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gdb
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===
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功能强大的程序调试器
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## 补充说明
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**gdb命令** 包含在GNU的gcc开发套件中，是功能强大的程序调试器。GDB中的命令固然很多，但我们只需掌握其中十个左右的命令，就大致可以完成日常的基本的程序调试工作。
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<table border="0" cellpadding="0" cellspacing="0">
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<tbody>
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<tr>
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<th>命令</th>
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<th>解释</th>
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<th>示例</th>
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</tr>
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<tr>
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<td>file <文件名></td>
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<td>加载被调试的可执行程序文件。
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因为一般都在被调试程序所在目录下执行GDB，因而文本名不需要带路径。</td>
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<td>(gdb) file gdb-sample</td>
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</tr>
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<tr>
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<td>r</td>
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<td>Run的简写，运行被调试的程序。
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如果此前没有下过断点，则执行完整个程序；如果有断点，则程序暂停在第一个可用断点处。</td>
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<td>(gdb) r</td>
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</tr>
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<tr>
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<td>c</td>
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<td>Continue的简写，继续执行被调试程序，直至下一个断点或程序结束。</td>
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<td>(gdb) c</td>
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</tr>
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<tr>
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<td>b <行号>
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b <函数名称>
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b *<函数名称>
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b *<代码地址> d [编号]</td>
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<td>b: Breakpoint的简写，设置断点。两可以使用“行号”“函数名称”“执行地址”等方式指定断点位置。
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其中在函数名称前面加“*”符号表示将断点设置在“由编译器生成的prolog代码处”。如果不了解汇编，可以不予理会此用法。 d: Delete breakpoint的简写，删除指定编号的某个断点，或删除所有断点。断点编号从1开始递增。</td>
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<td>(gdb) b 8
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(gdb) b main
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(gdb) b *main
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(gdb) b *0x804835c (gdb) d</td>
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</tr>
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<tr>
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<td>s, n</td>
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<td>s: 执行一行源程序代码，如果此行代码中有函数调用，则进入该函数；
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n: 执行一行源程序代码，此行代码中的函数调用也一并执行。 s 相当于其它调试器中的“Step Into (单步跟踪进入)”；
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n 相当于其它调试器中的“Step Over (单步跟踪)”。 这两个命令必须在有源代码调试信息的情况下才可以使用（GCC编译时使用“-g”参数）。</td>
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<td>(gdb) s
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(gdb) n</td>
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</tr>
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<tr>
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<td>si, ni</td>
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<td>si命令类似于s命令，ni命令类似于n命令。所不同的是，这两个命令（si/ni）所针对的是汇编指令，而s/n针对的是源代码。</td>
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<td>(gdb) si
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(gdb) ni</td>
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</tr>
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<tr>
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<td>p <变量名称></td>
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<td>Print的简写，显示指定变量（临时变量或全局变量）的值。</td>
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<td>(gdb) p i
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(gdb) p nGlobalVar</td>
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</tr>
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<tr>
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<td>display ... undisplay <编号></td>
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<td>display，设置程序中断后欲显示的数据及其格式。
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例如，如果希望每次程序中断后可以看到即将被执行的下一条汇编指令，可以使用命令
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“display /i $pc”
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其中 $pc 代表当前汇编指令，/i 表示以十六进行显示。当需要关心汇编代码时，此命令相当有用。 undispaly，取消先前的display设置，编号从1开始递增。</td>
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<td>(gdb) display /i $pc (gdb) undisplay 1</td>
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</tr>
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<tr>
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<td>i</td>
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<td>info的简写，用于显示各类信息，详情请查阅“help i”。</td>
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<td>(gdb) i r</td>
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</tr>
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<tr>
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<td>q</td>
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<td>Quit的简写，退出GDB调试环境。</td>
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<td>(gdb) q</td>
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</tr>
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<tr>
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<td>help [命令名称]</td>
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<td>GDB帮助命令，提供对GDB名种命令的解释说明。
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如果指定了“命令名称”参数，则显示该命令的详细说明；如果没有指定参数，则分类显示所有GDB命令，供用户进一步浏览和查询。</td>
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<td>(gdb) help</td>
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</tr>
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</tbody>
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</table>
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### 语法
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```
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gdb(选项)(参数)
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```
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```
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-cd：设置工作目录；
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-q：安静模式，不打印介绍信息和版本信息；
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-d：添加文件查找路径；
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-x：从指定文件中执行GDB指令；
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-s：设置读取的符号表文件。
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```
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### 参数
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文件：二进制可执行程序。
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### 实例
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以下是linux下dgb调试的一个实例，先给出一个示例用的小程序，C语言代码：
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```
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#include <stdio.h>
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int nGlobalVar = 0;
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int tempFunction(int a, int b)
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{
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printf("tempFunction is called, a = %d, b = %d /n", a, b);
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return (a + b);
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}
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int main()
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{
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int n;
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n = 1;
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n++;
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n--;
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nGlobalVar += 100;
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nGlobalVar -= 12;
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printf("n = %d, nGlobalVar = %d /n", n, nGlobalVar);
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n = tempFunction(1, 2);
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printf("n = %d", n);
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return 0;
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}
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```
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请将此代码复制出来并保存到文件 gdb-sample.c 中，然后切换到此文件所在目录，用GCC编译之：
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```
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gcc gdb-sample.c -o gdb-sample -g
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```
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在上面的命令行中，使用 -o 参数指定了编译生成的可执行文件名为 gdb-sample，使用参数 -g 表示将源代码信息编译到可执行文件中。如果不使用参数 -g，会给后面的GDB调试造成不便。当然，如果我们没有程序的源代码，自然也无从使用 -g 参数，调试/跟踪时也只能是汇编代码级别的调试/跟踪。
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下面“gdb”命令启动GDB，将首先显示GDB说明，不管它：
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```
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GNU gdb Red Hat Linux (5.3post-0.20021129.18rh)
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Copyright 2003 free Software Foundation, Inc.
