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TechTip: Filter Your Text in RPG IV

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A filter is a program or procedure that takes some text and modifies it in some way. The text to modify is passed as a parameter, and the modified value is returned. Filters are flexible in that they can be easily combined. The result of one filter can be directly used as the parameter to another.

All three of the examples presented here follow the same model: The RPG IV procedures take one varying-length character parameter and return a varying-length character value. Within the procedure, there's a loop over each character of the input. The character is changed if necessary and is then appended to the result.

All three examples demonstrate modifying some text that will be used as input to some other program, an increasingly common programming technique these days. These filters are necessary to ensure that the text is valid input to the other program.

At the end of this tip, I've included two files (Figures 1 and 2). File FILTERS includes the procedures and instructions for use. File TESTFILTS demonstrates how to use the filters.

The first procedure, "QUOTE()", should be used when dynamically constructing SQL queries. For example, you might want to build a query where some value is included in the query as a character literal. Not only are apostrophes needed at the beginning and end, but since apostrophes delimit literals, something must be done with apostrophes within the value. To have a syntactically correct literal, apostrophes within the literal must be doubled. Procedure "QUOTE()" does just that.

The second procedure, "ESCAPE_HTML()", is needed for applications that create HTML documents. Like the apostrophe within SQL literals, certain characters have special meaning within HTML documents. In particular, the less-than and greater-than characters (< and >) bracket HTML tags. When an HTML browser encounters a <, it assumes it has reached an HTML tag. Any text between the < and the > is treated as an HTML tag and is not presented to the user. If you insert text from a database into an HTML document without escaping the special characters, some important text may be invisible to the client. Likewise, the ampersand (&) signals the start of an HTML character entity, so that character needs special treatment too. The solution is to convert the characters <, >, and & to the appropriate character entities:

< = <
> = >
& = &

The final example illustrates a common situation in CGI programming. CGI programs often dynamically build URLs. For example, "http://www.mysite.ca/req?userid=hans&pw=glorp." But notice again that some characters have special meanings within the URL. The question mark (?) identifies the beginning of the parameters. The ampersand (&) separates individual parameters. And the equals sign (=) separates the parameter name from the parameter value. In addition, spaces are not allowed within the string. If you construct a query string with one of the special characters within the text of a parameter, a CGI program may justifiably end up confused. So what do we do if we want one of these special characters within a URL parameter value?

The solution is to convert any "non-safe" character to a hex code prefixed by a percent sign (%), yet another character with special meaning! You've probably seen URLs containing lots of funny characters like %25 or %5E. Procedure ENCODE_QUERY() converts the "non-safe" characters to their appropriate hex value suitable for use in a CGI query string.

These days, when more and more programs construct text intended to be processed by some other program, it is vitally important to build that text in a form that's acceptable to the other program. At best, the program will just abend, and the bug will become obvious to the programmer before the application is made public. Worse, incorrect text could be displayed to clients. At the extreme, errors in constructing URLs could potentially open a server to hacking.

--Hans Boldt
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Figure 1: The Procedures and Instructions for Use

      //*********************************************************************//
      // Module:  FILTERS                                                    //
      //---------------------------------------------------------------------//
      // Procedures that convert a string.                                   //
      //                                                                     //
      // Exported procedures:                                                //
      //                                                                     //
      // quote           -- Convert string value into quoted string.         //
      //                                                                     //
      // escape_html     -- Convert string value into escaped HTML text.     //
      //                                                                     //
      // encode_query    -- Convert string value into URL-encoded value.     //
      //                                                                     //
      //---------------------------------------------------------------------//
      //                                                                     //
      // Usage:                                                              //
      //                                                                     //
      // Include the prototypes into your code by coding the following       //
      // within the D-Specs of your program:                                 //
      //                                                                     //
      //   D/define INCLUDE_PROTOTYPES                                       //
      //   D/include FILTERS                                                 //
      //   D/undefine INCLUDE_PROTOTYPES                                     //
      //                                                                     //
      // Create a FILTERS service program for the procedures                 //
      //                                                                     //
      // Create a binding directory:                                         //
      //   CRTBNDDIR BNDDIR(yourlib/FILTERS)                                 //
      //                                                                     //
      // Add a binding directory entry:                                      //
      //   ADDBNDDIRE BNDDIR(yourlib/FILTERS) OBJ((yourlib/FILTERS))         //
      //                                                                     //
      //*********************************************************************//

