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CSE 452: Programming Languages
Subprograms
2Organization of Programming Languages-Cheng (Fall 2004)
Outline
? Subprograms?Parameter passing?Type checking?Using multidimensional arrays as parameters?Using subprograms as parameters?Overloaded subprograms?Generic subprograms? Implementation
3Organization of Programming Languages-Cheng (Fall 2004)
Parameter Passing
? Pass-by-value? Pass-by-result? Pass-by-value-result? Pass-by-reference? Pass-by-name
2
4Organization of Programming Languages-Cheng (Fall 2004)
Parameter Passing in PL
? Fortran? Always use inout-mode model of parameter passing? Before Fortran 77, mostly used pass-by -reference? Later implementations mostly use pass-by -value-result
? C ? mostly pass by value? Pass-by -reference is achieved using pointers as
parametersint *p = { 1, 2, 3 };void change( int *q) {
q[0] = 4;}main() {
change(p); /* p[0] = 4 after calling the change function */
}
5Organization of Programming Languages-Cheng (Fall 2004)
Parameter Passing in PL
? C? Pass-by reference: value of pointer is copied to the
called function and nothing is copied back
#include < stdio.h>void swap (int *p, int *q){
int *temp;temp = p;p = q;q = temp;
}main() {
int p[] = {1, 2, 3};int q[] = {4, 5, 6};int i;
swap (p, q); }
6Organization of Programming Languages-Cheng (Fall 2004)
Parameter Passing in PL
? C++ ? includes a special pointer type called a reference type
void GetData(double &Num1, const int &Num2) {int temp;for (int i=0; i<Num2; i++) {
cout << “Enter a number: “;cin >> temp;if (temp > Num1)
{ Num1 = temp; return; }}
? Num1 and Num2 are passed by reference? const modifier prevents a function from changing the
values of reference parameters? Referenced parameters are implicitly dereferenced? Why do we need a constant reference parameter?
3
7Organization of Programming Languages-Cheng (Fall 2004)
Parameter Passing in PL
? Ada? Reserved words: in, out, in out (in is the default mode)
procedure temp(A : in out Integer; B : in Integer; C : in Integer )
? out mode can be assigned but not referenced? in mode can be referenced but not assigned
? in out can be both referenced and assigned ? Fortran
? Semantic modes are declared using Intent attributeSubroutine temp(A, B, C)
Integer, Intent(Inout) :: A
Integer, Intent(In) :: BInteger, Intent(Out) :: C
8Organization of Programming Languages-Cheng (Fall 2004)
Parameter Passing in PL
? Perl?Actual parameters are implicitly placed in a
predefined array named @_sub foo {
local $i, $a=0, $b = 1;for ($i=0; $i<scalar(@_); $i++) {
$a = $a + $_[$i];$b = $b * $_[$i];
}return ($a, $b);
}…($a, $b) = foo(1, 2, 3);
9Organization of Programming Languages-Cheng (Fall 2004)
Type Checking
? Ansi C: users can choose whether parameters should be type-checked
#include <stdio.h>
double count1(x)double x; // avoids type checking
{ return x * 2; } // may generate nonsense: count1(y)
double count2(double x) // prototype method{ return x * 2; } // can coerce actual params: count2(y)
main() {double x = 30.0;int y = 30;printf("count1 : %f\n", count1(x));printf("count2 : %f\n", count2(x));printf("count1 : %f\n", count1(y));printf("count2 : %f\n", count2(y));
}
Output:
count1 : 60.000000
count2 : 60.000000
count1 : 0.000000
count2 : 60.000000
4
10Organization of Programming Languages-Cheng (Fall 2004)
Implementing Parameter Passing
Code
Data
Heap
Stack
Memory contents
program code
global and static data
Dynamically allocated variables
local data
11Organization of Programming Languages-Cheng (Fall 2004)
Implementing Parameter Passing
? Pass by Value? Values copied into stack locations? Stack locations serve as storage for corresponding
formal parameters
? Pass by Result? Implemented opposite of pass-by -value? Values assigned to actual parameters are placed in the
stack, where they can be retrieved by calling program unit upon termination of called subprogram
? Pass by Value Result? Stack location for parameters is initialized by by the
call and then copied back to actual parameters upon termination of called subprogram
12Organization of Programming Languages-Cheng (Fall 2004)
Implementing Parameter Passing
? Pass by Reference ? Regardless of type of parameter, put the address in the
stack? For literals, address of literal is put in the stack
? For expressions, compiler must build code to evaluate expression before the transfer of control to the called subprogram? Address of memory cell in which code places the result of its
evaluation is then put in the stack
? Compiler must make sure to prevent called subprogram from changing parameters that are literals or expressions
