使用 GDB 來為 C API 擴充功能和 CPython 內部偵錯¶
本文件解釋如何將 Python GDB 擴充功能 python-gdb.py 與 GDB 偵錯器一起使用來為 CPython 擴充功能和 CPython 直譯器本身偵錯。
在為崩潰或死鎖等低階問題幀錯時，低階偵錯器（例如 GDB）對於診斷和修正問題非常有用。預設情況下，GDB（或其任何前端）不支援特定於 CPython 直譯器的高階資訊。
python-gdb.py 擴充功能將 CPython 直譯器資訊新增至 GDB。此擴充有助於內省 (introspect) 目前執行的 Python 函式的堆疊。給定一個由 PyObject* 指標表示的 Python 物件，擴充功能會顯示該物件的型別和值。
正在開發 CPython 擴充功能或修補用 C 編寫之 CPython 部分的開發人員可以使用本文件來學習如何將 python-gdb.py 擴充功能與 GDB 一起使用。
備註
本文件假設你熟悉 GDB 和 CPython C API 的基礎知識。它整合了 devguide 和 Python wiki 的指引。
先決條件¶
你需要有：
GDB 7 或更之後的版本。（對於早期版本的 GDB，請參閱 Python 3.11 或更早版本的原始程式碼中的
Misc/gdbinit。）對 Python 和你在偵錯的任何擴充功能來說的與 GDB 相容的偵錯資訊。
python-gdb.py擴充。
該擴充功能是用 Python 建置的，但可能會單獨發布或根本不發布。以下我們將一些常見系統的提示作為範例。請注意，即使說明與你的系統匹配，它們也可能已過時。
使用從原始碼建置的 Python 進行設定¶
當你從原始程式碼建立 CPython 時，偵錯資訊應該可用，並且建置應該將 python-gdb.py 檔案新增至儲存庫的根目錄中。
若要啟用支援，你必須將包含 python-gdb.py 的目錄新增至 GDB 的 "auto-load-safe-path"。如果你還沒有這樣做，最新版本的 GDB 將列印警告，其中包含有關如何執行此操作的說明。
備註
如果你沒有看到適合你的 GDB 版本的說明，請將其放入你的設定檔中（~/.gdbinit 或 ~/.config/gdb/gdbinit）：
add-auto-load-safe-path /path/to/cpython
你也可以新增多個路徑，要以 : 分隔。
從 Linux 發行版設定 Python¶
大多數 Linux 系統在名為 python-debuginfo、python-dbg 或類似的套件中提供系統 Python 的偵錯資訊。例如：
Fedora：
sudo dnf install gdb sudo dnf debuginfo-install python3
Ubuntu：
sudo apt install gdb python3-dbg
在最近的幾個 Linux 系統上，GDB 可以使用 debuginfod 自動下載偵錯符號。但是這不會安裝 python-gdb.py 擴充功能；你通常需要另外安裝偵錯資訊套件。
使用偵錯建置與使用開發模式¶
為了更輕鬆地進行偵錯，你可能需要：
使用 Python 的偵錯建置。（從原始碼建置時，請使用
configure --with-pydebug。在 Linux 發行版上，安裝並執行諸如python-debug或python-dbg之類的套件（如果可用））。使用 runtime 開發模式 (
-X dev)。
兩者都啟用額外的斷言 (assertion) 並停用一些最佳化。有時這會隱藏你試圖想尋找的錯誤，但在大多數情況下，它們會使過程變得更容易。
使用 python-gdb 擴充功能¶
載入擴充功能後，它提供兩個主要功能：Python 值的漂亮列印器和附加命令。
漂亮列印器¶
這是啟用此擴充功能時 GDB 回溯 (backtrace) 的樣子（有被截斷）：
#0 0x000000000041a6b1 in PyObject_Malloc (nbytes=Cannot access memory at address 0x7fffff7fefe8
) at Objects/obmalloc.c:748
#1 0x000000000041b7c0 in _PyObject_DebugMallocApi (id=111 'o', nbytes=24) at Objects/obmalloc.c:1445
#2 0x000000000041b717 in _PyObject_DebugMalloc (nbytes=24) at Objects/obmalloc.c:1412
#3 0x000000000044060a in _PyUnicode_New (length=11) at Objects/unicodeobject.c:346
#4 0x00000000004466aa in PyUnicodeUCS2_DecodeUTF8Stateful (s=0x5c2b8d "__lltrace__", size=11, errors=0x0, consumed=
0x0) at Objects/unicodeobject.c:2531
#5 0x0000000000446647 in PyUnicodeUCS2_DecodeUTF8 (s=0x5c2b8d "__lltrace__", size=11, errors=0x0)
at Objects/unicodeobject.c:2495
#6 0x0000000000440d1b in PyUnicodeUCS2_FromStringAndSize (u=0x5c2b8d "__lltrace__", size=11)
at Objects/unicodeobject.c:551
#7 0x0000000000440d94 in PyUnicodeUCS2_FromString (u=0x5c2b8d "__lltrace__") at Objects/unicodeobject.c:569
#8 0x0000000000584abd in PyDict_GetItemString (v=
