EEE 243B Applied Computer Programming

1. EEE 243BApplied Computer Programming Memory model for C programs Pointer arithmetic

2. Review • How do we access the fields in a structure? • If I have a pointer what operator do I use to dereference and access a field at the same time? • If I declare a variable as static inside a function what does that do for me? Maj JGA Beaulieu & Capt MWP LeSauvage

3. Outline • Memory model for a program in C • Pointer arithmetic Maj JGA Beaulieu & Capt MWP LeSauvage

4. Memory model • A program in C has three main segments: • Code segment (your program) • Heap (AKA data segment) • Stack (automatic segment) Maj JGA Beaulieu & Capt MWP LeSauvage

5. Memory model Stack } Empty space; the final frontier Heap Code Maj JGA Beaulieu & Capt MWP LeSauvage

6. Dude… Where’s my Var? Memory model Maj JGA Beaulieu & Capt MWP LeSauvage

7. Memory model • Dude…Where is my var? int i = 0; void main (void) { int j = 0; } int Fctn () { int l = 0; static int k = 0; } Stack Heap Code Maj JGA Beaulieu & Capt MWP LeSauvage

8. Memory model • Dude…Where is my var? int i = 0; void main (void) { int j = 0; j = j + 5; } int Fctn () { int l = 0; static int k = 0; k = k +10; } Stack Heap Code Maj JGA Beaulieu & Capt MWP LeSauvage

9. Memory model • Dude…Where is my mem? void main (void) { int *iPtr; … iPtr = (int *)malloc(10 * sizeof(int)); } Stack Heap Code Maj JGA Beaulieu & Capt MWP LeSauvage

10. Pointer arithmetic • You have already seen or even used at least one pointer arithmetic operation int *pInt = NULL; … pInt++; //Move my pointer ahead by one • Ahead by one, but by one what? Maj JGA Beaulieu & Capt MWP LeSauvage

11. Pointer arithmetic • Pointers have types because they point to a type. This is important when we do pointer arithmetic: int aOfInts[5] = {1,2,3,4,5}; int *pInt = aOfInts; … pInt++; //This moves the pointer ahead //by two bytes • Each square is a byte • in memory. @init After pInt++ Maj JGA Beaulieu & Capt MWP LeSauvage

12. Pointer arithmetic • And: char aOfChars[3] = {'a', 'b', 'c', 'd', 'e'}; char *pChar = aOfChars; … pChar++; //This moves the pointer // ahead by one byte • Each square is a byte • in memory. @init After pChar++ Maj JGA Beaulieu & Capt MWP LeSauvage

13. Pointer arithmetic • The same goes for every other pointer type! • If you are pointing to the STUDENT type-defined structure: STUDENT *ptrToStu = aMilColStudent; … ptrToStu++; //This moves the pointer //ahead by sizeof(STUDENT) //bytes Maj JGA Beaulieu & Capt MWP LeSauvage

14. Pointer arithmetic • There are only a few arithmetic operations that you can do on pointers: int *pInt; • Unary: ++pInt, --pInt • Postfix: pInt++, pInt-- • Adding an index (an int) to a pointer: pInt + 5 (advances by 5 positions) • Subtracting an index: pInt – 5 • Subtracting pointers: pInt1 – pInt2 • Gives the number of positions between two pointers – useful to calculate offsets in arrays (distance between elements) Maj JGA Beaulieu & Capt MWP LeSauvage

15. Pointer arithmetic • You cannot add, multiply or divide two pointers (it makes no point to it ): • What good could come out of it? • Pointedly, something that is pointless. Maj JGA Beaulieu & Capt MWP LeSauvage

16. Quiz Time • What is the difference between the heap and the stack for a program? • Given the program answer stack or heap: //A program int i = 0; //Where? void main (void) { int j = 0; //Where? } int Fctn () { static int k = 0;//Where? int l = 0; //Where? } Maj JGA Beaulieu & Capt MWP LeSauvage

17. Quiz Time • Draw where the pointers are in the array: int aOfInts[5] = {1,2,3,4,5}; int *pInt = aOfInts; Int *pInt2 = NULL; pInt = pInt + 4; pInt2 = pInt--; aOfInts[0] = pInt – pInt2; //What is in //array aOfInts[0]? Maj JGA Beaulieu & Capt MWP LeSauvage