Add a "Debugging your Cython program" page to the User Guide

src/userguide/debugging.rst

+.. highlight:: cython
+
+.. _debugging:
+
+**********************************
+Debugging your Cython program
+**********************************
+
+Cython comes with an extension for the GNU Debugger that helps users debug 
+Cython code. To use this functionality, you will need to install gdb 7.2 or
+higher, build with Python support (linked to Python 2.5 or higher). 
+
+The debugger will need debug information that the Cython compiler can export.
+This can be achieved from within the setup
+script by passing ``pyrex_debug=True`` to your Cython Extenion class::
+
+    from Cython.Distutils import extension
+    
+    ext = extension.Extension('source', 'source.pyx', pyrex_debug=True)
+    setup(..., ext_modules=[ext)]
+
+With this approach debug information can be enabled on a per-module basis.
+Another (easier) way is to simply pass the ``--pyrex-debug`` flag as a command
+line argument::
+
+    python setup.py build_ext --pyrex-debug
+
+For development it's often easy to use the ``--inplace`` flag also, which makes
+distutils build your project "in place", i.e., not in a separate `build`
+directory.
+
+When invoking Cython from the command line directly you can have it write
+debug information using the ``--debug`` flag::
+
+    cython --debug myfile.pyx
+
+.. note:: The debugger is new in the upcoming release of Cython, 0.13.1.
+          Currently, it can be cloned from hg.cython.org/cython-gdb.
+
+Running the Debugger
+=====================
+.. highlight:: bash
+
+To run the Cython debugger and have it import the debug information exported 
+by Cython, run ``cygdb`` in the build directory::
+
+    $ python setup.py build_ext --pyrex-debug --inplace
+    $ cygdb
+    GNU gdb (GDB) 7.2
+    ...
+    (gdb)
+
+When using the Cython debugger, it's preferable that you build and run your code
+with an interpreter that is compiled with debugging symbols (i.e. configured
+with ``--with-pydebug`` or compiled with the ``-g`` CFLAG). If your Python is 
+installed and managed by your package manager you probably need to install debug
+support separately, e.g. for ubuntu::
+
+    $ sudo apt-get install python-dbg
+    $ python-dbg setup.py build_ext --pyrex-debug --inplace
+
+Then you need to run your script with ``python-dbg`` also.
+
+You can also pass additional arguments to gdb::
+
+    $ cygdb /path/to/build/directory/ GDBARGS
+
+i.e.::
+    
+    $ cygdb . --args python-dbg mainscript.py
+
+To tell cygdb not to import any debug information, supply ``--`` as the first
+argument::
+
+    $ cygdb --
+
+Using the Debugger
+===================
+The Cython debugger comes with a set of commands that support breakpoints,
+stack inspection, source code listing, stepping, stepping over, etc. Most
+of these commands are analogous to their respective gdb command.
+
+.. function:: cy break breakpoints...
+    
+    Break in a Python, Cython or C function. First it will look for a Cython
+    function with that name, if cygdb doesn't know about a function (or method)
+    with that name, it will set a (pending) C breakpoint. The ``-p`` option can
+    be used to specify a Python breakpoint.
+
+    Breakpoints can be set for either the function or method name, or they can
+    be fully "qualified", which means that the entire "path" to a function is
+    given::
+
+        (gdb) cy break cython_function_or_method
+        (gdb) cy break packagename.modulename.cythonfunction
+        (gdb) cy break packagename.modulename.ClassName.cythonmethod
+        (gdb) cy break c_function
+
+    You can also break on Cython line numbers::
+
+        (gdb) cy break packagename.modulename:14
+        (gdb) cy break :14
+
+    Python breakpoints currently support names of the module (not the entire
+    package path) and the function or method::
+
+        (gdb) cy break -p pythonmodule.python_function_or_method
+        (gdb) cy break -p python_function_or_method
+
+.. function:: cy step
+
+    Step through Python, Cython or C code. Python, Cython and C functions
+    called directly from Cython code are considered relevant and will be
+    stepped into.
+
+.. function:: cy next
+
+    Step over Python, Cython or C code.
+
+.. function:: cy run
+    
+    Run the program. The default interpreter is the interpreter cygdb is run
+    with (but can be overridden using gdb's ``file`` command).
+
+.. function:: cy cont
+
+    Continue the program.
+
+.. function:: cy up
+              cy down
+
+    Go up and down the stack to what is considered a relevant frame.
+
+.. function:: cy bt
+              cy backtrace
+
+    Print a traceback of all frames considered relevant. The ``-a`` option
+    makes it print the full traceback (all C frames).
+
+.. function:: cy print varname
+
+    Print a local or global Cython, Python or C variable (depending on the 
+    context).
+
+.. function:: cy list
+
+    List the source code surrounding the current line.
+
+.. function:: cy locals
+              cy globals
+
+    Print all the local and global variables and their values.
+
+.. function:: cy import FILE...
+
+    Import debug information from files given as arguments. The easiest way to
+    import debug information is to use the cygdb command line tool.
+

src/userguide/index.rst

@@ -19,6 +19,7 @@ Contents:
    limitations
    pyrex_differences
    early_binding_for_speed
+   debugging
 
 Indices and tables
 ------------------