16_heap.md

name: main

.aim[Systems: A Heap o'Trouble]

<style> .aim { font-size: .75em; border-bottom: 1px solid lightgray; margin: 1px; } .remark-inline-code { background-color: lightgray; border-radius: 3px; padding-left: 2px; padding-right: 2px; } h4 { font-size: 1.5em; margin: 1px; } </style>

---

template: main

Stack memory vs Heap memory

• Every program can have its own stack and heap.

--

• Stack memory
  • Stores all normally declared variables (including pointers and structs), arrays and function calls.

--

• Functions are pushed onto the stack in the order they are called, and popped off when completed.

--

• When a function is popped off the stack, the stack memory associated with it is released.

--

• Heap memory
  • Stores dynamically allocated memory.

--

• Data will remain in the heap until it is manually released. (or the program terminates)

---

template: main

Dynamic memory allocation

• malloc(size_t x)
  • Allocates x bytes of heap memory.

--

• Returns the address at the beginning of the allocation

--

• Returns a void *

  int *p;
  p = malloc( 5 * sizeof(int) );
  

--

• free(void * p)
  • Releases dynamically allocated memory.
  • Has one parameter, a pointer to the beginning of a dynamically allocated block of memory.
  • Every call to malloc/calloc should have a corresponding call to free.

---

template: main

• calloc(size_t n, size_t x)
  • Allocates n * x bytes of memory, ensuring every bit is 0.
  • Works like malloc in all other ways

    int *p;
    p = calloc( 5, sizeof(int) );
    

--

• realloc(void *p, size_t x)
  • Changes the amount of memory allocated for a block to x bytes.
  • p must point to the beginning of a block.

--

• Returns a pointer to the beginning of the block.

--

• If x is smaller than the original size of the allocation, the extra bytes will be released.

--

• If x is larger than the original size then either:
  • If there is enough space at the end of the original allocation, the original allocation will be updated.
  • If there is not enough space, a new allocation will be created, containing all the original values. The original allocation will be freed.