ON-units, file I/O and running PL/I on z/OS

Conditions and ON-units

When something notable happens, PL/I raises a condition. If you have established an ON-unit for it, that code runs; if not, the language's default action applies. This is PL/I's built-in error handling, decades older than try/catch in other languages.

 DCL EOF BIT(1) INIT('0'B);
 ON ENDFILE(INFILE) EOF = '1'B;

 ON CONVERSION BEGIN;
   PUT SKIP LIST('BAD NUMERIC DATA:', ONSOURCE());
   BAD_COUNT = BAD_COUNT + 1;
   GO TO NEXT_RECORD;
 END;

 ON ERROR BEGIN;
   ON ERROR SYSTEM;
   PUT SKIP LIST('UNEXPECTED ERROR, ONCODE', ONCODE());
   CALL PLIDUMP('TFC');
 END;

• ONSOURCE() returns the string that failed conversion and ONCODE() returns a number identifying the condition. Look up ONCODE values in the PL/I documentation for your compiler level.
• ON ERROR SYSTEM; inside the ERROR ON-unit stops a second error from looping back into the same handler.
• PLIDUMP writes a diagnostic dump; the option string controls its contents.
• Some conditions are enabled with a prefix, for example (SUBSCRIPTRANGE): in front of a procedure, often used in test.

Record I/O

Record I/O moves whole records without conversion, the usual choice for batch files. The file name in the program is matched to a DD statement of the same name in the JCL, unless the TITLE option names a different DD.

 DCL INFILE  FILE RECORD INPUT;
 DCL OUTFILE FILE RECORD OUTPUT;
 DCL INREC   CHAR(80);

 OPEN FILE(INFILE), FILE(OUTFILE);
 READ FILE(INFILE) INTO(INREC);
 DO WHILE (EOF = '0'B);
   CALL PROCESS(INREC);
   WRITE FILE(OUTFILE) FROM(INREC);
   READ FILE(INFILE) INTO(INREC);
 END;
 CLOSE FILE(INFILE), FILE(OUTFILE);

PL/I also supports keyed access to VSAM files with KEY and KEYFROM. The KEY condition is raised for problems such as a record not found.

Stream I/O

Stream I/O treats a file as a stream of characters and converts values as it reads or writes. PUT LIST writes values with default formatting; PUT EDIT uses explicit formats such as A(20) for characters and F(9,2) for a fixed number. It is convenient for reports and SYSPRINT diagnostics, but slower than record I/O for big files.

Compiling and running under z/OS

• IBM supplies cataloged procedures for compile, compile-and-bind and compile-bind-and-go; most sites wrap them in their own procs or build tools.
• By default the compiler reads source from column 2 to column 72 (the MARGINS option), so code starting in column 1 is ignored.
• Runtime output from PUT goes to SYSPRINT, and LE messages and dumps appear in the job output just as for COBOL.
• The compile listing, with its attribute and cross-reference tables, is your map when debugging an abend offset.

PL/I for COBOL maintainers