NS编程与调试 － gdb调试工具

gdb调试工具类似于vc中的debug工具。目前有命令行的和窗口界面的。窗口界面是DDD。我只用了命令行的。

用gdb调试ns2，要注意几点：
1，安装gdb，下载gdb包，安装了就是了。
2，改写Makefile，使之称为debug版本，具体做法：
在代码里面添加调试信息：修改Makefile(没有任何后缀的)将里面
CCOPT =     #如果是ns-allinone-2.28,这里是CCOPT = -O2
STATIC =
LDFLAGS = $(STATIC)
LDOUT = -o $(BLANK)
改变为：
CCOPT = -g   #如果是ns-allinone-2.28,这里修改为 CCOPT = -O2 -g
STATIC =
LDFLAGS = $(STATIC)
LDOUT = -o $(BLANK)
修改的东西是粉红色字体所示
然后重新编译
注意：以下步骤一个都不能少
Make clean
Make depend
Make

3，调试：
建议把我们的测试脚本写在ns-allinone-2.**/ns-2.XX/mytclscript/下面
当调试时，首先进入ns-2.XX文件夹   [tengda@localhost ~]$ cd ns-allinone-2.31/ns-2.31/
然后 [tengda@localhost ns-2.31]$ gdb ns
之后会出现
Copyright (C) 2006 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i386-redhat-linux-gnu"...Using host libthread_db library "/lib/libthread_db.so.1".
 
(gdb)
然后在该平台上运行我们的脚本，可以用cd 命令进入我们脚本所在文件夹。运行我们的脚本：
(gdb) run mbs.tcl

这样就可以进行调试了。具体调试过程与命令，我转载如下：

＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝

GDB（GNU 项目调试器）可以让您了解程序在执行时“内部” 究竟在干些什么，以及在程序发生崩溃的瞬间正在做什么。

GDB 做以下 4 件主要的事情来帮助您捕获程序中的 bug：

• 在程序启动之前指定一些可以影响程序行为的变量或条件
• 在某个指定的地方或条件下暂停程序
• 在程序停止时检查已经发生了什么
• 在程序执行过程中修改程序中的变量或条件，这样就可以体验修复一个 bug 的成果，并继续了解其他 bug

 

要调试的程序可以是使用 C、C++、Pascal、Objective-C 以及其他很多语言编写的。GDB 的二进制文件名是 gdb。

gdb 中有很多命令。使用 help 命令可以列出所有的命令，以及关于如何使用这些命令的介绍。下表给出了最常用的 GDB 命令。

表 1. gdb 中最常用的命令

命令	说明	例子
help	显示命令类别	help - 显示命令类别 help breakpoints - 显示属于 breakpoints 类别的命令 help break - 显示 break 命令的解释
run	启动所调试的程序	?
kill	终止正在调试的程序的执行	通常这会在要执行的代码行已经超过了您想要调试的代码时使用。执行 kill 会重置断点，并从头再次运行这个程序
cont	所调试的程序运行到一个断点、异常或单步之后，继续执行	?
info break	显示当前的断点或观察点	?
break	在指定的行或函数处设置断点	break 93 if i=8 - 当变量 i 等于 8 时，在第 93 行停止程序执行
Step	单步执行程序，直到它到达一个不同的源代码行。您可以使用 s 来代表 step 命令	?
Next	与 step 命令类似，只是它不会“单步跟踪到”子例程中	?
print	打印一个变量或表达式的值	print pointer - 打印变量指针的内容 print *pointer - 打印指针所指向的数据结构的内容
delete	删除某些断点或自动显示表达式	delete 1 - 删除断点 1。断点可以通过 info break 来显示
watch	为一个表达式设置一个观察点。当表达式的值发生变化时，这个观察点就会暂停程序的执行	?
where	打印所有堆栈帧的栈信息	where - 不使用参数，输出当前线程的堆栈信息 where all - 输出当前线程组中所有线程的堆栈信息 where threadindex - 输出指定线程的堆栈信息
attach	开始查看一个已经运行的进程	attach - 附加到进程 process_id 上。process_id 可以使用 ps 命令找到
info thread	显示当前正在运行的线程	?
thread apply threadno command	对一个线程运行 gdb 命令	thread apply 3 where - 对线程 3 运行 where 命令
Thread threadno	选择一个线程作为当前线程	?

