Getting Involved

The plugin’s web site is this GitHub repository:

The primary place for discussion of the plugin is the mailing list:

A pre-built version of the HTML documentation can be seen at:

The project’s mailing list is here:

Ideas for using the plugin

Here are some ideas for possible uses of the plugin. Please email the plugin’s mailing list if you get any of these working (or if you have other ideas!). Some guesses as to the usefulness and difficulty level are given in parentheses after some of the ideas. Some of them might require new attributes, methods and/or classes to be added to the plugin (to expose more of GCC internals), but you can always ask on the mailing list if you need help.

  • extend the libcpychecker code to add checking for the standard C library. For example, given this buggy C code:

    int foo() {
         FILE *src, *dst;
         src = fopen("source.txt", "r");
         if (!src) return -1;
         dst = fopen("dest.txt", "w");
         if (!dst) return -1;  /* <<<< BUG: this error-handling leaks "src" */
         /* etc, copy src to dst (or whatever) */

    it would be great if the checker could emit a compile-time warning about the buggy error-handling path above (or indeed any paths through functions that leak FILE*, file descriptors, or other resources). The way to do this (I think) is to add a new Facet subclass to libcpychecker, analogous to the CPython facet subclass that already exists (though the facet handling is probably rather messy right now). (useful but difficult, and a lot of work)

  • extend the libcpychecker code to add checking for other libraries. For example:

    • reference-count checking within glib and gobject

    (useful for commonly-used C libraries but difficult, and a lot of work)

  • detection of C++ variables with non-trivial constructors that will need to be run before main - globals and static locals (useful, ought to be fairly easy)

  • finding unused parameters in definitions of non-virtual functions, so that they can be removed - possibly removing further dead code. Some care would be needed for function pointers. (useful, ought to be fairly easy)

  • detection of bad format strings (see e.g. )

  • compile gcc’s own test suite with the cpychecker code, to reuse their coverage of C and thus shake out more bugs in the checker (useful and easy)

  • a new PyPy gc root finder, running inside GCC (useful for PyPy, but difficult)

  • reimplement GCC-XML in Python (probably fairly easy, but does anyone still use GCC-XML now that GCC supports plugins?)

  • .gir generation for GObject Introspection (unknown if the GNOME developers are actually interested in this though)

  • create an interface that lets you view the changing internal representation of each function as it’s modified by the various optimization pases: lets you see which passes change a given function, and what the changes are (might be useful as a teaching tool, and for understanding GCC)

  • add array bounds checking to C (to what extent can GCC already do this?)

  • taint mode for GCC! e.g. detect usage of data from network/from disk/etc; identify certain data as untrusted, and track how it gets used; issue a warning (very useful, but very difficult: how does untainting work? what about pointers and memory regions? is it just too low-level?)

  • implement something akin to PyPy’s pygame-based viewer, for viewing control flow graphs and tree structures: an OpenGL-based GUI giving a fast, responsive UI for navigating the data - zooming, panning, search, etc. (very useful, and fairly easy)

  • generation of pxd files for Cython (useful for Cython, ought to be fairly easy)

  • reverse-engineering a .py or .pyx file from a .c file: turning legacy C Python extension modules back into Python or Cython sources (useful but difficult)

Tour of the C code

The plugin’s C code heavily uses Python’s extension API, and so it’s worth knowing this API if you’re going to hack on this part of the project. A good tutorial for this can be seen here:

and detailed notes on it are here:

Most of the C “glue” for creating classes and registering their methods and attributes is autogenerated. Simple C one-liners tend to appear in the autogenerated C files, whereas longer implementations are broken out into a hand-written C file.

Adding new methods and attributes to the classes requires editing the appropriate generate-*.py script to wire up the new entrypoint. For very simple attributes you can embed the C code directly there, but anything that’s more than a one-liner should have its implementation in the relevant C file.

For example, to add new methods to a gcc.Cfg you’d edit:

  • to add the new methods and attributes to the relevant tables of callbacks
  • gcc-python-wrappers.h to add declarations of the new C functions
  • gcc-python-cfg.c to add the implementations of the new C functions

Please try to make the API “Pythonic”.

My preference with getters is that if the implementation is a simple field lookup, it should be an attribute (the “getter” is only implicit, existing at the C level):


whereas if getting the result involves some work, it should be an explicit method of the class (where the “getter” is explicit at the Python level):


Using the plugin to check itself

Given that the cpychecker code implements new error-checking for Python C code, and that the underlying plugin is itself an example of such code, it’s possible to build the plugin once, then compile it with itself (using CC=gcc-with-cpychecker as a Makefile variable:

$ make CC=/path/to/a/clean/build/of/the/plugin/gcc-with-cpychecker

Unfortunately it doesn’t quite compile itself cleanly right now.

