More on FunctionsCS-2301 B-term More on Functions CS-2301, System Programming for Non-majors (Slides include materials from The C Programming Language, 2 nd ed., by Kernighan and Ritchie and from C: How to Program, 5 th ed., by Deitel and Deitel)

More on FunctionsCS-2301 B-term Data Storage in Memory Variables may be automatic or static Automatic variables may only be declared within functions and compound statements Storage allocated when function is entered Storage is released when function returns Arguments and result are (somewhat) like automatic variables Storage is allocated and initialized by caller of function Storage is released after function returns to caller.

More on FunctionsCS-2301 B-term Static variables may be declared within or outside of functions Storage allocated when program is initialized Storage is released when program exits Static variables outside of functions may be visible to linker Compiler sets aside storage for all static variables at compiler or link time Values retained across function calls Initialization must evaluate to compile-time constant Static Data

More on FunctionsCS-2301 B-term Static Variable Examples int j;//static, visible to linker & all functs static float f; // not visible to linker, visible to // to all functions in this program int fcn (float a, int b) { // nothing inside of {} is visible to linker int i = b;//automatic double g;//automatic static double h;//static, not visible to // linker; value retained from call to call body – may access j, f, a, b, i, g, h }//int fcn( … )

More on FunctionsCS-2301 B-term Static Variable Examples (continued) int j;//static, visible to linker & all functs static float f; // not visible to linker, visible to // to all functions in this program int fcn (float a, int b) { // nothing inside of {} is visible to linker int i = b;//automatic double g;//automatic static double h;//static, not visible to // linker; value retained from call to call body – may access j, f, a, b, i, g, h }//int fcn( … ) Declaration outside any function:– always static static storage class:– not visible to linker

More on FunctionsCS-2301 B-term Static Variable Examples (continued) int j;//static, visible to linker & all functs static float f; // not visible to linker, visible to // to all functions in this program int fcn (float a, int b) { // nothing inside of {} is visible to linker int i = b;//automatic double g;//automatic static double h;//static, not visible to // linker; value retained from call to call body – may access j, f, a, b, i, g, h }//int fcn( … ) Inside function:– default is automatic static storage class:– not visible to linker

More on FunctionsCS-2301 B-term Static Variable Examples (continued) int j;//static, visible to linker & all functs static float f; // not visible to linker, visible to // to all functions in this program int fcn (float a, int b) { // nothing inside of {} is visible to linker int i = b;//automatic double g;//automatic static double h;//static, not visible to // linker; value retained from call to call body – may access j, f, a, b, i, g, h }//int fcn( … ) Note: value of h is retained from one call to next

More on FunctionsCS-2301 B-term Extern Variables int j;//static, visible to linker & all functs static float f; // not visible to linker, visible to // to all functions in this program extern float p; // static, defined in another program int fcn (float a, int b) { // nothing inside of {} is visible to linker int i = b;//automatic double g;//automatic static double h;//static, not visible to // linker; value retained from call to call body – may access j, f, a, b, i, g, h, p }//int fcn( … ) extern storage class:– a static variable defined in another C program

More on FunctionsCS-2301 B-term Questions?

More on FunctionsCS-2301 B-term Automatic Variables Arguments & Results Allocated on The Stack Definition – The Stack –A last-in, first-out data structure provided by the operating system for each running program –For temporary storage of automatic variables, arguments, function results, and other stuff Used by all modern programming languages –Even assembly languages for modern processors –… but not Fortran or Cobol!

More on FunctionsCS-2301 B-term Automatic Variables Allocated when function or compound statement is entered Released when function or compound statement is exited Values not retained from execution to next

More on FunctionsCS-2301 B-term Arguments and Results Arguments are values calculated by caller of function Placed on The Stack by caller for use by function Function may assign new value to argument, but … …caller never looks at argument values again! Result is storage allocated by caller On The Stack Assigned by return statement of function For use by caller Arguments & result are removed by caller After function returns, after caller has absorbed return value

More on FunctionsCS-2301 B-term Typical Implementation of The Stack Linear region of memory Stack Pointer “growing” downward Each time some information is pushed onto The Stack, pointer moves downward Each time info is popped off of The Stack, pointer moves back upward

More on FunctionsCS-2301 B-term Typical Memory for Running Program (Windows & Linux) 0x xFFFFFFFF address space program code (text) static data heap (dynamically allocated) stack (dynamically allocated) PC SP

More on FunctionsCS-2301 B-term Typical Memory for Running Program (Windows & Linux) 0x xFFFFFFFF address space program code (text) static data heap (dynamically allocated) stack (dynamically allocated) PC SP Heap to be discussed later in course

More on FunctionsCS-2301 B-term How it works Imagine the following program:– int factorial(int n){ … /* body of factorial function */ … }// factorial Imagine also the caller:– int x = factorial(100); What does compiled code look like?