如果一个程序崩溃了，并生成了一个 core 文件，您可以查看 core 文件来判断进程结束时的状态。使用下面的命令启动 gdb：

# gdb programname corefilename 

要调试一个 core 文件，您需要可执行程序、源代码文件以及 core 文件。要对一个 core 文件启动 gdb，请使用 -c 选项：

# gdb -c core programname 

gdb 会显示是哪行代码导致这个程序产生了核心转储。

默 认情况下，核心转储在 Novell 的 SUSE LINUX Enterprise Server 9（SLES 9）和 Red Hat? Enterprise Linux Advanced Server（RHEL AS 4）上都是禁用的。要启用核心转储，请以 root 用户的身份在命令行中执行 ulimit –c unlimited。

清单 8 中的例子阐述了如何使用 gdb 来定位程序中的 bug。清单 8 是一段包含 bug 的 C++ 代码。

清单 8 中的 C++ 程序试图构建 10 个链接在一起的数字框（number box），例如：

图 1. 一个包含 10 个链接在一起的数字框的列表

然后试图从这个列表中逐个删除数字框。

编译并运行这个程序，如下所示：

清单 9. 编译并运行这个程序

                
# g++ -g -o gdbtest1 gdbtest1.cpp
# ./gdbtest1
Number Box "0" created
Number Box "1" created
Number Box "2" created
Number Box "3" created
Number Box "4" created
Number Box "5" created
Number Box "6" created
Number Box "7" created
Number Box "8" created
Number Box "9" created
list created
Number Box "9" deleted
Segmentation fault

正如您可以看到的一样，这个程序会导致段错误。调用 gdb 来看一下这个问题，如下所示：

清单 10. 调用 gdb

                
# gdb ./gdbtest1
GNU gdb 6.2.1
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you 
are welcome to change it and/or distribute copies of it under certain 
conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for 
details.
This GDB was configured as "ppc-suse-linux"...Using host libthread_db 
library "/lib/tls/libthread_db.so.1".
(gdb)

您知道段错误是在数字框 "9" 被删除之后发生的。执行 run 和 where 命令来精确定位段错误发生在程序中的什么位置。

清单 11. 执行 run 和 where 命令

                
(gdb) run
Starting program: /root/test/gdbtest1 
Number Box "0" created
Number Box "1" created
Number Box "2" created
Number Box "3" created
Number Box "4" created
Number Box "5" created
Number Box "6" created
Number Box "7" created
Number Box "8" created
Number Box "9" created
list created
Number Box "9" deleted
Program received signal SIGSEGV, Segmentation fault.
0x10000f74 in NumBox::GetNext (this=0x0) at gdbtest1.cpp:14
14              NumBox*GetNext() const { return Next; }
(gdb) where
#0  0x10000f74 in NumBox::GetNext (this=0x0) at gdbtest1.cpp:14
#1  0x10000d10 in NumChain::RemoveBox (this=0x10012008, 

item_to_remove=@0xffffe200) at gdbtest1.cpp:63
#2  0x10000978 in main (argc=1, argv=0xffffe554) at gdbtest1.cpp:94
(gdb)

跟踪信息显示这个程序在第 14 行 NumBox::GetNext (this=0x0) 接收到一个段错误。这个数字框上 Next 指针的地址是 0x0，这对于一个数字框来说是一个无效的地址。从上面的跟踪信息可以看出，GetNext 函数是由 63 行调用的。看一下在 gdbtest1.cpp 的 63 行附近发生了什么：

清单 12. gdbtest1.cpp

                
     54                       } else {
     55                               temp->SetNext (current->GetNext());
     56                               delete temp;
     57                               temp = 0;
     58                               return 0;
     59                       }
     60               }
     61               current = 0;
     62               temp = current;
     63               current = current->GetNext();
     64       }
     65 
     66       return -1;