{'Yuck': <type at remote 0xad4730>, '__builtins__': <module at remote 0x7ffff7fd5ee8>, '__file__': 'Lib/test/crashers/nasty_eq_vs_dict.py', '__package__': None, 'y': <Yuck(i=0) at remote 0xaacd80>, 'dict': {0: 0, 1: 1, 2: 2, 3: 3}, '__cached__': None, '__name__': '__main__', 'z': <Yuck(i=0) at remote 0xaace60>, '__doc__': None}, key=
0x5c2b8d "__lltrace__") at Objects/dictobject.c:2171
請注意 PyDict_GetItemString 的字典引數如何顯示為其 repr()，而不是不透明的 PyObject * 指標。
該擴充功能透過為 PyObject * 型別的值提供自訂列印例程來運作。如果需要存取物件較低階的詳細資訊，請將值轉換為適當型別的指標。例如：
(gdb) p globals
$1 = {'__builtins__': <module at remote 0x7ffff7fb1868>, '__name__':
'__main__', 'ctypes': <module at remote 0x7ffff7f14360>, '__doc__': None,
'__package__': None}
(gdb) p *(PyDictObject*)globals
$2 = {ob_refcnt = 3, ob_type = 0x3dbdf85820, ma_fill = 5, ma_used = 5,
ma_mask = 7, ma_table = 0x63d0f8, ma_lookup = 0x3dbdc7ea70
<lookdict_string>, ma_smalltable = {{me_hash = 7065186196740147912,
me_key = '__builtins__', me_value = <module at remote 0x7ffff7fb1868>},
{me_hash = -368181376027291943, me_key = '__name__',
me_value ='__main__'}, {me_hash = 0, me_key = 0x0, me_value = 0x0},
{me_hash = 0, me_key = 0x0, me_value = 0x0},
{me_hash = -9177857982131165996, me_key = 'ctypes',
me_value = <module at remote 0x7ffff7f14360>},
{me_hash = -8518757509529533123, me_key = '__doc__', me_value = None},
{me_hash = 0, me_key = 0x0, me_value = 0x0}, {
me_hash = 6614918939584953775, me_key = '__package__', me_value = None}}}
請注意，漂亮列印器其實並不呼叫 repr()。對於基本型別，他們嘗試緊密匹配其結果。
一個可能令人困惑的地方是，某些型別的自訂列印器看起來很像 GDB 標準類型的內建列印器。例如，Python int (PyLongObject*) 的漂亮列印器給出的表示法無法與常規機器層級整數之其一區分：
(gdb) p some_machine_integer
$3 = 42
(gdb) p some_python_integer
$4 = 42
The internal structure can be revealed with a cast to PyLongObject*:
(gdb) p *(PyLongObject*)some_python_integer
$5 = {ob_base = {ob_base = {ob_refcnt = 8, ob_type = 0x3dad39f5e0}, ob_size = 1},
ob_digit = {42}}
使用 str 型別時也可能會出現類似的困惑，其中的輸出看起來很像對於 char * 的 gdb 內建列印器 ：
(gdb) p ptr_to_python_str
$6 = '__builtins__'
str 實例的漂亮列印器預設使用單引號（Python 的 repr 對於字串也是如此），而 char * 值的標準列印器使用雙引號並包含十六進位位址：
(gdb) p ptr_to_char_star
$7 = 0x6d72c0 "hello world"
同樣，可以透過轉換為 PyUnicodeObject* 來揭示實作細節：
(gdb) p *(PyUnicodeObject*)$6
$8 = {ob_base = {ob_refcnt = 33, ob_type = 0x3dad3a95a0}, length = 12,
str = 0x7ffff2128500, hash = 7065186196740147912, state = 1, defenc = 0x0}
py-list¶
該擴充功能新增了一個
py-list命令，該命令列出了所選執行緒中當前 frame 的 Python 原始程式碼（如果有）。當前的列會標有 ">"：(gdb) py-list 901 if options.profile: 902 options.profile = False 903 profile_me() 904 return 905 >906 u = UI() 907 if not u.quit: 908 try: 909 gtk.main() 910 except KeyboardInterrupt: 911 # properly quit on a keyboard interrupt...使用
py-list START列出 Python 原始碼中不同的列號，使用py-list START,END列出 Python 原始碼中特定範圍的列。
py-up 和 py-down¶
