c++ - 应用程序中的运行时错误 - 分段失败

Question

我正在处理在偶数位置显示字符的简单应用程序，但是当我输入时抛出运行时错误“应用程序收到 SIGSEGV 信号。分段失败”。我不知道哪里错了。我多次分析了我的代码，但找不到任何相关的解决方案。

我的代码:

#include <cstdio>
#include <iostream>
#include <string>

using namespace std;

int main()
{
    string s;
    scanf("%s",&s);
    int length = s.length();
    for(int i = 0; i < length; i++)
    {
        if(i%2==0)printf("%s",s[i]);
    } 
    return 0;
}

GDB 调试器信息:

**
->->pre-prompt
(gdb) 
->->prompt

->->post-prompt
Reading symbols from C:\Users\user\Desktop\par.exe...done.

->->breakpoints-invalid

->->breakpoints-invalid

->->pre-prompt
(gdb) 
->->prompt

->->post-prompt
Source directories searched: C:/Program Files/Dev-Cpp/MinGW32/lib;$cdir;$cwd

->->pre-prompt
(gdb) 
->->prompt

->->post-prompt
Source directories searched: C:/Program Files/Dev-Cpp/MinGW32/include;C:/Program Files/Dev-Cpp/MinGW32/lib;$cdir;$cwd

->->pre-prompt
(gdb) 
->->prompt

->->post-prompt
Source directories searched: C:/Program Files/Dev-Cpp/MinGW32/include;C:/Program Files/Dev-Cpp/MinGW32/include;C:/Program Files/Dev-Cpp/MinGW32/lib;$cdir;$cwd

->->pre-prompt
(gdb) 
->->prompt

->->post-prompt

->->pre-prompt
(gdb) 
->->prompt

->->post-prompt

->->pre-prompt
(gdb) 
->->prompt

->->post-prompt

->->pre-prompt
(gdb) 
->->prompt

->->post-prompt
Working directory C:\Users\user\Desktop.

->->pre-prompt
(gdb) 
->->prompt

->->post-prompt
Starting program: C:\Users\user\Desktop\par.exe 
[New Thread 5328.0x13c0]

->->new-thread

->->breakpoints-invalid

->->breakpoints-invalid

->->breakpoints-invalid

->->breakpoints-invalid

->->breakpoints-invalid

->->frames-invalid

->->breakpoints-invalid

->->breakpoints-invalid

->->breakpoints-invalid

->->breakpoints-invalid

->->breakpoints-invalid

->->breakpoints-invalid

->->starting

->->signal

Program received signal 
->->signal-name
SIGSEGV
->->signal-name-end
, 
->->signal-string
Segmentation fault
->->signal-string-end
.

->->frame-begin 0 0x41f036

->->frame-address
0x0041f036
->->frame-address-end
 in 
->->frame-function-name
std::string::length() const
->->frame-args
 ()

->->frame-end

->->stopped

->->pre-prompt
(gdb) 
->->prompt

->->post-prompt

->->error

->->pre-prompt
(gdb) 
->->prompt

->->error-begin
No symbol "include" in current context.

->->post-prompt
eax            0x64616441   1684104257
ecx            0x22ff04 2293508
edx            0x77a60958   2007370072
ebx            0x7ffdf000   2147348480
esp            0x22feec 0x22feec
ebp            0x22ff18 0x22ff18
esi            0x0  0
edi            0x0  0
eip            0x41f036 0x41f036 <std::string::length() const+2>
eflags         0x10206  [ PF IF RF ]
cs             0x1b 27
ss             0x23 35
ds             0x23 35
es             0x23 35
fs             0x3b 59
gs             0x0  0

->->pre-prompt
(gdb) 
->->prompt

->->post-prompt

->->pre-prompt
(gdb) 
->->prompt

->->post-prompt
Dump of assembler code for function _ZNKSs6lengthEv:
   0x0041f034 <+0>: mov    (%ecx),%eax
=> 0x0041f036 <+2>: mov    -0xc(%eax),%eax
   0x0041f039 <+5>: ret    
   0x0041f03a <+6>: nop
   0x0041f03b <+7>: nop
End of assembler dump.

->->pre-prompt
(gdb) 
->->prompt

->->post-prompt

->->frame-begin 0 0x41f036
#0  
->->frame-address
0x0041f036
->->frame-address-end
 in 
->->frame-function-name
std::string::length() const
->->frame-args
 ()

->->frame-end

->->frame-begin 1 0x4013ee
#1  
->->frame-address
0x004013ee
->->frame-address-end
 in 
->->frame-function-name
main
->->frame-args
 ()
->->frame-source-begin
 at 
->->frame-source-file
C:\Users\user\Desktop\par.cpp
->->frame-source-file-end
:
->->frame-source-line
11
->->frame-source-end


->->frame-end

->->pre-prompt
(gdb) 
->->prompt

->->post-prompt

->->error

->->pre-prompt
(gdb) 
->->prompt

->->error-begin
No symbol "length" in current context.
**

CPU 信息:

   0x0041f034 <+0>: mov    (%ecx),%eax
=> 0x0041f036 <+2>: mov    -0xc(%eax),%eax
   0x0041f039 <+5>: ret    
   0x0041f03a <+6>: nop
   0x0041f03b <+7>: nop

Answer 1

scanf 设计用于 char 数组而不是 std::string，并且要打印一个字符，您必须使用 %c 而不是 %s:

string s;
scanf("%s", &s);
           ^^^^
         //Undefined behavior invokes!

...

 printf("%s",s[i]);
        ^^^^
      //Undefined behavior again!

如果你真的想使用scanf，你应该使用中间的char数组:

char str[BIG_ENOUGHT];
scanf("%s", &str);

std::string s(str);

最好改用cin和cout:

cin >> s;

...

cout << s[i];