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A C99 preprocessor written in pure Python

Project description

A C99 preprocessor written in pure Python

(C) 2018-2021 Niall Douglas and (C) 2007-2020 David Beazley

PyPI: Github: API reference docs:

Travis master branch all tests passing for Python v2, v3 and PyPy v2, v3: travis

A pure universal Python C (pre-)preprocessor implementation very useful for pre-preprocessing header only C++ libraries into single file includes and other such build or packaging stage malarky. The implementation can be used as a Python module (see API reference) or as a command line tool pcpp which can stand in for a conventional C preprocessor (i.e. it’ll accept similar arguments). Works great under PyPy, and you can expect performance gains of between 0.84x and 2.62x (average = 2.2x, median = 2.31x).

Your includes can be benchmarked for heft in order to improve your build times! See the --times and --filetimes options, and you can see graphs from pcpp for the C++ STLs at

A very unique facility of this C preprocessor is partial preprocessing so you can programmatically control how much preprocessing is done by pcpp and how much is done by the C or C++ compiler’s preprocessor. The ultimate control is by subclassing the Preprocessor class in Python from which you can do anything you like, however for your convenience the pcpp command line tool comes with the following canned partial preprocessing algorithms:


Pass through but still execute #defines and #undefs if not always removed by preprocessor logic. This ensures that including the output sets exactly the same macros as if you included the original, plus include guards work.


If an #include is not found, pass it through unmodified. This is very useful for passing through includes of system headers.


This is one of the most powerful pass through algorithms. If an expression passed to #if (or its brethern) contains an unknown macro, expand the expression with known macros and pass through unexecuted, and then pass through the remaining block. Each #elif is evaluated in turn and if it does not contain unknown macros, it will be executed immediately. Finally, any #else clause is always passed through unexecuted. Note that include guards normally defeat this algorithm, so those are specially detected and ignored.


A major use case for pcpp is as a preprocessor for the doxygen reference documentation tool whose preprocessor is unable to handle any preprocessing of any complexity. pcpp can partially execute the preprocessing which doxygen is incapable of, thus generating output which produces good results with doxygen. Hence the ability to pass through comments containing doxygen markup is very useful.


Don’t expand __DATE__, __TIME__, __FILE__, __LINE__ nor __COUNTER__.


Don’t expand those #include whose arguments match the supplied regular expression into the output, however still execute those includes. This lets you generate output with macros from nested includes expanded, however those #include matching the regular expression are passed through into the output.

Standards (non-)compliance

pcpp passes a very slightly modified edition of the mcpp unit test suite. The only modifications done were to disable the digraph and trigraphs tests. It also passes the list of “preprocessor torture” expansion fragments in the C11 standard, correctly expanding some very complex recursive macro expansions where expansions cause new macro expansions to be formed. In this, it handily beats the MSVC preprocessor and ought to handle most C99 preprocessor metaprogramming. If you compare its output side-by-side to that of GCC or clang’s preprocessor, results are extremely close indeed with blank line collapsing being the only difference.

As of v1.30 (Oct 2020), a proper yacc based expression evaluator for #if expressions is used which is standards conforming, and fixes a large number of problems found in the previous Python eval() based expression evaluator.

A full, detailed list of known non-conformance with the C99 standard is below. Pull requests with bug fixes and new unit tests for the fix are welcome.

If you are on Python 2, files are parsed as strings, and unicode is not supported. On Python 3, input and output files can have your choice of encoding, and you can hook file open to inspect the encoding using chardet.

Note that most of this preprocessor was written originally by David Beazley to show off his excellent Python Lex-Yacc library PLY ( and is hidden in there without being at all obvious given the number of Stack Overflow questions which have asked for a pure Python C preprocessor implementation. This implementation fixes a lot of conformance bugs (the original was never intended to rigidly adhere to the C standard) and adds in a test suite based on the C11 preprocessor torture samples plus the mcpp preprocessor test suite. Still, this project would not be possible without David’s work, so please take off your hat and give a bow towards him.

Command line tool pcpp:

The help from the command line tool pcpp:

usage: pcpp [-h] [-o [path]] [-D macro[=val]] [-U macro] [-N macro] [-I path]
            [--passthru-defines] [--passthru-unfound-includes]
            [--passthru-unknown-exprs] [--passthru-comments]
            [--passthru-magic-macros] [--passthru-includes <regex>]
            [--disable-auto-pragma-once] [--line-directive [form]] [--debug]
            [--time] [--filetimes [path]] [--compress]
            [--assume-input-encoding <encoding>]
            [--output-encoding <encoding>] [--write-bom] [--version]
            [input [input ...]]