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GDB is free software, covered by the GNU General Public License, and you are
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welcome to change it and/or distribute copies of it under certain conditions.
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type "show copying" to see the conditions.
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There is absolutely no warranty for GDB. Type "show warranty" for details.
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This GDB was configured as "i386-redhat-linux-gnu".
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(gdb)
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```
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上面最后一行“(gdb)”为GDB内部命令引导符，等待用户输入GDB命令。
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下面使用“file”命令载入被调试程序 gdb-sample（这里的 gdb-sample 即前面 GCC 编译输出的可执行文件）：
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```
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(gdb) file gdb-sample
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Reading symbols from gdb-sample...done.
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```
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上面最后一行提示已经加载成功。
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下面使用“r”命令执行（Run）被调试文件，因为尚未设置任何断点，将直接执行到程序结束：
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```
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(gdb) r
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Starting program: /home/liigo/temp/test_jmp/test_jmp/gdb-sample
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n = 1, nGlobalVar = 88
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tempFunction is called, a = 1, b = 2
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n = 3
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Program exited normally.
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```
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下面使用“b”命令在 main 函数开头设置一个断点（Breakpoint）：
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```
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(gdb) b main
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Breakpoint 1 at 0x804835c: file gdb-sample.c, line 19.
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```
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上面最后一行提示已经成功设置断点，并给出了该断点信息：在源文件 gdb-sample.c 第19行处设置断点；这是本程序的第一个断点（序号为1）；断点处的代码地址为 0x804835c（此值可能仅在本次调试过程中有效）。回过头去看源代码，第19行中的代码为“n = 1”，恰好是 main 函数中的第一个可执行语句（前面的“int n;”为变量定义语句，并非可执行语句）。
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再次使用“r”命令执行（Run）被调试程序：
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```
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(gdb) r
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Starting program: /home/liigo/temp/gdb-sample
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Breakpoint 1, main () at gdb-sample.c:19
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19 n = 1;
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```
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程序中断在gdb-sample.c第19行处，即main函数是第一个可执行语句处。
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上面最后一行信息为：下一条将要执行的源代码为“n = 1;”，它是源代码文件gdb-sample.c中的第19行。
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下面使用“s”命令（Step）执行下一行代码（即第19行“n = 1;”）：
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|
```
|
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|
(gdb) s
|
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|
|
20 n++;
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|
```
|
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|
上面的信息表示已经执行完“n = 1;”，并显示下一条要执行的代码为第20行的“n++;”。
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既然已经执行了“n = 1;”，即给变量 n 赋值为 1，那我们用“p”命令（Print）看一下变量 n 的值是不是 1 ：
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|
```
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|
(gdb) p n
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|
$1 = 1
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|
```
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果然是 1。（$1大致是表示这是第一次使用“p”命令——再次执行“p n”将显示“$2 = 1”——此信息应该没有什么用处。）
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|
下面我们分别在第26行、tempFunction 函数开头各设置一个断点（分别使用命令“b 26”“b tempFunction”）：
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|
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|
```
|
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|
(gdb) b 26
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|
Breakpoint 2 at 0x804837b: file gdb-sample.c, line 26.
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|
(gdb) b tempFunction
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|
Breakpoint 3 at 0x804832e: file gdb-sample.c, line 12.
