memory model - cs 2022: introduction to c · i int *my ptr = &my var; i pointer arithmetic:...

Memory ModelCS 2022: Introduction to C

Instructor: Hussam Abu-Libdeh

Cornell University(based on slides by Saikat Guha)

Fall 2009, Lecture 4

Memory Model CS 2022, Fall 2009, Lecture 4

Pointers Recap

I int *ptr;

I Pointers are variables that store memoryaddresses of other variables

I Type of variable pointed to depends on type ofpointer:

I int *ptr points to an integer variable,char *ptr points to character variable

I Can cast between pointer types:my int ptr = (int *) my other ptr;

I void *ptr has an unspecified type; must be castto a type before used

Pointers RecapI Two main operaitons:

I * dereference: get the value at the memorylocation stored in a pointer

I & address of : get the address of a variableI int *my ptr = &my var;

I Pointer arithmetic: directly manipulate apointer’s content to access other locations

I Use with caution!: can access bad areas ofmemory and cause a crash

I However, it is useful in accessing and manipulatingdata structures

I Can have pointers to pointersI int **my 2d array;

Memory

I Program codeI Function variables

I ArgumentsI Local variablesI Return location

I Global VariablesI Statically AllocatedI Dynamically

Allocated

The Stack

Stores

I Function local variables

I Temporary variables

I Arguments for next function call

I Where to return when function ends

The Stack

Managed by compiler

I One stack frame each time function called

I Created when function called

I Stacked on top (under) one another

I Destroyed at function exit

The Stack

int fact(int n) {int res;if (n == 1)

return 1;res = fact(n-1);return n * res;

int main() {int res = fact(5);return 0;

The Stack

Stack games

I Locate the stack

I Find the direction ofstack growth

I Finding size of stackframe

What can go wrong

I Run out of stackspace

I Unintentionallychange values on thestack

I In some otherfunction’s frame

I Even return addressfrom function

I Access memory evenafter frame isdeallocated

Memory Recap

I Program codeI Function variables

I ArgumentsI Local variablesI Return location

I Global VariablesI Statically AllocatedI Dynamically

Allocated

HeapNeeded for long-term storage that needs to persistacross multiple function calls.

Managed by programmer

I Created by ptr = malloc(size)

I Destroyed by free(ptr)

MUST check the return value from malloc

MUST explicitly free memory when no longer in use

The Heap

int main() {int *p, *q, *r;

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

sizeof(int) * 10);r = (int *)malloc(sizeof(int));

if (p == NULL || !q || !r) {... do cleanup ...return 1;

free(p);... do other stuff ...

The Heap

Heap games

I Locate the heap

I How freespace ismanaged

I Find how memory isallocated

I How isfragmentationavoided

What can go wrong

I Run out of heapspace malloc returns 0

I Unintentionallychange other heapdata

I Or clobber heapmetadata

I Access memory afterfree’d

I free memory twice

I Create a memoryleak

memory model - cs 2022: introduction to c · i int *my ptr = &my var; i pointer arithmetic:...

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