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.

Including files

The %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. %INSCAN and %XINSCAN operate similarly, except that the name of the file to be included is specified by a preprocessor expression.

Listing control

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.
• %PUSH, %POP save and restore the current status of %PRINT/%NOPRINT on a pushdown stack and restore it, respectively.

Preprocessor operation

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.

 %IF preprocessor-expression
 %THEN preprocessor unit1
 %ELSE preprocessor-unit2

Preprocessor built-ins

These are the built-ins for IBM's PL/I for MVS and VM compiler.^{[2]: 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 COUNTER and increases by one for each subsequent call.
• INDEX — same as PL/I builtin INDEX.
• LENGTH — same as PL/I builtin LENGTH.
• PARMSET — PARMSET(p) returns '1'b if the argument p was set in the current call to this preprocessor procedure, otherwise '0'b.
• SUBSTR — same as PL/I builtin SUBSTR.

Example

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.^[3] 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;

The statement 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;

Evolution

A 1964 report on "NPL",^{[4]: 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.
• %assignment
• %null statement
• %IF compile_time_comparison THEN unit [ELSE unit] – this causes one or the other unit to be included in the source.
• %GOTO

"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.^{[5]: pp.131–133} All mention of preprocessor procedures was omitted. The following compile-time statements were specified:

• %DECLARE
• %assignment
• %null statement
• %IF compile_time_comparison THEN GOTO label – No ELSE clause was defined.
• %GOTO

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).^{[6]: pp.132–139 [7]: pp.154–162} Fixed-length character variables were gone. New statements added were:

• %ACTIVATE
• %DEACTIVATE
• %DO [preprocessor_variable = preprocessor_expression TO preprocessor_expression [BY preprocessor_expression]]
• RETURN in a compile-time procedure only.
• %INCLUDE
• %IF – the %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^[8][9] 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.

See also

• C preprocessor

References

External links

• IBM Corporation (October 2009). "Enterprise PL/I Language Reference (SC27-1460-09): Chapter 21. Preprocessor Facilities" (PDF). 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 2017-12-06. Comparison of preprocessor features