A function is a reusable block of code that can be called from different parts of a program. Functions accept parameters as input and may return a value. When a function is called, its parameters and local variables are stored on the stack. Each function call creates a stack frame that contains its parameters, local variables, and return address. This allows functions to maintain separate variable scopes while sharing the call stack.

1. How Functions Work Saumil Shah www.net-square.com

2. Introduction

3. # who am i Saumil Shah CEO Net-square. Hacker, Speaker, Trainer, Author. M.S. Computer Science Purdue University. Google: "saumil" LinkedIn: saumilshah

4. Preview

5. What is a function?

6. What is a function? A function is a special SUBROUTINE

7. What is a function? A function is a special SUBROUTINE Re-usable block of code Can be called from anywhere in the program

8. What is a function? A function is a special SUBROUTINE Re-usable block of code Can be called from anywhere in the program Program control jumps to the subroutine... ...and returns to the next statement after completing the subroutine

9. Anything else?

10. Anything else? A function accepts parameters A function returns a value

11. Anything else? A function accepts parameters A function returns a value It may also have LOCAL variables...

12. Anything else? A function accepts parameters A function returns a value It may also have LOCAL variables... ...created when function is invoked, and destroyed when the function returns. Scope limited to that function only.

13. An example - add(x, y) int add(int x, int y) { int sum; sum = x + y; return(sum); }

14. An example - add(x, y) Parameters int add(int x, int y) { int sum; sum = x + y; return(sum); } Local Variable Return Value

15. Where are all the values stored? How are parameters passed? Where are local variables stored?

16. Where are all the values stored? How are parameters passed? Where are local variables stored? It is all accomplished using the STACK!

17. Where are all the values stored? How are parameters passed? Where are local variables stored? It is all accomplished using the STACK! Parameters are pushed on the stack before calling the function. Local variables are stored in stack memory as well.

18. Calling a function

19. add(x, y) 1 PROLOGUE 2 Local Variables BODY 3 s = add(3, 4) EPILOGUE Return Calling a function 4

20. add(x, y) PROLOGUE Push 4 Local Variables Push 3 BODY CALL add EPILOGUE RET Calling a function

21. CALL does two things: add CALL add RET Calling a function

22. CALL does two things: add Push EIP on the stack Jump to the function's address CALL add RET Calling a function

23. add CALL add RET Calling a function CALL does two things: Push EIP on the stack Jump to the function's address RET simply pops the saved EIP value.

24. How does it all fit together?

25. How does it all fit together? Let's see what happens on the stack.

26. How does it all fit together? Let's see what happens on the stack. ESP is the stack pointer. It always points to the top of the stack.

27. In the beginning ESP points to the top of the stack, as usual ... ESP ... EBP

28. In the beginning ESP points to the top of the stack, as usual EBP is the frame pointer (called Base Pointer). It points to regions within the stack. ... ESP ... EBP

29. Push the parameters For add(3,4) we push 3 and 4 on the stack. 3 ESP 4 ... ... EBP

30. CALL add CALL pushes the current EIP on the stack... ...and jumps to add() Saved EIP ESP 3 4 ... ... EBP

31. Prologue The Prologue saves the old frame pointer (EBP) and sets EBP to top of stack. Old EBP EBP ESP Saved EIP 3 4 ... ...

32. Prologue The Prologue saves the old frame pointer (EBP) and sets EBP to top of stack. Old EBP EBP ESP What's a FRAME? Saved EIP 3 4 ... ...

33. Prologue The Prologue saves the old frame pointer (EBP) and sets EBP to top of stack. Old EBP EBP ESP What's a FRAME? Saved EIP 3 We shall discuss the frame a bit later. 4 ... ...

34. Local Variables Local variables are created in the stack memory. sum ESP Old EBP EBP Saved EIP 3 4 ... ...

35. Frame for add() The Stack Frame The stack memory used by a function is termed as its STACK FRAME sum ESP Old EBP EBP Saved EIP 3 4 ... ... Frame for main()

36. Functions and Frames Each function call results in a new frame being created on the stack. func1() frame for func1 ESP

37. Functions and Frames Each function call results in a new frame being created on the stack. func1() frame for func2 ESP func2() frame for func1

38. Functions and Frames Each function call results in a new frame being created on the stack. frame for func3 ESP func1() frame for func2 func2() frame for func1 func3()

39. frame for func2 frame for func1 Functions and Frames When a function returns, the frame is "unwound" or "collapsed". func1() ESP func2() func3()

40. Functions and Frames And as new functions get invoked, new frames get created. frame for func4 ESP func1() frame for func2 func2() frame for func1 func3() func4()

41. The Frame Pointer EBP is the frame pointer (base pointer). sum Old EBP EBP Saved EIP 3 4 ... ...

42. The Frame Pointer EBP is the frame pointer (base pointer). sum local var Old EBP EBP Local variables and Parameters are RELATIVE to the frame pointer. Saved EIP 3 param 1 4 param 2 ... ...

43. The Frame Pointer EBP is the frame pointer (base pointer). sum EBP - 4 Old EBP EBP Local variables and Parameters are RELATIVE to the frame pointer. Saved EIP 3 EBP + 8 4 EBP - n: Local vars EBP + n: Parameters EBP + 12 ... ...

44. Epilogue The Epilogue cleans up the stack frame. Local variables are effectively destroyed. sum Old EBP ESP EBP Saved EIP 3 4 ... ...

45. Epilogue The Epilogue cleans up the stack frame. Local variables are effectively destroyed. sum Old EBP POP EBP. Restores EBP back to the old frame. Saved EIP ESP 3 4 ... ... EBP

46. Epilogue The Epilogue cleans up the stack frame. Local variables are effectively destroyed. sum Old EBP POP EBP. Restores EBP back to the old frame. Saved EIP ESP 3 4 Stack pointer now points to where EIP was saved before CALL add(). ... ... EBP

47. Return! RET instruction pops the saved EIP value back into the EIP register. sum Old EBP Saved EIP ESP 3 4 ... ... EBP

48. Return! RET instruction pops the saved EIP value back into the EIP register. EIP sum Old EBP Program control is returns to the next statement after add() Saved EIP ESP 3 4 ... ... EBP

49. Return! RET instruction pops the saved EIP value back into the EIP register. EIP sum Old EBP Program control is returns to the next statement after add() Saved EIP 3 ESP 4 ESP shifts down by one word. ... ... EBP

50. Key Concepts

51. Review

52. HOW FUNCTIONS WORK saumil@net-square.com