      /if not defined(INCLUDE_PROTOTYPES)
     H nomain
      /endif

      //-----------------------------------//
      // Prototypes for FILTERS procedures //
      //-----------------------------------//

     D quote           pr          2002a   varying
     D   string                    1000a   const varying

     D escape_html     pr          5000a   varying
     D   string                    1000a   const varying

     D encode_query    pr          4000a   varying
     D   string                    1000a   const varying


      /if defined(INCLUDE_PROTOTYPES)
      /eof
      /endif


      //*********************************************************************//
      // Exported procedure:  quote                                          //
      //---------------------------------------------------------------------//
      // Return parameter as quoted string.                                  //
      //                                                                     //
      // Parameters:                                                         //
      //    I: string    -- string to quote                                  //
      //                                                                     //
      // Globals:                                                            //
      //    (none)                                                           //
      //                                                                     //
      // Returns:                                                            //
      //    -- quoted string                                                 //
      //---------------------------------------------------------------------//
      // Notes:                                                              //
      //                                                                     //
      //                                                                     //
      //                                                                     //
      //                                                                     //
      //                                                                     //
      //                                                                     //
      //*********************************************************************//
     P quote           b                   export
     D quote           pi          2002a   varying
     D   string                    1000a   const varying

     D result          s           2002a   varying
     D i               s             10i 0
     D ch              s              2a   varying
      /free
         // Prime result
         result = '''';

         // Loop for each character of input parameter
         for i = 1 to %len(string);
            ch = %subst(string:i:1);

            // Convert apostrophe into doubled apostrophe
            if ch = '''';
               ch = '''''';
            endif;

            // Append character to result
            %len(result) = %len(result) + %len(ch);
            %subst(result:%len(result)-%len(ch)+1) = ch;
         endfor;
         %len(result) = %len(result) + 1;
         %subst(result:%len(result)) = '''';

         return result;
      /end-free
     P quote           e



      //*********************************************************************//
      // Exported procedure:  escape_html                                    //
      //---------------------------------------------------------------------//
      // Convert HTML special characters into character entities.            //
      //                                                                     //
      // Parameters:                                                         //
      //    I: string    -- string to escape                                 //
      //                                                                     //
      // Globals:                                                            //
      //    (none)                                                           //
      //                                                                     //
      // Returns:                                                            //
      //    -- esaped HTML string                                            //
      //---------------------------------------------------------------------//
      // Notes:                                                              //
      //                                                                     //
      // 1) This procedure is used to properly escape text characters in     //
      //    HTML documents.  Characters '<', '>', and '&' must be converted  //
      //    to the appropriate character entities <, >, and &      //
      //    (respectively).  Failure to do so may mean that some text is     //
      //    not rendered by HTML browsers.                                   //
      //                                                                     //
      //*********************************************************************//
     P escape_html     b                   export
     D escape_html     pi          5000a   varying
     D   string                    1000a   const varying

     D result          s           5000a   varying
     D i               s             10i 0
     D ch              s              5a   varying
      /free
         // Prime result
         result = '';

         // Loop for each character of input parameter
         for i = 1 to %len(string);
            ch = %subst(string:i:1);

            // Convert special characters into character entities
            if ch = '<';
               ch = '<';
            elseif ch = '>';
               ch = '>';
            elseif ch = '&';
               ch = '&';
            endif;

