1:mod:`faulthandler` --- Dump the Python traceback 2================================================= 3 4.. module:: faulthandler 5 :synopsis: Dump the Python traceback. 6 7.. versionadded:: 3.3 8 9---------------- 10 11This module contains functions to dump Python tracebacks explicitly, on a fault, 12after a timeout, or on a user signal. Call :func:`faulthandler.enable` to 13install fault handlers for the :const:`SIGSEGV`, :const:`SIGFPE`, 14:const:`SIGABRT`, :const:`SIGBUS`, and :const:`SIGILL` signals. You can also 15enable them at startup by setting the :envvar:`PYTHONFAULTHANDLER` environment 16variable or by using the :option:`-X` ``faulthandler`` command line option. 17 18The fault handler is compatible with system fault handlers like Apport or the 19Windows fault handler. The module uses an alternative stack for signal handlers 20if the :c:func:`sigaltstack` function is available. This allows it to dump the 21traceback even on a stack overflow. 22 23The fault handler is called on catastrophic cases and therefore can only use 24signal-safe functions (e.g. it cannot allocate memory on the heap). Because of 25this limitation traceback dumping is minimal compared to normal Python 26tracebacks: 27 28* Only ASCII is supported. The ``backslashreplace`` error handler is used on 29 encoding. 30* Each string is limited to 500 characters. 31* Only the filename, the function name and the line number are 32 displayed. (no source code) 33* It is limited to 100 frames and 100 threads. 34* The order is reversed: the most recent call is shown first. 35 36By default, the Python traceback is written to :data:`sys.stderr`. To see 37tracebacks, applications must be run in the terminal. A log file can 38alternatively be passed to :func:`faulthandler.enable`. 39 40The module is implemented in C, so tracebacks can be dumped on a crash or when 41Python is deadlocked. 42 43 44Dumping the traceback 45--------------------- 46 47.. function:: dump_traceback(file=sys.stderr, all_threads=True) 48 49 Dump the tracebacks of all threads into *file*. If *all_threads* is 50 ``False``, dump only the current thread. 51 52 .. versionchanged:: 3.5 53 Added support for passing file descriptor to this function. 54 55 56Fault handler state 57------------------- 58 59.. function:: enable(file=sys.stderr, all_threads=True) 60 61 Enable the fault handler: install handlers for the :const:`SIGSEGV`, 62 :const:`SIGFPE`, :const:`SIGABRT`, :const:`SIGBUS` and :const:`SIGILL` 63 signals to dump the Python traceback. If *all_threads* is ``True``, 64 produce tracebacks for every running thread. Otherwise, dump only the current 65 thread. 66 67 The *file* must be kept open until the fault handler is disabled: see 68 :ref:`issue with file descriptors <faulthandler-fd>`. 69 70 .. versionchanged:: 3.5 71 Added support for passing file descriptor to this function. 72 73 .. versionchanged:: 3.6 74 On Windows, a handler for Windows exception is also installed. 75 76.. function:: disable() 77 78 Disable the fault handler: uninstall the signal handlers installed by 79 :func:`enable`. 80 81.. function:: is_enabled() 82 83 Check if the fault handler is enabled. 84 85 86Dumping the tracebacks after a timeout 87-------------------------------------- 88 89.. function:: dump_traceback_later(timeout, repeat=False, file=sys.stderr, exit=False) 90 91 Dump the tracebacks of all threads, after a timeout of *timeout* seconds, or 92 every *timeout* seconds if *repeat* is ``True``. If *exit* is ``True``, call 93 :c:func:`_exit` with status=1 after dumping the tracebacks. (Note 94 :c:func:`_exit` exits the process immediately, which means it doesn't do any 95 cleanup like flushing file buffers.) If the function is called twice, the new 96 call replaces previous parameters and resets the timeout. The timer has a 97 sub-second resolution. 98 99 The *file* must be kept open until the traceback is dumped or 100 :func:`cancel_dump_traceback_later` is called: see :ref:`issue with file 101 descriptors <faulthandler-fd>`. 102 103 This function is implemented using a watchdog thread. 104 105 .. versionchanged:: 3.7 106 This function is now always available. 107 108 .. versionchanged:: 3.5 109 Added support for passing file descriptor to this function. 110 111.. function:: cancel_dump_traceback_later() 112 113 Cancel the last call to :func:`dump_traceback_later`. 114 115 116Dumping the traceback on a user signal 117-------------------------------------- 118 119.. function:: register(signum, file=sys.stderr, all_threads=True, chain=False) 120 121 Register a user signal: install a handler for the *signum* signal to dump 122 the traceback of all threads, or of the current thread if *all_threads* is 123 ``False``, into *file*. Call the previous handler if chain is ``True``. 124 125 The *file* must be kept open until the signal is unregistered by 126 :func:`unregister`: see :ref:`issue with file descriptors <faulthandler-fd>`. 127 128 Not available on Windows. 129 130 .. versionchanged:: 3.5 131 Added support for passing file descriptor to this function. 132 133.. function:: unregister(signum) 134 135 Unregister a user signal: uninstall the handler of the *signum* signal 136 installed by :func:`register`. Return ``True`` if the signal was registered, 137 ``False`` otherwise. 138 139 Not available on Windows. 140 141 142.. _faulthandler-fd: 143 144Issue with file descriptors 145--------------------------- 146 147:func:`enable`, :func:`dump_traceback_later` and :func:`register` keep the 148file descriptor of their *file* argument. If the file is closed and its file 149descriptor is reused by a new file, or if :func:`os.dup2` is used to replace 150the file descriptor, the traceback will be written into a different file. Call 151these functions again each time that the file is replaced. 152 153 154Example 155------- 156 157Example of a segmentation fault on Linux with and without enabling the fault 158handler: 159 160.. code-block:: shell-session 161 162 $ python3 -c "import ctypes; ctypes.string_at(0)" 163 Segmentation fault 164 165 $ python3 -q -X faulthandler 166 >>> import ctypes 167 >>> ctypes.string_at(0) 168 Fatal Python error: Segmentation fault 169 170 Current thread 0x00007fb899f39700 (most recent call first): 171 File "/home/python/cpython/Lib/ctypes/__init__.py", line 486 in string_at 172 File "<stdin>", line 1 in <module> 173 Segmentation fault 174 175