Tuesday, July 17, 2012

.NET Memory management - stack and heap


 
for more info refer to above URL
 
Stack:
  • Stored in computer RAM like the heap.
  • Variables created on the stack will go out of scope and automatically deallocate.
  • Much faster to allocate in comparison to variables on the heap.
  • Implemented with an actual stack data structure.
  • Stores local data, return addresses, used for parameter passing
  • Can have a stack overflow when too much of the stack is used. (mostly from inifinite (or too much) recursion, very large allocations)
  • Data created on the stack can be used without pointers.
  • You would use the stack if you know exactly how much data you need to allocate before compile time and it is not too big.
  • Usually has a maximum size already determined when your program starts
Heap:
  • Stored in computer RAM like the stack.
  • Variables on the heap must be destroyed manually and never fall out of scope. The data is freed with delete, delete[] or free
  • Slower to allocate in comparison to variables on the stack.
  • Used on demand to allocate a block of data for use by the program.
  • Can have fragmentation when there are a lot of allocations and deallocations
  • In C++ data created on the heap will be pointed to by pointers and allocated with new or malloc
  • Can have allocation failures if too big of a buffer is requested to be allocated.
  • You would use the heap if you don't know exactly how much data you will need at runtime or if you need to allocate a lot of data.
  • Responsible for memory leaks
Example:
int foo()
{

  char *pBuffer; //<--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];

   }//<-- buffer is deallocated here, pBuffer is not
}//<--- oops there's a memory leak, I should have called delete[] pBuffer;

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