cpython/Doc/library/faulthandler.rst

:mod:`faulthandler` --- Dump the Python traceback
=================================================

.. module:: faulthandler
   :synopsis: Dump the Python traceback.

This module contains functions to dump Python tracebacks explicitly, on a fault,
after a timeout, or on a user signal. Call :func:`faulthandler.enable` to
install fault handlers for the :const:`SIGSEGV`, :const:`SIGFPE`,
:const:`SIGABRT`, :const:`SIGBUS`, and :const:`SIGILL` signals. You can also
enable them at startup by setting the :envvar:`PYTHONFAULTHANDLER` environment
variable or by using :option:`-X` ``faulthandler`` command line option.

The fault handler is compatible with system fault handlers like Apport or the
Windows fault handler. The module uses an alternative stack for signal handlers
if the :c:func:`sigaltstack` function is available. This allows it to dump the
traceback even on a stack overflow.

The fault handler is called on catastrophic cases and therefore can only use
signal-safe functions (e.g. it cannot allocate memory on the heap). Because of
this limitation traceback dumping is minimal compared to normal Python
tracebacks:

* Only ASCII is supported. The ``backslashreplace`` error handler is used on
  encoding.
* Each string is limited to 100 characters.
* Only the the filename, the function name and the line number are
  displayed. (no source code)
* It is limited to 100 frames and 100 threads.

By default, the Python traceback is written to :data:`sys.stderr`. To see
tracebacks, applications must be run in the terminal. A log file can
alternatively be passed to :func:`faulthandler.enable`.

The module is implemented in C, so tracebacks can be dumped on a crash or when
Python is deadlocked.

.. versionadded:: 3.3


Dump the traceback
------------------

.. function:: dump_traceback(file=sys.stderr, all_threads=True)

   Dump the traceback of all threads into *file*. If *all_threads* is ``True``,
   produce tracebacks for every running thread. Otherwise, dump only the current
   thread.


Fault handler state
-------------------

.. function:: enable(file=sys.stderr, all_threads=True)

   Enable the fault handler: install handlers for the :const:`SIGSEGV`,
   :const:`SIGFPE`, :const:`SIGABRT`, :const:`SIGBUS` and :const:`SIGILL`
   signals to dump the Python traceback. If *all_threads* is ``True``,
   produce tracebacks for every running thread. Otherwise, dump only the current
   thread.

.. function:: disable()

   Disable the fault handler: uninstall the signal handlers installed by
   :func:`enable`.

.. function:: is_enabled()

   Check if the fault handler is enabled.


Dump the tracebacks after a timeout
-----------------------------------

.. function:: dump_tracebacks_later(timeout, repeat=False, file=sys.stderr, exit=False)

   Dump the tracebacks of all threads, after a timeout of *timeout* seconds, or
   every *timeout* seconds if *repeat* is ``True``.  If *exit* is ``True``, call
   :c:func:`_exit` with status=1 after dumping the tracebacks.  (Note
   :c:func:`_exit` doesn't flush file buffers.) If the function is called twice,
   the new call replaces previous parameters and resets the timeout. The timer
   has a sub-second resolution.

   This function is implemented using a watchdog thread and therefore is not
   available if Python is compiled with threads disabled.

.. function:: cancel_dump_tracebacks_later()

   Cancel the last call to :func:`dump_tracebacks_later`.


Dump the traceback on a user signal
-----------------------------------

.. function:: register(signum, file=sys.stderr, all_threads=True)

   Register a user signal: install a handler for the *signum* signal to dump
   the traceback of all threads, or of the current thread if *all_threads* is
   ``False``, into *file*.

   Not available on Windows.

.. function:: unregister(signum)

   Unregister a user signal: uninstall the handler of the *signum* signal
   installed by :func:`register`. Return ``True`` if the signal was registered,
   ``False`` otherwise.

   Not available on Windows.


File descriptor issue
---------------------

:func:`enable`, :func:`dump_tracebacks_later` and :func:`register` keep the
file descriptor of their *file* argument. If the file is closed and its file
descriptor is reused by a new file, or if :func:`os.dup2` is used to replace
the file descriptor, the traceback will be written into a different file. Call
these functions again each time that the file is replaced.


Example
-------

Example of a segmentation fault on Linux: ::

    $ python -q -X faulthandler
    >>> import ctypes
    >>> ctypes.string_at(0)
    Fatal Python error: Segmentation fault

    Current thread 0x00007fb899f39700:
      File "/home/python/cpython/Lib/ctypes/__init__.py", line 486 in string_at
      File "<stdin>", line 1 in <module>
    Segmentation fault