linux进阶19——GDB（五）：单步调试命令（next、step和until）
一段c语言程序:
#include
int print(int num){
int ret = num * num;
return ret;
}
int myfunc(int num){
int i = 1;
int sum = 0;
while(i <= num){
sum += print(i);
i++;
}
return sum;
}
int main(){
int num =0;
scanf("%d", &num);
int result = myfunc(num);
printf("%d", result);
return 0;
} 1. next命令
1.1 功能
next 是最常用来进行单步调试的命令，其最大的特点是当遇到包含调用函数的语句时，无论函数内部包含多少行代码，next 指令都会一步执行完。也就是说，对于调用的函数来说，next 命令只会将其视作一行代码。
1.2 语法
(gdb) next count参数 count 表示单步执行多少行代码，默认为 1 行。
1.3 示例
[root@localhost day5]# gcc test1.c -o test1 -g
[root@localhost day5]# ls
test1 test1.c
[root@localhost day5]# gdb test1 -q
Reading symbols from /home/gdb/day5/test1...done.
(gdb) l 0
1 #include
2 int print(int num){
3 int ret = num * num;
4 return ret;
5 }
6 int myfunc(int num){
7 int i = 1;
8 int sum = 0;
9 while(i <= num){
10 sum += print(i);
(gdb)
11 i++;
12 }
13 return sum;
14 }
15 int main(){
16 int num =0;
17 scanf("%d", &num);
18 int result = myfunc(num);
19 printf("%d", result);
20 return 0;
(gdb)
21 }
(gdb)
Line number 22 out of range; test1.c has 21 lines.
(gdb) b 16
Breakpoint 1 at 0x4005c8: file test1.c, line 16.
(gdb) r
Starting program: /home/gdb/day5/test1
Breakpoint 1, main () at test1.c:16
16 int num =0;
Missing separate debuginfos, use: debuginfo-install glibc-2.17-323.el7_9.x86_64
(gdb) n 2
3
18 int result = myfunc(num);
(gdb) n <---------跳过myfunc函数，直接到19行的代码
19 printf("%d", result);
(gdb) n
20 return 0;
(gdb) p result
$1 = 14
(gdb)
2. step命令
2.1 功能
通常情况下，step 命令和 next 命令的功能相同，都是单步执行程序。不同之处在于，当 step 命令所执行的代码行中包含函数时，会进入该函数内部，并在函数第一行代码处停止执行。
2.2 语法
(gdb) step count2.3 示例
[root@localhost day5]# clear
[root@localhost day5]# gcc test1.c -o test1 -g
[root@localhost day5]# gdb test1 -q
Reading symbols from /home/gdb/day5/test1...done.
(gdb) l 0
1 #include
2 int print(int num){
3 int ret = num * num;
4 return ret;
5 }
6 int myfunc(int num){
7 int i = 1;
8 int sum = 0;
9 while(i <= num){
10 sum += print(i);
(gdb)
11 i++;
12 }
13 return sum;
14 }
15 int main(){
16 int num =0;
17 scanf("%d", &num);
18 int result = myfunc(num);
19 printf("%d", result);
20 return 0;
(gdb)
21 }
(gdb) b 18
Breakpoint 1 at 0x4005e5: file test1.c, line 18.
(gdb) r
Starting program: /home/gdb/day5/test1
3
Breakpoint 1, main () at test1.c:18
18 int result = myfunc(num);
Missing separate debuginfos, use: debuginfo-install glibc-2.17-323.el7_9.x86_64
(gdb) step <------------程序进入mufunc函数，并在第一行代码处停止
myfunc (num=3) at test1.c:7
7 int i = 1;
(gdb) 3. until命令
3.1 语法
1、(gdb) until
2、(gdb) until location其中，参数 location 为某一行代码的行号。until 命令可以简写为 u 命令。
不带参数的 until 命令，可以使 GDB 调试器快速运行完当前的循环体，并运行至循环体外停止。注意，until 命令并非任何情况下都会发挥这个作用，只有当执行至循环体尾部（最后一行代码）时，until 命令才会发生此作用；反之，until 命令和 next 命令的功能一样，只是单步执行程序。
3.2 示例
[root@localhost day5]# gdb test1 -q
Reading symbols from /home/gdb/day5/test1...done.
(gdb) l 0
1 #include
2 int print(int num){
3 int ret = num * num;
4 return ret;
5 }
6 int myfunc(int num){
7 int i = 1;
8 int sum = 0;
9 while(i <= num){
10 sum += print(i);
(gdb)
11 i++;
12 }
13 return sum;
14 }
15 int main(){
16 int num =0;
17 scanf("%d", &num);
18 int result = myfunc(num);
19 printf("%d", result);
20 return 0;
(gdb)
21 }
(gdb)
Line number 22 out of range; test1.c has 21 lines.
(gdb) b 17
Breakpoint 1 at 0x4005cf: file test1.c, line 17.
(gdb) r
Starting program: /home/gdb/day5/test1
Breakpoint 1, main () at test1.c:17
17 scanf("%d", &num);
Missing separate debuginfos, use: debuginfo-install glibc-2.17-323.el7_9.x86_64
(gdb) u
3
18 int result = myfunc(num);
(gdb) step
myfunc (num=3) at test1.c:7
7 int i = 1;
(gdb) u
8 int sum = 0;
(gdb) u
9 while(i <= num){
(gdb) u
10 sum += print(i);
(gdb) u
11 i++;
(gdb) u
9 while(i <= num){
(gdb) u <-------------重点:快速执行完循环体
13 return sum;
(gdb) p sum
$1 = 14
(gdb) until 命令还可以后跟某行代码的行号，以指示 GDB 调试器直接执行至指定位置后停止。举个例子：
[root@localhost day5]# gdb test1 -q
Reading symbols from /home/gdb/day5/test1...done.
(gdb) l 0
1 #include
2 int print(int num){
3 int ret = num * num;
4 return ret;
5 }
6 int myfunc(int num){
7 int i = 1;
8 int sum = 0;
9 while(i <= num){
10 sum += print(i);
(gdb)
11 i++;
12 }
13 return sum;
14 }
15 int main(){
16 int num =0;
17 scanf("%d", &num);
18 int result = myfunc(num);
19 printf("%d", result);
20 return 0;
(gdb)
21 }
(gdb)
Line number 22 out of range; test1.c has 21 lines.
(gdb) b 17
Breakpoint 1 at 0x4005cf: file test1.c, line 17.
(gdb) r
Starting program: /home/gdb/day5/test1
Breakpoint 1, main () at test1.c:17
17 scanf("%d", &num);
Missing separate debuginfos, use: debuginfo-install glibc-2.17-323.el7_9.x86_64
(gdb) until 19 <-------------------直接执行至第19行停止
3
main () at test1.c:19
19 printf("%d", result);
(gdb) p result <---------------打印result结果
$1 = 14
(gdb) 可以看到，通过执行 until 19 命令，GDB 调试器直接从第 17 行代码处执行至指定的第 19 行。