threaded application  this also means that there has to be some coordination between the threads so that they don   t try to access and manipulate the same piece s  of memory in the heap at the same time    Which is faster     the stack or the heap  And why    The stack is much faster than the heap   This is because of the way that memory is allocated on the stack   Allocating memory on the stack is as simple as moving the stack pointer up    For people new to programming  it   s probably a good idea to use the stack since it   s easier   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   It   s better to use the heap when you know that you will need a lot of memory for your data  or you just are not sure how much memory you will need  like with a dynamic array   Java Memory Model  The stack is the area of memory where local variables  including method parameters  are stored  When it comes to object variables  these are merely references  pointers  to the actual objects on the heap  Every time an object is instantiated  a chunk of heap memory is set aside to hold the data  state  of that object  Since objects can contain other objects  some of this data can in fact hold references to those nested objects

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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  In C    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   or C  data created on the heap will be pointed to by pointers and allocated with new or malloc respectively  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    Example   int foo       char  pBuffer     lt --nothing allocated yet  excluding the pointer itself  which is allocated here on the stack     bool b   true     Allocated on the stack    if b            Create 500 bytes on the stack     char buffer 500          Create 500 bytes on the heap     pBuffer   new char 500           lt -- buffer is deallocated here  pBuffer is not     lt --- oops there s a memory leak  I should have called delete   pBuffer

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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   The Memory Management Glossary web page has a diagram of this memory layout   The stack and heap are traditionally located at opposite ends of the process s virtual address space  The stack grows automatically when accessed  up to a size set by the kernel  which can be adjusted with setrlimit RLIMIT STACK         The heap grows when the memory allocator invokes the brk   or sbrk   system call  mapping more pages of physical memory into the process s virtual address space    In systems without virtual memory  such as some embedded systems  the same basic layout often applies  except the stack and heap are fixed in size  However  in other embedded systems  such as those based on Microchip PIC microcontrollers   the program stack is a separate block of memory that is not addressable by data movement instructions  and can only be modified or read indirectly through program flow instructions  call  return  etc    Other architectures  such as Intel Itanium processors  have multiple stacks  In this sense  the stack is an element of the CPU architecture

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I think many other people have given you mostly correct answers on this matter   One detail that has been missed  however  is that the  heap  should in fact probably be called the  free store    The reason for this distinction is that the original free store was implemented with a data structure known as a  binomial heap    For that reason  allocating from early implementations of malloc   free   was allocation from a heap   However  in this modern day  most free stores are implemented with very elaborate data structures that are not binomial heaps

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I have something to share  although the major points are already covered   Stack     Very fast access  Stored in RAM  Function calls are loaded here along with the local variables and function parameters passed  Space is freed automatically when program goes out of a scope  Stored in sequential memory    Heap   Slow access comparatively to Stack  Stored in RAM  Dynamically created variables are stored here  which later requires freeing the allocated memory after use  Stored wherever memory allocation is done  accessed by pointer always    Interesting note    Should the function calls had been stored in heap  it would had resulted in 2 messy points      Due to sequential storage in stack  execution is faster  Storage in heap would have resulted in huge time consumption thus making the whole program execute slower  If functions were stored in heap  messy storage pointed by pointer   there would have been no way to return to the caller address back  which stack gives due to sequential storage in memory