            // Append character to result
            %len(result) = %len(result) + %len(ch);
            %subst(result:%len(result)-%len(ch)+1) = ch;
         endfor;

         return result;
      /end-free
     P escape_html     e



      //*********************************************************************//
      // Exported procedure:  encode_query                                   //
      //---------------------------------------------------------------------//
      // Encode CGI query string text.                                       //
      //                                                                     //
      // Parameters:                                                         //
      //    I: string    -- string to encode                                 //
      //                                                                     //
      // Globals:                                                            //
      //    (none)                                                           //
      //                                                                     //
      // Returns:                                                            //
      //    -- URL-encoded string                                            //
      //---------------------------------------------------------------------//
      // Notes:                                                              //
      //                                                                     //
      // 1) This procedure is used to ensure that the elements of a CGI      //
      //    query string are properly encoded.  Certain characters, such as  //
      //    '?', '+', '=', '%', and '&' (among others) have special meaning  //
      //    in a URL.  And so, if a parameter passed in a query string       //
      //    contains one of the special characters, it must be properly      //
      //    encoded.  The character is encoded in the form '%xx', where      //
      //    'xx' represents the hex value of the character in ASCII form.    //
      //                                                                     //
      // 2) Characters are converted from EBCDIC to ASCII by assuming that   //
      //    the first byte of a UCS2 character is an ASCII character.  This  //
      //    "trick" assumes the EBCDIC character is in the job CCSID (or     //
      //    the default job CCSID).                                          //
      //                                                                     //
      //*********************************************************************//
     P encode_query    b                   export
     D encode_query    pi          4000a   varying
     D   string                    1000a   const varying

     D result          s           4000a   varying
     D i               s             10i 0
     D ch              s              4a   varying

     D safe            c                   'ABCDEFGHIJKLMNOPQRSTUVWXYZ-
     D                                     abcdefghijklmnopqrstuvwxyz-
     D                                     0123456789_.-'
     D hex             s             16a   inz('0123456789ABCDEF')
     D conv            ds                  qualified
     D   ucschar                      1c
     D      ucslow                    3u 0 overlay(ucschar:2)

      /free
         // Prime result
         result = '';

         // Loop for each character of input parameter
         for i = 1 to %len(string);
            ch = %subst(string:i:1);

            // No conversion necessary if character is already "safe"
            if %scan(ch:safe) = 0;

               // Convert "non-safe" characters into special codes
               if ch = ' ';
                  // Space is encoded by '+'
                  ch = '+';
               else;
                  // Other non-safe chars converted to '%xx'
                  conv.ucschar = %ucs2(ch);
                  ch = '%' + %subst(hex: %div(conv.ucslow:16)+1: 1)
                     + %subst(hex: %rem(conv.ucslow:16)+1: 1);
               endif;
            endif;

            // Append character to result
            %len(result) = %len(result) + %len(ch);
            %subst(result:%len(result)-%len(ch)+1) = ch;
         endfor;

         return result;
      /end-free
     P encode_query    e

Figure 2: How to Use the Filters

     H dftactgrp(*no) bnddir('FILTERS')
      /define INCLUDE_PROTOTYPES
      /include FILTERS
      /undefine INCLUDE_PROTOTYPES
     D str             s             50a   varying
      /free
         str = quote('string ''literal''');
         dsply str;
         str = escape_html('abc<>&def');
         dsply str;
         str = encode_query('abcdefghij');
         dsply str;
         str = encode_query('abc %&= def');
         dsply str;
         str = encode_query('abc %&=?&#(@)_@'' def ');
         dsply str;
         *inlr = *on;
      /end-free

Hans Boldt
Hans Boldt worked as a software developer for 26 years at the IBM Canada Laboratory in Toronto , mainly on S/38, AS/400, and iSeries RPG compilers. From 2004 to 2007, he worked on the PL/X compiler on the zSeries, which gave him an increased appreciation for the iSeries. With a B.Sc. (Honours) in Computing and Information Science from Queen's University, Hans' interests range from programming languages to Web design to Linux. His other interests include photography, model railroading, and stained glass art. 

 

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