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178 lines
6.1 KiB
178 lines
6.1 KiB
:mod:`faulthandler` --- Dump the Python traceback
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=================================================
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.. module:: faulthandler
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:synopsis: Dump the Python traceback.
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.. versionadded:: 3.3
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----------------
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This module contains functions to dump Python tracebacks explicitly, on a fault,
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after a timeout, or on a user signal. Call :func:`faulthandler.enable` to
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install fault handlers for the :const:`SIGSEGV`, :const:`SIGFPE`,
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:const:`SIGABRT`, :const:`SIGBUS`, and :const:`SIGILL` signals. You can also
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enable them at startup by setting the :envvar:`PYTHONFAULTHANDLER` environment
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variable or by using the :option:`-X` ``faulthandler`` command line option.
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The fault handler is compatible with system fault handlers like Apport or the
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Windows fault handler. The module uses an alternative stack for signal handlers
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if the :c:func:`sigaltstack` function is available. This allows it to dump the
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traceback even on a stack overflow.
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The fault handler is called on catastrophic cases and therefore can only use
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signal-safe functions (e.g. it cannot allocate memory on the heap). Because of
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this limitation traceback dumping is minimal compared to normal Python
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tracebacks:
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* Only ASCII is supported. The ``backslashreplace`` error handler is used on
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encoding.
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* Each string is limited to 500 characters.
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* Only the filename, the function name and the line number are
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displayed. (no source code)
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* It is limited to 100 frames and 100 threads.
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* The order is reversed: the most recent call is shown first.
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By default, the Python traceback is written to :data:`sys.stderr`. To see
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tracebacks, applications must be run in the terminal. A log file can
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alternatively be passed to :func:`faulthandler.enable`.
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The module is implemented in C, so tracebacks can be dumped on a crash or when
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Python is deadlocked.
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The :ref:`Python Development Mode <devmode>` calls :func:`faulthandler.enable`
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at Python startup.
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Dumping the traceback
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---------------------
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.. function:: dump_traceback(file=sys.stderr, all_threads=True)
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Dump the tracebacks of all threads into *file*. If *all_threads* is
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``False``, dump only the current thread.
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.. versionchanged:: 3.5
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Added support for passing file descriptor to this function.
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Fault handler state
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-------------------
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.. function:: enable(file=sys.stderr, all_threads=True)
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Enable the fault handler: install handlers for the :const:`SIGSEGV`,
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:const:`SIGFPE`, :const:`SIGABRT`, :const:`SIGBUS` and :const:`SIGILL`
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signals to dump the Python traceback. If *all_threads* is ``True``,
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produce tracebacks for every running thread. Otherwise, dump only the current
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thread.
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The *file* must be kept open until the fault handler is disabled: see
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:ref:`issue with file descriptors <faulthandler-fd>`.
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.. versionchanged:: 3.5
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Added support for passing file descriptor to this function.
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.. versionchanged:: 3.6
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On Windows, a handler for Windows exception is also installed.
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.. function:: disable()
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Disable the fault handler: uninstall the signal handlers installed by
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:func:`enable`.
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.. function:: is_enabled()
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Check if the fault handler is enabled.
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Dumping the tracebacks after a timeout
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--------------------------------------
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.. function:: dump_traceback_later(timeout, repeat=False, file=sys.stderr, exit=False)
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Dump the tracebacks of all threads, after a timeout of *timeout* seconds, or
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every *timeout* seconds if *repeat* is ``True``. If *exit* is ``True``, call
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:c:func:`_exit` with status=1 after dumping the tracebacks. (Note
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:c:func:`_exit` exits the process immediately, which means it doesn't do any
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cleanup like flushing file buffers.) If the function is called twice, the new
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call replaces previous parameters and resets the timeout. The timer has a
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sub-second resolution.
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The *file* must be kept open until the traceback is dumped or
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:func:`cancel_dump_traceback_later` is called: see :ref:`issue with file
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descriptors <faulthandler-fd>`.
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This function is implemented using a watchdog thread.
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.. versionchanged:: 3.7
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This function is now always available.
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.. versionchanged:: 3.5
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Added support for passing file descriptor to this function.
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.. function:: cancel_dump_traceback_later()
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Cancel the last call to :func:`dump_traceback_later`.
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Dumping the traceback on a user signal
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--------------------------------------
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.. function:: register(signum, file=sys.stderr, all_threads=True, chain=False)
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Register a user signal: install a handler for the *signum* signal to dump
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the traceback of all threads, or of the current thread if *all_threads* is
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``False``, into *file*. Call the previous handler if chain is ``True``.
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The *file* must be kept open until the signal is unregistered by
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:func:`unregister`: see :ref:`issue with file descriptors <faulthandler-fd>`.
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Not available on Windows.
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.. versionchanged:: 3.5
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Added support for passing file descriptor to this function.
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.. function:: unregister(signum)
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Unregister a user signal: uninstall the handler of the *signum* signal
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installed by :func:`register`. Return ``True`` if the signal was registered,
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``False`` otherwise.
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Not available on Windows.
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.. _faulthandler-fd:
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Issue with file descriptors
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---------------------------
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:func:`enable`, :func:`dump_traceback_later` and :func:`register` keep the
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file descriptor of their *file* argument. If the file is closed and its file
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descriptor is reused by a new file, or if :func:`os.dup2` is used to replace
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the file descriptor, the traceback will be written into a different file. Call
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these functions again each time that the file is replaced.
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Example
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-------
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Example of a segmentation fault on Linux with and without enabling the fault
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handler:
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.. code-block:: shell-session
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$ python3 -c "import ctypes; ctypes.string_at(0)"
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Segmentation fault
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$ python3 -q -X faulthandler
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>>> import ctypes
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>>> ctypes.string_at(0)
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Fatal Python error: Segmentation fault
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Current thread 0x00007fb899f39700 (most recent call first):
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File "/home/python/cpython/Lib/ctypes/__init__.py", line 486 in string_at
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File "<stdin>", line 1 in <module>
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Segmentation fault
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