More on FunctionsCS-2301 B-term 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

More on FunctionsCS-2301 B-term 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 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

More on FunctionsCS-2301 B-term 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? We will explain recursion shortly

More on FunctionsCS-2301 B-term Answer: The Stack Calling function Push return address, space for result, and arguments onto stack Jump to called function Called function Push registers and automatic storage space onto stack Do work of the routine, store result in space pushed above Pop registers and automatic storage off stack Jump to return address left by calling routine

More on FunctionsCS-2301 B-term Typical Address Space (Windows & Linux) 0x xFFFFFFFF Memory address space program code (text) static data heap (dynamically allocated) stack (dynamically allocated) PC SP

More on FunctionsCS-2301 B-term Note Through the magic of operating systems, each running program has its own memory –Complete with stack & everything else Called a process Windows, Linux, Unix, etc.

More on FunctionsCS-2301 B-term Note (continued) Not necessarily true in small, embedded systems Real-time & control systems Mobile phone & PDA Remote sensors, instrumentation, etc. Multiple running programs share a memory Each in own partition with own stack Barriers to prevent each from corrupting others

More on FunctionsCS-2301 B-term Shared Physical Memory OS Kernel stack Process 1 stack Process 2 0x x0000FFFF Physical memory stack Process 3

More on FunctionsCS-2301 B-term Questions?

More on FunctionsCS-2301 B-term Why a Stack? Engineering reason – Computer architectures without stacks are not “rich” enough to support demands of modern computing Multiple running programs at the same time Complex interactions among running programs Modern computer languages Mathematical reason – To support recursive programs Not often by ordinary programmers with ordinary skills Some problems are too hard to solve without it Most notably, the compiler!

More on FunctionsCS-2301 B-term Why a Stack? Engineering reason – Computer architectures without stacks are not “rich” enough to support demands of modern computing Multiple running programs at the same time Complex interactions among running programs Modern computer languages Mathematical reason – To support recursive programs Not often by ordinary programmers with ordinary skills Some problems are too hard to solve without it Most notably, the compiler!

More on FunctionsCS-2301 B-term Why a Stack? Engineering reason – Computer architectures without stacks are not “rich” enough to support demands of modern computing Multiple running programs at the same time Complex interactions among running programs Modern computer languages Mathematical reason – To support recursive programs Not often used by programmers with ordinary skills, but … Some problems are too hard to solve without recursion Most notably, the compiler!

More on FunctionsCS-2301 B-term Why a Stack? Engineering reason – Computer architectures without stacks are not “rich” enough to support demands of modern computing Multiple running programs at the same time Complex interactions among running programs Modern computer languages Mathematical reason – To support recursive programs Not often used by programmers with ordinary skills, but … Some problems are too hard to solve without it Also, advanced games and robotics with “intelligent” objects

More on FunctionsCS-2301 B-term Definition – Recursion When a function calls itself, directly or indirectly When a mathematical or programmatic concept is defined in terms of itself E.g., definition of compound statement in §A9.3, p.223

More on FunctionsCS-2301 B-term Definition – Recursion When a function calls itself, directly or indirectly When a mathematical or programmatic concept is defined in terms of itself E.g., definition of compound statement in §A9.3, p.223 In both cases, something is needed to break the circularity

More on FunctionsCS-2301 B-term Example – Towers of Hanoi Move stack of disks from one peg to another Move one disk at a time Larger disk may never be on top of smaller disk

More on FunctionsCS-2301 B-term Tower of Hanoi Program #include void move (int disks, int a, int c, int b); int main() { int n; printf ("How many disks?"); scanf ("%d", &n); printf ("\n"); move (n, 1, 3, 2); return 0; }// main /* PRE: n >= 0; a, b, and c represent some order of the distinct integers 1, 2, 3 POST: the function displays the individual moves necessary to move n disks from needle a to needle c, using needle b as a temporary storage needle */ void move (int disks, int a, int c, int b) { if (disks > 0) { move (disks-1, a, b, c); printf ("Move one disk from %d to %d\n", a, c); move (disks-1, b, c, a); }// if (disks > 0 return; }//move

