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python-3.7.4-docs-html/_sources/library/faulthandler.rst.txt

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+:mod:`faulthandler` --- Dump the Python traceback
+=================================================
+
+.. module:: faulthandler
+   :synopsis: Dump the Python traceback.
+
+.. versionadded:: 3.3
+
+----------------
+
+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 the :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 500 characters.
+* Only the filename, the function name and the line number are
+  displayed. (no source code)
+* It is limited to 100 frames and 100 threads.
+* The order is reversed: the most recent call is shown first.
+
+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.
+
+
+Dumping the traceback
+---------------------
+
+.. function:: dump_traceback(file=sys.stderr, all_threads=True)
+
+   Dump the tracebacks of all threads into *file*. If *all_threads* is
+   ``False``, dump only the current thread.
+
+   .. versionchanged:: 3.5
+      Added support for passing file descriptor to this function.
+
+
+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.
+
+   The *file* must be kept open until the fault handler is disabled: see
+   :ref:`issue with file descriptors <faulthandler-fd>`.
+
+   .. versionchanged:: 3.5
+      Added support for passing file descriptor to this function.
+
+   .. versionchanged:: 3.6
+      On Windows, a handler for Windows exception is also installed.
+
+.. function:: disable()
+
+   Disable the fault handler: uninstall the signal handlers installed by
+   :func:`enable`.
+
+.. function:: is_enabled()
+
+   Check if the fault handler is enabled.
+
+
+Dumping the tracebacks after a timeout
+--------------------------------------
+
+.. function:: dump_traceback_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` exits the process immediately, which means it doesn't do any
+   cleanup like flushing 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.
+
+   The *file* must be kept open until the traceback is dumped or
+   :func:`cancel_dump_traceback_later` is called: see :ref:`issue with file
+   descriptors <faulthandler-fd>`.
+
+   This function is implemented using a watchdog thread and therefore is not
+   available if Python is compiled with threads disabled.
+
+   .. versionchanged:: 3.5
+      Added support for passing file descriptor to this function.
+
+.. function:: cancel_dump_traceback_later()
+
+   Cancel the last call to :func:`dump_traceback_later`.
+
+
+Dumping the traceback on a user signal
+--------------------------------------
+
+.. function:: register(signum, file=sys.stderr, all_threads=True, chain=False)
+
+   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*. Call the previous handler if chain is ``True``.
+
+   The *file* must be kept open until the signal is unregistered by
+   :func:`unregister`: see :ref:`issue with file descriptors <faulthandler-fd>`.
+
+   Not available on Windows.
+
+   .. versionchanged:: 3.5
+      Added support for passing file descriptor to this function.
+
+.. 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.
+
+
+.. _faulthandler-fd:
+
+Issue with file descriptors
+---------------------------
+
+:func:`enable`, :func:`dump_traceback_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 with and without enabling the fault
+handler:
+
+.. code-block:: shell-session
+
+    $ python3 -c "import ctypes; ctypes.string_at(0)"
+    Segmentation fault
+
+    $ python3 -q -X faulthandler
+    >>> import ctypes
+    >>> ctypes.string_at(0)
+    Fatal Python error: Segmentation fault
+
+    Current thread 0x00007fb899f39700 (most recent call first):
+      File "/home/python/cpython/Lib/ctypes/__init__.py", line 486 in string_at
+      File "<stdin>", line 1 in <module>
+    Segmentation fault
+