A pure universal Python C (pre-)preprocessor implementation very useful for
pre-preprocessing header only C++ libraries into single file includes and
other such build or packaging stage malarky.

positional arguments:
  input                 Files to preprocess (use '-' for stdin)

optional arguments:
  -h, --help            show this help message and exit
  -o [path]             Output to a file instead of stdout
  -D macro[=val]        Predefine name as a macro [with value]
  -U macro              Pre-undefine name as a macro
  -N macro              Never define name as a macro, even if defined during
                        the preprocessing.
  -I path               Path to search for unfound #include's
  --passthru-defines    Pass through but still execute #defines and #undefs if
                        not always removed by preprocessor logic
                        Pass through #includes not found without execution
                        Unknown macros in expressions cause preprocessor logic
                        to be passed through instead of executed by treating
                        unknown macros as 0L
  --passthru-comments   Pass through comments unmodified
                        Pass through double underscore magic macros unmodified
  --passthru-includes <regex>
                        Regular expression for which #includes to not expand.
                        #includes, if found, are always executed
                        Disable the heuristics which auto apply #pragma once
                        to #include files wholly wrapped in an obvious include
                        guard macro
  --line-directive [form]
                        Form of line directive to use, defaults to #line,
                        specify nothing to disable output of line directives
  --debug               Generate a pcpp_debug.log file logging execution
  --time                Print the time it took to #include each file
  --filetimes [path]    Write CSV file with time spent inside each included
                        file, inclusive and exclusive
  --compress            Make output as small as possible
  --assume-input-encoding <encoding>
                        The text encoding to assume inputs are in
  --output-encoding <encoding>
                        The text encoding to use when writing files
  --write-bom           Prefix any output with a Unicode BOM
  --version             show program's version number and exit

Note that so pcpp can stand in for other preprocessor tooling, it ignores any
arguments it does not understand.

Quick demo of pass through mode

Let us look at an example for pass through mode. Here is the original:

#if !defined(__cpp_constexpr)
#if __cplusplus >= 201402L
#define __cpp_constexpr 201304  // relaxed constexpr
#define __cpp_constexpr 190000
#if __cpp_constexpr >= 201304
#define BOOSTLITE_CONSTEXPR constexpr

pcpp test.h --passthru-defines --passthru-unknown-exprs will output:

#if !defined(__cpp_constexpr)
#if __cplusplus >= 201402
#define __cpp_constexpr 201304
#define __cpp_constexpr 190000
#if __cpp_constexpr >= 201304
#define BOOSTLITE_CONSTEXPR constexpr

This is because __cpp_constexpr was not defined, so because of the --passthru-unknown-exprs flag we pass through everything inside that if block unexecuted i.e. defines and undefs are NOT executed by pcpp. Let’s define __cpp_constexpr:

pcpp test.h --passthru-defines --passthru-unknown-exprs -D __cpp_constexpr

#line 8 "test.h"


So, big difference now. We execute the entire first if block as __cpp_constexpr is now defined, thus leaving whitespace. Let’s try setting __cpp_constexpr a bit higher:

pcpp test.h --passthru-defines --passthru-unknown-exprs -D __cpp_constexpr=201304

#line 8 "test.h"

#define BOOSTLITE_CONSTEXPR constexpr


As you can see, the lines related to the known __cpp_constexpr are executed and removed, passing through any if blocks with unknown macros in the expression.

What if you want a macro to be known but undefined? The -U (to undefine) flag has an obvious meaning in pass through mode in that it makes a macro no longer unknown, but known to be undefined.

pcpp test.h --passthru-defines --passthru-unknown-exprs -U __cpp_constexpr

#if __cplusplus >= 201402
#define __cpp_constexpr 201304
#define __cpp_constexpr 190000



Here __cpp_constexpr is known to be undefined so the first clause executes, but __cplusplus is unknown so that entire block is passed through unexecuted. In the next test comparing __cpp_constexpr to 201304 it is still known to be undefined, and so 0 >= 201304 is the expressions tested which is false, hence the following stanza is removed entirely.

Helping pcpp using source code annotation

You can achieve a great deal using -D (define), -U (undefine) and -N (never define) on the command line, but for more complex preprocessing it gets hard to pass through the correct logic without some source code annotation.

pcpp lets you annotate which part of an if block being passed through due to use of unknown macros to also be executed in addition to the pass through. For this use __PCPP_ALWAYS_FALSE__ or __PCPP_ALWAYS_TRUE__ which tells pcpp to temporarily start executing the passed through preprocessor commands e.g.

#if !defined(__cpp_constexpr)
#if __cplusplus >= 201402L
#define __cpp_constexpr 201304
#elif !__PCPP_ALWAYS_FALSE__     // pcpp please execute this next block
#define __cpp_constexpr 190000
#if __cpp_constexpr >= 201304
#define BOOSTLITE_CONSTEXPR constexpr

Note that __PCPP_ALWAYS_FALSE__ will always be false in any other preprocessor, and it is also false in pcpp. However, it causes pcpp to execute the define of __cpp_constexpr to 190000:

pcpp test.h --passthru-defines --passthru-unknown-exprs

#if !defined(__cpp_constexpr)
#if __cplusplus >= 201402
#define __cpp_constexpr 201304
#elif 1
#define __cpp_constexpr 190000


This is one way of marking up #else clauses so they always execute in a normal preprocessor and also pass through with execution with pcpp. You can, of course, also place || __PCPP_ALWAYS_FALSE__ in any #if stanza to cause it to be passed through with execution, but not affect the preprocessing logic otherwise.

What’s implemented by the Preprocessor class:

  • Digraphs and Trigraphs

  • line continuation operator ‘\

  • C99 correct elimination of comments and maintenance of whitespace in output.

  • __DATE__, __TIME__, __FILE__, __LINE__. Note that __STDC__ et al are NOT defined by default, you need to define those manually before starting preprocessing.

  • __COUNTER__, a very common extension

  • Object #define

  • Function #define macro(...)

    • Retokenisation and reexpansion after expansion is C99 compliant.

  • #undef

  • #include "path", <path> and PATH

  • defined operator

  • C operators:

    • +, -, !, ~

    • *, /, %

    • +, -

    • <<, >>

    • <, <=, >, >=

    • ==, !=

    • &

    • ^

    • |

    • &&

    • ||

    • x ? y : z (partial support, see known bugs)

  • #if, #ifdef, #ifndef, #elif, #else, #endif

  • Stringizing operator #

  • Token pasting operator ##

  • #pragma once, a very common extension

Additionally implemented by pcpp command line tool:

  • #error (default implementation prints to stderr and increments the exit code)

  • #warning (default implementation prints to stderr)

Not implemented yet (donations of code welcome):

  • #pragma anything other than once.

  • _Pragma used to emit preprocessor calculated #pragma.

  • #line num, num "file" and NUMBER FILE.

Known bugs (ordered from worst to least worst):

None presently known.

Customising your own preprocessor:

See the API reference docs at

You can find an example of overriding the on_*() processing hooks at


v1.30 (29th October 2021):

  • Thanks to a 5km limit covid lockdown in my country, a public holiday where we were supposed to be away meant I was stuck at home instead. I took the full day to finish the branch which is a proper C preprocessor expression evaluator based on ‘s yacc module. This was a very long outstanding piece of work which had been in progress for nearly two years. It just needed a full day of my time to get it done, and now it is indeed done at long last.

  • BREAKING CHANGE: Thanks to the new expression evaluator, fix a long standing bug where unknown function macros in expressions were parsed as 0(0) which obviously enough does not work. Fixing this changes how the on_unknown_macro_in_expr() hook works, and there is now an added on_unknown_macro_function_in_expr() hook.

  • Add a new passthru option --passthru-includes which enables selected #include to be passed through, in addition to being executed. Thanks to schra for suggesting this, including a PR. The original implementation had some subtle corner case bugs, thanks to trelau for reporting those.

  • Fix a token expansion ordering bug whereby if a function macro used the same macro in more than one argument, expansion in one argument evaluation caused overly eager expansion in later argument evaluations. This fix ought to fix pcpp’s ability to parse Boost (untested). Thanks to joaquintides for reporting this.

  • Now that pcpp no longer ever calls eval(), pcpp is PyPy compatible and is probably also compatible with Pyston (untested). Typical speedup is about 2.2x-2.3x, though it can also be slower occasionally for some inputs. PyPy compatibility is now being tested by CI to ensure it remains working going forth.