More on FunctionsCS-2301 B-term Tower of Hanoi Program #include void move (int disks, int a, int c, int b); int main() { int n; printf ("How many disks?"); scanf ("%d", &n); printf ("\n"); move (n, 1, 3, 2); return 0; }// main /* PRE: n >= 0; a, b, and c represent some order of the distinct integers 1, 2, 3 POST: the function displays the individual moves necessary to move n disks from needle a to needle c, using needle b as a temporary storage needle */ void move (int disks, int a, int c, int b) { if (disks > 0){ move (disks-1, a, b, c); printf ("Move one disk from %d to %d\n", a, c); move (disks-1, b, c, a); }// if (disks > 0 return; }//move The function main – gets number of disks and invokes function move

More on FunctionsCS-2301 B-term Tower of Hanoi Program #include void move (int disks, int a, int c, int b); int main() { int n; printf ("How many disks?"); scanf ("%d", &n); printf ("\n"); move (n, 1, 3, 2); return 0; }// main /* PRE: n >= 0; a, b, and c represent some order of the distinct integers 1, 2, 3 POST: the function displays the individual moves necessary to move n disks from needle a to needle c, using needle b as a temporary storage needle */ void move (int disks, int a, int c, int b) { if (disks > 0){ move (disks-1, a, b, c); printf ("Move one disk from %d to %d\n", a, c); move (disks-1, b, c, a); }// if (disks > 0 return; }//move The function move – where the action is

More on FunctionsCS-2301 B-term Tower of Hanoi Program #include void move (int disks, int a, int c, int b); int main() { int n; printf ("How many disks?"); scanf ("%d", &n); printf ("\n"); move (n, 1, 3, 2); return 0; }// main /* PRE: n >= 0; a, b, and c represent some order of the distinct integers 1, 2, 3 POST: the function displays the individual moves necessary to move n disks from needle a to needle c, using needle b as a temporary storage needle */ void move (int disks, int a, int c, int b) { if (disks > 0){ move (disks-1, a, b, c); printf ("Move one disk from %d to %d\n", a, c); move (disks-1, b, c, a); }// if (disks > 0 return; }//move First move all but one of the disks to temporary peg

More on FunctionsCS-2301 B-term Tower of Hanoi Program #include void move (int disks, int a, int c, int b); int main() { int n; printf ("How many disks?"); scanf ("%d", &n); printf ("\n"); move (n, 1, 3, 2); return 0; }// main /* PRE: n >= 0; a, b, and c represent some order of the distinct integers 1, 2, 3 POST: the function displays the individual moves necessary to move n disks from needle a to needle c, using needle b as a temporary storage needle */ void move (int disks, int a, int c, int b) { if (disks > 0){ move (disks-1, a, b, c); printf ("Move one disk from %d to %d\n", a, c); move (disks-1, b, c, a); }// if (disks > 0 return; }//move Next, move the remaining disk to the destination peg

More on FunctionsCS-2301 B-term Tower of Hanoi Program #include void move (int disks, int a, int c, int b); int main() { int n; printf ("How many disks?"); scanf ("%d", &n); printf ("\n"); move (n, 1, 3, 2); return 0; }// main /* PRE: n >= 0; a, b, and c represent some order of the distinct integers 1, 2, 3 POST: the function displays the individual moves necessary to move n disks from needle a to needle c, using needle b as a temporary storage needle */ void move (int disks, int a, int c, int b) { if (disks > 0){ move (disks-1, a, b, c); printf ("Move one disk from %d to %d\n", a, c); move (disks-1, b, c, a); }// if (disks > 0 return; }//move Finally, move disks from temporary to destination peg

More on FunctionsCS-2301 B-term Tower of Hanoi Program #include void move (int disks, int a, int c, int b); int main() { int n; printf ("How many disks?"); scanf ("%d", &n); printf ("\n"); move (n, 1, 3, 2); return 0; }// main /* PRE: n >= 0; a, b, and c represent some order of the distinct integers 1, 2, 3 POST: the function displays the individual moves necessary to move n disks from needle a to needle c, using needle b as a temporary storage needle */ void move (int disks, int a, int c, int b) { if (disks > 0){ move (disks-1, a, b, c); printf ("Move one disk from %d to %d\n", a, c); move (disks-1, b, c, a); }// if (disks > 0 return; }//move Notice that move calls itself twice, but with one fewer disks each time

More on FunctionsCS-2301 B-term Implementation on The Stack n SP Stack entry for main …

More on FunctionsCS-2301 B-term Implementation on The Stack (continued) n SP Stack entry for main … return address a = 1 c = 3 b = 2 disks = n Args for call to move