py-up和py-down命令類似於 GDB 的常規up和down命令，但嘗試在 CPython frame 層級移動，而不是 C frame。GDB 並不總是能夠讀取相關的 frame 資訊，這取決於編譯 CPython 的最佳化等級。在內部，這些指令會尋找正在執行預設 frame 計算 (evaluation) 函式（即 CPython 中迴圈的核心位元組碼直譯器）的 C frame，並尋找相關
PyFrameObject *的值。它們在執行緒內發出（於 C 層級的）frame 編號。
例如：
(gdb) py-up #37 Frame 0x9420b04, for file /usr/lib/python2.6/site-packages/ gnome_sudoku/main.py, line 906, in start_game () u = UI() (gdb) py-up #40 Frame 0x948e82c, for file /usr/lib/python2.6/site-packages/ gnome_sudoku/gnome_sudoku.py, line 22, in start_game(main=<module at remote 0xb771b7f4>) main.start_game() (gdb) py-up Unable to find an older python frame所以現在我們處於 Python 堆疊的頂端。
frame 編號與 GDB 標準
backtrace指令顯示的 frame 編號相對應。此指令會跳過不執行 Python 程式碼的 C frame。回到下面：
(gdb) py-down #37 Frame 0x9420b04, for file /usr/lib/python2.6/site-packages/gnome_sudoku/main.py, line 906, in start_game () u = UI() (gdb) py-down #34 (unable to read python frame information) (gdb) py-down #23 (unable to read python frame information) (gdb) py-down #19 (unable to read python frame information) (gdb) py-down #14 Frame 0x99262ac, for file /usr/lib/python2.6/site-packages/gnome_sudoku/game_selector.py, line 201, in run_swallowed_dialog (self=<NewOrSavedGameSelector(new_game_model=<gtk.ListStore at remote 0x98fab44>, puzzle=None, saved_games=[{'gsd.auto_fills': 0, 'tracking': {}, 'trackers': {}, 'notes': [], 'saved_at': 1270084485, 'game': '7 8 0 0 0 0 0 5 6 0 0 9 0 8 0 1 0 0 0 4 6 0 0 0 0 7 0 6 5 0 0 0 4 7 9 2 0 0 0 9 0 1 0 0 0 3 9 7 6 0 0 0 1 8 0 6 0 0 0 0 2 8 0 0 0 5 0 4 0 6 0 0 2 1 0 0 0 0 0 4 5\n7 8 0 0 0 0 0 5 6 0 0 9 0 8 0 1 0 0 0 4 6 0 0 0 0 7 0 6 5 1 8 3 4 7 9 2 0 0 0 9 0 1 0 0 0 3 9 7 6 0 0 0 1 8 0 6 0 0 0 0 2 8 0 0 0 5 0 4 0 6 0 0 2 1 0 0 0 0 0 4 5', 'gsd.impossible_hints': 0, 'timer.__absolute_start_time__': <float at remote 0x984b474>, 'gsd.hints': 0, 'timer.active_time': <float at remote 0x984b494>, 'timer.total_time': <float at remote 0x984b464>}], dialog=<gtk.Dialog at remote 0x98faaa4>, saved_game_model=<gtk.ListStore at remote 0x98fad24>, sudoku_maker=<SudokuMaker(terminated=False, played=[], batch_siz...(truncated) swallower.run_dialog(self.dialog) (gdb) py-down #11 Frame 0x9aead74, for file /usr/lib/python2.6/site-packages/gnome_sudoku/dialog_swallower.py, line 48, in run_dialog (self=<SwappableArea(running=<gtk.Dialog at remote 0x98faaa4>, main_page=0) at remote 0x98fa6e4>, d=<gtk.Dialog at remote 0x98faaa4>) gtk.main() (gdb) py-down #8 (unable to read python frame information) (gdb) py-down Unable to find a newer python frame我們現在處於 Python 堆疊的底部。
請注意，在 Python 3.12 及更高版本中，同一個 C 堆疊 frame 可用於多個 Python 堆疊 frame。這意味著
