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Simple Perl (Python/C/C++) preprocessor

This simple preprocessor handles in-comments include directives. Such include statements that are contained inside comments provide an additional level of code organization, above Perl's require and use, Python's import, or C/C++ #include native mechanisms. However, both these mechanisms (native and pcpp) can coexist together. Or, the pcpp can be used instead of the native mechanism.

In the following Perl and C examples the "abc" code will be copy-pasted into the source code by the pcpp, while the "xyz" code will be handled by native include mechanisms:

# Perl usage
# include "abc.pl"
require "xyz.pl";
# Python usage
# include "abc.py"
import xyz
// C/C++ usage
// include "abc.h"
# include "xyz.h";

The pcpp is a simple tool, although this description seems long, it only handles:

  • include directives,
  • removal of triple-comments,
  • nothing else.

Why to "compile" Perl or Python?

To "use" the pcpp pre-processed code it is necessary to run the pcpp first, like:

pcpp abc.in.pl > abc.pl
perl abc.pl

This looks as if we first "compile" the bunch of Perl source files into the "executable" Perl file and only then run it. Similarly, a C code has to be compiled before it can be executed. It is a disadvantage as it requires the extra step pcpp abc.in.pl. But the following step perl abc.pl becomes simpler:

  • now the abc.pl doesn't require the dependent .pl files to be installed,
  • the abc.pl stays an interpreted language code that can be fixed if needed (comments stay in as well).

The same holds for Python. Pcpp-ing is some kind of simple code amalgamation or a "static linker" for Perl/Python. In C/C++ the pcpp is useful if you want to hide something from the cpp preprocessor.

Another reason to use pcpp instead of native require/use/import is that the pcpp processing logic is simpler. One more reason is that such include process cannot be skewed at the use-time and run-time.

In-comment directives

Hiding the pcpp directives in comments allows to avoid conflicts with the main language interpreter/compiler/preprocessor, with IDEs or editing modes. Further, this increases the number of comments ;-)

Multiple filenames in the single include directive line are allowed, quoting is optional, whitespace between the hash and the "include" word is optional:

#include abc.pl xyz.pl "efg.pl"

The additional text after the filename(s) is a comment and is ignored, filenames have to be identified by the .pl (or .py/.h) suffix or have to be quoted for the parser to find the beginning of the comment:

# include abc.pl xyz.pl comment text which is ignored
# include abc.py xyz.py # visualy more noticeable comment
// include abc.h comment in C/C++

Multiple hashes (slashes) are not accepted, the following code are just comments without any effect:

## include abc.pl
### include abc.pl
#### include abc.pl
/// include abc.h
//// include abc.h

Included content in the pcpp output is "watermarked" by the # included and # end comments/directives (in C/C++ // included and // end):

# included "abc.pl"
...
# end "abc.pl"

Indentation of the include statement is propagated into output. For instance a two-space indentation of the hash of the include statement in perl code:

some(code);
  # include abc.pl

will lead to adding two spaces to the original indentation in the included file:

some(code);
  # included abc.pl
  originally_unindented_abc_pl(code);
  following_line();
  ...

The language is autodetected according to the suffix of the input file: .pl, .py, .c, or .c++.

Paths resolving

Include paths can be specified as a filename only # include abc.pl or specifying also a part of the path # include xy/abc.pl or # include yz/abc.pl to distinguish between equal filenames in different directories.

The path resolving algorithm distinguishes between files included from the top-level source or from a file which is included. Path resolving algorithm considers paths relative to the current working directory (CWD) or to the directory of the currently parsed file. The algorithm is:

  1. try direct path for the top-level file includes (CWD relative),
  2. try path relative to currently active included file (to allow incorporation of whole code trees with relative includes working),
  3. try CWD-relative paths even for not-top-level files (to allow programmer to think about files as relative from CWD),
  4. try to find files recursively in any subdirectory of the CWD in the depth order (to allow to skip dirnames for unique filenames),
  5. strip the directory part from the included file name, and try to find it just by the filename (just a plan-B for wrong dirnames when moving files).

In the case of conflict, i.e. # include "abc.pl" where two abc.pl files are available, the first one is chosen: ./abc.pl is the direct path so it has a higher priority than the xy/abc.pl.

