A C99 preprocessor written in pure Python
A C99 preprocessor written in pure Python
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).
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
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
#includeis 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
#elifis evaluated in turn and if it does not contain unknown macros, it will be executed immediately. Finally, any
#elseclause 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.
pcpp passes a modified edition of the mcpp unit test suite. Modifications done were to clarify ternary operators with extra brackets, plus those testing the unusual special quirks in expression evaluation (see detailed description below). 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.
The most non-conforming part is
parsing (donations of a proper yacc based parser for executing
#if expressions based on
http://www.dabeaz.com/ply/ are welcome). In practice, in most real world code, you
won’t notice the departures and if you do, the application of extra brackets to
group subexpressions so Python’s
eval() executes right will fix it.
A full, detailed list of known non-conformance with the C99 standard is below. We have been told that pcpp does not pass the Boost.Wave preprocessor test suite, but the chances of that biting most people is low. If it does, pull requests with bug fixes and new unit tests for the fix are welcome.
Note that most of this preprocessor was written originally by David Beazley to show off his excellent Python Lex-Yacc library PLY (http://www.dabeaz.com/ply/) and is hidden in there without being at all obvious given the number of Stackoverflow 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] [--disable-auto-pragma-once] [--line-directive [form]] [--debug] [--version] [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 File to preprocess 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 --passthru-unfound-includes Pass through #includes not found without execution --passthru-unknown-exprs 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 --disable-auto-pragma-once 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 --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 and any files it cannot open.
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 #else #define __cpp_constexpr 190000 #endif #endif #ifndef BOOSTLITE_CONSTEXPR #if __cpp_constexpr >= 201304 #define BOOSTLITE_CONSTEXPR constexpr #endif #endif #ifndef BOOSTLITE_CONSTEXPR #define BOOSTLITE_CONSTEXPR #endif
pcpp test.h --passthru-defines --passthru-unknown-exprs will output:
#if !defined(__cpp_constexpr) #if __cplusplus >= 201402 #define __cpp_constexpr 201304 #else #define __cpp_constexpr 190000 #endif #endif #ifndef BOOSTLITE_CONSTEXPR #if __cpp_constexpr >= 201304 #define BOOSTLITE_CONSTEXPR constexpr #endif #endif #ifndef BOOSTLITE_CONSTEXPR #define BOOSTLITE_CONSTEXPR #endif
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" #ifndef BOOSTLITE_CONSTEXPR #endif #ifndef BOOSTLITE_CONSTEXPR #define BOOSTLITE_CONSTEXPR #endif
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" #ifndef BOOSTLITE_CONSTEXPR #define BOOSTLITE_CONSTEXPR constexpr #endif
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 #else #define __cpp_constexpr 190000 #endif #ifndef BOOSTLITE_CONSTEXPR #endif #ifndef BOOSTLITE_CONSTEXPR #define BOOSTLITE_CONSTEXPR #endif
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 #endif #endif #ifndef BOOSTLITE_CONSTEXPR #if __cpp_constexpr >= 201304 #define BOOSTLITE_CONSTEXPR constexpr #endif #endif #ifndef BOOSTLITE_CONSTEXPR #define BOOSTLITE_CONSTEXPR #endif
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 #endif #endif #ifndef BOOSTLITE_CONSTEXPR #endif #ifndef BOOSTLITE_CONSTEXPR #define BOOSTLITE_CONSTEXPR #endif
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.
__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
- Retokenisation and reexpansion after expansion is C99 compliant.
- C operators:
+, -, !, ~
*, /, %
<, <=, >, >=
x ? y : z(partial support, see known bugs)
- 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):
#pragmaanything other than once.
_Pragmaused to emit preprocessor calculated #pragma.
Known bugs (ordered from worst to least worst):
- Expression evaluation is a bit broken
#ifexpressions are evaluated by converting them into Python expressions and calling
eval()on them. This works surprisingly well most of the time, but because Python is not C, corner cases break. These are the known such broken corner cases:
- Unary operator evaluation will break for evil expressions such as
-!+!9because logical NOT in Python results in a boolean, not an integer, and a unary plus or negative boolean is invalid syntax in Python
- Similarly expressions which assume that boolean operations output either
a zero or a one will fail e.g.
(2 || 3) == 0
- Python has no concept of an unsigned integer and C expressions relying
on unsigned integer semantics will fail badly e.g.
-1 <= 0Uis supposed to be evaluated as false in the C preprocessor, but it will be evaluated as true under this implementation. To be honest if your preprocessor logic is relying on those sorts of behaviours, you should rewrite it.
- Without a back tracking parser, the C ternary operator is hard to accurately
convert into a Python ternary operation, so you need to help it by using one
of these two forms:
(x) ? y : z(z gets evaluated according to Python not C precedence)
(x ? y : z)(preferred, evaluates correctly, we inject brackets around the subexpessions before sending to Python)
Code donations of a proper lexing parser based on http://www.dabeaz.com/ply/ are welcome!
- Unary operator evaluation will break for evil expressions such as
- We do not pass the Boost.Wave preprocessor test suite
- A lot of bugs have been fixed since this was reported, however the chances are that pcpp still doesn’t pass it. A TODO is to port the Wave test suite to Python and find out how bad things are. We suspect that any failures will be in highly estoric use cases i.e. known illegal input. If you only use valid input then we expect you generally won’t have trouble.
Customising your own preprocessor:
See the API reference docs at https://ned14.github.io/pcpp/
You can find an example of overriding the on_*() processing hooks at https://github.com/ned14/pcpp/blob/master/pcpp/pcpp_cmd.py
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):
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