Subprograms - PowerPoint PPT Presentation

1 / 31

About This Presentation

Title:

Subprograms

Description:

the value of the parameter is accessible by the callee at any time. Changes to that value will only be known by the caller after the subprogram ends ... – PowerPoint PPT presentation

Number of Views:68

Avg rating:3.0/5.0

Slides: 32

Provided by: juancarl

Category:

more less

Transcript and Presenter's Notes

Title: Subprograms

1
Subprograms

Juan Carlos Guzmán
CS 3123 Programming Languages Concepts
Southern Polytechnic State University

2
Subprogram

Process abstraction
Definition
Header
interface
parameters
also known as
prototype
signature
Body
implementation
Call

int fact(int x)
if (x0)
return 1
else
return xfact(x-1)

3
Kinds of subprograms

Procedure
a collection of statements in a parameterized
computation
communication caller-callee (called subprogram)
using parameters, and data accessible by both
no meaningful return value
Functions
Same as procedures, but return values

4
Parameter Passing

Formal parameters
those that appear in the subprograms header
Actual parameters
those that are used to call the function
Positional vs keyword parameters
(same issues as tuples vs. records)

5
Parameter Passing Styles

Direction of communication
1) actual parameters to formal parameters (in
mode)
2) formal parameters to actual parameters (out
mode)
3) both (inout mode)
Communication between the caller and the callee
a) at the onset of the call
b) throughout the call
c) when the call ends

6
Parameter Passing Styles

Pass by value (1a)
the value of the parameter is accessible by the
callee at any time
Pass by result (2c)
the value of the parameter is accessible by the
caller after the subprogram ends
Pass by value-result (3ac)
the value of the parameter is accessible by the
callee at any time. Changes to that value will
only be known by the caller after the subprogram
ends
Pass by reference (3b)
value of the parameter is shared between the
caller and the callee
Pass by name (3b)
more complex than the rest...

7
Examples

swap(int x, int y)
int z
z x
x y
y z
main()
int a5
int b7
swap(a,b)
printf("ad, bd",a,b)

Pass by value
a5, b7
Pass by value-result
a7, b5
Pass by reference
a7, b5

8
Examples

int a5
f(int x)
x
printf("xd, ad,x,a)
main()
f(a)
printf(", ad",a)

Pass by value
x6, a5, a5
Pass by value-result
x6, a5, a6
Pass by reference
x6, a6, a6

9
Examples

swap(int x, int y)
int z
z x
x y
y z
main()
int i0
int A31,2,3
swap(i,Ai)
print(id, Ad",i,A)

Pass by value
i0, A1,2,3
Pass by value-result
i1, A0,2,3
Pass by reference
i1, A0,2,3
Pass by name
i1, A1,0,3

10
Subprogram Implementation

Compilation of programs written in high-level
languages usually have several segments at run
time
Code
code (main, subprograms) resides here
Global data
all global declarations reside here
persistent declarations (static) reside here
Stack
keeps data local to subprograms
Heap
keeps dynamically allocated data

11
Stack

The stack keeps info local to subprograms
It is conformed of records
Each record should contain
arguments
local variables
temporaries
control info
Records are of different size!

Not accessible from the program
12
Elements ofan Activation Record Instance

Arguments
fixed or variable number of arguments
interface caller-callee
Local Variables
Temporaries
Added by the compiler

Control Information
Where to find non-local data
static link
Previous ARI
dynamic link
Return address
what to be executed after this routine ends

13
Activation Records
Temporaries
Local Vars
Control Info
Static Link Dynamic Link
Pointer to Code
Arguments
14
Sample Program

program SAMPLE
procedure C forward
procedure A(flag boolean)
procedure B
...
A(false)
end B
begin A
if flag
then B
else C
...
end A

procedure C
procedure D
... ??????? ?
end D
begin C
...
D
end C
begin SAMPLE
A(true)
...
end SAMPLE

15
Runtime Stack at ?

program SAMPLE
procedure C forward
procedure A(flag boolean)
procedure B
...
A(false)
end B
begin A
if flag
then B
else C
...
end A
procedure C
procedure D
... ??????? ?
end D
begin C

Globals
D
C
A
B
Heap
A
Sample
Static Link Dynamic Link
Pointer to Code
O/S
16
Run of the Sample Program

program SAMPLE
procedure C forward
procedure A(flag boolean)
procedure B
...
A(false)
end B
begin A
if flag
then B
else C
...
end A
procedure C
procedure D
... ??????? ?
end D
begin C

Globals
Heap
Sample
Static Link Dynamic Link
Pointer to Code
?
O/S
17
Run of the Sample Program

program SAMPLE
procedure C forward
procedure A(flag boolean)
procedure B
...
A(false)
end B
begin A
if flag
then B
else C
...
end A
procedure C
procedure D
... ??????? ?
end D
begin C

Globals
?
Heap
A
Sample
Static Link Dynamic Link
Pointer to Code
O/S
18
Run of the Sample Program

program SAMPLE
procedure C forward
procedure A(flag boolean)
procedure B
...
A(false)
end B
begin A
if flag
then B
else C
...
end A
procedure C
procedure D
... ??????? ?
end D
begin C

Globals
?
B
Heap
A
Sample
Static Link Dynamic Link
Pointer to Code
O/S
19
Run of the Sample Program

program SAMPLE
procedure C forward
procedure A(flag boolean)
procedure B
...
A(false)
end B
begin A
if flag
then B
else C
...
end A
procedure C
procedure D
... ??????? ?
end D
begin C

Globals
?
A
B
Heap
A
Sample
Static Link Dynamic Link
Pointer to Code
O/S
20
Run of the Sample Program

program SAMPLE
procedure C forward
procedure A(flag boolean)
procedure B
...
A(false)
end B
begin A
if flag
then B
else C
...
end A
procedure C
procedure D
... ??????? ?
end D
begin C

Globals
C
A
B
Heap
A
?
Sample
Static Link Dynamic Link
Pointer to Code
O/S
21
Run of the Sample Program

program SAMPLE
procedure C forward
procedure A(flag boolean)
procedure B
...
A(false)
end B
begin A
if flag
then B
else C
...
end A
procedure C
procedure D
... ??????? ?
end D
begin C

Globals
D
C
A
B
Heap
A
?
Sample
Static Link Dynamic Link
Pointer to Code
O/S
22
Display

Accessing nonlocal, nonglobal variables requires
several indirections through the static chain
The display solves this--it keeps all needed
environments in an array for easy access
There is no display element for global data

23
Stack at ?, using Display

program SAMPLE
procedure C forward
procedure A(flag boolean)
procedure B
...
A(false)
end B
begin A
if flag
then B
else C
...
end A
procedure C
procedure D
... ??????? ?
end D
begin C

Globals
D
C
A
B
Heap
A
?
Display
Sample
Dynamic Link Pointer to Code
O/S
24
Issues on Display

The display size is computed at compile time--it
is the largest lexical depth in the program
The need for static chain is eliminated, but now
there is the need to keep the old display
pointer

25
Stack at ?, using Display