Test suite

There are three test suites:

  • a minimal test suite which is used before the plugin itself is built. This verifies that the cpybuilder code works.
  • make test-suite (aka a test harness and suite which was written for this project. See the notes below on patches.
  • make testcpychecker and a suite based on Python’s unittest module

Debugging the plugin’s C code

The gcc binary is a harness that launches subprocesses, so it can be fiddly to debug. Exactly what it launches depend on the inputs and options. Typically, the subprocesses it launches are (in order):

  • cc1 or cc1plus: The C or C++ compiler, generating a .s assember file.
  • as: The assembler, converting a .s assembler file to a .o object file.
  • collect2: The linker, turning one or more .o files into an executable (if you’re going all the way to building an a.out-style executable).

The easiest way to debug the plugin is to add these parameters to the gcc command line (e.g. to the end):

-wrapper gdb,--args

Note the lack of space between the comma and the –args.


./gcc-with-python examples/ test.c -wrapper gdb,--args

This will invoke each of the subprocesses in turn under gdb: e.g. cc1, as and collect2; the plugin runs with cc1 (cc1plus for C++ code).

For example:

$ ./gcc-with-cpychecker -c -I/usr/include/python2.7 demo.c -wrapper gdb,--args

GNU gdb (GDB) Fedora
Reading symbols from /usr/libexec/gcc/x86_64-redhat-linux/4.8.2/cc1...Reading symbols from /usr/lib/debug/usr/libexec/gcc/x86_64-redhat-linux/4.8.2/cc1.debug...done.
(gdb) run

Another way to do it is to add “-v” to the gcc command line (verbose), so that it outputs the commands that it’s running. You can then use this to launch:


to debug the subprocess that actually loads the Python plugin.

For example:

$ gcc -v -fplugin=$(pwd)/ test.c

on my machine emits this:

Using built-in specs.
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl= --enable-bootstrap --enable-shared --enable-threads=posix --enable-checking=release --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object --enable-linker-build-id --enable-languages=c,c++,objc,obj-c++,java,fortran,ada,go,lto --enable-plugin --enable-java-awt=gtk --disable-dssi --with-java-home=/usr/lib/jvm/java-1.5.0-gcj- --enable-libgcj-multifile --enable-java-maintainer-mode --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --disable-libjava-multilib --with-ppl --with-cloog --with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux
Thread model: posix
gcc version 4.6.1 20110908 (Red Hat 4.6.1-9) (GCC)
COLLECT_GCC_OPTIONS='-v' '-fplugin=/home/david/coding/gcc-python/gcc-python/contributing/' '' '-mtune=generic' '-march=x86-64'
 /usr/libexec/gcc/x86_64-redhat-linux/4.6.1/cc1 -quiet -v -iplugindir=/usr/lib/gcc/x86_64-redhat-linux/4.6.1/plugin test.c -iplugindir=/usr/lib/gcc/x86_64-redhat-linux/4.6.1/plugin -quiet -dumpbase test.c -mtune=generic -march=x86-64 -auxbase test -version -fplugin=/home/david/coding/gcc-python/gcc-python/contributing/ -o /tmp/cc1Z3b95.s
(output of the script follows)

This allows us to see the line in which cc1 is invoked: in the above example, it’s the final line before the output from the script:

/usr/libexec/gcc/x86_64-redhat-linux/4.6.1/cc1 -quiet -v -iplugindir=/usr/lib/gcc/x86_64-redhat-linux/4.6.1/plugin test.c -iplugindir=/usr/lib/gcc/x86_64-redhat-linux/4.6.1/plugin -quiet -dumpbase test.c -mtune=generic -march=x86-64 -auxbase test -version -fplugin=/home/david/coding/gcc-python/gcc-python/contributing/ -o /tmp/cc1Z3b95.s

We can then take this line and rerun this subprocess under gdb by adding gdb –args to the front like this:

$ gdb --args /usr/libexec/gcc/x86_64-redhat-linux/4.6.1/cc1 -quiet -v -iplugindir=/usr/lib/gcc/x86_64-redhat-linux/4.6.1/plugin test.c -iplugindir=/usr/lib/gcc/x86_64-redhat-linux/4.6.1/plugin -quiet -dumpbase test.c -mtune=generic -march=x86-64 -auxbase test -version -fplugin=/home/david/coding/gcc-python/gcc-python/contributing/ -o /tmp/cc1Z3b95.s

This approach to obtaining a debuggable process doesn’t seem to work in the presence of ccache, in that it writes to a temporary directory with a name that embeds the process ID each time, which then gets deleted. I’ve worked around this by uninstalling ccache, but apparently setting:


before invoking gcc -v ought to also work around this.