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In Short  A stack is used for static memory allocation and a heap for dynamic memory allocation  both stored in the computer s RAM     In Detail  The Stack  The stack is a  LIFO   last in  first out  data structure  that is managed and optimized by the CPU quite closely  Every time a function declares a new variable  it is  pushed  onto the stack  Then every time a function exits  all of the variables pushed onto the stack by that function  are freed  that is to say  they are deleted   Once a stack variable is freed  that region of memory becomes available for other stack variables   The advantage of using the stack to store variables  is that memory is managed for you  You don t have to allocate memory by hand  or free it once you don t need it any more  What s more  because the CPU organizes stack memory so efficiently  reading from and writing to stack variables is very fast   More can be found here     The Heap  The heap is a region of your computer s memory that is not managed automatically for you  and is not as tightly managed by the CPU  It is a more free-floating region of memory  and is larger   To allocate memory on the heap  you must use malloc   or calloc    which are built-in C functions  Once you have allocated memory on the heap  you are responsible for using free   to deallocate that memory once you don t need it any more   If you fail to do this  your program will have what is known as a memory leak  That is  memory on the heap will still be set aside  and won t be available to other processes   As we will see in the debugging section  there is a tool called Valgrind that can help you detect memory leaks   Unlike the stack  the heap does not have size restrictions on variable size  apart from the obvious physical limitations of your computer   Heap memory is slightly slower to be read from and written to  because one has to use pointers to access memory on the heap  We will talk about pointers shortly   Unlike the stack  variables created on the heap are accessible by any function  anywhere in your program  Heap variables are essentially global in scope   More can be found here     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  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  Elements 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     You can use the stack if you know exactly how much data you need to allocate before compile time  and it is not too big  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   In a multi-threaded situation each thread will have its own completely independent stack  but they will share the heap  The stack is thread specific and the heap is application specific  The stack is important to consider in exception handling and thread executions   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      At run-time  if the application needs more heap  it can allocate memory from free memory and if the stack needs memory  it can allocate memory from free memory allocated memory for the application   Even  more detail is given here and here     Now come to your question s answers      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   More can be found here      What is their scope    Already given in top       You can use the stack if you know exactly how much data you need to allocate before compile time  and it is not too big  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     More can be found in here      What determines the size of each of them    The size of the stack is set by OS when a thread is created  The size of the heap is set on application startup  but it can grow as space is needed  the allocator requests more memory from the operating system       What makes one faster    Stack allocation is much faster since all it really does is move the stack pointer  Using memory pools  you can get comparable performance out of heap allocation  but that comes with a slight added complexity and its own headaches   Also  stack vs  heap is not only a performance consideration  it also tells you a lot about the expected lifetime of objects   Details can be found from here

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The Stack When you call a function the arguments to that function plus some other overhead is put on the stack  Some info  such as where to go on return  is also stored there  When you declare a variable inside your function  that variable is also allocated on the stack    Deallocating the stack is pretty simple because you always deallocate in the reverse order in which you allocate  Stack stuff is added as you enter functions  the corresponding data is removed as you exit them  This means that you tend to stay within a small region of the stack unless you call lots of functions that call lots of other functions  or create a recursive solution    The Heap The heap is a generic name for where you put the data that you create on the fly  If you don t know how many spaceships your program is going to create  you are likely to use the new  or malloc or equivalent  operator to create each spaceship  This allocation is going to stick around for a while  so it is likely we will free things in a different order than we created them    Thus  the heap is far more complex  because there end up being regions of memory that are unused interleaved with chunks that are - memory gets fragmented  Finding free memory of the size you need is a difficult problem  This is why the heap should be avoided  though it is still often used    Implementation Implementation of both the stack and heap is usually down to the runtime   OS  Often games and other applications that are performance critical create their own memory solutions that grab a large chunk of memory from the heap and then dish it out internally to avoid relying on the OS for memory    This is only practical if your memory usage is quite different from the norm - i e for games where you load a level in one huge operation and can chuck the whole lot away in another huge operation   Physical location in memory This is less relevant than you think because of a technology called Virtual Memory which makes your program think that you have access to a certain address where the physical data is somewhere else  even on the hard disc    The addresses you get for the stack are in increasing order as your call tree gets deeper  The addresses for the heap are un-predictable  i e implimentation specific  and frankly not important

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Since some answers went nitpicking  I m going to contribute my mite   Surprisingly  no one has mentioned that multiple  i e  not related to the number of running OS-level threads  call stacks are to be found not only in exotic languages  PostScript  or platforms  Intel Itanium   but also in fibers  green threads and some implementations of coroutines   Fibers  green threads and coroutines are in many ways similar  which leads to much confusion   The difference between fibers and green threads is that the former use cooperative multitasking  while the latter may feature either cooperative or preemptive one  or even both   For the distinction between fibers and coroutines  see here   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   When using fibers  green threads or coroutines  you usually have a separate stack per function   Technically  not just a stack but a whole context of execution is per function  Most importantly  CPU registers   For every thread there re as many stacks as there re concurrently running functions  and the thread is switching between executing each function according to the logic of your program  When a function runs to its end  its stack is destroyed  So  the number and lifetimes of stacks are dynamic and are not determined by the number of OS-level threads   Note that I said  usually have a separate stack per function   There re both stackful and stackless implementations of couroutines  Most notable stackful C   implementations are Boost Coroutine and Microsoft PPL s async await   However  C   s resumable functions  a k a   async and await    which were proposed to C  17  are likely to use stackless coroutines    Fibers proposal to the C   standard library is forthcoming  Also  there re some third-party libraries  Green threads are extremely popular in languages like Python and Ruby

