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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.