  • Fix internal preprocessor error and failure to insert newlines before #include caused by certain sequence of line continuations in a macro. Thanks to dslijepcevic for reporting this.

v1.22 (19th October 2020):

  • Fix bug where outputting to stdout did not combine with anything which printed to stdout. Thanks to Fondesa for reporting this.

  • Fix extra newlines being inserted after a multiline comment. Thanks to virtuald for sending a PR fixing this.

  • Fix not being able to actually specify an empty line directive. Thanks to kuri65536 for sending a PR fixing this.

  • Update ply submodule to latest from trunk.

  • Emit line continuations as tokens, rather than collapsing lines during parsing. Thanks to MathieuDuponchelle for the pull request implementing this.

  • Enable parsing and emission of files in arbitrary text encodings. This is supported in Python 3 or later only. Thanks to MathieuDuponchelle for the suggestion.

  • Fix bad regex for parsing floats, so now floats are correctly tokenised. Thanks to LynnKirby for reporting this.

  • BREAKING CHANGE: Passthrough for #include MACRO was not supported. This was not intentional, and to fix it required modifying the on_include_not_found() customisation point which is a source breaking change. Thanks to schra for reporting this.

v1.21 (30th September 2019):

  • Fix bug where token pasting two numeric tokens did not yield a numeric token. Thanks to Sei-Lisa for reporting this.

  • BREAKING CHANGE: Paths emitted by pcpp into #line directives now are relative to the working directory of the process when Preprocessor is initialised. This includes added search paths - files included from those locations will be emitted with a sequence of ../ to relativise the path emitted. If no path exists between the working directory and the path of the file being emitted, an absolute path is emitted instead.

    If you wish to disable this new behaviour, or use different behaviour, you can customise the new rewrite_paths member variable of Preprocessor.

  • Fix bug where __LINE__ was expanding into the line number of its definition instead of its use. Thanks to Sei-Lisa for reporting this.

  • Add --passthru-magic-macros command line option.

  • BREAKING CHANGE: The PreprocessorHooks and OutputDirective interface has changed. One now must specify the kind of OutputDirective abort one wants, and one can now both ignore AND remove directives. on_directive_handle() and on_directive_unknown() now take an extra parameter precedingtoks, these are the tokens from the # up to the directive.

  • Fix a corner case where FUNC(void)foo() expanded to voidfoo() and not void foo() which is a very common non-conforming extension of the C preprocessor. Thanks to OmegaDoom for reporting this.

  • Add tokens for all the C operators, to help implementation of an expression evaluator.

  • Updated embedded ply to HEAD (2019-04-25)

  • Fix #include not working if no -I parameters were supplied. Thanks to csm10495 for reporting this.

v1.20 (7th January 2019):

  • Now supports character literals in expressions. Thanks to untaugh for the pull request adding this.

  • Stopped the default collapsing of whitespace in output, and made it optional via a new command line option --compress.

  • Fixed extraneous whitespace in --passthru-comments caused by multiline comments. Thanks to p2k for reporting this.

  • Fixed bug where defining a macro via string did not set the source attribute in the token. Thanks to ZedThree for reporting this.

  • Stop triggering an exception when no arguments are supplied to pcpp. Thanks to virtuald for reporting this.

  • Rebase onto PLY latest from Dec 28th 2018 ( Latest PLY no longer works using pypi packaging, David wants people to include the source of PLY directly. pcpp does this via a git submodule, and has setuptools bundle the submodule.

  • Add a formal LICENSE.txt file, as requested by Sei-Lisa.

  • Fix failure to issue #line directive for first include file in a file. Thanks to Sei-Lisa for reporting this.

v1.1 (19th June 2018):

  • Added the --times and --filetimes features.

  • Fix bug where macros containing operator defined were not being expanded properly.

  • Added the ability to accept multiple inputs, they are concatenated into the output.

  • Fix bug where lines beginning with # and no contents caused an internal preprocessor error.

  • Fix bug where the macro expansion par par##ext was expanding into parext parext.

v1.01 (21st Feb 2018):

  • Fix bug where in pass through mode, an #elif in an #if block inside an #if block in ifpassthru was failing to be passed through.

  • Downgraded failure to evaluate an expression to a warning.

  • Fix missing Readme.rst in pypi package.

v1.00 (13th Mar 2017):

First release

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