py-up和py-down可以一次移動多個 Python frame。例如：(gdb) py-up #6 Frame 0x7ffff7fb62b0, for file /tmp/rec.py, line 5, in recursive_function (n=0) time.sleep(5) #6 Frame 0x7ffff7fb6240, for file /tmp/rec.py, line 7, in recursive_function (n=1) recursive_function(n-1) #6 Frame 0x7ffff7fb61d0, for file /tmp/rec.py, line 7, in recursive_function (n=2) recursive_function(n-1) #6 Frame 0x7ffff7fb6160, for file /tmp/rec.py, line 7, in recursive_function (n=3) recursive_function(n-1) #6 Frame 0x7ffff7fb60f0, for file /tmp/rec.py, line 7, in recursive_function (n=4) recursive_function(n-1) #6 Frame 0x7ffff7fb6080, for file /tmp/rec.py, line 7, in recursive_function (n=5) recursive_function(n-1) #6 Frame 0x7ffff7fb6020, for file /tmp/rec.py, line 9, in <module> () recursive_function(5) (gdb) py-up Unable to find an older python frame
py-bt¶
py-bt指令嘗試顯示目前執行緒的 Python 層級回溯。例如：
(gdb) py-bt #8 (unable to read python frame information) #11 Frame 0x9aead74, for file /usr/lib/python2.6/site-packages/gnome_sudoku/dialog_swallower.py, line 48, in run_dialog (self=<SwappableArea(running=<gtk.Dialog at remote 0x98faaa4>, main_page=0) at remote 0x98fa6e4>, d=<gtk.Dialog at remote 0x98faaa4>) gtk.main() #14 Frame 0x99262ac, for file /usr/lib/python2.6/site-packages/gnome_sudoku/game_selector.py, line 201, in run_swallowed_dialog (self=<NewOrSavedGameSelector(new_game_model=<gtk.ListStore at remote 0x98fab44>, puzzle=None, saved_games=[{'gsd.auto_fills': 0, 'tracking': {}, 'trackers': {}, 'notes': [], 'saved_at': 1270084485, 'game': '7 8 0 0 0 0 0 5 6 0 0 9 0 8 0 1 0 0 0 4 6 0 0 0 0 7 0 6 5 0 0 0 4 7 9 2 0 0 0 9 0 1 0 0 0 3 9 7 6 0 0 0 1 8 0 6 0 0 0 0 2 8 0 0 0 5 0 4 0 6 0 0 2 1 0 0 0 0 0 4 5\n7 8 0 0 0 0 0 5 6 0 0 9 0 8 0 1 0 0 0 4 6 0 0 0 0 7 0 6 5 1 8 3 4 7 9 2 0 0 0 9 0 1 0 0 0 3 9 7 6 0 0 0 1 8 0 6 0 0 0 0 2 8 0 0 0 5 0 4 0 6 0 0 2 1 0 0 0 0 0 4 5', 'gsd.impossible_hints': 0, 'timer.__absolute_start_time__': <float at remote 0x984b474>, 'gsd.hints': 0, 'timer.active_time': <float at remote 0x984b494>, 'timer.total_time': <float at remote 0x984b464>}], dialog=<gtk.Dialog at remote 0x98faaa4>, saved_game_model=<gtk.ListStore at remote 0x98fad24>, sudoku_maker=<SudokuMaker(terminated=False, played=[], batch_siz...(truncated) swallower.run_dialog(self.dialog) #19 (unable to read python frame information) #23 (unable to read python frame information) #34 (unable to read python frame information) #37 Frame 0x9420b04, for file /usr/lib/python2.6/site-packages/gnome_sudoku/main.py, line 906, in start_game () u = UI() #40 Frame 0x948e82c, for file /usr/lib/python2.6/site-packages/gnome_sudoku/gnome_sudoku.py, line 22, in start_game (main=<module at remote 0xb771b7f4>) main.start_game()The frame numbers correspond to those displayed by GDB's standard
backtracecommand.