第 61 行 current=0 将这个指针设置为一个无效的地址，这正是产生段错误的根源。注释掉第 61 行，将其保存为 gdbtest2.cpp，然后编译并重新运行。

清单 13. 再次运行程序（gdbtest2.cpp）

                
# g++ -g -o gdbtest2 gdbtest2.cpp
# ./gdbtest2
Number Box "0" created
Number Box "1" created
Number Box "2" created
Number Box "3" created
Number Box "4" created
Number Box "5" created
Number Box "6" created
Number Box "7" created
Number Box "8" created
Number Box "9" created
list created
Number Box "9" deleted
Number Box "0" deleted

这个程序现在可以成功完成而不会出现段错误了。然而，结果并不像我们预期的一样：程序在删除 Number Box "9"之后删除了 Number Box "0"，而不像我们期望的一样删除 Number Box "8,"。使用 gdb 再次来看一下。

清单 14. 再次使用 gdb 进行查看

                
# gdb ./gdbtest2
GNU gdb 6.2.1
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you 
are welcome to change it and/or distribute copies of it under certain 
conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for 
details.
This GDB was configured as "ppc-suse-linux"...Using host libthread_db 
library "/lib/tls/libthread_db.so.1".
(gdb) break 94 if i==8
Breakpoint 1 at 0x10000968: file gdbtest2.cpp, line 94.
(gdb) run
Starting program: /root/test/gdbtest2 
Number Box "0" created
Number Box "1" created
Number Box "2" created
Number Box "3" created
Number Box "4" created
Number Box "5" created
Number Box "6" created
Number Box "7" created
Number Box "8" created
Number Box "9" created
list created
Number Box "9" deleted
Breakpoint 1, main (argc=1, argv=0xffffe554) at gdbtest2.cpp:94
94                      list ->RemoveBox(i);

您可能希望找出为什么这个程序删除的是 Number Box 0，而不是 Number Box 8，因此需要在您认为程序会删除 Number Box 8 的地方停止程序。设置这个断点：break 94 if i==8，可以在 i 等于 8 时在第 94 行处停止程序。然后单步跟踪到 RemoveBox() 函数中。

清单 15. 单步跟踪到 RemoveBox() 函数中

                
(gdb) s
38                      NumBox *temp = 0;  
(gdb) s
40                      while (current != 0) {
(gdb) print pointer
$1 = (NumBox *) 0x100120a8
 (gdb) print *pointer
$2 = {Num = 0, Next = 0x0}
(gdb)

指针早已指向了 Number Box "0"，因此这个 bug 可能就存在于程序删除 Number Box "9" 的地方。要在 gdb 中重新启动这个程序，请使用 kill 删除原来的断点，然后添加一个 i 等于 9 时的新断点，然后再次运行这个程序。

清单 16. 在 gdb 中重新启动程序

                
(gdb) kill
Kill the program being debugged? (y or n) y
(gdb) info break
Num Type           Disp Enb Address    What
1   breakpoint     keep y   0x10000968 in main at gdbtest2.cpp:94
        stop only if i == 8
        breakpoint already hit 1 time
(gdb) delete 1
(gdb) break 94 if i==9
Breakpoint 2 at 0x10000968: file gdbtest2.cpp, line 94.
(gdb) run
Starting program: /root/test/gdbtest2 
Number Box "0" created
Number Box "1" created
Number Box "2" created
Number Box "3" created
Number Box "4" created
Number Box "5" created
Number Box "6" created
Number Box "7" created
Number Box "8" created
Number Box "9" created
list created
Breakpoint 2, main (argc=1, argv=0xffffe554) at gdbtest2.cpp:94
94                      list ->RemoveBox(i);
(gdb)

当这一次单步跟踪 RemoveBox() 函数时，要特别注意 list->pointer 正在指向哪一个数字框，因为 bug 可能就在于 list->pointer 开始指向 Number Box "0" 的地方。请使用 display *pointer 命令来查看，这会自动显示这个函数。

