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

• 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
Display
null
Sample
null
Prev. Display Ptr. Dynamic
Link Pointer to Code
?
O/S
26
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
?
Heap
A
Display
null
Sample
Prev. Display Ptr. Dynamic
Link Pointer to Code
O/S
27
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
?
B
Heap
A
Display
Sample
Prev. Display Ptr. Dynamic
Link Pointer to Code
O/S
28
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
?
A
B
Heap
A
Display
Sample
Prev. Display Ptr. Dynamic
Link Pointer to Code
O/S
29
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
C
A
B
Heap
A
Display
?
Sample
Prev. Display Ptr. Dynamic
Link Pointer to Code
O/S
30
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
Prev. Display Ptr. Dynamic
Link Pointer to Code
O/S
31
Other Issues

• First activation record is treated specially
• it should contain info to return to the O/S
• global variables should not be put there--they
  should be in the globals segment