↑ 點選藍字 關注視學算法
作者丨薛定谔的喵@知乎
來源丨https://zhuanlan.zhihu.com/p/152274203
編輯丨極市平台
你還在用GDB調試程式嗎？
如果是，那麼我們是同道中人。但是你知道GDB有一個很強大的功能，Python scripting嘛？
如果是的，那麼恭喜你，你是一個大牛。
本文主要講述如何使用Python來提高你的GDB調試技能， 讓你從繁重的重複的工作裡面掙脫出來呼吸新鮮空氣。
首先，第一件事，使用gdb7.x以上的版本，最好9.x的。因為Python的支援是從gdb7.0（2009年？）開始的。
進入正題
gdb本來就支援自定義腳本輔助調試，為什麼還要用Python腳本呢？因為自定義腳本的文法比較老，不如寫Python歡快。如果你喜歡用原來的自定義腳本方法，那也是可以的。
借助Python，你可以将難看的資料變得好看，
借助Python，你可以将重複的工作變成一個指令，
借助Python，你可以更快的調試bug，
借助Python，你可以裝逼，哈哈哈……
将難看的資料變得好看
以下面的代碼為例：
#include <map>
#include <iostream>
#include <string>
using namespace std;
int main() {
std::map<string, string> lm;
lm["good"] = "heart";
// 檢視map 裡面内容
std::cout<<lm["good"];
}
當代碼運作到std<<cout時, 你想檢視map裡面的内容，如果沒有python和自定義的腳本，print lm看到的是
$2 = {_M_t = {
_M_impl = {<std::allocator<std::_Rb_tree_node<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >> = {<__gnu_cxx::new_allocator<std::_Rb_tree_node<std::pair<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >> = {<No data fields>}, <No data fields>}, <std::_Rb_tree_key_compare<std::less<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >> = {
_M_key_compare = {<std::binary_function<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool>> = {<No data fields>}, <No data fields>}}, <std::_Rb_tree_header> = {_M_header = {
_M_color = std::_S_red, _M_parent = 0x55555556eeb0,
_M_left = 0x55555556eeb0, _M_right = 0x55555556eeb0},
_M_node_count = 1}, <No data fields>}}}
但是當你在gdb9.2裡面輸入print lm的時候，你看到的将是
(gdb) p lm
$3 = std::map with 1 element = {["good"] = "heart"}
map裡面有什麼一清二楚。這是因為gdb9.x自帶了一系列标準庫的Python pretty priniter。如果你使用的是gdb7.x，那麼你可以手動的導入這些pretty printer實作同樣的效果。具體步驟如下：
下載下傳pretty printer: svn co svn://http://gcc.gnu.org/svn/gcc/trunk/libstdc++-v3/python
在gdb裡面輸入(将路徑改成你下載下傳的路徑)：
python
import sys
sys.path.insert(0, '/home/maude/gdb_printers/python')
from libstdcxx.v6.printers import register_libstdcxx_printers
register_libstdcxx_printers (None)
end
這樣你就可以放心使用了~
詳細請看：
https://sourceware.org/gdb/wiki/STLSupport
https://codeyarns.com/2014/07/17/how-to-enable-pretty-printing-for-stl-in-gdb/
将重複的工作變成一個指令
比如在調試的時候，你知道目前棧指向一個字元串，但是你不知道具體在哪裡，你想周遊這個棧将它找出來，那麼你可以借助Python自定義一個指令"stackwalk"，這個指令可以直接Python代碼周遊棧，将字元串找出來。
#####################################################
# Usage: to load this to gdb run:
# (gdb) source ..../path/to/<script_file>.py
import gdb
class StackWalk(gdb.Command):
def __init__(self):
# This registers our class as "StackWalk"
super(StackWalk, self).__init__("stackwalk", gdb.COMMAND_DATA)
def invoke(self, arg, from_tty):
# When we call "StackWalk" from gdb, this is the method
# that will be called.
print("Hello from StackWalk!")