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The most important point is that heap and stack are generic terms for ways in which memory can be allocated   They can be implemented in many different ways  and the terms apply to the basic concepts    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      The simplicity of a stack is that you do not need to maintain a table containing a record of each section of allocated memory  the only state information you need is a single pointer to the end of the stack   To allocate and de-allocate  you just increment and decrement that single pointer   Note  a stack can sometimes be implemented to start at the top of a section of memory and extend downwards rather than growing upwards  In a heap  there is no particular order to the way items are placed   You can reach in and remove items in any order because there is no clear  top  item     Heap allocation requires maintaining a full record of what memory is allocated and what isn t  as well as some overhead maintenance to reduce fragmentation  find contiguous memory segments big enough to fit the requested size  and so on   Memory can be deallocated at any time leaving free space   Sometimes a memory allocator will perform maintenance tasks such as defragmenting memory by moving allocated memory around  or garbage collecting - identifying at runtime when memory is no longer in scope and deallocating it     These images should do a fairly good job of describing the two ways of allocating and freeing memory in a stack and a heap   Yum    To what extent are they controlled by the OS or language runtime   As mentioned  heap and stack are general terms  and can be implemented in many ways   Computer programs typically have a stack called a call stack which stores information relevant to the current function such as a pointer to whichever function it was called from  and any local variables   Because functions call other functions and then return  the stack grows and shrinks to hold information from the functions further down the call stack   A program doesn t really have runtime control over it  it s determined by the programming language  OS and even the system architecture   A heap is a general term used for any memory that is allocated dynamically and randomly  i e  out of order   The memory is typically allocated by the OS  with the application calling API functions to do this allocation   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  What is their scope   The call stack is such a low level concept that it doesn t relate to  scope  in the sense of programming   If you disassemble some code you ll see relative pointer style references to portions of the stack  but as far as a higher level language is concerned  the language imposes its own rules of scope   One important aspect of a stack  however  is that once a function returns  anything local to that function is immediately freed from the stack   That works the way you d expect it to work given how your programming languages work   In a heap  it s also difficult to define   The scope is whatever is exposed by the OS  but your programming language probably adds its rules about what a  scope  is in your application   The processor architecture and the OS use virtual addressing  which the processor translates to physical addresses and there are page faults  etc   They keep track of what pages belong to which applications   You never really need to worry about this  though  because you just use whatever method your programming language uses to allocate and free memory  and check for errors  if the allocation freeing fails for any reason   What determines the size of each of them   Again  it depends on the language  compiler  operating system and architecture   A stack is usually pre-allocated  because by definition it must be contiguous memory   The language compiler or the OS determine its size   You don t store huge chunks of data on the stack  so it ll be big enough that it should never be fully used  except in cases of unwanted endless recursion  hence   stack overflow   or other unusual programming decisions   A heap is a general term for anything that can be dynamically allocated   Depending on which way you look at it  it is constantly changing size   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  What makes one faster   The stack is faster because all free memory is always contiguous   No list needs to be maintained of all the segments of free memory  just a single pointer to the current top of the stack   Compilers usually store this pointer in a special  fast register for this purpose   What s more  subsequent operations on a stack are usually concentrated within very nearby areas of memory  which at a very low level is good for optimization by the processor on-die caches

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The Stack When you call a function the arguments to that function plus some other overhead is put on the stack  Some info  such as where to go on return  is also stored there  When you declare a variable inside your function  that variable is also allocated on the stack    Deallocating the stack is pretty simple because you always deallocate in the reverse order in which you allocate  Stack stuff is added as you enter functions  the corresponding data is removed as you exit them  This means that you tend to stay within a small region of the stack unless you call lots of functions that call lots of other functions  or create a recursive solution    The Heap The heap is a generic name for where you put the data that you create on the fly  If you don t know how many spaceships your program is going to create  you are likely to use the new  or malloc or equivalent  operator to create each spaceship  This allocation is going to stick around for a while  so it is likely we will free things in a different order than we created them    Thus  the heap is far more complex  because there end up being regions of memory that are unused interleaved with chunks that are - memory gets fragmented  Finding free memory of the size you need is a difficult problem  This is why the heap should be avoided  though it is still often used    Implementation Implementation of both the stack and heap is usually down to the runtime   OS  Often games and other applications that are performance critical create their own memory solutions that grab a large chunk of memory from the heap and then dish it out internally to avoid relying on the OS for memory    This is only practical if your memory usage is quite different from the norm - i e for games where you load a level in one huge operation and can chuck the whole lot away in another huge operation   Physical location in memory This is less relevant than you think because of a technology called Virtual Memory which makes your program think that you have access to a certain address where the physical data is somewhere else  even on the hard disc    The addresses you get for the stack are in increasing order as your call tree gets deeper  The addresses for the heap are un-predictable  i e implimentation specific  and frankly not important