py-print¶
py-print命令查找 Python 名稱並嘗試列印它。它在當前執行緒中尋找局部變數，然後是全域變數，最後是內建變數：(gdb) py-print self local 'self' = <SwappableArea(running=<gtk.Dialog at remote 0x98faaa4>, main_page=0) at remote 0x98fa6e4> (gdb) py-print __name__ global '__name__' = 'gnome_sudoku.dialog_swallower' (gdb) py-print len builtin 'len' = <built-in function len> (gdb) py-print scarlet_pimpernel 'scarlet_pimpernel' not found如果目前 C frame 對應多個 Python frame，則
py-print僅考慮第一個。
py-locals¶
py-locals命令尋找所選執行緒中當前 Python frame 內的所有 Python 局部變數，並列印它們的表示：(gdb) py-locals self = <SwappableArea(running=<gtk.Dialog at remote 0x98faaa4>, main_page=0) at remote 0x98fa6e4> d = <gtk.Dialog at remote 0x98faaa4>如果目前 C frame 對應於多個 Python frame，則會顯示所有這些 frame 的局部變數：
(gdb) py-locals Locals for recursive_function n = 0 Locals for recursive_function n = 1 Locals for recursive_function n = 2 Locals for recursive_function n = 3 Locals for recursive_function n = 4 Locals for recursive_function n = 5 Locals for <module>
與 GDB 指令一起使用¶
擴充命令補充了 GDB 的內建命令。例如，你可以將 py-bt 顯示的 frame 編號與 frame 命令一同使用來跳到所選執行緒中的特定 frame，如下所示：
(gdb) py-bt
(output snipped)
#68 Frame 0xaa4560, for file Lib/test/regrtest.py, line 1548, in <module> ()
main()
(gdb) frame 68
#68 0x00000000004cd1e6 in PyEval_EvalFrameEx (f=Frame 0xaa4560, for file Lib/test/regrtest.py, line 1548, in <module> (), throwflag=0) at Python/ceval.c:2665
2665 x = call_function(&sp, oparg);
(gdb) py-list
1543 # Run the tests in a context manager that temporary changes the CWD to a
1544 # temporary and writable directory. If it's not possible to create or
1545 # change the CWD, the original CWD will be used. The original CWD is
1546 # available from test_support.SAVEDCWD.
1547 with test_support.temp_cwd(TESTCWD, quiet=True):
>1548 main()
info threads 命令將為你提供行程內的執行緒串列，你可以使用 thread 命令選擇不同的執行緒：
(gdb) info threads
105 Thread 0x7fffefa18710 (LWP 10260) sem_wait () at ../nptl/sysdeps/unix/sysv/linux/x86_64/sem_wait.S:86
104 Thread 0x7fffdf5fe710 (LWP 10259) sem_wait () at ../nptl/sysdeps/unix/sysv/linux/x86_64/sem_wait.S:86
* 1 Thread 0x7ffff7fe2700 (LWP 10145) 0x00000038e46d73e3 in select () at ../sysdeps/unix/syscall-template.S:82
你可以使用 thread apply all COMMAND（或簡短地用 t a a COMMAND）在所有執行緒上執行命令。使用 py-bt 你可以看到每個執行緒在 Python 層級正在做什麼：
(gdb) t a a py-bt
Thread 105 (Thread 0x7fffefa18710 (LWP 10260)):
#5 Frame 0x7fffd00019d0, for file /home/david/coding/python-svn/Lib/threading.py, line 155, in _acquire_restore (self=<_RLock(_Verbose__verbose=False, _RLock__owner=140737354016512, _RLock__block=<thread.lock at remote 0x858770>, _RLock__count=1) at remote 0xd7ff40>, count_owner=(1, 140737213728528), count=1, owner=140737213728528)
self.__block.acquire()
#8 Frame 0x7fffac001640, for file /home/david/coding/python-svn/Lib/threading.py, line 269, in wait (self=<_Condition(_Condition__lock=<_RLock(_Verbose__verbose=False, _RLock__owner=140737354016512, _RLock__block=<thread.lock at remote 0x858770>, _RLock__count=1) at remote 0xd7ff40>, acquire=<instancemethod at remote 0xd80260>, _is_owned=<instancemethod at remote 0xd80160>, _release_save=<instancemethod at remote 0xd803e0>, release=<instancemethod at remote 0xd802e0>, _acquire_restore=<instancemethod at remote 0xd7ee60>, _Verbose__verbose=False, _Condition__waiters=[]) at remote 0xd7fd10>, timeout=None, waiter=<thread.lock at remote 0x858a90>, saved_state=(1, 140737213728528))
