A Short Digression Stacks and Heaps

1. A Short DigressionStacks and Heaps CS-502, Operating SystemsFall 2007 (Slides include materials from Operating System Concepts, 7th ed., by Silbershatz, Galvin, & Gagne and from Modern Operating Systems, 2nd ed., by Tanenbaum) Stacks and Heaps

2. Digression: the “Stack” • Imagine the following program:– int factorial(int n){ if (n <= 1) return (1); else int y = factorial(n-1); return (y * n); } • Imagine also the caller:– int x = factorial(100); • What does compiled code look like? Stacks and Heaps

3. Compiled code: the caller int x = factorial(100); • Put the value “100” somewhere that factorial can find • Put the current program counter somewhere so that factorial can return to the right place in caller • Provide a place to put the result, so that caller can find it Stacks and Heaps

4. Compiled code: factorial function • Save the caller’s registers somewhere • Get the argument n from the agreed-upon place • Set aside some memory for local variables and intermediate results – i.e., y, n - 1 • Do whatever it was programmed to do • Put the result where the caller can find it • Restore the caller’s registers • Transfer back to the program counter saved by the caller Stacks and Heaps

5. Question: Where is “somewhere”? • So that caller can provide as many arguments as needed (within reason)? • So that called routine can decide at run-time how much temporary space is needed? • So that called routine can call any other routine, potentially recursively? Stacks and Heaps

6. Answer: a “Stack” • Stack – a linear data structure in which items are added and removed in last-in, first-out order. • Calling program • Push arguments & return address onto stack • After return, pop result off stack Stacks and Heaps

7. “Stack” (continued) • Called routine • Push registers and return address onto stack • Push temporary storage space onto stack • Do work of the routine • Pop registers and temporary storage off stack • Leave result on stack • Return to address left by calling routine Stacks and Heaps

8. Stack (continued) • Definition: context – the region of the stack that provides the execution environment of a particular call to a function • Implementation • Usually, a linear piece of memory and a stack pointer contained in a (fixed) register • Occasionally, a linked list • Recursion • Stack discipline allows multiple contexts for the same function in the stack at the same time Stacks and Heaps

9. Stacks in Modern Systems • All modern programming languages require a stack • Fortran and Cobol did not (non-recursive) • All modern processors provide a designated stack pointer register • All modern process address spaces provide room for a stack • Able to grow to a large size • May grow upward or downward Stacks and Heaps

10. 0xFFFFFFFF stack (dynamically allocated) SP heap (dynamically allocated) Virtual address space static data program code (text) PC 0x00000000 Process Address Space (Typical) See also Silbershatz, figure 3.1 Stacks and Heaps

11. Stacks in Multi-threaded Environments • Every thread requires its own stack • Separate from all other stacks • Each stack may grow separately • Address space must be big enough to accommodate stacks for all threads Stacks and Heaps

12. Stacks in Multi-threaded Address Space thread 1 stack SP (T1) 0xFFFFFFFF thread 2 stack SP SP (T2) thread 3 stack SP (T3) Virtual address space heap static data 0x00000000 PC code (text) PC (T2) PC (T1) PC (T3) Stacks and Heaps

13. Stacks in Multi-threaded Environments • Every thread requires its own stack • Separate from all other stacks • Each stack may grow separately • Address space must be big enough to accommodate stacks for all threads • Some small or RT operating systems are equivalent to multi-threaded environments Stacks and Heaps

14. Heap • A place for allocating memory that is not part of last-in, first-out discipline • I.e., dynamically allocated data structures that survive function calls • E.g., strings in C • new objects in C++, Java, etc. Stacks and Heaps

15. 0xFFFFFFFF stack (dynamically allocated) SP heap (dynamically allocated) Virtual address space static data program code (text) PC 0x00000000 Process Address Space (Typical) See also Silbershatz, figure 3.1 Stacks and Heaps

16. Dynamically Allocating from Heap • malloc() – POSIX standard function • Allocates a chunk of memory of desired size • Remembers size • Returns pointer • free () – POSIX standard function • Returns previously allocated chunk to heap for reallocation • Assumes that pointer is correct! Stacks and Heaps

17. Dynamically Allocating from Heap • malloc() – POSIX standard function • Allocates a chunk of memory of desired size • Remembers size • Returns pointer • free () – POSIX standard function • Returns previously allocated chunk to heap for reallocation • Assumes that pointer is correct! • Storage leak – failure to free something Stacks and Heaps

18. Heaps in Modern Systems • Many modern programming languages require a heap • C++, Java, etc. • NOT Fortran • Typical process environment • Grows toward stack • Multi-threaded environments • All threads share the same heap • Data structures may be passed from one thread to another. Stacks and Heaps

19. Heap Heap in Multi-threaded Address Space thread 1 stack SP (T1) 0xFFFFFFFF thread 2 stack SP SP (T2) thread 3 stack SP (T3) Virtual address space heap static data 0x00000000 PC code (text) PC (T2) PC (T1) PC (T3) Stacks and Heaps

20. What’s this? Stacks in Multi-threaded Address Space thread 1 stack SP (T1) 0xFFFFFFFF thread 2 stack SP SP (T2) thread 3 stack SP (T3) Virtual address space heap static data 0x00000000 PC code (text) PC (T2) PC (T1) PC (T3) Stacks and Heaps

21. Questions? Stacks and Heaps