《软件调试的艺术》笔记--检查和设置变量

1.使用print命令查看变量值

使用print命令（简写为p）能够查看变量值。

使用例如以下的程序1进行測试。

#include <stdio.h>

struct node{
        int index;
        struct node* next;
};

int main(void) {
        struct node head;
        head.index = 1;
        head.next = NULL;

        int i;
        for(i=0; i<3; i++){
                head.index++;
        }

        return 0;
}

(gdb) b main
Breakpoint 1 at 0x804839a: file a.c, line 10.
(gdb) r
Starting program: /root/a.out

Breakpoint 1, main () at a.c:10
10 head.index = 1;
(gdb) n
11 head.next = NULL;
(gdb)
14 for(i=0; i<3; i++){
(gdb)
15 head.index++;
(gdb)
14 for(i=0; i<3; i++){
(gdb) p head
$1 = {index = 2, next = 0x0}
(gdb) n
15 head.index++;
(gdb)
14 for(i=0; i<3; i++){
(gdb)
15 head.index++;
(gdb)
14 for(i=0; i<3; i++){
(gdb) p head
$3 = {index = 4, next = 0x0}

print命令同意指定可选的格式，比如

p/x y

会以十六进制格式显示变量，而不是十进制。其它经常使用的格式为c（字符），s（字符串），f（浮点型）

2.使用display命令查看变量值

每次遇到断点，仅仅须要键入print命令就行查看变量值，可是，假设知道会在每次遇到断点时键入这个命令，那么使用

display命令（简写为disp）能节省很多其它的时间。这个命令要求GDB在运行每次有暂停时就输出指定条目。

使用上面的程序1输出的调试的结果例如以下：

(gdb) b main
Breakpoint 1 at 0x804839a: file a.c, line 10.
(gdb) b 15
Breakpoint 2 at 0x80483b1: file a.c, line 15.
(gdb) r
Starting program: /root/a.out

Breakpoint 1, main () at a.c:10
10 head.index = 1;
(gdb) disp head
1: head = {index = -1208168460, next = 0x80483d0}
(gdb) c
Continuing.

Breakpoint 2, main () at a.c:15
15 head.index++;
1: head = {index = 1, next = 0x0}
(gdb)
Continuing.

Breakpoint 2, main () at a.c:15
15 head.index++;
1: head = {index = 2, next = 0x0}
(gdb)
Continuing.

Breakpoint 2, main () at a.c:15
15 head.index++;
1: head = {index = 3, next = 0x0}
(gdb)

能够暂时禁用某个显示项，比如：

dis disp 1

暂时禁用显示列表中的条目1，条目能够通过 info disp命令检查。

要又一次启用条目，使用enable

enable disp 1

要全然删除条目，使用undisplay（简写为undisp）

undisp 1

3.使用call命令调用函数

使用call函数能够调用被调试程序的函数。

使用以下的程序2进行演示：

#include <stdio.h>

struct node{
        int index;
        char* content;
        struct node* next;
};

void display(struct node head){
        printf("head.index=%d,head.content=%s\n",head.index,head.content);
}

int main(void) {
        struct node head;
        head.index = 1;
        head.content = "yes";
        head.next = NULL;

        int i;
        for(i=0; i<3; i++){
                head.index++;
        }

        display(head);
        return 0;
}

(gdb) b main
Breakpoint 1 at 0x80483f0: file a.c, line 15.
(gdb) r
Starting program: /root/a.out

Breakpoint 1, main () at a.c:15
15 head.index = 1;
(gdb) n
16 head.content = "yes";
(gdb)
17 head.next = NULL;
(gdb)
20 for(i=0; i<3; i++){
(gdb)
21 head.index++;
(gdb) call display(head)
head.index=1,head.content=yes

4.使用ptype查看变量类型

使用程序2进行调试：

(gdb) b main
Breakpoint 1 at 0x804842a: file a.c, line 16.
(gdb) r
Starting program: /root/a.out

Breakpoint 1, main () at a.c:16
16 head.index = 1;
(gdb) ptype head
type = struct node {
int index;
char *content;
struct node *next;
}
(gdb)

5.监视局部变量

使用info locals命令得到当前栈帧中全部局部变量的值列表。

使用程序2进行调试：

(gdb) b main
Breakpoint 1 at 0x804842a: file a.c, line 16.
(gdb) r
Starting program: /root/a.out