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Others have answered the broad strokes pretty well  so I ll throw in a few details    Stack and heap need not be singular  A common situation in which you have more than one stack is if you have more than one thread in a process   In this case each thread has its own stack  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  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  This memory won t survive your return statement  but it s useful for a scratch buffer  Making a huge temporary buffer on Windows that you don t use much of is not free  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  If you access memory more than one page off the end of the stack you will crash   Example    void myfunction        char big 10000000         Do something that only uses for first 1K of big 99  of the time

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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   The Memory Management Glossary web page has a diagram of this memory layout   The stack and heap are traditionally located at opposite ends of the process s virtual address space  The stack grows automatically when accessed  up to a size set by the kernel  which can be adjusted with setrlimit RLIMIT STACK         The heap grows when the memory allocator invokes the brk   or sbrk   system call  mapping more pages of physical memory into the process s virtual address space    In systems without virtual memory  such as some embedded systems  the same basic layout often applies  except the stack and heap are fixed in size  However  in other embedded systems  such as those based on Microchip PIC microcontrollers   the program stack is a separate block of memory that is not addressable by data movement instructions  and can only be modified or read indirectly through program flow instructions  call  return  etc    Other architectures  such as Intel Itanium processors  have multiple stacks  In this sense  the stack is an element of the CPU architecture

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Simply  the stack is where local variables get created  Also  every time you call a subroutine the program counter  pointer to the next machine instruction  and any important registers  and sometimes the parameters get pushed on the stack  Then any local variables inside the subroutine are pushed onto the stack  and used from there   When the subroutine finishes  that stuff all gets popped back off the stack  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   The heap is the area of memory dynamic memory allocations are made out of  explicit  new  or  allocate  calls   It is a special data structure that can keep track of blocks of memory of varying sizes and their allocation status   In  classic  systems RAM was laid out such that the stack pointer started out at the bottom of memory  the heap pointer started out at the top  and they grew towards each other  If they overlap  you are out of RAM  That doesn t work with modern multi-threaded OSes though  Every thread has to have its own stack  and those can get created dynamicly

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What is a stack  A stack is a pile of objects  typically one that is neatly arranged     Stacks in computing architectures are regions of memory where data is added or removed in a last-in-first-out manner   In a multi-threaded application  each thread will have its own stack    What is a heap  A heap is an untidy collection of things piled up haphazardly     In computing architectures the heap is an area of dynamically-allocated memory that is managed automatically by the operating system or the memory manager library   Memory on the heap is allocated  deallocated  and resized regularly during program execution  and this can lead to a problem called fragmentation   Fragmentation occurs when memory objects are allocated with small spaces in between that are too small to hold additional memory objects   The net result is a percentage of the heap space that is not usable for further memory allocations    Both together   In a multi-threaded application  each thread will have its own stack  But  all the different threads will share the heap   Because the different threads share the heap in a multi-threaded application  this also means that there has to be some coordination between the threads so that they don   t try to access and manipulate the same piece s  of memory in the heap at the same time    Which is faster     the stack or the heap  And why    The stack is much faster than the heap   This is because of the way that memory is allocated on the stack   Allocating memory on the stack is as simple as moving the stack pointer up    For people new to programming  it   s probably a good idea to use the stack since it   s easier   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   It   s better to use the heap when you know that you will need a lot of memory for your data  or you just are not sure how much memory you will need  like with a dynamic array   Java Memory Model  The stack is the area of memory where local variables  including method parameters  are stored  When it comes to object variables  these are merely references  pointers  to the actual objects on the heap  Every time an object is instantiated  a chunk of heap memory is set aside to hold the data  state  of that object  Since objects can contain other objects  some of this data can in fact hold references to those nested objects