# get the register
rbp = gdb.parse_and_eval('$rbp')
rsp = gdb.parse_and_eval('$rsp')
ptr = rsp
ppwc = gdb.lookup_type('wchar_t').pointer().pointer()
while ptr < rbp:
try:
print('pointer is {}'.format(ptr))
print(gdb.execute('wc_print {}'.format(ptr.cast(ppwc).dereference())))
print('===')
except:
pass
ptr += 8
# This registers our class to the gdb runtime at "source" time.
StackWalk()
Note: wc_print是我寫的另外一個簡單Python指令，用于列印給定位址的寬字元串，具體實作留作習題~
更快地調試bug
當你調試多線程的時候，你發現callstack 一堆，而且好多都是重複的，如果它們可以自動去重或者折疊多好，這樣你隻需要關注一小部分。好消息！Python可以讓你用一個指令就可以輕松搞定。而且已經有人寫好了相應的代碼，你隻需要導入即可。詳細介紹請看：
https://fy.blackhats.net.au/blog/html/2017/08/04/so_you_want_to_script_gdb_with_python.html
# From https://fy.blackhats.net.au/blog/html/2017/08/04/so_you_want_to_script_gdb_with_python.html
#####################################################
#
# Usage: to load this to gdb run:
# (gdb) source ..../path/to/debug_naughty.py
#
# To have this automatically load, you need to put the script
# in a path related to your binary. If you make /usr/sbin/foo,
# You can ship this script as:
# /usr/share/gdb/auto-load/ <PATH TO BINARY>
# /usr/share/gdb/auto-load/usr/sbin/foo
#
# This will trigger gdb to autoload the script when you start
# to acces a core or the live binary from this location.
#
import gdb
class StackFold(gdb.Command):
def __init__(self):
super(StackFold, self).__init__("stackfold", gdb.COMMAND_DATA)
def invoke(self, arg, from_tty):
# An inferior is the 'currently running applications'. In this case we only
# have one.
stack_maps = {}
# This creates a dict where each element is keyed by backtrace.
# Then each backtrace contains an array of "frames"
#
inferiors = gdb.inferiors()
for inferior in inferiors:
for thread in inferior.threads():
try:
# Change to our threads context
thread.switch()
# Get the thread IDS
(tpid, lwpid, tid) = thread.ptid
gtid = thread.num
# Take a human readable copy of the backtrace, we'll need this for display later.
o = gdb.execute('bt', to_string=True)
# Build the backtrace for comparison
backtrace = []
gdb.newest_frame()
cur_frame = gdb.selected_frame()
while cur_frame is not None:
if cur_frame.name() is not None:
backtrace.append(cur_frame.name())
cur_frame = cur_frame.older()
# Now we have a backtrace like ['pthread_cond_wait@@GLIBC_2.3.2', 'lazy_thread', 'start_thread', 'clone']
# dicts can't use lists as keys because they are non-hashable, so we turn this into a string.
# Remember, C functions can't have spaces in them ...
s_backtrace = ' '.join(backtrace)
# Let's see if it exists in the stack_maps
if s_backtrace not in stack_maps:
stack_maps[s_backtrace] = []
# Now lets add this thread to the map.
stack_maps[s_backtrace].append({'gtid': gtid, 'tpid' : tpid, 'bt': o} )
except Exception as e:
print(e)
# Now at this point we have a dict of traces, and each trace has a "list" of pids that match. Let's display them
for smap in stack_maps:
# Get our human readable form out.
o = stack_maps[smap][0]['bt']
for t in stack_maps[smap]:
# For each thread we recorded
print("Thread %s (LWP %s))" % (t['gtid'], t['tpid']))
print(o)
# This registers our class to the gdb runtime at "source" time.
StackFold()
等等！還有好多，畢竟Python圖靈完備，隻要GDB提供相應的API,你想要啥都能實作。
會了這些，你就可以向新手裝逼去了~
References
https://undo.io/resources/gdb-watchpoint/python-gdb/
https://codeyarns.com/2014/07/17/how-to-enable-pretty-printing-for-stl-in-gdb/
覺得有用麻煩給個在看啦~
