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CHAPTER 9. FUNCTIONS AND PARAMETERS using barcode creator for .net vs 2010 control to generate, create datamatrix 2d barcode image in .net vs 2010 applications. Microsoft Windows Official Website Call-by-Value-and-Return We can combine call-b y-value and call-by-return to achieve two-way communication between called and caller, with no accessing restrictions inside the subroutine. At rst look, call-by-value-andreturn seems more complex than call-by-reference, and it seems to o er no advantages. While this is true in an isolated program on a simple machine, it does not hold in more complex environments.

Two factors, hardware complexity and concurrency, make call-by-value-and-return the preferred mechanism. Consider a machine with a partitioned memory, or a program that is being executed on a networked system. In such hardware environments, nearby memory is fast (and therefore cheap) to access, and more distant memory costs more.

A function that executes using only local storage will be more e cient than one that accesses data in another memory segment. When call-by-value is used, the data is copied from the caller s memory into the local memory. Although this copying operation takes time, much more time may be saved by avoiding out-of-segment references to the parameter.

Call-by-value is simply more e cient once the argument has been copied. This is true on any partitioned architecture, even an IBM PC, although the savings may be minor. However, for a program on a network that is processing data stored at a remote location, the savings would be substantial, and the di erence in response time would be important.

The di erence between call-by-reference and call-by-value-and-return is very important in an application where two or more processes concurrently access the same data object. Suppose that processes P1 and P2 both have received argument ARG as a reference parameter. Using call-byvalue-and-return, the answer is always one of three things: If the rst process to begin, P1, ends before the second one, P2, starts, the nal value in ARG is just the result of ordinary, sequential execution.

If P1 starts, then P2 starts, then P1 returns, then P2 returns, the result of P1 s execution is wiped out. The value in ARG is the answer from P2, as if P1 had never been called. If P1 starts, then P2 starts, then P2 returns, then P1 returns, the result of P2 s execution is wiped out.

The value in ARG is the answer from P1, as if P2 had never been called. In any case, the nal result is some value that was correctly and meaningfully computed. It may seem very undesirable to have the possibility that the result of calling a procedure could be totally wiped out.

However, that situation is better than the alternative; call-by-reference can cause real trouble in a concurrent application. If both P1 and P2 read and modify ARG several times, and if their actions are interspersed, the nal value of ARG can be completely unpredictable. 4 Worst of all, the value left in ARG could be di erent from any legal value that could be computed by either process alone, or both processes executed in either sequence.

The modern call-by-value-and-return semantics should be used for Ada because Ada was specifically designed to be used: On any machine architecture.. Most textbooks on operating systems cover this problem in a chapter on concurrency. 9.2. WHAT DOES AN ARGUMENT MEAN Exhibit 9.16. A swap VS .

NET DataMatrix routine using reference parameters in Pascal. The VAR keyword is used before the parameter name to indicate a reference parameter. Assuming that j and k are integers, an appropriate call would be: swap1 (j, k); PROCEDURE swap1 (VAR a, b: integer); VAR t:integer; BEGIN t:=a; a:=b; b:=t END;.

On networks of machines. With concurrent, multitasked applications. Call-by-Pointer Call-by-pointer is a subcase of call-by-value. The contents of the pointer variable in the calling program is an address. During parameter passing this address is copied into the corresponding pointer variable in the stack frame for the subroutine.

Now two pointers, one in each stack frame, point at the same object. Because the argument is the address of a storage object belonging to the calling program, it can be used to return a value from the subroutine. Unlike a by-reference parameter, though, a pointer parameter must be explicitly dereferenced within the subprogram.

Three versions of a swap subroutine are given here in two languages. These illustrate the similarities and di erences between call-by-reference and call-by-pointer. In all three, the programmer indicates that a variable address, not a value, is passed into the subroutine so that the swapped values can be passed outward.

Exhibit 9.16 shows a procedure named swap1 which uses call-byreference. Exhibit 9.

17 shows the storage allocated for execution of swap1. The subprogram stack frames for call-by-pointer [Exhibit 9.21] and call-by-reference contain the same information, but that information has a di erent meaning because of the extra automatic dereference that accompanies call-by-reference.

When call-by-pointer is used to implement the same process, the programmer must write an explicit dereference symbol each time the pointer parameter is used, as shown in Exhibits 9.18 and 9.20.

Contrast Exhibit 9.17 to Exhibit 9.21.

Note the double binding arrows in the former, and the single pointer arrows in the latter. These semantic di erences account for and correspond to the absence of explicit dereference symbols in Exhibit 9.16 and their presence ( in Pascal and * in C) in Exhibits 9.

18 and 9.20. Moreover, although a by-reference binding cannot be changed, the address stored in the pointer parameter can be changed by an assignment statement, as shown on the last line of Exhibit 9.

22. Use of call-by-pointer is severely restricted in Pascal. Since Pascal pointers can never point at objects on the stack, one can only use call-by-pointer to process dynamically allocated storage.

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