Stack and Heap Memory and Dynamic Memory | CMSC 212, Study Guides, Projects, Research of Computer Science

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Announcements

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Project #2 Deadline– Extended to Tuesday 3/15/05 8:00 PM

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Midterm #2 returned at end of class today

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Reading– Chapter 11 (Today)– Chapter 14 (Tuesday)

Stack and Heap Memory

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Stack Memory– Allocated and de-allocated in a specific order

• last allocated is the first to be freed
  • Useful for representing program local variables z^

Heap Memory– Can allocate and de-allocate in any order

Stack Top Free Space

Allocated Space

Heap Memory Management

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Allocating memory:– void *malloc(size_t requestedSize);

• allocate requestedSize bytes of memory
  • void *calloc(size_t requestCount, size_t objectSize);
    • allocate requestCount objects of size objectSize bytes each• set the requested memory to zero

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Type Information– malloc/calloc return void *– need to convert to desired type using cast operator

Example

T p;p = (T) malloc(sizeof(T));if (!p) {

exit(-1); /* often nothing can do on failure / } / use of p */void do something(int k) {

int i, *myInts;myInts = (int *) calloc(sizeof(int),k);if (!myInts) exit(-1);for (i=0; i < k; i++) {

myInts[i] = anotherFunc(i, k); }

Memory Deallocation

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free(void *p);– returns space pointed to by p– does

not

change p

• good idea p=NULL; right after deallocation.

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Using Free– always free memory when done with it– never free things that are not allocated by malloc/calloc:

• int a = 3;• int *p = &a;• free(p);

/* error, p points to something not allocated */

• Never dereference a pointer after free is called:
  • free(p);• if (*p) { …}

/* error, pointer is no longer valid */

What if you need more memory?

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Could– malloc new space– copy data from old space to new– free old space

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C provides a short cut– void *realloc(void *ptr, size_t new_size);– allocates new space (larger than old)– copies old data and frees it– returns pointer to new space

Common Dynamic Memory Errors

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Derefencing NULL pointers– int *foo = NULL;– foo = 3; / ERROR */

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Forgetting to check return value of malloc/calloc

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Using malloc and not initializing values

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Going past end of allocated space– very easy when using array syntax and loops

for (i=0; i

k; i++) {

myInts[i] = anotherFunc(i, k); }^

Goes past end of previous example

More Common Memory Errors

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Calling Free on something not from heapint *ptr; j;j = 3;ptr = &j;free(ptr);

/* ERROR: ptr doe not point to something in heap */

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Calling Free on other than the start of allocationint *ptr;ptr = calloc(sizeof(int), 10);free (ptr + 3);

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Using memory that has been freedT *curr;free (curr);curr = curr->next;

/* ERROR: curr has already been freed */

Pointers and Functions

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Parameters– can use const to prevent functions from changing valuesint frob(const char *data, int size) {

if (data[0] == 'Q') { .. }

/* OK: just using the data */

data[2] = 42

/* Error: can't modify data */

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Functions can return pointer typesint *getStuff() {

int *ptr;ptr = calloc(sizeof(int), 42);return ptr; }

Blocks of Memory

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C supports operations on blocks of memory– might be an array, a struct, an array of structs, etc.

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Copying– void *memcpy(void *dest, const void *src, size_t len);

• copies memory pointed to by src to memory pointed to by dest - copies len bytes of memory• returns the value of src -^

void *memmove(void *dest, const void *src, size_t len);• copies memory pointed to by src to memory pointed to by

dest

• copies len bytes of memory• returns the value of src• handles the case if dest and src overlap

Common Mt #1 Mistakes

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#1– Program Counter

• advances one location per instruction, branch changes
  • Function Prototype
    • defines name, paramter types, and return types• allows compiler to type check functions in separate files

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#2– last integer (+2 is the value 4)

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#3– Explaining two passes– Describing memory field & that a new op-code was needed

Common Mt #1 Mistakes (cont.)

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#4– Expressions can be valid even if they don't refer to scalars

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#5– finding comma, and not overwriting parts of string that are

needed

• printing lastname firstname not firstname lastname z^

#6– Make example - go re-read how to create a make file– cp copies one file, doesn't combine two– Review basic UNIX commands