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Adding 'Value' to Your Prototypes

December 2004 | by Jon Parisand Susan Gantner

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Let's examine a simple C-function prototype that illustrates the use of the VALUE keyword and allows us to point out a few other C-function idiosyncrasies. The C

function to calculate the sine of a number, and in fact those for the whole of the math library, are simple and work well for this example despite having limited

applicability in most RPG applications. This is what the prototype for the sine function looks like when translated to RPG:

D Sine PR 8F ExtProc('sin')

D Double 8F Value

Note the use the EXTPROC keyword. We don't often use that keyword when calling RPG subprocedures, because the compiler supplies it by default. However, we

need to use it when calling C functions, because most are spelled in lower case. Since the RPG compiler upper cases every name that isn't in quotation marks, we

must use the EXTPROC keyword to ensure that the name of the function is correct ly identified as 'sin.' It also provides a good excuse to override the name to one

that's actually spelled correctly and makes more sense in our program logic. (After all, isn't there enough 'sin' in the world already?) If we hadn't specified

EXTPROC, the compiler would have translated the procedure name from 'Sine' to SINE.

You may also have noticed the use of the floating point (F) data type-something else we don't often use in RPG. The 8F definition corresponds to what C calls a

"double," meaning double-precision floating point. Don't be put of f by this. Although floating point is widely used in C functions, you won't have to worry about it or

define fields in floating-point format other than within prototypes. Why? For one thing, the VALUE keyword allows us to specify any numeric field, literal or function as

the input parameter . Under the covers, the compiler converts it to floating point. Similarly, although the functions return value is a lso specified as floating point, the

normal rules of RPG apply and the EVAL convert it to any format required, just as a packed-numeric return value can be assigned to a signed-numeric field. Its all

automatic.

Now that our prototype is in place, we can use the sine function just as easily as if IBM had added it to the RPG language. In fact, the only real difference is that if

IBM had added it, it would have been %SINE! A simple invocation of the function would look like this:

D X S 9p 2

D SineX S 7s 2

C Eval SineX = Sine(X)

Look-no floating-point fields.

Null-Terminated Strings

The C language doesn't deal well w ith the fixed-length character fields that are an RPG staple. Instead, if a character value is to be passed to a C function, its

typically expected as a null-terminated str ing-a variable-length character f ield that contains a null (X'00') denoting the end of the data. Actually, to be specific, C

typically expects that a pointer to a null-terminated str ing will be passed. To simplify coexistence with C functions, the compiler writers provided Options(*String),

which allows the compiler to do the tedious task of copying data to a temporary field, adding the X'00' to the end and passing the temporary fields address. To see

how this works, consider this example:

D Command S 40A Inz('command goes here') Varying

D CallSystem Pr 10I 0 ExtProc('system')

D CmdString * Value Options(*String)

D ExceptionMsg S 7A Import('_EXCP_MSGID')

C If CallSystem(Command) > 0

* Command failed

This is an example of using the C-system function. It serves a similar purpose to the QCMDEXC API. One of its advantages is that it returns a value that tells you

whether the command executed successfully. Because of this we can invoke the command as part of an IF statement, thereby executing the command and test ing


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the result in one step. If you want to know why the command failed, you can access the CPF message ID by importing the _EXCP_MSGID variable as shown

above.

Next page: >>

Page 12 3

Jon Paris is a technical editor with IBM Systems Magazine and co-owner of Partner400.

More Articles From Jon Paris

Susan Gantner is a technical editor with IBM Systems Magazine and co-owner of Partner400.

More Articles From Susan Gantner

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