Subprogram

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Subprograms as computational abstractions

Basic notions

General view of a subprogram

A subprogram´s data

Functions and procedures

Example of a function

General form of a function definition

Formal parameters of functions

Actual parameters and function calls

Calling a function in a main program

General form of a main program

Rewriting the factorial function into a procedure

General form of a procedure definition

Generalizing parameter passing

Models of Parameter Passing

Parameter Passing Methods

• Implementation Models:

1. Pass-by-value (in mode)– Either by physical move or access path– Disadvantages of access path method:

• Must write-protect in the called subprogram

• Accesses cost more (indirect addressing)

– Disadvantages of physical move:• Requires more storage (duplicated space)

• Cost of the moves (if the parameter is large)

Parameter Passing Methods

2. Pass-by-result (out mode)– Local’s value is passed back to the caller

– Physical move is usually used

– Disadvantages:

• If value is passed, time and space

• In both cases, order dependence may be a problem

e.g.

procedure sub1(y: int, z: int);

...

sub1(x, x);– Value of x in the caller depends on order of assignments at the return

Parameter Passing Methods

3. Pass-by-value-result (inout mode)– Physical move, both ways– Also called pass-by-copy– Disadvantages:

• Those of pass-by-result• Those of pass-by-value

Parameter Passing Methods

4. Pass-by-reference (inout mode)– Pass an access path– Also called pass-by-sharing– Advantage: passing process is efficient (no copying

and no duplicated storage)– Disadvantages:

• Slower accesses

Parameter Passing Methods

• Pass-by-reference - disadvantages (cont)

b. Allows aliasing:i. Actual parameter collisions:

e.g. procedure sub1(a: int, b: int);

...

sub1(x, x);

Example of Parameter Passing

• Consider the following C function

void swap1(int a, int b){

int temp = a;

a = b;

b = temp;

}• Suppose this function is called with

swap1(c,d);

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Example of Parameter Passing

Consider the following C function (pass by reference)

void swap2(int *a, int *b){

int temp = *a;

*a = *b;

*b = temp;

}

swap2 can be called with

swap2(&c, &d);

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Example of Parameter Passing

Consider the following C++ function (pass by reference)

void swap3(int &a, int &b){

int temp = a;

a = b;

b = temp;

}

swap3 can be called with

swap3(c, d);

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Memory states during program execution

1. At program start 2. Immediately after entering the procedddd 3. During the execution of the body 4. Immediately before leaving the procedur e 5. Immediately after leaving the procedur e

A procedure with in/out parameters

Handling of in/out parameters

Memory states during program execution

Memory states during program execution

1. At program start 2. Immediately after entering the procedddd 3. During the execution of the body 4. Immediately before leaving the procedur e 5. Immediately after leaving the procedur e

Subprogram calls: control flow

Execution trace

Call stack

ทั้ #มา• dddddddd ddddddddddd : RWTH

Aachen University• Concepts of programming

languages : Sebesta, Robert W.