||This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (February 2012)|
The PL/I preprocessor is the preprocessor for the PL/I computer programming language. The preprocessor interprets a subset of the full PL/I language to perform source file inclusion, conditional compilation, and macro expansion.
The preprocessor language has a PL/I-like syntax with preprocessor statements and preprocessor procedures prefixed with a percent symbol (
%). Listing-control statements, which supply formatting commands for the compiler listing, are usually considered preprocessor statements and also begin with
%. Preprocessor statements are imbedded in and operate on input text. The input text is normally a PL/I program, but is agnostic to the grammar of PL/I, so the preprocessor can also be used independently to process other kinds of text files.
The preprocessor is not specified as part of standard PL/I, but most PL/I implementations accept the language of the IBM preprocessor.
%INCLUDE preprocessor statement is used to include the text of another file, which may also contain preprocessor directives. The latest IBM compilers also provide an
%XINCLUDE directive, which has the effect of including the specified file only if it has not already been included.
%XINSCAN operate similarly, except that the name of the file to be included is specified by a preprocessor expression.
Listing control statements provide instructions for formatting both the listing generated by the preprocessor and the listing generated by the compiler.
%PRINT;causes the printing of listings of the following text to be started or resumed.
%NOPRINT;causes the printing of the listings of the following text to be suppressed.
%PAGE;causes a new page to be started in the listings.
%SKIP [(n)];causes n lines to be skipped in the listings. If n is omitted the default is one line.
%POPsave and restore the current status of
%NOPRINTon a pushdown stack and restore it, respectively.
The preprocessor operates by scanning the input text and recognizing declared preprocessor names, also called preprocessor identifiers. The text is copied to the preprocessor output with the preprocessor names replaced with their current values. The name may represent a call to a preprocessor procedure (macro). Replacement text may be rescanned by the preprocessor for possible additional replacement.
Preprocessor data types
Preprocessor data may be declared to be
CHARACTER, a character string with no maximum length, or
FIXED an integer number of up to five decimal digits. A preprocessor builtin is a predefined procedure operating on preprocessor data. A preprocessor expression is an expression consisting only of preprocessor names, references to preprocessor procedures or builtins, and decimal or character constants. There are no
BIT variables, but a
BIT result may be obtained by comparison. The expression in
%IF evaluates to
BIT. All PL/I operators are allowed except exponentiation.
%DECLAREestablishes an identifier as a preprocessor variable, either
- %assignment assigns a value to a preprocessor identifier.
%ACTIVATEmakes a preprocessor identifier active, that is, eligible for replacement when encountered in the input text.
%DEACTIVATEmakes a preprocessor ineligible for replacement.
%DOheads a preprocessor
DO-group, which is used to group statements and possibly specify iteration. A preprocessor
DO-group can contain any combination of preprocessor statements and input text.
%PROCEDUREheads a preprocessor procedure, a set of preprocessor statements that functions as a macro returning a value when its name is encountered in the input text.
%SELECTheads a preprocessor
%ENDterminates a preprocessor
SELECT-group, or preprocessor procedure.
%GO TO) causes the preprocessor to continue its scan at the specified preprocessor label, either a preprocessor statement or an arbitrary point in the input text.
%IFcontrols the flow of the preprocessor scan according to the value of a preprocessor expression.
%IF preprocessor-expression %THEN preprocessor unit1 %ELSE preprocessor-unit2
The preprocessor-units can be any single preprocessor statement or a preprocessor
%ITERATEtransfers control to the
%ENDof the containing preprocessor
DO-group, ending the current iteration and beginning the next if needed.
%LEAVEterminates any remaining iterations of the containing preprocessor
DO-group transfers control to the
%NOTEgenerates a user-specified preprocessor diagnostic message.
%nullis a preprocessor statement consisting only of an optional statement label and a semicolon (
;). It does nothing, but serves as a place-holder where a required statement is not needed.
%REPLACEallows immediate replacement of a name by a character or fixed expression. The name does not have to be a declared preprocessor identifier.
A preprocessor procedure is a subroutine executed by the preprocessor. The procedure is delimited by
%END statements and can contain only preprocessor statements, without the leading
%. It is invoked as a function reference from open code, outside of any preprocessor procedure, or from another preprocessor procedure, and returns a
FIXED value. When the procedure is invoked from open code the arguments are passed by name, that is they are interpreted as character strings delimited by commas or a right parenthesis, all leading, trailing, or embedded blanks are significant and considered part of the argument.:pp.508–509
These are the built-ins for IBM's PL/I for MVS and VM compiler.:pp.404–406 There can be considerable difference in the built-ins provided among preprocessors of various PL/I compilers.