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|
```
|
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|
使用“c”命令继续（Continue）执行被调试程序，程序将中断在第二 个断点（26行），此时全局变量 nGlobalVar 的值应该是 88；再一次执行“c”命令，程序将中断于第三个断点（12行，tempFunction 函数开头处），此时tempFunction 函数的两个参数 a、b 的值应分别是 1 和 2：
|
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|
|
|
```
|
|
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|
|
(gdb) c
|
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|
|
|
Continuing.
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|
Breakpoint 2, main () at gdb-sample.c:26
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|
26 printf("n = %d, nGlobalVar = %d /n", n, nGlobalVar);
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|
(gdb) p nGlobalVar
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|
$2 = 88
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|
|
(gdb) c
|
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|
|
Continuing.
|
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|
|
n = 1, nGlobalVar = 88
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|
|
Breakpoint 3, tempFunction (a=1, b=2) at gdb-sample.c:12
|
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|
12 printf("tempFunction is called, a = %d, b = %d /n", a, b);
|
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|
(gdb) p a
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|
$3 = 1
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|
(gdb) p b
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|
|
$4 = 2
|
|
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|
|
|
```
|
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|
|
上面反馈的信息一切都在我们预料之中~~
|
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|
再一次执行“c”命令（Continue），因为后面再也没有其它断点，程序将一直执行到结束：
|
|
|
|
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|
|
|
|
|
|
|
```
|
|
|
|
|
|
(gdb) c
|
|
|
|
|
|
Continuing.
|
|
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|
|
|
tempFunction is called, a = 1, b = 2
|
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|
|
n = 3
|
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|
|
Program exited normally.
|
|
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|
|
|
```
|
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|
|
有时候需要看到编译器生成的汇编代码，以进行汇编级的调试或跟踪，又该如何操作呢？
|
|
|
|
|
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|
|
这就要用到display命令“display /i $pc”了（此命令前面已有详细解释）：
|
|
|
|
|
|
|
|
|
|
|
|
```
|
|
|
|
|
|
(gdb) display /i $pc
|
|
|
|
|
|
(gdb)
|
|
|
|
|
|
```
|
|
|
|
|
|
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|
|
此后程序再中断时，就可以显示出汇编代码了：
|
|
|
|
|
|
|
|
|
|
|
|
```
|
|
|
|
|
|
(gdb) r
|
|
|
|
|
|
Starting program: /home/liigo/temp/test_jmp/test_jmp/gdb-sample
|
|
|
|
|
|
|
|
|
|
|
|
Breakpoint 1, main () at gdb-sample.c:19
|
|
|
|
|
|
19 n = 1;
|
|
|
|
|
|
1: x/i $pc 0x804835c <main+16>: movl $0x1,0xfffffffc(%ebp)
|
|
|
|
|
|
```
|
|
|
|
|
|
|
|
|
|
|
|
看到了汇编代码，“n = 1;”对应的汇编代码是“movl $0x1,0xfffffffc(%ebp)”。
|
|
|
|
|
|
|
|
|
|
|
|
并且以后程序每次中断都将显示下一条汇编指定（“si”命令用于执行一条汇编代码——区别于“s”执行一行C代码）：
|
|
|
|
|
|
|
|
|
|
|
|
```
|
|
|
|
|
|
(gdb) si
|
|
|
|
|
|
20 n++;
|
|
|
|
|
|
1: x/i $pc 0x8048363 <main+23>: lea 0xfffffffc(%ebp),%eax
|
|
|
|
|
|
(gdb) si
|
|
|
|
|
|
0x08048366 20 n++;
|
|
|
|
|
|
1: x/i $pc 0x8048366 <main+26>: incl (%eax)
|
|
|
|
|
|
(gdb) si
|
|
|
|
|
|
21 n--;
|
|
|
|
|
|
1: x/i $pc 0x8048368 <main+28>: lea 0xfffffffc(%ebp),%eax
|
|
|
|
|
|
(gdb) si
|
|
|
|
|
|
0x0804836b 21 n--;
|
|
|
|
|
|
1: x/i $pc 0x804836b <main+31>: decl (%eax)
|
|
|
|
|
|
(gdb) si
|
|
|
|
|
|
23 nGlobalVar += 100;
|
|
|
|
|
|
1: x/i $pc 0x804836d <main+33>: addl $0x64,0x80494fc
|
|
|
|
|
|
```
|
|
|
|
|
|
|
|
|
|
|
|
接下来我们试一下命令“b *<函数名称>”。
|
|
|
|
|
|
|
|
|
|
|
|
为了更简明，有必要先删除目前所有断点（使用“d”命令——Delete breakpoint）：
|
|
|
|
|
|
|
|
|
|
|
|
```
|
|
|
|
|
|
(gdb) d
|
|
|
|
|
|
Delete all breakpoints? (y or n) y
|
|
|
|
|
|
(gdb)
|
|
|
|
|
|
```
|
|
|
|
|
|
|
|
|
|
|
|
当被询问是否删除所有断点时，输入“y”并按回车键即可。
|
|
|
|
|
|
|
|
|
|
|
|
下面使用命令“b *main”在 main 函数的 prolog 代码处设置断点（prolog、epilog，分别表示编译器在每个函数的开头和结尾自行插入的代码）：
|
|
|
|
|
|
|
|
|
|
|
|
```
|
|
|
|
|
|
(gdb) b *main
|
|
|
|
|
|
Breakpoint 4 at 0x804834c: file gdb-sample.c, line 17.
