Author: liigo
Original link: http://blog.csdn.net/liigo/archive/2006/01/17/582231.aspx
Date: January 16, 2006
This article is written for programmers and program enthusiasts who mainly work under the Windows operating system and need to develop some cross-platform software.
GDB is a powerful program debugging tool based on the command line under the UNIX/LINUX operating system released by the GNU open source organization.
There are many commands in GDB, but we only need to master about ten of them to roughly complete the daily basic program debugging work.
| Order | explain | example |
| file <filename> | Load the executable program file to be debugged. Because GDB is generally executed in the directory where the debugged program is located, the text name does not need to include a path. |
(gdb) file gdb-sample |
| r | Short for Run, run the program being debugged. If no breakpoint has been set before, the entire program is executed; if there is a breakpoint, the program pauses at the first available breakpoint. |
(gdb) r |
| c | Shorthand for Continue, continue to execute the debugged program until the next breakpoint or the end of the program. | (gdb) c |
| b <line number> b <function name> b *<function name> b *<code address> d [number] |
b: Shorthand for Breakpoint, set a breakpoint. Two, you can use "line number", "function name", "execution address" and other ways to specify the breakpoint position. Adding a "*" symbol in front of the function name means setting the breakpoint at "the prolog code generated by the compiler". If you don't know assembly, you can ignore this usage. d: Shorthand for Delete breakpoint, delete a breakpoint with a specified number, or delete all breakpoints. Breakpoint numbers start at 1 and increment. |
(gdb) b 8 (gdb) b main (gdb) b *main (gdb) b *0x804835c (gdb) d |
| s, n | s: Execute a line of source code, if there is a function call in this line of code, enter the function; n: Execute a line of source code, and execute the function call in this line of code. s is equivalent to "Step Into" in other debuggers; These two commands can only be used when there is source code debugging information (use the "-g" parameter when compiling with GCC). |
(gdb) s (gdb) n |
| si, ni | The si command is similar to the s command, and the ni command is similar to the n command. The difference is that these two commands (si/ni) are aimed at assembly instructions, while s/n is aimed at source code. | (gdb) is (gdb) by |
| p <variable name> | Shorthand for Print, which displays the value of a specified variable (temporary or global). | (gdb) p i (gdb) p nGlobalVar |
| display ... undisplay <number> |
display, set the data and its format to be displayed after the program is interrupted. For example, if you want to see the next assembly instruction to be executed every time the program is interrupted, you can use the command "display /i $pc" , where $pc represents the current assembly instruction, and /i represents display in sixteen. This command is useful when you need to care about assembly code. undispaly, cancel the previous display setting, and the number will increase from 1. |
(gdb) display /i $pc (gdb) undisplay 1 |
| i | Shorthand for Info, used to display various information, please refer to "help i" for details. | (gdb) i r |
| q | Shorthand for Quit, exit the GDB debugging environment. | (gdb) q |
| help [command name] | GDB help command, providing explanations for GDB name commands. If the "command name" parameter is specified, the detailed description of the command will be displayed; if no parameter is specified, all GDB commands will be displayed in categories for users to further browse and query. |
(gdb) help display |
Not much nonsense, let's start to practice.
First give a small program for example, C language code, the simplicity cannot be simpler:
| 1 |
//此程序仅作为“GDB十分钟教程”的示例代码, by liigo |
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：
| gcc gdb-sample.c -o gdb-sample -g |
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。
下面“gdb”命令启动GDB，将首先显示GDB说明，不管它：
| GNU gdb Red Hat Linux (5.3post-0.20021129.18rh) Copyright 2003 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". (gdb) |
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。
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）：
| (gdb) file gdb-sample Reading symbols from gdb-sample...done. |
上面最后一行提示已经加载成功。
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：
| (gdb) r Starting program: /home/liigo/temp/test_jmp/test_jmp/gdb-sample n = 1, nGlobalVar = 88 tempFunction is called, a = 1, b = 2 n = 3 Program exited normally. |
下面使用“b”命令在 main 函数开头设置一个断点（Breakpoint）：
| (gdb) b main Breakpoint 1 at 0x804835c: file gdb-sample.c, line 19. |
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）。
再次使用“r”命令执行（Run）被调试程序：
| (gdb) r Starting program: /home/liigo/temp/gdb-sample Breakpoint 1, main () at gdb-sample.c:19 19 n = 1; |
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。
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。
下面使用“s”命令（Step）执行下一行代码（即第19行“n = 1;”）：
| (gdb) s 20 n++; |
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++;”。
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 ：
| (gdb) p n $1 = 1 |
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。）
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”）：
| (gdb) b 26 Breakpoint 2 at 0x804837b: file gdb-sample.c, line 26. (gdb) b tempFunction Breakpoint 3 at 0x804832e: file gdb-sample.c, line 12. |
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：
| (gdb) c Continuing. Breakpoint 2, main () at gdb-sample.c:26 26 printf("n = %d, nGlobalVar = %d /n", n, nGlobalVar); (gdb) p nGlobalVar $2 = 88 (gdb) c Continuing. n = 1, nGlobalVar = 88 Breakpoint 3, tempFunction (a=1, b=2) at gdb-sample.c:12 12 printf("tempFunction is called, a = %d, b = %d /n", a, b); (gdb) p a $3 = 1 (gdb) p b $4 = 2 |
上面反馈的信息一切都在我们预料之中，哈哈~~~
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：
| (gdb) c Continuing. tempFunction is called, a = 1, b = 2 n = 3 Program exited normally. |
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？
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) |
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) y |