清单 17. 使用 display *pointer 命令进行监视

                
Breakpoint 2, main (argc=1, argv=0xffffe554) at gdbtest2.cpp:94
94            list ->RemoveBox(i);
(gdb) s
NumChain::RemoveBox (this=0x10012008, item_to_remove=@0xffffe200) 

at gdbtest2.cpp:37
37            NumBox *current = pointer;
(gdb) display *pointer
1: *this->pointer = {Num = 9, Next = 0x10012098}
(gdb) s
38            NumBox *temp = 0;  
1: *this->pointer = {Num = 9, Next = 0x10012098}
(gdb) s
40            while (current != 0) {
1: *this->pointer = {Num = 9, Next = 0x10012098}
(gdb) s
41                    if (current->GetValue() == item_to_remove) {
1: *this->pointer = {Num = 9, Next = 0x10012098}
(gdb) s
NumBox::GetValue (this=0x100120a8) at gdbtest2.cpp:16
16            const T& GetValue () const { return Num; }
(gdb) s
NumChain::RemoveBox (this=0x10012008, item_to_remove=@0xffffe200) 
 
at gdbtest2.cpp:42
42                       if (temp == 0) { 
1: *this->pointer = {Num = 9, Next = 0x10012098}
(gdb) s
44                               if (current->GetNext() == 0) {
1: *this->pointer = {Num = 9, Next = 0x10012098}
(gdb) s
NumBox::GetNext (this=0x100120a8) at gdbtest2.cpp:14
14            NumBox*GetNext() const { return Next; }
(gdb) s
NumChain::RemoveBox (this=0x10012008, item_to_remove=@0xffffe200) 

at gdbtest2.cpp:50
50                                            delete current;
1: *this->pointer = {Num = 9, Next = 0x10012098}
(gdb) s
~NumBox (this=0x100120a8) at gdbtest2.cpp:10
10            std::cout << "Number Box " <<"\""  << GetValue() 
 
<<"\""  <<" deleted" << std::endl;
(gdb) s
NumBox::GetValue (this=0x100120a8) at gdbtest2.cpp:16
16            const T& GetValue () const { return Num; }
(gdb) s
Number Box "9" deleted
~NumBox (this=0x100120a8) at gdbtest2.cpp:11
11            Next = 0;
(gdb) s
NumChain::RemoveBox (this=0x10012008, item_to_remove=@0xffffe200) 

at gdbtest2.cpp:51
51                                           current = 0;
1: *this->pointer = {Num = 0, Next = 0x0}
(gdb) s
53                                    return 0;
1: *this->pointer = {Num = 0, Next = 0x0}
(gdb) s
0x10000d1c      66                      return -1;
1: *this->pointer = {Num = 0, Next = 0x0}

从上面的跟踪过程中，您可以看到 list->pointer 在删除 Number Box "9" 之后指向了 Number Box "0"。这个逻辑并不正确，因为在删除 Number Box "9" 之后，list->pointer 应该指向的是 Number Box "8"。现在非常显然我们应该在第 50 行之前添加一条语句 pointer = pointer->GetNext();，如下所示：

清单 18. 在第 50 行之前添加一条 pointer = pointer->GetNext(); 语句

                
     49                     } else {
     50                             pointer = pointer->GetNext();
     51                             delete current;
     52                             current = 0;
     53                      }
     54                      return 0;

将新修改之后的程序保存为 gdbtest3.cpp，然后编译并再次运行。

清单 19. 再次运行程序（gdbtest3.cpp）

                
# g++ -g -o gdbtest3 gdbtest3.cpp
# ./gdbtest3
Number Box "0" created
Number Box "1" created
Number Box "2" created
Number Box "3" created
Number Box "4" created
Number Box "5" created
Number Box "6" created
Number Box "7" created
Number Box "8" created
Number Box "9" created
list created
Number Box "9" deleted
Number Box "8" deleted
Number Box "7" deleted
Number Box "6" deleted
Number Box "5" deleted
Number Box "4" deleted
Number Box "3" deleted
Number Box "2" deleted
Number Box "1" deleted
Number Box "0" deleted