self._acquire_restore(saved_state)
#12 Frame 0x7fffb8001a10, for file /home/david/coding/python-svn/Lib/test/lock_tests.py, line 348, in f ()
cond.wait()
#16 Frame 0x7fffb8001c40, for file /home/david/coding/python-svn/Lib/test/lock_tests.py, line 37, in task (tid=140737213728528)
f()
Thread 104 (Thread 0x7fffdf5fe710 (LWP 10259)):
#5 Frame 0x7fffe4001580, for file /home/david/coding/python-svn/Lib/threading.py, line 155, in _acquire_restore (self=<_RLock(_Verbose__verbose=False, _RLock__owner=140737354016512, _RLock__block=<thread.lock at remote 0x858770>, _RLock__count=1) at remote 0xd7ff40>, count_owner=(1, 140736940992272), count=1, owner=140736940992272)
self.__block.acquire()
#8 Frame 0x7fffc8002090, for file /home/david/coding/python-svn/Lib/threading.py, line 269, in wait (self=<_Condition(_Condition__lock=<_RLock(_Verbose__verbose=False, _RLock__owner=140737354016512, _RLock__block=<thread.lock at remote 0x858770>, _RLock__count=1) at remote 0xd7ff40>, acquire=<instancemethod at remote 0xd80260>, _is_owned=<instancemethod at remote 0xd80160>, _release_save=<instancemethod at remote 0xd803e0>, release=<instancemethod at remote 0xd802e0>, _acquire_restore=<instancemethod at remote 0xd7ee60>, _Verbose__verbose=False, _Condition__waiters=[]) at remote 0xd7fd10>, timeout=None, waiter=<thread.lock at remote 0x858860>, saved_state=(1, 140736940992272))
self._acquire_restore(saved_state)
#12 Frame 0x7fffac001c90, for file /home/david/coding/python-svn/Lib/test/lock_tests.py, line 348, in f ()
cond.wait()
#16 Frame 0x7fffac0011c0, for file /home/david/coding/python-svn/Lib/test/lock_tests.py, line 37, in task (tid=140736940992272)
f()
Thread 1 (Thread 0x7ffff7fe2700 (LWP 10145)):
#5 Frame 0xcb5380, for file /home/david/coding/python-svn/Lib/test/lock_tests.py, line 16, in _wait ()
time.sleep(0.01)
#8 Frame 0x7fffd00024a0, for file /home/david/coding/python-svn/Lib/test/lock_tests.py, line 378, in _check_notify (self=<ConditionTests(_testMethodName='test_notify', _resultForDoCleanups=<TestResult(_original_stdout=<cStringIO.StringO at remote 0xc191e0>, skipped=[], _mirrorOutput=False, testsRun=39, buffer=False, _original_stderr=<file at remote 0x7ffff7fc6340>, _stdout_buffer=<cStringIO.StringO at remote 0xc9c7f8>, _stderr_buffer=<cStringIO.StringO at remote 0xc9c790>, _moduleSetUpFailed=False, expectedFailures=[], errors=[], _previousTestClass=<type at remote 0x928310>, unexpectedSuccesses=[], failures=[], shouldStop=False, failfast=False) at remote 0xc185a0>, _threads=(0,), _cleanups=[], _type_equality_funcs={<type at remote 0x7eba00>: <instancemethod at remote 0xd750e0>, <type at remote 0x7e7820>: <instancemethod at remote 0xd75160>, <type at remote 0x7e30e0>: <instancemethod at remote 0xd75060>, <type at remote 0x7e7d20>: <instancemethod at remote 0xd751e0>, <type at remote 0x7f19e0...(truncated)
_wait()