GDB是GNU開源組織發布的一個強大的UNIX下的程序調試工具，GDB主要可幫助工程師完成下面4個方面的功能：
int add(int a, int b)
{
return a + b;
}
main()
{
int sum[10] =
{
0, 0, 0, 0, 0, 0, 0, 0, 0, 0
} ;
int i;
int array1[10] =
{
48, 56, 77, 33, 33, 11, 226, 544, 78, 90
};
int array2[10] =
{
85, 99, 66, 0x199, 393, 11, 1, 2, 3, 4
};
for (i = 0; i < 10; i++)
{
sum[i] = add(array1[i], array2[i]);
}
}
使用命令gcc –g gdb_example.c –o gdb_example編譯上述程序，得到包含調試信息的二進制文件example，執行gdb gdb_example命令進入調試狀態：
[root@localhost driver_study]# gdb gdb_example 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)
(gdb) list 15
int array1[10] =
{
48, 56, 77, 33, 33, 11, 226, 544, 78, 90
};
int array2[10] =
{
85, 99, 66, 0x199, 393, 11, 1, 2, 3, 4
};
(gdb) list main
{
return a + b;
}
main()
{
int sum[10];
int i;
int array1[10] =
(gdb) break add
Breakpoint 1 at 0x80482f7: file gdb_example.c, line 3.
(gdb) run
Starting program: /driver_study/gdb_example
Breakpoint 1, add (a=48, b=85) at gdb_example.c:3
warning: Source file is more recent than executable.
3 return a + b;
(gdb) next
4 }
(gdb) next
main () at gdb_example.c:23
23 for (i = 0; i < 10; i++)
(gdb) next
25 sum[i] = add(array1[i], array2[i]);
(gdb) print sum
$1 = {133, 0, 0, 0, 0, 0, 0, 0, 0, 0}
(gdb) break 25 Breakpoint 1 at 0x8048362: file gdb_example.c, line 25. (gdb) run Starting program: /driver_study/gdb_example Breakpoint 1, main () at gdb_example.c:25 25 sum[i] = add(array1[i], array2[i]); (gdb) step add (a=48, b=85) at gdb_example.c:3 3 return a + b;單步執行的更復雜用法包括：
continue [ignore-count] c [ignore-count] fg [ignore-count]ignore-count表示忽略其後多少次斷點。 假設我們設置了函數斷點add()，並watch i，則在continue過程中，每次遇到add()函數或i發生變化，程序就會停住，如：
(gdb) continue Continuing. Hardware watchpoint 3: i Old value = 2 New value = 3 0x0804838d in main () at gdb_example.c:23 23 for (i = 0; i < 10; i++) (gdb) continue Continuing. Breakpoint 1, main () at gdb_example.c:25 25 sum[i] = add(array1[i], array2[i]); (gdb) continue Continuing. Hardware watchpoint 3: i Old value = 3 New value = 4 0x0804838d in main () at gdb_example.c:23 23 for (i = 0; i < 10; i++)
print <expr> print /<f> <expr><expr>是表達式，是被調試的程序中的表達式，<f>是輸出的格式，比如，如果要把表達式按16進制的格式輸出，那麼就是/x。在表達式中，有幾種GDB所支持的操作符，它們可以用在任何一種語言中，“@”是一個和數組有關的操作符，“::”指定一個在文件或是函數中的變量，“{<type>} <addr>”表示一個指向內存地址<addr>的類型為type的一個對象。
(gdb) print sum
$2 = {133, 155, 0, 0, 0, 0, 0, 0, 0, 0}
(gdb) next
Breakpoint 1, main () at gdb_example.c:25
25 sum[i] = add(array1[i], array2[i]);
(gdb) next
23 for (i = 0; i < 10; i++)
(gdb) print sum
$3 = {133, 155, 143, 0, 0, 0, 0, 0, 0, 0}
當需要查看一段連續內存空間的值的時間，可以使用GDB的“@”操作符，“@”的左邊是第一個內存地址，“@”的右邊則是想查看內存的長度。例如如下動態申請的內存：(gdb) watch i Hardware watchpoint 3: i (gdb) next 23 for (i = 0; i < 10; i++) (gdb) next Hardware watchpoint 3: i Old value = 0 New value = 1 0x0804838d in main () at gdb_example.c:23 23 for (i = 0; i < 10; i++) (gdb) next Breakpoint 1, main () at gdb_example.c:25 25 sum[i] = add(array1[i], array2[i]); (gdb) next 23 for (i = 0; i < 10; i++) (gdb) next Hardware watchpoint 3: i Old value = 1 New value = 2 0x0804838d in main () at gdb_example.c:23 23 for (i = 0; i < 10; i++)
main()
{
char *c = "hello world";
printf("%s\n", c);
}
我們在
char *c = "hello world";下一行設置斷點後：
(gdb) l
1 main()
2 {
3 char *c = "hello world";
4 printf("%s\n", c);
5 }
(gdb) b 4
Breakpoint 1 at 0x100000f17: file main.c, line 4.
(gdb) r
Starting program: /Users/songbarry/main
Reading symbols for shared libraries +. done
Breakpoint 1, main () at main.c:4
4 printf("%s\n", c);
可以通過多種方式看C指向的字符串：(gdb) p c $1 = 0x100000f2e "hello world"方法2：
(gdb) x/s 0x100000f2e 0x100000f2e: "hello world"
(gdb) p (char *)0x100000f2e $3 = 0x100000f2e "hello world"
(gdb) p *(char *)0x100000f2e='H' $4 = 72 'H'
(gdb) p c $5 = 0x100000f2e "Hello world"
(gdb) set $v0 = 0x004000000 (gdb) set $epc = 0xbfc00000
(gdb) set {unsigned int}0x8048a51=0x0
(gdb) set {unsigned int}0x100000f2e=0x0
(gdb) x/10cb 0x100000f2e
0x100000f2e: 0 '\0' 0 '\0' 0 '\0' 0 '\0' 111 'o' 32 ' ' 119 'w' 111 'o'
0x100000f36: 114 'r' 108 'l'
(gdb) p c
$10 = 0x100000f2e ""
return return <expression>
info line tst.c:func
(gdb) disassemble func Dump of assembler code for function func: 0x8048450 <func>: push %ebp 0x8048451 <func+1>: mov %esp,%ebp 0x8048453 <func+3>: sub $0x18,%esp 0x8048456 <func+6>: movl $0x0,0xfffffffc(%ebp) ... End of assembler dump.