这才是我们期望的结果。

多线程环境

在 GDB 中有一些特殊的命令可以用于多线程应用程序的调试。下面这个例子给出了一个死锁情况，并介绍了如何使用这些命令来检查多线程应用程序中的问题：

清单 20. 多线程的例子

                
#include 
#include "pthread.h>
pthread_mutex_t AccountA_mutex;
pthread_mutex_t AccountB_mutex;
struct BankAccount {
     char account_name[1];
     int balance;
};
struct BankAccount  accountA = {"A", 10000 };
struct BankAccount  accountB = {"B", 20000 };
void * transferAB (void* amount_ptr) {
     int amount = *((int*)amount_ptr);
     pthread_mutex_lock(&AccountA_mutex);
     if (accountA.balance < amount)   {
             printf("There is not enough memory in Account A!\n");
             pthread_mutex_unlock(&AccountA_mutex);
             pthread_exit((void *)1);
     }
     accountA.balance -=amount;
     sleep(1);
     pthread_mutex_lock(&AccountB_mutex);
     accountB.balance +=amount;
     pthread_mutex_unlock(&AccountA_mutex); 
     pthread_mutex_unlock(&AccountB_mutex);
}
void * transferBA (void* amount_ptr) {
     int amount = *((int*)amount_ptr);
     pthread_mutex_lock(&AccountB_mutex);
     if (accountB.balance < amount)   {
             printf("There is not enough memory in Account B!\n");
             pthread_mutex_unlock(&AccountB_mutex);
             pthread_exit((void *)1);
     }
     accountB.balance -=amount;
     sleep(1);
     pthread_mutex_lock(&AccountA_mutex);
     accountA.balance +=amount;
     pthread_mutex_unlock(&AccountB_mutex);
     pthread_mutex_unlock(&AccountA_mutex);
}
int main(int argc, char* argv[]) {
     int             threadid[4];
     pthread_t       pthread[4];
     int             transfer_amount[4] = {100, 200, 300, 400};
     int             final_balanceA, final_balanceB;
     final_balanceA=accountA.balance-transfer_amount[0]-

transfer_amount[1]+transfer_amount[2]+transfer_amount[3];
     final_balanceB=accountB.balance+transfer_amount[0]

+transfer_amount[1]-transfer_amount[2]-transfer_amount[3];
     if (threadid[0] = pthread_create(&pthread[0], NULL, transferAB, 
(void*)&transfer_amount[0]) " 0) {
             perror("Thread #0 creation failed.");
             exit (1);
        }
     if (threadid[1] = pthread_create(&pthread[1], NULL, transferAB, 
(void*)&transfer_amount[1]) " 0) {
             perror("Thread #1 creation failed.");
             exit (1);
     }
     if (threadid[2] = pthread_create(&pthread[2], NULL, transferBA, 
(void*)&transfer_amount[2]) < 0) {
             perror("Thread #2 creation failed.");
             exit (1);
     }
     if (threadid[3] = pthread_create(&pthread[3], NULL, transferBA, 
(void*)&transfer_amount[3]) < 0) {
             perror("Thread #3 creation failed.");
             exit (1);
     }
     printf("Transitions are in progress..");
     while ((accountA.balance != final_balanceA) && (accountB.balance 
!= final_balanceB)) {
             printf("..");
     }
     printf("\nAll the  money is transferred !!\n");
}

