需求
想要了解模組之間函式呼叫的關係時, 與其一層層比對多個類別之間的呼叫關係, 不如直接在最後一個呼叫函式放中斷點, 直接顯示 backtrace。但是當函式裡有太多參數或 template 時, backtrace 的 frame 訊息會變得很長, 不易閱讀。我的目的只是找出呼叫的函式名稱、檔名和行數, 函式帶的參數反而是困擾。
作法一: 用 gdb.execute()
一個簡單的作法是截取 gdb 的輸出, 然後解析文字去掉不要的部份:
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| import gdb | |
| import re | |
| class ShorternBacktraceCommand(gdb.Command): | |
| '''Show a backtrace without argument info in each frame.''' | |
| def __init__(self): | |
| super(ShorternBacktraceCommand, self).__init__ ("bt", | |
| gdb.COMMAND_SUPPORT, | |
| gdb.COMPLETE_NONE) | |
| def invoke(self, arg, from_tty): | |
| if not arg: | |
| arg = '' | |
| raw = gdb.execute("backtrace %s" % arg, True, True) | |
| lines = raw.split('\n') | |
| for i, line in enumerate(lines): | |
| if not line: | |
| continue | |
| tokens = line.split() | |
| # first line format: e.g., #0 A::hello (...) at a.cpp:8 | |
| # the rest : e.g., #2 0x0..0 in A::foo (...) at a.cpp:18 | |
| func_index = 1 if i == 0 else 3 | |
| print ('\033[1;33m%2s\033[m %s at %s' | |
| '' % (tokens[0], tokens[func_index], tokens[-1])) | |
| ShorternBacktraceCommand() |
Btw, 上面的作法還順便幫行首的標號上色。
但是, 使用 cgdb 時會無法運作, 理由是 cgdb 使用 GDB MI, gdb.execute('backtrace') 的結果不是原本看到的格式, 難以解析。
作法二: 用 gdb.Frame() API
只好改用中規中矩的方式逐一讀取 frame, 取出需要的資訊:
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| import gdb | |
| class ShorternBacktraceCommand(gdb.Command): | |
| '''Show a backtrace without argument info in each frame.''' | |
| def __init__(self): | |
| super(ShorternBacktraceCommand, self).__init__ ("bt", | |
| gdb.COMMAND_SUPPORT, | |
| gdb.COMPLETE_NONE) | |
| def invoke(self, arg, from_tty): | |
| num = 0; | |
| try: | |
| num = int(arg) | |
| except Exception, e: | |
| pass | |
| lines = [] | |
| f = gdb.newest_frame() | |
| fn = 0 | |
| while f is not None: | |
| symtab_and_line = gdb.Frame.find_sal(f) | |
| frame_name = gdb.Frame.name(f) | |
| if frame_name: | |
| args = [ | |
| fn, | |
| frame_name, | |
| symtab_and_line.symtab.filename, | |
| symtab_and_line.line, | |
| ] | |
| else: | |
| args = [fn, '??', 'unknown', 0] | |
| lines.append('#%2d %s at %s:%s' % tuple(args)) | |
| f = gdb.Frame.older(f) | |
| fn += 1 | |
| if num > 0: | |
| lines = lines[:num] | |
| elif num < 0: | |
| lines = lines[len(lines) + num:] | |
| for line in lines: | |
| print line | |
| ShorternBacktraceCommand() |
將上面的 script 存到 /path/to/gdb/scripts/backtrace.py, 接著在 $HOME/.gdbinit 裡加入以下設定:
python sys.path.insert(0, '/path/to/gdb/scripts') import backtrace end
之後就能用 bt 顯示精簡後的 backtrace 了, 也方便手動複製貼上到筆記裡。以下是一個輸出例子:
(gdb) bt # 0 A::hello at a.cpp:8 # 1 A::bar at a.cpp:13 # 2 A::foo at a.cpp:18 # 3 main at a.cpp:25
Btw, 若是需求比較簡單, 可以試看看 Print Settings, 有些選項可以改變 backtrace 顯示的訊息。
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