I’ve also been running into this error from gdb:

[Thread debugging using libthread_db enabled]
Cannot find new threads: generic error

Apparently this happens when debugging a process that uses dlopen to load a library that pulls in libpthread (as does gcc when loading in my plugin), and a workaround is to link cc1 with -lpthread

The workaround I’ve been using (to avoid the need to build my own gcc) is to use LD_PRELOAD, either like this: gdb --args ARGS GO HERE...

or this:

(gdb) set environment LD_PRELOAD

Handy tricks

Given a (PyGccTree*) named “self”:

(gdb) call debug_tree(self->t)

will use GCC’s prettyprinter to dump the embedded (tree*) and its descendants to stderr; it can help to put a breakpoint on that function too, to explore the insides of that type.


The project doesn’t have any copyright assignment requirement: you get to keep copyright in any contributions you make, though AIUI there’s an implicit licensing of such contributions under the GPLv3 or later, given that any contribution is a derived work of the plugin, which is itself licensed under the GPLv3 or later. I’m not a lawyer, though.

The Python code within the project is intended to be usable with both Python 2 and Python 3 without running 2to3: please stick to the common subset of the two languages. For example, please write print statements using parentheses:


Under Python 2 this is a print statement with a parenthesized number: (42) whereas under Python 3 this is an invocation of the print function.

Please try to stick PEP-8 for Python code, and to PEP-7 for C code (rather than the GNU coding conventions).

In C code, I strongly prefer to use multiline blocks throughout, even where single statements are allowed (e.g. in an “if” statement):

if (foo()) {

as opposed to:

if (foo())

since this practice prevents introducing bugs when modifying such code, and the resulting “diff” is much cleaner.

A good patch ought to add test cases for the new code that you write, and documentation.

The test cases should be grouped in appropriate subdirectories of “tests”. Each new test case is a directory with an:

  • input.c (or for C++)
  • exercising the relevant Python code
  • stdout.txt containing the expected output from the script.

For more realistic examples of test code, put them below tests/examples; these can be included by reference from the docs, so that we have documentation that’s automatically verified by, and users can use this to see the relationship between source-code constructs and the corresponding Python objects.

More information can be seen in

By default, will invoke all the tests. You can pass it a list of paths and it run all tests found in those paths and below.

You can generate the “gold” stdout.txt by hacking up this line in

out.check_for_diff(out.actual, err.actual, p, args, 'stdout', 0)

so that the final 0 is a 1 (the “writeback” argument to check_for_diff). There may need to be a non-empty stdout.txt file in the directory for this to take effect though.

Unfortunately, this approach over-specifies the selftests, making them rather “brittle”. Improvements to this approach would be welcome.

To directly see the GCC command line being invoked for each test, and to see the resulting stdout and stderr, add –show to the arguments of

For example:

$ python tests/plugin/diagnostics --show
tests/plugin/diagnostics: gcc -c -o tests/plugin/diagnostics/output.o -fplugin=/home/david/coding/gcc-python-plugin/ -fplugin-arg-python-script=tests/plugin/diagnostics/ -Wno-format tests/plugin/diagnostics/input.c
tests/plugin/diagnostics/input.c: In function 'main':
tests/plugin/diagnostics/input.c:23:1: error: this is an error (with positional args)
tests/plugin/diagnostics/input.c:23:1: error: this is an error (with keyword args)
tests/plugin/diagnostics/input.c:25:1: warning: this is a warning (with positional args) [-Wdiv-by-zero]
tests/plugin/diagnostics/input.c:25:1: warning: this is a warning (with keyword args) [-Wdiv-by-zero]
tests/plugin/diagnostics/input.c:23:1: error: a warning with some embedded format strings %s and %i
tests/plugin/diagnostics/input.c:25:1: warning: this is an unconditional warning [enabled by default]
tests/plugin/diagnostics/input.c:25:1: warning: this is another unconditional warning [enabled by default]
expected error was found: option must be either None, or of type gcc.Option
tests/plugin/diagnostics/input.c:23:1: note: This is the start of the function
tests/plugin/diagnostics/input.c:25:1: note: This is the end of the function
1 success; 0 failures; 0 skipped


We use Sphinx for documentation, which makes it easy to keep the documentation up-to-date. For notes on how to document Python in the .rst form accepted by Sphinx, see e.g.: