多线程调试必杀技

作者：破砂锅 (转)

开源的GDB被广泛使用在Linux、OSX、Unix和各种嵌入式系统（例如手机），这次它又带给我们一个惊喜。

多线程调试之痛

调试器（如VS2008和老版GDB）往往只支持all-stop模式，调试多线程程序时，如果某个线程断在一个断点上，你的调试器会让整个程序freeze，直到你continue这个线程，程序中的其他线程才会继续运行。这个限制使得被调试的程序不能够像真实环境中那样运行--当某个线程断在一个断点上，让其他线程并行运行。

GDBv7.0引入的non-stop模式使得这个问题迎刃而解。在这个模式下，

当某个或多个线程断在一个断点上，其他线程仍会并行运行
你可以选择某个被断的线程，并让它继续运行

让我们想象一下，有了这个功能后

当其他线程断在断点上时，程序里的定时器线程可以正常的运行了，从而避免不必要得超时
当其他线程断在断点上时，程序里的watchdog线程可以正常的运行了，从而避免嵌入式硬件以为系统崩溃而重启
可以控制多个线程运行的顺序，从而重现deadlock场景了。由于GDB可以用python脚本驱动调试，理论上可以对程序在不同的线程运行顺序下进行自动化测试。

因此，non-stop模式理所当然成为多线程调试“必杀技”。这2009年下半年之后发布的Linux版本里都带有GDBv7.0之后的版本。很好奇，不知道VS2010里是不是也支持类似的调试模式了。

演示GDB的non-stop模式

让破砂锅用一个C++小程序在Ubuntu Linux 09.10下demo这个必杀技。虽然我的demo使用命令行版gdb，如果你喜欢图形化的调试器，Eclipse2009年5月之后的版本可以轻松的调用这个功能，详情参见Eclipse参见http://live.eclipse.org/node/723

1. 编译以下程序nonstop

1 // gdb non-stop mode demo 2 // build instruction: g++ -g -o nonstop nonstop.cpp -lboost_thread 3 4 #include <iostream> 5 #include <boost/thread/thread.hpp> 6 7 struct op 8 { 9 op(int id): m_id(id) {} 10 11 void operator()() 12 { 13 std::cout << m_id << " begin" << std::endl; 14 std::cout << m_id << " end" << std::endl; 15 } 16 17 int m_id; 18 }; 19 20 int main(int argc, char ** argv) 21 { 22 boost::thread t1(op(1)), t2(op(2)), t3(op(3)); 23 t1.join(); t2.join(); t3.join(); 24 return 0; 25 } 26

2. 把一下3行添加到~/.gdbinit来打开non-stop模式

set target-async 1 set pagination off set non-stop on

3. 启动gdb,设断点,运行.可以看到主线程1是running,3个子线程都断在断点上,而不是只有一个子线程断在断点上.

~/devroot/nonstop$ gdb ./nonstop GNU gdb (GDB) 7.0-ubuntu Reading symbols from /home/frankwu/devroot/nonstop/nonstop...done. (gdb) break 14 Breakpoint 1 at 0x402058: file nonstop.cpp, line 14. (gdb) break 24 Breakpoint 3 at 0x401805: file nonstop.cpp, line 24. (gdb) run Starting program: /home/frankwu/devroot/nonstop/nonstop [Thread debugging using libthread_db enabled] [New Thread 0x7ffff6c89910 (LWP 2762)] [New Thread 0x7ffff6488910 (LWP 2763)] 1 begin Breakpoint 1, op::operator() (this=0x605118) at nonstop.cpp:14 14 std::cout << m_id << " end" << std::endl; 2 begin Breakpoint 1, op::operator() (this=0x605388) at nonstop.cpp:14 14 std::cout << m_id << " end" << std::endl; [New Thread 0x7ffff5c87910 (LWP 2764)] 3 begin Breakpoint 1, op::operator() (this=0x605618) at nonstop.cpp:14 14 std::cout << m_id << " end" << std::endl; (gdb) info threads 4 Thread 0x7ffff5c87910 (LWP 2764) op::operator() (this=0x605618) at nonstop.cpp:14 3 Thread 0x7ffff6488910 (LWP 2763) op::operator() (this=0x605388) at nonstop.cpp:14 2 Thread 0x7ffff6c89910 (LWP 2762) op::operator() (this=0x605118) at nonstop.cpp:14 * 1 Thread 0x7ffff7fe3710 (LWP 2759) (running)

4. 让线程3继续运行,注意我顾意把主线程1也continue,这是我发现的workaround,否则gdb不能切回thread 1.

(gdb) thread apply 3 1 continue
Thread 3 (Thread 0x7ffff6488910 (LWP 2763)): Continuing.
Thread 1 (Thread 0x7ffff7fe3710 (LWP 2759)): Continuing. Cannot execute this command while the selected thread is running. 2 end [Thread 0x7ffff6488910 (LWP 2763) exited]
warning: Unknown thread 3.
Thread 1 (Thread 0x7ffff7fe3710 (LWP 2759)): Continuing. Cannot execute this command while the selected thread is running. (gdb) info threads 4 Thread 0x7ffff5c87910 (LWP 2764) op::operator() (this=0x605618) at nonstop.cpp:14 2 Thread 0x7ffff6c89910 (LWP 2762) op::operator() (this=0x605118) at nonstop.cpp:14 * 1 Thread 0x7ffff7fe3710 (LWP 2759) (running)

5. 让另外两个线程继续运行而结束,主线程断在第24行,最后结束.

(gdb) thread apply 4 2 1 continue
Thread 4 (Thread 0x7ffff5c87910 (LWP 2764)): Continuing.
Thread 2 (Thread 0x7ffff6c89910 (LWP 2762)): Continuing.
Thread 1 (Thread 0x7ffff7fe3710 (LWP 2759)): Continuing. Cannot execute this command while the selected thread is running. 3 end 1 end [Thread 0x7ffff5c87910 (LWP 2764) exited] [Thread 0x7ffff6c89910 (LWP 2762) exited]
Breakpoint 3, main (argc=1, argv=0x7fffffffe348) at nonstop.cpp:24 24 return 0; (gdb) continue Thread 1 (Thread 0x7ffff7fe3710 (LWP 2759)): Continuing.
Program exited normally.

参考资料

Debugging with GDB

Reverse Debugging, Multi-Process and Non-Stop Debugging Come to the CDT