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In the following C  code  public void Method1         int i   4      int y   2      class1 cls1   new class1        Here s how the memory is managed    Local Variables that only need to last as long as the function invocation go in the stack  The heap is used for variables whose lifetime we don t really know up front but we expect them to last a while  In most languages it s critical that we know at compile time how large a variable is if we want to store it on the stack    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  In many languages the heap is garbage collected to find objects  such as the cls1 object  that no longer have any references    In Java  most objects go directly into the heap  In languages like C   C    structs and classes can often remain on the stack when you re not dealing with pointers   More information can be found here   The difference between stack and heap memory allocation  laquo   timmurphy org  and here    Creating Objects on the Stack and Heap  This article is the source of picture above  Six important  NET concepts  Stack  heap  value types  reference types  boxing  and unboxing - CodeProject  but be aware it may contain some inaccuracies

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From WikiAnwser   Stack  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   This chain of suspended function calls is the stack  because elements in the stack  function calls  depend on each other   The stack is important to consider in exception handling and thread executions   Heap  The heap is simply the memory used by programs to store variables  Element of the heap  variables  have no dependencies with each other and can always be accessed randomly at any time

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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... up to Microsoft and its Intermediate Language (IL) that changed the paradigm to have a OO virtual machine assembly language. So we'll be able to have some CLI/CIL CPU in the future (one project of MS).

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 .NET

Stack vs Heap: Know the Difference

Static class memory allocation where it is stored C#

What and where are the stack and heap?

https://en.wikipedia.org/wiki/Memory_management

https://en.wikipedia.org/wiki/Stack_register

Assembly language resources:

Assembly Programming Tutorial

Intel® 64 and IA-32 Architectures Software Developer Manuals

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Others have answered the broad strokes pretty well  so I ll throw in a few details    Stack and heap need not be singular  A common situation in which you have more than one stack is if you have more than one thread in a process   In this case each thread has its own stack  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  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  This memory won t survive your return statement  but it s useful for a scratch buffer  Making a huge temporary buffer on Windows that you don t use much of is not free  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  If you access memory more than one page off the end of the stack you will crash   Example    void myfunction        char big 10000000         Do something that only uses for first 1K of big 99  of the time

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OK  simply and in short words  they mean ordered and not ordered      Stack  In stack items  things get on the top of each-other  means gonna be faster and more efficient to be processed       So there is always an index to point the specific item  also processing gonna be faster  there is relationship between the items as well      Heap  No order  processing gonna be slower and values are messed up together with no specific order or index    there are random and there is no relationship between them    so execution and usage time could be vary     I also create the image below to show how they may look like

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You can do some interesting things with the stack   For instance  you have functions like alloca  assuming you can get past the copious warnings concerning its use   which is a form of malloc that specifically uses the stack  not the heap  for memory   That said  stack-based memory errors are some of the worst I ve experienced   If you use heap memory  and you overstep the bounds of your allocated block  you have a decent chance of triggering a segment fault    Not 100   your block may be incidentally contiguous with another that you have previously allocated    But since variables created on the stack are always contiguous with each other  writing out of bounds can change the value of another variable   I have learned that whenever I feel that my program has stopped obeying the laws of logic  it is probably buffer overflow

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stack  heap and data of each process in virtual memory

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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    It s the region of memory below the stack pointer register  which can be set as needed   The stack is also used for passing arguments to subroutines  and also for preserving the values in registers before calling subroutines   The heap is a portion of memory that is given to an application by the operating system  typically through a syscall like malloc   On modern OSes this memory is a set of pages that only the calling process has access to   The size of the stack is determined at runtime  and generally does not grow after the program launches   In a C program  the stack needs to be large enough to hold every variable declared within each function   The heap will grow dynamically as needed  but the OS is ultimately making the call  it will often grow the heap by more than the value requested by malloc  so that at least some future mallocs won t need to go back to the kernel to get more memory   This behavior is often customizable   Because you ve allocated the stack before launching the program  you never need to malloc before you can use the stack  so that s a slight advantage there   In practice  it s very hard to predict what will be fast and what will be slow in modern operating systems that have virtual memory subsystems  because how the pages are implemented and where they are stored is an implementation detail

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From WikiAnwser   Stack  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   This chain of suspended function calls is the stack  because elements in the stack  function calls  depend on each other   The stack is important to consider in exception handling and thread executions   Heap  The heap is simply the memory used by programs to store variables  Element of the heap  variables  have no dependencies with each other and can always be accessed randomly at any time