使用 gcc 来编译这个程序，如下所示：

# gcc -g -o gdbtest2 gdbtest2.c -L/lib/tls -lpthread

程序 gdbtest2 会挂起，不会返回一条 All the money is transferred !! 消息。

将 gdb 附加到正在运行的进程上，从而了解这个进程内部正在发生什么。

清单 21. 将 gdb 附加到正在运行的进程上

                
# ps -ef |grep gdbtest2
root      9510  8065  1 06:30 pts/1    00:00:00 ./gdbtest2
root      9516  9400  0 06:30 pts/4    00:00:00 grep gdbtest2
# gdb -pid 9510
GNU gdb 6.2.1
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you 
are welcome to change it and/or distribute copies of it under certain 
conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for 
details.
This GDB was configured as "ppc-suse-linux".
Attaching to process 9510
Reading symbols from /root/test/gdbtest2...done.
Using host libthread_db library "/lib/tls/libthread_db.so.1".
Reading symbols from /lib/tls/libpthread.so.0...done.
[Thread debugging using libthread_db enabled]
[New Thread 1073991712 (LWP 9510)]
[New Thread 1090771744 (LWP 9514)]
[New Thread 1086577440 (LWP 9513)]
[New Thread 1082383136 (LWP 9512)]
[New Thread 1078188832 (LWP 9511)]
Loaded symbols for /lib/tls/libpthread.so.0
Reading symbols from /lib/tls/libc.so.6...done.
Loaded symbols for /lib/tls/libc.so.6
Reading symbols from /lib/ld.so.1...done.
Loaded symbols for /lib/ld.so.1
0x0ff4ac40 in __write_nocancel () from /lib/tls/libc.so.6
(gdb) info thread
  5 Thread 1078188832 (LWP 9511)  0x0ffe94ec in __lll_lock_wait () 
from /lib/tls/libpthread.so.0
  4 Thread 1082383136 (LWP 9512)  0x0ffe94ec in __lll_lock_wait () 
from /lib/tls/libpthread.so.0
  3 Thread 1086577440 (LWP 9513)  0x0ffe94ec in __lll_lock_wait () 
from /lib/tls/libpthread.so.0
  2 Thread 1090771744 (LWP 9514)  0x0ffe94ec in __lll_lock_wait () 
from /lib/tls/libpthread.so.0
  1 Thread 1073991712 (LWP 9510)  0x0ff4ac40 in __write_nocancel () 
from /lib/tls/libc.so.6
(gdb)

从 info thread 命令中，我们可以了解到除了主线程（thread #1）之外的所有线程都在等待函数 __lll_lock_wait () 完成。

使用 thread apply threadno where 命令来查看每个线程到底运行到了什么地方：

清单 22. 查看每个线程运行到了什么地方

                
(gdb) thread apply 1 where
Thread 1 (Thread 1073991712 (LWP 9510)):
#0  0x0ff4ac40 in __write_nocancel () from /lib/tls/libc.so.6
#1  0x0ff4ac28 in __write_nocancel () from /lib/tls/libc.so.6
Previous frame identical to this frame (corrupt stack?)
#0  0x0ff4ac40 in __write_nocancel () from /lib/tls/libc.so.6
(gdb) thread apply 2 where
Thread 2 (Thread 1090771744 (LWP 9514)):
#0  0x0ffe94ec in __lll_lock_wait () from /lib/tls/libpthread.so.0
#1  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0
#2  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0
#3  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0
#4  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0
Previous frame inner to this frame (corrupt stack?)
#0  0x0ff4ac40 in __write_nocancel () from /lib/tls/libc.so.6
(gdb) thread apply 3 where
Thread 3 (Thread 1086577440 (LWP 9513)):
#0  0x0ffe94ec in __lll_lock_wait () from /lib/tls/libpthread.so.0
#1  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0
#2  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0
#3  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0
#4  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0
Previous frame inner to this frame (corrupt stack?)
#0  0x0ff4ac40 in __write_nocancel () from /lib/tls/libc.so.6
(gdb) thread apply 4 where
Thread 4 (Thread 1082383136 (LWP 9512)):
#0  0x0ffe94ec in __lll_lock_wait () from /lib/tls/libpthread.so.0
#1  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0
#2  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0
#3  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0
#4  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0
Previous frame inner to this frame (corrupt stack?)
#0  0x0ff4ac40 in __write_nocancel () from /lib/tls/libc.so.6
 (gdb) thread apply 5 where
Thread 5 (Thread 1078188832 (LWP 9511)):
#0  0x0ffe94ec in __lll_lock_wait () from /lib/tls/libpthread.so.0
#1  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0
#2  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0
#3  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0
#4  0x0ffe466c in pthread_mutex_lock () from /lib/tls/libpthread.so.0
Previous frame inner to this frame (corrupt stack?)
#0  0x0ff4ac40 in __write_nocancel () from /lib/tls/libc.so.6