Double includes are automatically avoided by the pcpp. Any included file is copy-pasted to the output only once, on the place of the first appearance of its include statement.

Missing include files are silently ignored (by default), or reported in the verbose mode (-v switch).

Triple comments

Pcpp preserves comments to allow the Perl or Python output code to be readable as best as possible to allow the output to be hacked/fixed.

However, to allow the programmer to request the removal of comments from the output, we introduce the "triple comments":

### this line will be removed from the Perl/Python code by pcpp
/// this line will be removed from the C/C++ code by pcpp
#### but this will be kept
## this will be kept too

Triple comments are removed together with preceding empty lines by pcpp.

Debugger problem

Unfortunately, Perl or Python don't know how to translate line numbers from pcpp processed code back to the original code. So debugging requires checking the pcpp processed code, not the code you wrote.

Uninclude

Watermarked pcpp output allows the removal of included parts and return to the original source code using the uninclude tool. This can be useful when building "libraries" which can recursively pack all dependencies, which can be stripped off when not needed (when already provided by another "library").

Uninclude of multi-level included files is flattened to a single level, for instance the following included content:

# included abc.pl
  xyz
  # included def.pl
    uid
  # end def.pl
  hjk
# end abc.pl

will be flattened by uninclude to:

# include abc.pl
# include def.pl

which when included back will become:

# included abc.pl
  xyz
  hjk
# end abc.pl
# included def.pl
  uid
# end def.pl

Pcpp in Makefile

Example to make xyz from its source xyz.pl and two included files:

xyz: xyz.pl inc1.pl inc2.pl
	echo '#!/usr/bin/perl' > $@
	pcpp $< >> $@
	@chmod 755 $@
	@sync # to ensure the result is saved before being used in the next rule
  1. generate #! interpreter identifier
  2. build xyz from xyz.pl
  3. make it executable
  4. sync the result before it is used by another makefile rule (otherwise it can be incomplete)

More complex example:

OUTPUT := xyz
DEPENDENCIES := $(shell pcpp -lp $(OUTPUT:%=%.pl))
SIGN := "$(PKGNAME) $(AUTHOR)"
DATE := $(shell date '+%Y-%m-%d')

$(OUTPUT): %: %.pl $(DEPENDENCIES) Makefile
	echo -e '#!/usr/bin/perl' > $@
	echo -e "# $@ generated from $(PKGNAME)/$< $(DATE)\n" >> $@
	echo -e '$$SIGN = $(SIGN);\n' >> $@
	pcpp $< >> $@
	@chmod 755 $@
	@sync
  • DEPENDENCIES are a list of files to be included obtained by pcpp -lp
  • SIGN is a variable made available from Makefile into the script

Dependency files

The pcpp -d target_name can be used to generate a dependency file for Makefile. Compared to the -lp option, the -d and -dd options also add the input file into the list and nonexistent files too. Nonexistent include files are files that will be generated by the Makefile. A full path is required for them to work properly (relative path is ok if it is complete). Example Makefile:

# require rebuild of the dependencies file .abc.d when processing abc.pl
%: %.pl .%.d
	echo -e '#!/usr/bin/perl' > $@
	pcpp -v $< >> $@

# save dependencies into .abc.d for the abc.pl source of the abc target
.%.d: %.pl
	pcpp -d $(<:%.pl=%) $< > $@

# include generated dependencies but don't fail if they are missing
-include .abc.d

Next table lists pcpp reporting options, and which files they report. Direct files are these loaded according to the command-line request, not by the include directive. Missing files are those not found, by requested by the include directive.

          | where  | direct | found | missing | redundant
----------------------------------------------------------
  -v/vv/l | stderr |  yes   |  yes  |  yes X    |   yes
-ln/l1/lp | stdout |   no   |  yes  |  no   X  |   no
   -d/-dd | stdout |  yes   |  yes  |  yes X   |   no X

See also

   pcpp -h
   uninclude -h

Installation

Files pcpp and uninclude are standalone Perl scripts, which can be copied to any /bin directory for a system-wide installation.

Example

The pcpp itself is processed by the pcpp, so its source code is an example of how to use the pcpp.


R.Jaksa 2008,2024