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A lot of answers are correct as concepts  but we must note that a stack is needed by the hardware  i e  microprocessor  to allow calling subroutines  CALL in assembly language      OOP guys will call it methods   On the stack you save return addresses and call   push   ret   pop is managed directly in hardware   You can use the stack to pass parameters   even if it is slower than using registers  would a microprocessor guru say or a good 1980s BIOS book       Without stack no microprocessor can work   we can t imagine a program  even in assembly language  without subroutines functions  Without the heap it can   An assembly language program can work without  as the heap is a OS concept  as malloc  that is a OS Lib call    Stack usage is faster as    Is hardware  and even push pop are very efficient  malloc requires entering kernel mode  use lock semaphore  or other synchronization primitives  executing some code and manage some structures needed to keep track of allocation

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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       The heap is a memory for items of which you can   t predetermine the   exact size and structure  Since objects and arrays can be mutated and   change at runtime  they have to go into the heap    Source  Academind

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I have moved this answer from another question that was more or less a dupe of this one    The answer to your question is implementation specific and may vary across compilers and processor architectures  However  here is a simplified explanation    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 list of blocks on the heap  This meta information about the blocks on the heap is also stored on the heap often in a small area just in front of every block  As the heap grows new blocks are often allocated from lower addresses towards higher addresses  Thus you can think of the heap as a heap of memory blocks that grows in size as memory is allocated  If the heap is too small for an allocation the size can often be increased by acquiring more memory from the underlying operating system  Allocating and deallocating many small blocks may leave the heap in a state where there are a lot of small free blocks interspersed between the used blocks  A request to allocate a large block may fail because none of the free blocks are large enough to satisfy the allocation request even though the combined size of the free blocks may be large enough  This is called heap fragmentation  When a used block that is adjacent to a free block is deallocated the new free block may be merged with the adjacent free block to create a larger free block effectively reducing the fragmentation of the heap      The stack   The stack often works in close tandem with a special register on the CPU named the stack pointer  Initially the stack pointer points to the top of the stack  the highest address on the stack   The CPU has special instructions for pushing values onto the stack and popping them back from the stack  Each push stores the value at the current location of the stack pointer and decreases the stack pointer   A pop retrieves the value pointed to by the stack pointer and then increases the stack pointer  don t be confused by the fact that adding a value to the stack decreases the stack pointer and removing a value increases it  Remember that the stack grows to the bottom   The values stored and retrieved are the values of the CPU registers  When a function is called the CPU uses special instructions that push the current instruction pointer  i e  the address of the code executing on the stack  The CPU then jumps to the function by setting the  instruction pointer to the address of the function called  Later  when the function returns  the old instruction pointer is popped from the stack and execution resumes at the code just after the call to the function  When a function is entered  the stack pointer is decreased to allocate more space on the stack for local  automatic  variables  If the function has one local 32 bit variable four bytes are set aside on the stack  When the function returns  the stack pointer is moved back to free the allocated area  If a function has parameters  these are pushed onto the stack before the call to the function  The code in the function is then able to navigate up the stack from the current stack pointer to locate these values  Nesting function calls work like a charm  Each new call will allocate function parameters  the return address and space for local variables and these activation records can be stacked for nested calls and will unwind in the correct way when the functions return  As the stack is a limited block of memory  you can cause a stack overflow by calling too many nested functions and or allocating too much space for local variables  Often the memory area used for the stack is set up in such a way that writing below the bottom  the lowest address  of the stack will trigger a trap or exception in the CPU  This exceptional condition can then be caught by the runtime and converted into some kind of stack overflow exception         Can a function be allocated on the heap instead of a stack    No  activation records for functions  i e  local or automatic variables  are allocated on the stack that is used not only to store these variables  but also to keep track of nested function calls   How the heap is managed is really up to the runtime environment  C uses malloc and C   uses new  but many other languages have garbage collection   However  the stack is a more low-level feature closely tied to the processor architecture  Growing the heap when there is not enough space isn t too hard since it can be implemented in the library call that handles the heap  However  growing the stack is often impossible as the stack overflow only is discovered when it is too late  and shutting down the thread of execution is the only viable option

User · Answer

In the 1980s  UNIX propagated like bunnies with big companies rolling their own  Exxon had one as did dozens of brand names lost to history  How memory was laid out was at the discretion of the many implementors   A typical C program was laid out flat in memory with an opportunity to increase by changing the brk   value  Typically  the HEAP was just below this brk value and increasing brk increased the amount of available heap   The single STACK was typically an area below HEAP which was a tract of memory containing nothing of value until the top of the next fixed block of memory  This next block was often CODE which could be overwritten by stack data in one of the famous hacks of its era   One typical memory block was BSS  a block of zero values  which was accidentally not zeroed in one manufacturer s offering  Another was DATA containing initialized values  including strings and numbers  A third was CODE containing CRT  C runtime   main  functions  and libraries   The advent of virtual memory in UNIX changes many of the constraints  There is no objective reason why these blocks need be contiguous  or fixed in size  or ordered a particular way now  Of course  before UNIX was Multics which didn t suffer from these constraints  Here is a schematic showing one of the memory layouts of that era

