Digression on Stack and Heaps CS-502 (EMC) Fall A Short Digression on Stacks and Heaps CS-502, Operating Systems Fall 2009 (EMC) (Slides include materials from Modern Operating Systems, 3 rd ed., by Andrew Tanenbaum and from Operating System Concepts, 7 th ed., by Silbershatz, Galvin, & Gagne)

Digression on Stack and Heaps CS-502 (EMC) Fall Heads Up This topic is not covered explicitly in the textbooks It’s something you are supposed to know … … but that often slips through the cracks!

Digression on Stack and Heaps CS-502 (EMC) Fall The Stack (from C background) The place where arguments of a function call are stored The place where registers of the calling function are saved The place where local data of called function is allocated Automatic data The place where called function leaves result for calling function Supports recursive function calls …

Digression on Stack and Heaps CS-502 (EMC) Fall Introduction: 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?

Digression on Stack and Heaps CS-502 (EMC) Fall Compiled code: the caller int x = factorial(100); Put the value “100” somewhere that factorial function can find Put the current program counter somewhere so that factorial function can return to the right place in calling function Provide a place to put the result, so that calling function can find it

Digression on Stack and Heaps CS-502 (EMC) Fall 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 factorial 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

Digression on Stack and Heaps CS-502 (EMC) Fall 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?

Digression on Stack and Heaps CS-502 (EMC) Fall Answer: a “Stack” Stack – a 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

Digression on Stack and Heaps CS-502 (EMC) Fall “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

Digression on Stack and Heaps CS-502 (EMC) Fall 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 (designated) register Occasionally, a linked list Recursion Stack discipline allows multiple contexts for the same function in the stack at the same time

Digression on Stack and Heaps CS-502 (EMC) Fall 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

Digression on Stack and Heaps CS-502 (EMC) Fall Process Address Space (Typical) 0x xFFFFFFFF Virtual address space program code (text) static data heap (dynamically allocated) stack (dynamically allocated) PC SP

Digression on Stack and Heaps CS-502 (EMC) Fall 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

Digression on Stack and Heaps CS-502 (EMC) Fall Stacks in Multi-threaded Address Space 0x xFFFFFFFF Virtual address space code (text) static data heap thread 1 stack PC (T2) SP (T2) thread 2 stack thread 3 stack SP (T1) SP (T3) PC (T1) PC (T3) SP PC

Digression on Stack and Heaps CS-502 (EMC) Fall 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. Anything allocated with malloc() or calloc()

Digression on Stack and Heaps CS-502 (EMC) Fall Process Address Space (Typical) 0x xFFFFFFFF Virtual address space program code (text) static data heap (dynamically allocated) stack (dynamically allocated) PC SP

Digression on Stack and Heaps CS-502 (EMC) Fall 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!

Digression on Stack and Heaps CS-502 (EMC) Fall 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

Digression on Stack and Heaps CS-502 (EMC) Fall Heaps in Modern Systems Many modern programming languages require a heap C++, Java, etc. NOT Fortran Typical process environment Heap grows toward stack — but never shrinks! Multi-threaded environments All threads share the same heap Data structures may be passed from one thread to another.

Digression on Stack and Heaps CS-502 (EMC) Fall Heap in Multi-threaded Address Space 0x xFFFFFFFF Virtual address space code (text) static data heap thread 1 stack PC (T2) SP (T2) thread 2 stack thread 3 stack SP (T1) SP (T3) PC (T1) PC (T3) SP PC Heap

Digression on Stack and Heaps CS-502 (EMC) Fall Stacks in Multi-threaded Address Space 0x xFFFFFFFF Virtual address space code (text) static data heap thread 1 stack PC (T2) SP (T2) thread 2 stack thread 3 stack SP (T1) SP (T3) PC (T1) PC (T3) SP PC What’s this?

Digression on Stack and Heaps CS-502 (EMC) Fall Questions?