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CHAPTER 9. FUNCTIONS AND PARAMETERS using vs .net toproduce data matrix ecc200 in web,windows application Visual Studio Development Tools and Languages Exhibit 9.5. visual .

net Data Matrix 2d barcode Meaningless calls on a variable-argument function in FORTH. 32 5 21 4 55 62 3 SumN 3 SumN ( Consumes one more parameter than was supplied, ) ( causing immediate or eventual stack underflow. ) ( Fails to consume a parameter, leaving garbage ) ( on the stack.

). Too many argu ments are supplied in the second meaningless call. This leaves garbage (the extra argument) on the stack. This garbage may or may not interfere with future program operation and can lead to a run-time error.

In any case, leaving garbage lying around is undesirable. Note that FORTH is a post x language and has no syntactic markers for the beginning and end of a parameter list. Moreover, there is no way to tell how many parameters will be used by a function without executing it on speci c data.

It is impossible, therefore, for a FORTH translator to help the programmer achieve semantic validity by checking that she or he has called his functions in meaningful ways. FORTH s stack operations are extremely low-level. Higher-level languages do not normally let the programmer manipulate the translator s internal data structures because the semantics of this kind of operation are unclear and unpredictable at compile time.

If the number of parameters actually supplied is fewer than the number needed, the program will crash. If too many parameters are supplied, junk will build up on the stack. Although C functions do specify how many parameters they expect, it is possible to call a C function with a di erent number of parameters.

In general, it would not be very useful to pass extra parameters to a function, but that is how the printf function works [Exhibit 9.6]. It uses the number of arguments speci ed by the information in its rst argument (which is the format speci cation) to determine how many more arguments to process.

. Exhibit 9.6. datamatrix 2d barcode for .

NET A call on a variable-argument function in C. The call on printf below has three arguments a format string and two numeric items. The format, in quotes, contains two % elds; each one directs that an integer is to be converted to ASCII and written to the output stream.

printf ("The sum is %d and the average is %d\n", sum, sum/count); This call will execute correctly because the number (and types) of conversions speci ed in the format matches the rest of the argument list. The format string is not actually checked at all at compile time. Rather, it is passed on to the run-time system to be interpreted when printf is called.

A mismatch in number or type between the format elds and the actual arguments will cause a run-time error or simply produce garbage.. 9.2. WHAT DOES AN ARGUMENT MEAN C will transl .net framework Data Matrix ate programs containing calls with missing or extra parameters, and such programs will behave much like similar programs in FORTH. All is well if the program actually uses the number of parameters that are supplied.

If too few are supplied, the program will either access garbage, causing unpredictable behavior, or it will stop with a stack under ow. Supplying too many parameters will not cause C to malfunction. In current C implementations, parameters are removed from the stack by the calling program, not by the subroutine.

This produces a longer calling sequence but avoids creating a serious problem by passing too many parameters. Excess parameters will sit on the stack unused until the subroutine exits, but then they will be removed. Junk will not pile up on the stack.

Pros and Cons. The examples cited previously help us answer the question, To what extent is it good and/or bad to permit argument lists of inde nite length Putting syntactic markers around parameter lists in a program makes them easier for humans to work with, and thus is good. Once these markers are required by a language, it is easy for a translator to check that the right number of parameters is used in every call.

Doing so prevents the programmer from writing nonsense that will cause programs to malfunction or crash. Thus a parameter checking mechanism in the translator can be a powerful aid to writing correct programs. On the other hand, there is a real cost involved.

Once in a while it is very useful to write a function, such as printf, that accepts a variable number of parameters. This can be done by the programmer in C but not in a language such as Pascal that requires well-formed function calls. The Pascal I/O procedures do accept a variable number of parameters, but they are prede ned and no functions like them can be de ned by the programmer.

Whether or not a restriction is severe depends partly on whether the useful things it prohibits can be achieved some other way. For example, a function in a list-oriented language might accomplish the same end more simply by taking a single argument that is a variable-length list. Thus the question of whether to require function calls to be well-formed can be reduced to making a set of value judgements about the relative importance of (1) semantic validity and (2) unrestricted exibility.

The answer also depends on what other mechanisms are included in the language. Most language designers placed a higher value on communicating and preserving semantic intent than the designers of FORTH and C, who valued exibility more highly. Neither of these languages enforces the very helpful restriction that a function must be called with a semantically meaningful number of parameters.

Both are considered to be systems programming languages, and both provide nearly full access to the machine hardware. Both permit programs to be written that can crash easily and in spectacular ways..

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