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To clarify  this answer has incorrect information  thomas fixed his answer after comments  cool       Other answers just avoid explaining what static allocation means  So I will explain the three main forms of allocation and how they usually relate to the heap  stack  and data segment below  I also will show some examples in both C C   and Python to help people understand    Static   AKA statically allocated  variables are not allocated on the stack  Do not assume so - many people do only because  static  sounds a lot like  stack   They actually exist in neither the stack nor the heap  The are part of what s called the data segment   However  it is generally better to consider  scope  and  lifetime  rather than  stack  and  heap    Scope refers to what parts of the code can access a variable  Generally we think of local scope  can only be accessed by the current function  versus global scope  can be accessed anywhere  although scope can get much more complex   Lifetime refers to when a variable is allocated and deallocated during program execution  Usually we think of static allocation  variable will persist through the entire duration of the program  making it useful for storing the same information across several function calls  versus automatic allocation  variable only persists during a single call to a function  making it useful for storing information that is only used during your function and can be discarded once you are done  versus dynamic allocation  variables whose duration is defined at runtime  instead of compile time like static or automatic    Although most compilers and interpreters implement this behavior similarly in terms of using stacks  heaps  etc  a compiler may sometimes break these conventions if it wants as long as behavior is correct  For instance  due to optimization a local variable may only exist in a register or be removed entirely  even though most local variables exist in the stack  As has been pointed out in a few comments  you are free to implement a compiler that doesn t even use a stack or a heap  but instead some other storage mechanisms  rarely done  since stacks and heaps are great for this    I will provide some simple annotated C code to illustrate all of this  The best way to learn is to run a program under a debugger and watch the behavior  If you prefer to read python  skip to the end of the answer        Statically allocated in the data segment when the program DLL is first loaded    Deallocated when the program DLL exits    scope - can be accessed from anywhere in the code int someGlobalVariable      Statically allocated in the data segment when the program is first loaded    Deallocated when the program DLL exits    scope - can be accessed from anywhere in this particular code file static int someStaticVariable       someArgument  is allocated on the stack each time MyFunction is called     someArgument  is deallocated when MyFunction returns    scope - can be accessed only within MyFunction   void MyFunction int someArgument            Statically allocated in the data segment when the program is first loaded        Deallocated when the program DLL exits        scope - can be accessed only within MyFunction       static int someLocalStaticVariable          Allocated on the stack each time MyFunction is called        Deallocated when MyFunction returns        scope - can be accessed only within MyFunction       int someLocalVariable          A  pointer  is allocated on the stack each time MyFunction is called        This pointer is deallocated when MyFunction returns        scope - the pointer can be accessed only within MyFunction       int  someDynamicVariable          This line causes space for an integer to be allocated in the heap        when this line is executed  Note this is not at the beginning of        the call to MyFunction    like the automatic variables        scope - only code within MyFunction   can access this space         through this particular variable          However  if you pass the address somewhere else  that code        can access it too     someDynamicVariable   new int           This line deallocates the space for the integer in the heap         If we did not write it  the memory would be  leaked          Note a fundamental difference between the stack and heap        the heap must be managed  The stack is managed for us      delete someDynamicVariable          In other cases  instead of deallocating this heap space you        might store the address somewhere more permanent to use later         Some languages even take care of deallocation for you    but        always it needs to be taken care of at runtime by some mechanism          When the function returns  someArgument  someLocalVariable        and the pointer someDynamicVariable are deallocated         The space pointed to by someDynamicVariable was already        deallocated prior to returning      return        Note that someGlobalVariable  someStaticVariable and    someLocalStaticVariable continue to exist  and are not    deallocated until the program exits    A particularly poignant example of why it s important to distinguish between lifetime and scope is that a variable can have local scope but static lifetime - for instance   someLocalStaticVariable  in the code sample above  Such variables can make our common but informal naming habits very confusing  For instance when we say  local  we usually mean  locally scoped automatically allocated variable  and when we say global we usually mean  globally scoped statically allocated variable   Unfortunately when it comes to things like  file scoped statically allocated variables  many people just say     huh       Some of the syntax choices in C C   exacerbate this problem - for instance many people think global variables are not  static  because of the syntax shown below   int var1     Has global scope and static allocation static int var2     Has file scope and static allocation  int main    return 0     Note that putting the keyword  static  in the declaration above prevents var2 from having global scope  Nevertheless  the global var1 has static allocation  This is not intuitive  For this reason  I try to never use the word  static  when describing scope  and instead say something like  file  or  file limited  scope  However many people use the phrase  static  or  static scope  to describe a variable that can only be accessed from one code file  In the context of lifetime   static  always means the variable is allocated at program start and deallocated when program exits   Some people think of these concepts as C C   specific  They are not  For instance  the Python sample below illustrates all three types of allocation  there are some subtle differences possible in interpreted languages that I won t get into here    from datetime import datetime  class Animal       FavoriteFood    Undefined     FavoriteFood is statically allocated      def PetAnimal self           curTime   datetime time datetime now      curTime is automatically allocatedion         print  Thank you for petting me  But it s     str curTime       you should feed me  My favorite food is     self  FavoriteFood   class Cat Animal        FavoriteFood    tuna    Note since we override  Cat class has its own statically allocated  FavoriteFood variable  different from Animal s  class Dog Animal        FavoriteFood    steak    Likewise  the Dog class gets its own static variable  Important to note - this one static variable is shared among all instances of Dog  hence it is not dynamic    if   name         main         whiskers   Cat     Dynamically allocated     fido   Dog     Dynamically allocated     rinTinTin   Dog     Dynamically allocated      whiskers PetAnimal       fido PetAnimal       rinTinTin PetAnimal        Dog  FavoriteFood    milkbones      whiskers PetAnimal       fido PetAnimal       rinTinTin PetAnimal      Output is    Thank you for petting me  But it s 13 05 02 255000  you should feed me  My favorite food is tuna   Thank you for petting me  But it s 13 05 02 255000  you should feed me  My favorite food is steak   Thank you for petting me  But it s 13 05 02 255000  you should feed me  My favorite food is steak   Thank you for petting me  But it s 13 05 02 255000  you should feed me  My favorite food is tuna   Thank you for petting me  But it s 13 05 02 255000  you should feed me  My favorite food is milkbones   Thank you for petting me  But it s 13 05 02 256000  you should feed me  My favorite food is milkbones