|
|
|
|
|
|
(gdb) r
|
|
|
|
|
|
The program being debugged has been started already.
|
|
|
|
|
|
Start it from the beginning? (y or n) y
|
|
|
|
|
|
Starting program: /home/liigo/temp/test_jmp/test_jmp/gdb-sample
|
|
|
|
|
|
|
|
|
|
|
|
Breakpoint 4, main () at gdb-sample.c:17
|
|
|
|
|
|
17 {
|
|
|
|
|
|
1: x/i $pc 0x804834c <main>: push %ebp
|
|
|
|
|
|
(gdb) si
|
|
|
|
|
|
0x0804834d 17 {
|
|
|
|
|
|
1: x/i $pc 0x804834d <main+1>: mov %esp,%ebp
|
|
|
|
|
|
(gdb) si
|
|
|
|
|
|
0x0804834f in main () at gdb-sample.c:17
|
|
|
|
|
|
17 {
|
|
|
|
|
|
1: x/i $pc 0x804834f <main+3>: sub $0x8,%esp
|
|
|
|
|
|
(gdb) si
|
|
|
|
|
|
0x08048352 17 {
|
|
|
|
|
|
1: x/i $pc 0x8048352 <main+6>: and $0xfffffff0,%esp
|
|
|
|
|
|
(gdb) si
|
|
|
|
|
|
0x08048355 17 {
|
|
|
|
|
|
1: x/i $pc 0x8048355 <main+9>: mov $0x0,%eax
|
|
|
|
|
|
(gdb) si
|
|
|
|
|
|
0x0804835a 17 {
|
|
|
|
|
|
1: x/i $pc 0x804835a <main+14>: sub %eax,%esp
|
|
|
|
|
|
(gdb) si
|
|
|
|
|
|
19 n = 1;
|
|
|
|
|
|
1: x/i $pc 0x804835c <main+16>: movl $0x1,0xfffffffc(%ebp)
|
|
|
|
|
|
```
|
|
|
|
|
|
|
|
|
|
|
|
此时可以使用“i r”命令显示寄存器中的当前值———“i r”即“Infomation Register”：
|
|
|
|
|
|
|
|
|
|
|
|
```
|
|
|
|
|
|
(gdb) i r
|
|
|
|
|
|
eax 0xbffff6a4 -1073744220
|
|
|
|
|
|
ecx 0x42015554 1107383636
|
|
|
|
|
|
edx 0x40016bc8 1073834952
|
|
|
|
|
|
ebx 0x42130a14 1108544020
|
|
|
|
|
|
esp 0xbffff6a0 0xbffff6a0
|
|
|
|
|
|
ebp 0xbffff6a8 0xbffff6a8
|
|
|
|
|
|
esi 0x40015360 1073828704
|
|
|
|
|
|
edi 0x80483f0 134513648
|
|
|
|
|
|
eip 0x8048366 0x8048366
|
|
|
|
|
|
eflags 0x386 902
|
|
|
|
|
|
cs 0x23 35
|
|
|
|
|
|
ss 0x2b 43
|
|
|
|
|
|
ds 0x2b 43
|
|
|
|
|
|
es 0x2b 43
|
|
|
|
|
|
fs 0x0 0
|
|
|
|
|
|
gs 0x33 51
|
|
|
|
|
|
```
|
|
|
|
|
|
|
|
|
|
|
|
当然也可以显示任意一个指定的寄存器值：
|
|
|
|
|
|
|
|
|
|
|
|
```
|
|
|
|
|
|
(gdb) i r eax
|
|
|
|
|
|
eax 0xbffff6a4 -1073744220
|
|
|
|
|
|
```
|
|
|
|
|
|
|
|
|
|
|
|
最后一个要介绍的命令是“q”，退出（Quit）GDB调试环境：
|
|
|
|
|
|
|
|
|
|
|
|
```
|
|
|
|
|
|
(gdb) q
|
|
|
|
|
|
The program is running. exit anyway? (y or n)
|
|
|
|
|
|
```
|
|
|
|
|
|
|
|
|
|
|
|
|