More on FunctionsCS-2301 B-term Tower of Hanoi Program #include void move (int disks, int a, int c, int b); int main() { int n; printf ("How many disks?"); scanf ("%d", &n); printf ("\n"); move (n, 1, 3, 2); return 0; }// main /* PRE: n >= 0; a, b, and c represent some order of the distinct integers 1, 2, 3 POST: the function displays the individual moves necessary to move n disks from needle a to needle c, using needle b as a temporary storage needle */ void move (int disks, int a, int c, int b) { if (disks > 0){ move (disks-1, a, b, c); printf ("Move one disk from %d to %d\n", a, c); move (disks-1, b, c, a); }// if (disks > 0 return; }//move

More on FunctionsCS-2301 B-term Implementation on The Stack (continued) n SP Stack entry for main … return address a = 1 c = 3 b = 2 disks = n Args for call to move return address a = 1 c = 2 b = 3 disks = n-1 Args for 2 nd call to move

More on FunctionsCS-2301 B-term Implementation on The Stack (continued) n SP Stack entry for main … return address a = 1 c = 3 b = 2 disks = n Args for call to move return address a = 1 c = 2 b = 3 disks = n-1 Args for 2 nd call to move Args for subsequent calls to move go here

More on FunctionsCS-2301 B-term Implementation on The Stack (continued) n SP Stack entry for main … return address a = 1 c = 3 b = 2 disks = n Args for call to move return address a = 1 c = 2 b = 3 disks = n-1 Args for 2 nd call to move Eventually, disks = 0, so no more calls to move All existing calls return

More on FunctionsCS-2301 B-term Implementation on The Stack (continued) n SP Stack entry for main … return address a = 1 c = 3 b = 2 disks = n Args for call to move Leaving stack like this again

More on FunctionsCS-2301 B-term Tower of Hanoi Program #include void move (int disks, int a, int c, int b); int main() { int n; printf ("How many disks?"); scanf ("%d", &n); printf ("\n"); move (n, 1, 3, 2); return 0; }// main /* PRE: n >= 0; a, b, and c represent some order of the distinct integers 1, 2, 3 POST: the function displays the individual moves necessary to move n disks from needle a to needle c, using needle b as a temporary storage needle */ void move (int disks, int a, int c, int b) { if (disks > 0){ move (disks-1, a, b, c); printf ("Move one disk from %d to %d\n", a, c); move (disks-1, b, c, a); }// if (disks > 0 return; }//move

More on FunctionsCS-2301 B-term Implementation on The Stack (continued) n SP Stack entry for main … return address a = 1 c = 3 b = 2 disks = n Args for call to move move now moves one disk from peg1 to peg3, and then calls move again!

More on FunctionsCS-2301 B-term Implementation on The Stack (continued) n SP Stack entry for main … return address a = 1 c = 3 b = 2 disks = n Args for call to move return address a = 3 c = 2 b = 1 disks = n-1 Args for call to move disks back from peg3 to peg2

More on FunctionsCS-2301 B-term Implementation on The Stack (continued) n SP Stack entry for main … return address a = 1 c = 3 b = 2 disks = n Args for call to move return address a = 3 c = 2 b = 1 disks = n-1 Args for call to move back Args for subsequent calls to move back go here

More on FunctionsCS-2301 B-term Implementation on The Stack (continued) n SP Stack entry for main … return address a = 1 c = 3 b = 2 disks = n Args for call to move return address a = 3 c = 2 b = 1 disks = n-1 Args for 2 nd call to move Eventually, disks = 0, so no more calls to move All existing calls return

More on FunctionsCS-2301 B-term Implementation on The Stack (continued) n SP Stack entry for main … return address a = 1 c = 3 b = 2 disks = n Args for call to move Leaving stack like this again, but with all disks moved to peg 3

More on FunctionsCS-2301 B-term Implementation on The Stack n SP Stack entry for main … move returns back to main

More on FunctionsCS-2301 B-term Fundamental Principle Stack provides a simple mechanism for implementing recursive functions Arbitrary numbers of intermediate functions Eventually, something must break the circularity of recursion Or stack will grow until it overflows memory!

More on FunctionsCS-2301 B-term Note on Recursive Functions Some simple recursive functions can be “unwound” into loops – e.g., int factorial(int n) { return (n <= 1) ? 1 : (n * factorial(n-1)); }//factorial is equivalent to int factorial(int n) { int product = 1; for (int k=1; k <= n; k++) product *= k; return product } //factorial

More on FunctionsCS-2301 B-term Note on Recursive Functions (continued) Other recursive functions are much too difficult to understand if unwound E.g., Tower of Hanoi Keeping track of which disk is on which peg requires superhuman intellect, very complex code!

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