? Access to formal parameters is by indirect addressing from the stack location of the address
5
13Organization of Programming Languages-Cheng (Fall 2004)
Implementing Parameter Passing
Main program calls sub(w,x,y,z) where w is passed by value, x is passed by result,
y is passed by value -result, and z is passed by reference
sub(w,x,y,z) sub(a,b,c,d)
val
res
val-res
ref
14Organization of Programming Languages-Cheng (Fall 2004)
Implementing Parameter Passing
? Pass by Name? run-time resident code segments or subprograms
evaluate the address of the parameter? called for each reference to the formal?Very expensive, compared to pass by reference
or value-result
15Organization of Programming Languages-Cheng (Fall 2004)
Multidimensional Arrays as Parameters
? C:? Uses row major order for matrices
address(mat[i, j]) = address(mat[0,0]) + i*num_columns + j
? Must specify num_columns but not num_rows
void fun (int matrix[][10]) { … }void main() {
int mat[5][10];fun(mat);…
}? Does not allow programmers to write function that
accepts different number of columns? Alternative: use pointers
6
16Organization of Programming Languages-Cheng (Fall 2004)
? Ada: type Mat_Type is array (Integer range <>
Integer range<>) of Float;Mat1 : Mat_Type(1..100, 1..20);
function Sumer(Mat : in Mat_Type) return Flat isSum : Float := 0.0;beginfor Row in Mat’range(1) loop
for Col in Mat’range(2) loopSum := Sum + Mat(Row, Col);
end loop;end loop;return Sum;
end Sumer;
Multidimensional Arrays as Parameters
No need to specify
size of array
Use range attribute
to obtain size of
arrray
17Organization of Programming Languages-Cheng (Fall 2004)
Multidimensional Arrays as Parameters
? Fortran?Array parameters must have declaration after the
header
Subroutine Sub (Matrix, Rows, Cols, Result)Integer, Intent(In) :: Rows, ColsReal, Dimension(Rows, Cols), Intent(In) :: MatrixReal, Intent (In) :: Result…
End Subroutine Sub
18Organization of Programming Languages-Cheng (Fall 2004)
Subprogram Names as Parameters
? Issues:1. Are parameter types checked?
? Early Pascal and FORTRAN 77 do not; later versions of Pascal andFORTRAN 90 do
? Ada does not allow subprogram parameters? Java does not allow method names to be passed as parameters? C and C++ - pass pointers to functions; parameters can be type
checked
2. What is the correct referencing environment for a subprogram that was sent as a parameter?? Environment of the call statement that enacts the passed subprogram
? Shallow binding? Environment of the definition of the passed subprogram
? Deep binding? Environment of the call statement that passed the subprogram as
actual parameter? Ad hoc binding (Has never been used)
7
19Organization of Programming Languages-Cheng (Fall 2004)
Subprogram Names as Parameters
function sub1() {var x;function sub2() {
alert(x);
};function sub3() {
var x;x = 3;sub4(sub2);
}function sub4(subx) {
var x;x = 4;subx();
};x = 1;sub3();
};
Shallow binding:? Referencing
environment of sub2 is that of sub4
Deep binding? Referencing
environment of sub2 is that of sub1
Ad-hoc binding? Referencing
environment of sub2 is that of sub3
20Organization of Programming Languages-Cheng (Fall 2004)
Overloaded Subprograms
? A subprogram that has the same name as another subprogram in the same referencing environment
? Every version of the overloaded subprogram must have a unique protocol? Must be different from others in the number, order, or
types of its parameters, or its return type (if it is a function)
? C++, Java, Ada, and C# include predefined overloaded subprograms – e.g., overloaded constructors in C++
? Overloaded subprograms with default parameters can lead to ambiguous subprogram calls
void foo( float b = 0.0 );void foo();…foo(); /* call is ambiguous; may lead to compilation error
*/
21Organization of Programming Languages-Cheng (Fall 2004)
Generic (Polymorphic) Subprograms
? Polymorphism: ? Increase reusability of software
?Types:?Ad hoc polymorphism = Overloaded subprogram?Parametric polymorphism
? Provided by a subprogram that takes a generic parameter that is used in a type expression
? Ada and C++ provide compile-time parametric polymorphism
8
22Organization of Programming Languages-Cheng (Fall 2004)
Generic Subprogramsgenerictype Index_Type is (<>);type Element_Type is private;type Vector is array (Integer range <>) of Element_Type;
procedure Generic_Sort(List : in out Vector);procedure Generic_Sort(List : in out Vector) isTemp : Element_Type;beginfor Top in List'First .. Index_Type’Pred(List’Last ) loop for Bottom in Index_Type’Succ(Top ) .. List’Last loop
if List(Top) > List(Bottom) thenTemp := List (Top);List(Top) := List(Bottom);List(Bottom) := Temp;
end if;end loop; -- for Bottom ...