每个线程都试图对一个互斥体进行加锁，但是这个互斥体却是不可用的，可能是因为有另外一个线程已经对其进行加锁了。从上面的证据我们可以判断程序中一定存在死锁。您还可以看到哪个线程现在拥有这个互斥体。

清单 23. 查看哪个线程拥有互斥体

                
(gdb) print AccountA_mutex
$1 = {__m_reserved = 2, __m_count = 0, __m_owner = 0x2527, 

__m_kind = 0, __m_lock  = {__status = 1, __spinlock = 0}}
(gdb) print 0x2527
$2 = 9511
(gdb) print AccountB_mutex
$3 = {__m_reserved = 2, __m_count = 0, __m_owner = 0x2529, 

__m_kind = 0, __m_lock = {__status = 1, __spinlock = 0}}
(gdb) print 0x2529
$4 = 9513
(gdb)  

从上面的命令中，我们可以看出 AccontA_mutex 是被线程 5（LWP 9511）加锁（拥有）的，而 AccontB_mutex 是被线程 3（LWP 9513）加锁（拥有）的。

为了解决上面的死锁情况，可以按照相同的顺序对互斥体进行加锁，如下所示：
清单 24. 按照相同的顺序对互斥体进行加锁

                    
.
.
void * transferAB (void* amount_ptr) {
        int amount = *((int*)amount_ptr);
        pthread_mutex_lock(&AccountA_mutex);
        pthread_mutex_lock(&AccountB_mutex);
        if (accountA.balance < amount)   {
                printf("There is not enough memory in Account A!\n");
                pthread_mutex_unlock(&AccountA_mutex);
                pthread_exit((void *)1);
        }
        accountA.balance -=amount;
        sleep(1);
        accountB.balance +=amount;
        pthread_mutex_unlock(&AccountA_mutex);
        pthread_mutex_unlock(&AccountB_mutex);
}
void * transferBA (void* amount_ptr) {
        int amount = *((int*)amount_ptr);
        pthread_mutex_lock(&AccountA_mutex);
        pthread_mutex_lock(&AccountB_mutex);
        if (accountB.balance < amount)   {
                printf("There is not enough memory in Account B!\n");
                pthread_mutex_unlock(&AccountB_mutex);
                pthread_exit((void *)1);
        }
        accountB.balance -=amount;
        sleep(1);
        accountA.balance +=amount;
        pthread_mutex_unlock(&AccountA_mutex);
        pthread_mutex_unlock(&AccountB_mutex);
}
.
.

或者对每个帐号单独进行加锁，如下所示：
清单 25. 对每个帐号单独进行加锁

                    
.
.
void * transferAB (void* amount_ptr) {
        int amount = *((int*)amount_ptr);
        pthread_mutex_lock(&AccountA_mutex);
        if (accountA.balance < amount)   {
                printf("There is not enough memory in Account A!\n");
                pthread_mutex_unlock(&AccountA_mutex);
                pthread_exit((void *)1);
        }
        accountA.balance -=amount;
        sleep(1);
        pthread_mutex_unlock(&AccountA_mutex);
        pthread_mutex_lock(&AccountB_mutex); 
        accountB.balance +=amount;
        pthread_mutex_unlock(&AccountB_mutex);
}
void * transferBA (void* amount_ptr) {
        int amount = *((int*)amount_ptr);
        pthread_mutex_lock(&AccountB_mutex);
        if (accountB.balance < amount)   {
                printf("There is not enough memory in Account B!\n");
                pthread_mutex_unlock(&AccountB_mutex);
                pthread_exit((void *)1);
        }
        accountB.balance -=amount;
        sleep(1);
        pthread_mutex_unlock(&AccountB_mutex);
        pthread_mutex_lock(&AccountA_mutex); 
        accountA.balance +=amount;
        pthread_mutex_unlock(&AccountA_mutex);
}
.
.
.

＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