User · Answer

I think many other people have given you mostly correct answers on this matter   One detail that has been missed  however  is that the  heap  should in fact probably be called the  free store    The reason for this distinction is that the original free store was implemented with a data structure known as a  binomial heap    For that reason  allocating from early implementations of malloc   free   was allocation from a heap   However  in this modern day  most free stores are implemented with very elaborate data structures that are not binomial heaps

User · Answer

In the 1980s  UNIX propagated like bunnies with big companies rolling their own  Exxon had one as did dozens of brand names lost to history  How memory was laid out was at the discretion of the many implementors   A typical C program was laid out flat in memory with an opportunity to increase by changing the brk   value  Typically  the HEAP was just below this brk value and increasing brk increased the amount of available heap   The single STACK was typically an area below HEAP which was a tract of memory containing nothing of value until the top of the next fixed block of memory  This next block was often CODE which could be overwritten by stack data in one of the famous hacks of its era   One typical memory block was BSS  a block of zero values  which was accidentally not zeroed in one manufacturer s offering  Another was DATA containing initialized values  including strings and numbers  A third was CODE containing CRT  C runtime   main  functions  and libraries   The advent of virtual memory in UNIX changes many of the constraints  There is no objective reason why these blocks need be contiguous  or fixed in size  or ordered a particular way now  Of course  before UNIX was Multics which didn t suffer from these constraints  Here is a schematic showing one of the memory layouts of that era

User · Answer

You can do some interesting things with the stack   For instance  you have functions like alloca  assuming you can get past the copious warnings concerning its use   which is a form of malloc that specifically uses the stack  not the heap  for memory   That said  stack-based memory errors are some of the worst I ve experienced   If you use heap memory  and you overstep the bounds of your allocated block  you have a decent chance of triggering a segment fault    Not 100   your block may be incidentally contiguous with another that you have previously allocated    But since variables created on the stack are always contiguous with each other  writing out of bounds can change the value of another variable   I have learned that whenever I feel that my program has stopped obeying the laws of logic  it is probably buffer overflow

[memory-management] What and where are the stack and heap?

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