COMPILETIME— returns the date and time of compilation as a character string such as "15 SEP 12 15:30:00" for September 15, 2012 3:30PM (local time).
COUNTER— returns a character string containing a number that is "00001" for the first call to
COUNTERand increases by one for each subsequent call.
INDEX— same as PL/I builtin
LENGTH— same as PL/I builtin
'1'bif the argument
pwas set in the current call to this preprocessor procedure, otherwise
SUBSTR— same as PL/I builtin
The following example for IBM PL/I for OS/2 illustrates the use of a preprocessor procedure to implement a C-like write statement for PL/I. The procedure would be called by coding the statement
uwrite file(filename) from(varying_string) count(byte_count); Byte_count is optional and defaults to the length of varying_string if omitted.
%uwrite: procedure keys (File, From, Count); dcl (File, From, Count, Number, Size) char; if parmset(File) & parmset(From) then; else do; note ('FILE and FROM must be specified!', 12); return; end; if parmset(Count) then Size = 'min(length(' || From || '), ' || Count || ')'; else Size = 'length(' || From || ')'; Number = Counter(); ans ('do;'); ans ('dcl Count' || Number || ' fixed bin (15);' ) skip; ans ('Count' || Number || ' = filewrite(' || File || ', ptradd(addr(' || From || '), 2)' || ', ' || Size || ');') skip; ans ('end;') skip; %end; %act uwrite;
uwrite file(file_name) from(var_str) count(64); generates the following:
do; dcl Count00001 fixed bin (15); Count00001 = filewrite(file_name, ptradd(addr(var_str), 2), min(length(var_str), 64)); end;
A 1964 report on "NPL",:pp.109–114 as PL/I was called at the time, provided that macro procedures, identified by the keyword
MACRO, could use the complete facilities of the language. The following compile-time statements were allowed in open code:
%DECLARE– both fixed-length and varying character strings were defined.
IF compile_time_comparison THEN unit [ELSE unit]– this causes one or the other unit to be included in the source.
"NPL" as defined in this manual was never implemented.
In 1965 an update to IBM's PL/I Language specification defined an even less ambitious preprocessor language.:pp.131–133 All mention of preprocessor procedures was omitted. The following compile-time statements were specified:
IF compile_time_comparison THEN GOTO label– No
ELSEclause was defined.
This language specification was again never implemented, however a 1966 revision of this manual restored preprocessor procedures with the now-current
%PROCEDURE ... %END syntax and brought the specification close to what was actually included in PL/I(F).:pp.132–139:pp.154–162 Fixed-length character variables were gone. New statements added were:
%DO [preprocessor_variable = preprocessor_expression TO preprocessor_expression [BY preprocessor_expression]]
RETURNin a compile-time procedure only.
IF compile_time_comparison THEN unit [ELSE unit]was restored.
A single compile-time builtin,
SUBSTR, was added.
Also in 1966 Robert Rosin published a pair of articles discussing development of the preprocessor. This development was based in a "SHARE XXVI Memo" from earlier the same year and a paper by Mark Elson. Rosin credits MAD as the only previous example of a macro processor in a high-level language.
- IBM Corporation (2005). Enterprise PL/I for z/OS PL/I for AIX WebSphere Developer for zSeries PL/I for Windows Language Reference.
- IBM Corporation (1995). IBM PL/I for MVS & VM Language Reference.
- Sturm, Eberhard. "UIO-Makros für Builtin-Funktionen fileread und filewrite". Retrieved January 22, 2012.
- IBM Corporation (1964). NPL Technical Report.
- IBM Corporation (1965). IBM Operating System/360 PL/I: Language Specifications (C28-6571-1).
- IBM Corporation (1966). IBM Operating System/360 PL/I: Language Specifications (C28-6571-3).
- IBM Corporation (1969). IBM System/360 PL/I Reference Manual (C28-8201-3).
- Rosin, Robert (August 1966). "PL/I Macro Processor - Progress Report". PL/I Bulletin (2). Retrieved January 22, 2013.
- Rosin, Robert (August 1966). "Macros in PL/I". PL/I Bulletin (2). Retrieved January 22, 2013.
- IBM Corporation (October 2009). "Enterprise PL/I Language Reference (SC27-1460-09): Chapter 21. Preprocessor Facilities". Retrieved Jan 19, 2012.
- Micro Focus International plc (2011). "Micro Focus Documentation: Open PL/I Macro Preprocessor". Retrieved Feb 14, 2012.
- Kednos Enterprises (2007). "Kednos PL/I for OpenVMS Systems Reference Manual: Chapter 10 Preprocessor". Retrieved Feb 14, 2012.
- Peter Flass (2010). "PL/I Preprocessor Wiki". Retrieved July 2014. Comparison of preprocessor features