Breakpoint 1, main () at a.c:16
16 head.index = 1;
(gdb) info locals
head = {index = -1209402059,
content = 0xb7ff0a50 "U\211\345WV1\366S\350N\212", next = 0x804849b}
i = -1208168460
(gdb) n
17 head.content = "yes";
(gdb)
18 head.next = NULL;
(gdb)
21 for(i=0; i<3; i++){
(gdb) info locals
head = {index = 1, content = 0x8048574 "yes", next = 0x0}
i = -1208168460
(gdb) n
22 head.index++;
(gdb) info local
head = {index = 1, content = 0x8048574 "yes", next = 0x0}
i = 0
(gdb)

6.检查动态数组

对以下的程序3进行调试。

#include <stdio.h>
#include <malloc.h>

int main(void){
        int a[3] = {1,2,3};
        int *b = (int*)malloc(3*sizeof(int));
        b[0] = 1;
        b[1] = 2;
        b[2] = 3;
        return 0;
}

(gdb) b main
Breakpoint 1 at 0x80483cd: file b.c, line 5.
(gdb) r
Starting program: /root/a.out

Breakpoint 1, main () at b.c:5
5 int a[3] = {1,2,3};
(gdb) n
6 int *b = (int*)malloc(3*sizeof(int));
(gdb)
7 b[0] = 1;
(gdb)
8 b[1] = 2;
(gdb)
9 b[2] = 3;
(gdb)
10 return 0;
(gdb) p a
$1 = {1, 2, 3}
(gdb) p b
$2 = (int *) 0x804b008
(gdb) p *b
$3 = 1

假设是动态数组，直接使用p命令仅仅能打印一个数。

使用例如以下命令，能够打印出整个数组：

(gdb) p *b@3
$1 = {1, 2, 3}

(gdb)

语法例如以下：

p *pointer@number_of_elements.

还有一种方式是使用类型强制转换：

(gdb) p (int [3])*b
$3 = {1, 2, 3}
(gdb)

7.设置变量

在gdb中，值的设置很easy，比如

set x=12

会将x的当前值改动为12.

能够通过gdb的set args命令设置程序的命令行參数，该命令能够须要下次运行run命令时才会发生变化。事实上仅仅要在调用

gdb的run命令时使用新參数就能达到相同的效果。

gdb提供info args命令来检查当前函数的參数。

使用以下的程序4进行调试：

#include <stdio.h>

void display(int i, char* argv)
{
        printf("argv[%d]=%s\n",i,argv);
}

int main(int argc, char* argv[])
{
        int i;
        for (i=0; i<argc; i++) {
                display(i,argv[i]);
        }
        return 0;
}

调试：

(gdb) set args 1 2 3
(gdb) b main
Breakpoint 2 at 0x80483f0: file a.c, line 11.
(gdb) b display
Note: breakpoint 1 also set at pc 0x80483ca.
Breakpoint 3 at 0x80483ca: file a.c, line 5.
(gdb) r
Starting program: /root/a.out 1 2 3

Breakpoint 2, main (argc=4, argv=0xbffff724) at a.c:11
11 for (i=0; i<argc; i++) {
(gdb) info args
argc = 4
argv = 0xbffff724
(gdb) c
Continuing.

Breakpoint 1, display (i=0, argv=0xbffff86a "/root/a.out") at a.c:5
5 printf("argv[%d]=%s\n",i,argv);
(gdb) c
Continuing.
argv[0]=/root/a.out

Breakpoint 1, display (i=1, argv=0xbffff876 "1") at a.c:5
5 printf("argv[%d]=%s\n",i,argv);
(gdb)
Continuing.
argv[1]=1

Breakpoint 1, display (i=2, argv=0xbffff878 "2") at a.c:5
5 printf("argv[%d]=%s\n",i,argv);
(gdb)
Continuing.
argv[2]=2

Breakpoint 1, display (i=3, argv=0xbffff87a "3") at a.c:5
5 printf("argv[%d]=%s\n",i,argv);
(gdb)
Continuing.
argv[3]=3

posted @ 2014-08-07 12:18 mengfanrong 阅读(228) 评论(0) 编辑收藏举报

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