C++ 为什么'；t gdb“；u"；命令退出循环？

我有一个非常简单的主函数，里面有一个for循环，如下所示：

#include<stdio.h> 
int main() 
{ 
   for(int i=0;i<30;++i) 
     printf("%d\n",i); 
   return 0; 
} 

然后我用gdb调试它：

$ gdb a.out 
GNU gdb (Ubuntu 7.11.1-0ubuntu1~16.04)... 
Reading symbols from a.out...done. 
(gdb) list 
1    #include<stdio.h> 
2    int main() 
3    { 
4      for(int i=0;i<30;++i) 
5        printf("%d\n",i); 
6      return 0; 
7    } 
(gdb) b 5 
Breakpoint 1 at 0x400537: file 4.c, line 5. 
(gdb) b 6 
Breakpoint 2 at 0x400555: file 4.c, line 6. 
(gdb) r 
Starting program: /home/a/cpp/a.out  

Breakpoint 1, main () at 4.c:5 
5        printf("%d\n",i); 
(gdb) p i 
$1 = 0 
(gdb) u 
0 
4      for(int i=0;i<30;++i) 
(gdb) u //not exiting for loop?

Breakpoint 1, main () at 4.c:5 
5        printf("%d\n",i); 
(gdb)  
1 
4      for(int i=0;i<30;++i) 
(gdb) u 

$gdb a.out
GNU gdb（Ubuntu 7.11.1-0ubuntu1~16.04）。。。
从a中读取符号…完成。
（gdb）清单
1#包括
2 int main（）
3    { 
4对于（inti=0；i来说，gdb似乎必须通过一次循环才能理解循环结构

(gdb) list
1   #include<stdio.h> 
2   int main() 
3   { 
4      for(int i=0;i<5;++i) 
5      {
6        printf("%d\n",i); 
7      }
8      return 0; 
9   } 
10  
(gdb) b main
Breakpoint 1 at 0x400535: file junk.cpp, line 4.
(gdb) b 8
Breakpoint 2 at 0x40055c: file junk.cpp, line 8.
(gdb) r
Starting program: /tmp/local/matcher_server/bin/a.out 

Breakpoint 1, main () at junk.cpp:4
4      for(int i=0;i<30;++i) 
(gdb) n
6        printf("%d\n",i); 
(gdb) n
0
4      for(int i=0;i<30;++i) 
(gdb) u
1
2
3
4

Breakpoint 2, main () at junk.cpp:8
8      return 0; 

[3,0]列是行号和列号。正如我们所看到的，循环导致行号是非顺序的，3,4,6,4

我怀疑，当程序第一次运行到第6行并给出“u”命令时，gdb对矮人符号中的循环感到困惑。但是在第二个循环中，它得到了正确的结果。可能是一个小错误或“u”命令是如何实现的

请注意，gdb在执行“u”命令时仍会遇到断点。在您的示例中，您需要删除printf上的断点

(gdb) list
1   #include<stdio.h> 
2   int main() 
3   { 
4      for(int i=0;i<5;++i) 
5      {
6        printf("%d\n",i); 
7      }
8      return 0; 
9   } 
10  
(gdb) b main
Breakpoint 1 at 0x400535: file junk.cpp, line 4.
(gdb) b 8
Breakpoint 2 at 0x40055c: file junk.cpp, line 8.
(gdb) r
Starting program: /tmp/local/matcher_server/bin/a.out 

Breakpoint 1, main () at junk.cpp:4
4      for(int i=0;i<30;++i) 
(gdb) n
6        printf("%d\n",i); 
(gdb) n
0
4      for(int i=0;i<30;++i) 
(gdb) u
1
2
3
4

Breakpoint 2, main () at junk.cpp:8
8      return 0; 

(gdb) disass
Dump of assembler code for function main():
   0x000000000040052d <+0>: push   %rbp
   0x000000000040052e <+1>: mov    %rsp,%rbp
   0x0000000000400531 <+4>: sub    $0x10,%rsp
   0x0000000000400535 <+8>: movl   $0x0,-0x4(%rbp)
   0x000000000040053c <+15>:    jmp    0x400556 <main()+41>
   0x000000000040053e <+17>:    mov    -0x4(%rbp),%eax
   0x0000000000400541 <+20>:    mov    %eax,%esi
   0x0000000000400543 <+22>:    mov    $0x4005f4,%edi
   0x0000000000400548 <+27>:    mov    $0x0,%eax
   0x000000000040054d <+32>:    callq  0x400410 <printf@plt>
=> 0x0000000000400552 <+37>:    addl   $0x1,-0x4(%rbp)
   0x0000000000400556 <+41>:    cmpl   $0x1d,-0x4(%rbp)
   0x000000000040055a <+45>:    jle    0x40053e <main()+17>
   0x000000000040055c <+47>:    mov    $0x0,%eax
   0x0000000000400561 <+52>:    leaveq 
   0x0000000000400562 <+53>:    retq   
End of assembler dump.

.debug_line: line number info for a single cu
Source lines (from CU-DIE at .debug_info offset 0x0000000b):
<pc>        [row,col] NS BB ET PE EB IS= DI= uri: "filepath"
NS new statement, BB new basic block, ET end of text sequence
PE prologue end, EB epilogue begin
IA=val ISA number, DI=val discriminator value
0x0040052d  [   3, 0] NS uri: "/tmp/local/matcher_server/bin/junk.cpp"
0x00400535  [   4, 0] NS
0x0040053e  [   6, 0] NS DI=0x2
0x00400552  [   4, 0] NS DI=0x2
0x00400556  [   4, 0] DI=0x1
0x0040055c  [   8, 0] NS
0x00400561  [   9, 0] NS
0x00400563  [   9, 0] NS ET