Special files are a means by which RPG programs can handle file types that are not native to the language.When any file operation is performed on a file defined as ‘special', such as open,close, read, write, etc,control is passed to a named program to perform the operation. The following code is a sample how a special file might be used to provide non-standard editing functions to show on a display file.

The RPG "F" spec for a Special file looks like the following:

      FSPECIAL O   F      20            SPECIAL       EDITCL         
      F                                              KPLIST PLEDIT   
    

The file type is SPECIAL and the extension is the name of the program that will process the file requests, (Open, Close, Read, Write, Update and Delete)

The File Extension spec is used to define the name of the Parameter List (PLIST) used to call the program. The program parameter list has some required parameters and you need only specify on the PLIST those parameters that are need over and above these.

The required parameters are:

Parm. Len. Description ------ ---- ----------------------------------------------------------------- Option A(1) where O=Open, C=Close, R=Read, W=Write, D=Delete and U=Update Status A(1) where 0=Normal, 1=Input EOF and 2=Error Error N(5,0) Error number placed in *RECORD of INFDS Area A(rcd-len) field defined with the same length as record length asspecified on the "F" spec.

In the example "F" spec above the record length was 20. The PLIST definition below uses one extra field the same length as the record length. So the PLIST would be specified as follows:

      C           PLEDIT    PLIST                        
      C                     PARM           AMTDSP 20     
    

The following "C" and "O" specs serve as an example of how you would write to a special file:

      C                     EXCPTEDIT            
    

      OSPECIAL E                EDIT                
      O                         AMT   J   20         
    

The RPG code fragments are part of a program that will return to the program a character field DSPAMT which is the fully edited representation of the numeric field AMT.

In this example, RPG is doing most of the work and the Special File Processing Program EDITCL actually recieves the fully edited amount, therefore the CL program needs only to copy the input parameter to the return parametr, Viz:

QCLSRC source: EDITCL (Sample Special file handling program)

/* ***************************************************************** */
/* * This program handles I/O to RPG Special Files                 * */
/* *                                                               * */
/* * Author: Trevor D. Seeney,   SENTINEX Inc.                     * */
/* ***************************************************************** */
      PGM        PARM(&OPTION &STATUS &ERROR &AREA &RETURN)            
      DCL        VAR(&OPTION) TYPE(*CHAR) LEN(1)                       
      DCL        VAR(&STATUS) TYPE(*CHAR) LEN(1)                       
      DCL        VAR(&ERROR) TYPE(*DEC) LEN(5 0)                       
      DCL        VAR(&AREA) TYPE(*CHAR) LEN(20)                        
      DCL        VAR(&RETURN) TYPE(*CHAR) LEN(20)                      
                                                                       
/* * Receive inut buffer                                           * */
     CHGVAR     VAR(&RETURN) VALUE(&AREA)                              

     RETURN                                                           
     ENDPGM                                                           

    

Although this is a very simple example it does enable a programmer to bring to the display screen the editing options that are available in printed form. With a slight modification to the CL program it is possible to change the color of the displayed amount by testing the sign of the amount and prefixing the return variable with a display attribute (for example, red for negative and dark blue for positive amounts). Another good use for this technique is to unpacked numeric data found in character strings.

I would like to give credit to Steve Ames for showing me this technique.

The complete RPG program and the CL program that inserts the color attributes is available on request by E-Mail or FAX.

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