Dynamic allocation and deallocation of memory:

1
Dynamic allocation and deallocation of memory
Chapter 4, Slide 1
2

• Memory management is a simple accounting of
  what process owns what part(s) of the memory.
• Memory allocation is making an entry in the
  accounting book that this segment is given to
  this process for keeps.
• Memory deallocation is an entry that this
  segment is not needed by this process and hence
  free.
• The operating system process manager usually
  keeps track of allocated blocks in a data
  structure called binary heap (in which each node
  is labeled by a label that is smaller than labels
  of all its descendants). Its purpose is to
  facilitate fast and efficient searching for a
  suitable free block.
• This heap is sometimes referred to as system
  heap or free store.

Chapter 4, Slide 2
3

• The process memory manager usually keeps a
  dynamic list of free segments.
• One of the implications is that every time your
  program requests more memory, the process memory
  manager has to search through the list to find a
  suitable segment if none is found, more memory
  must be requested from the operating system
  memory manager that must search the system heap
  for a suitable block and when delivered to the
  process memory manager, a suitable segment must
  be carved out from the freshly allocated block.
• The time delay cannot be determined. Thus, if a
  program does a significant number of allocations
  and deallocations, the unpredictable delays
  caused by these may affect the program's
  performance.
• These issues must be considered for real-time
  software systems and for all programs where
  performance is essential. We will look at these
  issues when we discuss the concept of allocation
  from arena.

Chapter 4, Slide 3
4
The memory manager in C programs is engaged
through various interfaces (standard functions)
include ltstdlib.hgt void malloc(size_t size)
The size of the allocated segment is at least
size, the contents are arbitrary!
include ltstdlib.hgt void calloc(size_t
nelem,size_t elsize)
The size of the allocated segment is at least
nelemelsize, the contents are cleared!
include ltstdlib.hgt void realloc(void
ptr,size_t size)
Chapter 4, Slide 4
5
If the reallocated segment can be extended, it is
done so, otherwise a new segment is created, the
contentrs of the old segment are copied to the
new one, and the old segment is
deallocated. Careless use of realloc() often
leads to dangling pointers! extending node c
Chapter 4, Slide 5
6
Extension was possible, all is fine!
Chapter 4, Slide 6
7
Extension was not possible, reallocation was
necessayr, right child of node a is dangling!
Chapter 4, Slide 7
8
Deallocation
include ltstdlib.hgt void free(void ptr)
End of slides for chapter 4
Chapter 4, Slide 8