cpython_lldb is an LLDB extension for debugging Python programs.
It can be useful for troubleshooting stuck threads and crashes in the interpreter or external libraries. Unlike most Python debuggers, LLDB allows attaching to a running process without instrumenting it in advance, as well as loading a process coredump to perform an offline (or post-mortem) analysis of a problem.
When analyzing the state of a Python process, normally you would only have
access to intepreter-level information: most variables would be of type
PyObject*, and stack traces would only contain CPython internal calls and
calls to library functions. Unless you are a CPython developer troubleshooting
the interpreter implementation, that is typically not very useful. This extension,
however, allows you to extract application-level information about execution of
a program: print the values of variables, list the source code, display Python
stack traces, etc.
While CPython already provides a similar extension for gdb out of the box, LLDB might be the debugger of choice on some operating systems, e.g. on Mac OS.
This extension requires CPython to be built with debugging symbols enabled, which is not the case for some Linux distros (notably Arch Linux). CPython official Docker images are known to work correctly, as they are used for integration testing.
cpython_lldb targets CPython 3.5+ and supports the following features:
- pretty-priting of built-in types (int, bool, float, bytes, str, none, tuple, list, set, frozenset, dict)
- printing of Python-level stack traces
- printing of local variables
- listing the source code
- walking up and down the Python call stack
If your version of LLDB is linked against system libpython, it's recommended that you install the extension to the user site packages directory and allow it to be loaded automatically on start of a new LLDB session:
$ python -m pip install --user cpython-lldb
$ echo "command script import cpython_lldb" >> ~/.lldbinit
$ chmod +x ~/.lldbinitAlternatively, you can install the extension to some other location and tell LLDB to load it from there, e.g. ~/.lldb:
$ mkdir -p ~/.lldb/cpython_lldb
$ python -m pip install --target ~/.lldb/cpython_lldb cpython-lldb
$ echo "command script import ~/.lldb/cpython_lldb/cpython_lldb.py" >> ~/.lldbinit
$ chmod +x ~/.lldbinitLLDB bundled with MacOS is linked with the system version of CPython which may not even be in your PATH. To locate the right version of the interpreter, use:
$ lldb --print-script-interpreter-infoThe output of the command above is a JSON with the following structure:
{
"executable":"/Library/.../Python3.framework/Versions/3.9/bin/python3",
"language":"python",
"lldb-pythonpath":"/Library/.../LLDB.framework/Resources/Python",
"prefix":"/Library/.../Python3.framework/Versions/3.9"
}
Where the value for "executable" is the CPython version that should be used to install
cpython_lldb for LLDB to be able to successfully import the script:
$(lldb --print-script-interpreter-info | jq -r .executable) -m pip install cpython_lldbStart a new LLDB session:
$ lldb /usr/bin/pythonor attach to an existing CPython process:
$ lldb /usr/bin/python -p $PIDIf you've followed the installation steps, the extension will now be automatically loaded on start of a new LLDB session and register some Python-specific commands:
(lldb) help
...
Current user-defined commands:
py-bt -- Print a Python-level call trace of the selected thread.
py-down -- Select a newer Python stack frame.
py-list -- List the source code of the Python module that is currently being executed.
py-locals -- Print the values of local variables in the selected Python frame.
py-up -- Select an older Python stack frame.
For more information on any command, type 'help <command-name>'.
All known PyObject's (i.e. built-in types) are automatically pretty-printed
when encountered, as if you tried to get a repr() of something in Python REPL,
e.g.:
(lldb) frame variable v
(PyObject *) v = 0x0000000100793c00 42
(lldb) p v->ob_type->tp_name
(const char *) $3 = 0x000000010017d42a "int"
Use py-bt to print a full application-level stack trace of the current thread, e.g.:
(lldb) py-bt
Traceback (most recent call last):
File "test.py", line 15, in <module>
fc()
File "test.py", line 12, in fc
fb()
File "test.py", line 8, in fb
fa()
File "test.py", line 2, in fa
abs(1)
Use py-up and py-down to select an older or a newer Python call stack frame, e.g.:
(lldb) py-up
File "/Users/malor/src/cpython/test.py", line 6, in cb
self.ca()
(lldb) py-up
File "/Users/malor/src/cpython/test.py", line 20, in f_static
c.cb()
(lldb) py-down
File "/Users/malor/src/cpython/test.py", line 6, in cb
self.ca()
(lldb) py-down
File "/Users/malor/src/cpython/test.py", line 3, in ca
abs(1)
(lldb) py-down
*** Newest frame
Use py-locals to print the values of local variables in the selected frame:
(lldb) py-locals
a = 42
args = (1, 2, 3)
b = [1, u'hello', u'\\u0442\\u0435\\u0441\\u0442']
c = ([1], 2, [[3]])
d = u'test'
e = {u'a': -1, u'b': 0, u'c': 1}
eggs = 42
kwargs = {u'foo': 'spam'}
spam = u'foobar'
Use py-list to list the source code that is currently being executed in the selected
Python frame, e.g.:
(lldb) py-list
1 SOME_CONST = 42
2
3
4 def fa():
>5 abs(1)
6 return 1
7
8
9 def fb():
10 1 + 1
The command also accepts optional start and end arguments that allow to
list the source code within a specific range of lines, e.g.:
(lldb) py-list 4
4 def fa():
>5 abs(1)
6 return 1
7
8
9 def fb():
10 1 + 1
11 fa()
12
13
14 def fc():
or:
(lldb) py-list 4 11
4 def fa():
>5 abs(1)
6 return 1
7
8
9 def fb():
10 1 + 1
11 fa()
CPython 2.7.x is not supported. There are currently no plans to support it in the future.
CPython debugging symbols are required. You can check if they are available as follows:
$ lldb /usr/bin/python
$ (lldb) type lookup PyObjectIf debugging symbols are not available, you'll see something like:
no type was found matching 'PyObject'Some Linux distros ship debugging symbols separately. To fix the problem on Debian / Ubuntu do:
$ sudo apt-get install python-dbgon CentOS / Fedora / RHEL do:
$ sudo yum install python-debuginfoOther distros, like Arch Linux, do not provide debugging symbols in the package repos. In this case, you would need to build CPython from source. Please refer to the official CPython or distro documentation for instructions.
Alternatively, you can use the official CPython Docker images.
Some Linux distros (notably Debian Stretch) are shipped with a version of LLDB in which using Python scripting triggers a segmentation fault when executing any non-trivial operation:
$ lldb
(lldb) script
Python Interactive Interpreter. To exit, type 'quit()', 'exit()' or Ctrl-D.
>>> import io
>>> Segmentation faultIt's recommended that you use the latest LLDB release from the official APT repo instead of the one shipped with your distro.
See this page for advice on troubleshooting LLDB.
Tests currently require make and docker to be installed.
To run the tests against the latest released CPython version, do:
$ make test
To run the tests against a specific CPython (or LLDB) version, do:
$ PY_VERSION=X.Y LLDB_VERSION=Z make test
Supported CPython versions are:
3.73.83.93.10
Supported LLDB versions:
7891011
Kudos to everyone who have contributed to this project!
- Marco Neumann
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