end loop; -- for Top ...end Generic_Sort;
Example:
procedure Integer_Sort is new Generic_Sort(
Index_Type => Integer;Element_Type => Integer;Vector => Int_Array);
23Organization of Programming Languages-Cheng (Fall 2004)
Generic Subprograms
template <class Type>void generic_sort(Type list[], int len) {
int top, bottom;Type temp;for (top = 0; top < len - 2; top++)for (bottom = top + 1; bottom < len - 1; bottom++) {if (list[top] > list[bottom]) {temp = list [top];list[top] = list[bottom];list[bottom] = temp;} //** end of for (bottom ...
} //** end of generic_sort
float flt_list[100]; ...generic_sort(flt_list, 100); // Implicit instantiation
24Organization of Programming Languages-Cheng (Fall 2004)
Implementing Subprograms
? The subprogram call and return operations are together called subprogram linkage
? Implementation of subprograms must be based on semantics of subprogram linkage
? Implementation:?Simple subprograms
? no recursion, use only static local variables
?Subprograms with stack-dynamic variables?Nested subprograms
9
25Organization of Programming Languages-Cheng (Fall 2004)
Simple Subprograms
? Simple ? subprograms are not nested and all local variables are static? Example: early versions of Fortran
? Call Semantics require the following actions:? Save execution status of current program unit? Carry out parameter passing process? Pass return address to the callee? Transfer control to the callee
? Return Semantics require the following actions:? If pass by value-result or out-mode, move values of those parameters
to the corresponding actual parameters? If subprogram is a function, move return value of function to a place
accessible to the caller? Restore execution status of caller? Transfer control back to caller
26Organization of Programming Languages-Cheng (Fall 2004)
Simple Subprograms
? Required Storage: ? Status information of the caller
? Parameters
? return address? functional value (if it is a function)
? Subprogram consists of 2 parts:? Subprogram code
? Subprogram data ? The format, or layout, of the noncode part of an executing
subprogram is called an activation record? An activation record instance (ARI) is a concrete example of
an activation record (the collection of data for a particular subprogram activation)
? Code and Activation record of a program with simple subprograms
?Activation record instance for simple subprograms has fixed size. Therefore, it can be statically allocated
?Since simple subprograms do not support recursion, there can be only one active version of a given subprogram
10
28Organization of Programming Languages-Cheng (Fall 2004)
Subprograms with Stack-Dynamic Variables
? Compiler must generate code to cause implicit allocation and deallocation of local variables
Local variables
Parameters
Dynamic link
Return address
Points to top of activation record instance of caller
Activation record instance
Run-time stackTop of the stack
Pointer to code segment of the caller and an offset address of the instruction following the call
29Organization of Programming Languages-Cheng (Fall 2004)
Subprograms with Stack-Dynamic Variables
void sub(float total, int part) {int list[4];float sum;…
}
Parameter
Parameter
Dynamic link
Return address
Local variable
Local variable
Local variable
Local variable
Local variable
total
part
list[0]
list[1]
list[2]
list[3]
sum
30Organization of Programming Languages-Cheng (Fall 2004)
Example: without Recursionvoid A(int X) {
int Y;…C(Y);
}void B(float R) {
int S, T;…A(S);…
}void C(int Q) {
…}void main() {
float P;…B(P);…
}
2
1
3
Collection of dynamic links present in the stack at any given time is called the dynamic chain
11
31Organization of Programming Languages-Cheng (Fall 2004)
Subprograms with Stack-Dynamic Variables
? Recursion adds possibility of multiple simultaneous activations of a subprogram?Each activation requires its own copy of formal
parameters and dynamically allocated local variables, along with return address
32Organization of Programming Languages-Cheng (Fall 2004)
Subprograms with Recursion
int factorial (int n) {…
if (n <= 1) return 1;
elsereturn n*factorial(n-1);
…
}void main() {
int value;value = factorial(3);…
}
33Organization of Programming Languages-Cheng (Fall 2004)
Subprograms with Recursion
int factorial (int n) {…
if (n <= 1) return 1;
elsereturn n*factorial(n-1);
…
}void main() {
int value;value = factorial(3);…
}
12
34Organization of Programming Languages-Cheng (Fall 2004)
Subprograms with Recursion
int factorial (int n) {…
if (n <= 1) return 1;
elsereturn n*factorial(n-1);
…
}void main() {
int value;value = factorial(3);…
}
35Organization of Programming Languages-Cheng (Fall 2004)
Subprograms with Recursion
int factorial (int n) {…
if (n <= 1) return 1;
elsereturn n*factorial(n-1);
…
}void main() {
int value;value = factorial(3);…
}
1
2
3
36Organization of Programming Languages-Cheng (Fall 2004)
Subprograms with Recursion
int factorial (int n) {…
if (n <= 1) return 1;
elsereturn n*factorial(n-1);
…
}void main() {
int value;value = factorial(3);…
}
1
2
3
13
37Organization of Programming Languages-Cheng (Fall 2004)
Subprograms with Recursion
int factorial (int n) {…
if (n <= 1) return 1;
elsereturn n*factorial(n-1);
…
}void main() {
int value;value = factorial(3);…
}
1
2
3
38Organization of Programming Languages-Cheng (Fall 2004)
Subprograms with Recursion
int factorial (int n) {…
if (n <= 1) return 1;
elsereturn n*factorial(n-1);
…
}void main() {
int value;value = factorial(3);…
}
1
2
3
39Organization of Programming Languages-Cheng (Fall 2004)
Nested Subprograms
? Support for static scoping? Implemented using static link (also called static
scope pointer), which points to the bottom of the activation record instance of its static parent
14
40Organization of Programming Languages-Cheng (Fall 2004)
Nested Subprograms? Static chain:
? links all static ancestors of executing subprogram? Static_depth
? an integer associated with static scope that indicates how deeply it is nested in outermost scope
? Chain offset? Difference between static_depth of procedure containing reference to
variable x and static_depth of procedure containing declaration of x
procedure A is procedure B is
procedure C is …
end; -- of C…
end; -- of B…
end; -- of A
?Static_depths of A, B, and C are 0, 1, and 2, respectively
?If procedure C references a variable declared in A, the chain_offset of that reference is 2
41Organization of Programming Languages-Cheng (Fall 2004)
Nested Subprograms
program MAIN_2;var X : integer;procedure BIGSUB;
var A, B, C : integer;procedure SUB1;
var A, D : integer;begin { SUB1 }A := B + C; <-----------------1end; { SUB1 }
procedure SUB2(X : integer);var B, E : integer;procedure SUB3;
var C, E : integer;begin { SUB3 }SUB1;E := B + A: <-------------2end; { SUB3 }
begin { SUB2 }SUB3;A := D + E; <----------------3end; { SUB2 }
begin { BIGSUB }SUB2(7);end; { BIGSUB }
beginBIGSUB;end. { MAIN_2 }
Calling sequence:
Main_2 calls BIGSUB
BIGSUB calls Sub2
Sub2 calls Sub3
Sub3 calls Sub1
Exampleprogram MAIN_2;var X : integer;procedure BIGSUB;
var A , B, C : integer;procedure SUB1;
var A , D : integer;begin { SUB1 }A := B + C; <-----------------1end; { SUB1 }
procedure SUB2(X : integer);var B, E : integer;procedure SUB3;
var C, E : integer;begin { SUB3 }SUB1;E := B + A: <-------------2end; { SUB3 }
begin { SUB2 }SUB3;A := D + E; <----------------3end; { SUB2 }
begin { BIGSUB }SUB2(7);end; { BIGSUB }
beginBIGSUB;end. { MAIN_2 }
References to A:1: (0,3) (local)2: (2,3) (two levels away)3: (1,3) (one level away)
15
43Organization of Programming Languages-Cheng (Fall 2004)
Nested Subprograms
? At position 1 in SUB1:? A - (0, 3) ============> (chain_offset, local_offset)? B - (1, 4)? C - (1, 5)
? At position 2 in SUB3:? E - (0, 4)? B - (1, 4)? A - (2, 3)
? At position 3 in SUB2:? A - (1, 3)? D - an error ====? ARI for sub1 has been removed? E - (0, 5)
44Organization of Programming Languages-Cheng (Fall 2004)
Nested Subprograms
? Drawbacks? A nonlocal reference is slow if the number of scopes
between the reference and the declaration of the referenced variable is large
? Time-critical code is difficult, because the costs of nonlocal references are hard to estimate
? Displays? Alternative to static chains? Store static links in a single array called display, instead
of storing in the activation records? Accesses to nonlocals require exactly two steps for every
access, regardless of the number of scope levels? Link to correct activation record is found using a statically
computed value called the display_offset? Compute local_offset within activation record instance
45Organization of Programming Languages-Cheng (Fall 2004)
Blocks
? User-specified local scope for variables{
int temp; temp = list[upper];list[upper] = list[lower];list[lower] = temp;
}
? Blocks can be implemented using static chain? Blocks are treated as parameterless subprograms that are
always called from same place in the program? Every block has an activation record? An instance is created every time a block is executed
? Alternative implementation? Amount of space can be allocated statically? Offsets of all block variables can be statically computed, so block
variables can be addressed exactly as if they were local variables
16
46Organization of Programming Languages-Cheng (Fall 2004)
Blocks
void main() {int x, y, z;while (…) {
int a, b, c;…while (…) {
int d, e;…
}}while (…) {
int f, g;…
}…
}Activation record
instance for Main
x
y
z
a and f
b and g
c
d
Variables occupy same locations
e
47Organization of Programming Languages-Cheng (Fall 2004)
Subprogram Implementation
? Activation record on the stack? Parameters? Return address? Local variables? Static link? Dynamic link
int factorial ( int n) {
…
if (n <= 1) return 1;
else
return n*factorial(n-1);
…
}
void main() {
int value;
value = factorial(3);
…}
48Organization of Programming Languages-Cheng (Fall 2004)
Subprogram Implementation
? Bad design of subprogram implementation may result in network security problems
? Buffer overflow attack?A type of vulnerability used by hackers to
compromise the integrity of a system
?Problem is due to ? Lack of safety feature in language design? bad coding by programmers
17
49Organization of Programming Languages-Cheng (Fall 2004)
Buffer overflow attack
? The effectiveness of the buffer overflow attack has been common knowledge in software circles since the 1980’s
? The Internet Worm used it in November 1988 to gain unauthorized access to many networks and systems nationwide
? Still used today by hacking tools to gain “root” access to otherwise protected computers
? The fix is a very simple change in the way we write array accesses; unfortunately, once code that has this vulnerability is deployed in the field, it is nearly impossible to stop a buffer overflow attack
50Organization of Programming Languages-Cheng (Fall 2004)
Overview of Buffer Overflow Attacks
? The buffer overflow attack exploits a common problem in many programs.
? In several high-level programming languages such as C, “boundary checking”, i.e. checking to see if the length of a variable you are copying is what you were expecting, is not done.
void myFunction(char *str) {
char bufferB[16];
strcpy(bufferB, str);
}
void main(){
char bufferA[256];
myFunction(bufferA);
}
51Organization of Programming Languages-Cheng (Fall 2004)
void myFunction(char *str) {
char bufferB[16];
strcpy(bufferB, str);
}
void main(){
char bufferA[256];
myFunction(bufferA);
}
Overview of Buffer Overflow Attacks
?main() passes a 256 byte array to myFunction(), and myFunction() copies it into a 16 byte array!
?Since there is no check on whether bufferB is big enough, the extra data overwrites other unknown space in memory.
?This vulnerability is the basis of buffer overflow attacks
?How is it used to harm a system? ? It modifies the system stack
18
52Organization of Programming Languages-Cheng (Fall 2004)
void main(){
char bufferA[256];
myFunction(bufferA);
}
bufferA
Overview of Buffer Overflow Attacks
Stack content
53Organization of Programming Languages-Cheng (Fall 2004)
void main(){
char bufferA[256];
myFunction(bufferA);
}
Overview of Buffer Overflow Attacks
void myFunction(char *str) {
char bufferB[16];
strcpy(bufferB, str);
}
bufferA
Return Address to Main
Dynamic link
Str
bufferB
Stack content
OS data
54Organization of Programming Languages-Cheng (Fall 2004)
void main(){
char bufferA[256];
myFunction(bufferA);
}
Overview of Buffer Overflow Attacks
void myFunction(char *str) {
char bufferB[16];
strcpy(bufferB, str);
}
bufferA
Return Address to Main
Dynamic link
Str
bufferB
Stack content
OS data
May overwrite the return address!!
This region is now
contaminated with data from str
19
55Organization of Programming Languages-Cheng (Fall 2004)
Overview of Buffer Overflow Attacks
bufferA
Stack content
Malicious Code
New Address
? If the content of str is carefully selected, we can point the return address to a piece of code we have written
? When the system returns from the function call, it will begin executing the malicious code
56Organization of Programming Languages-Cheng (Fall 2004)
A Possible Solution
void main(){
char bufferA[256];
myFunction(bufferA , 256);
}
void myFunction(char *str, int len)
{
char bufferB[16];
if (len <= 16)
strcpy(bufferB , str);
}
57Organization of Programming Languages-Cheng (Fall 2004)
Buffer Overflow Attack