分解一个简单的C ++程序

Question

I have a simple C++ program:

#include <iostream>
using namespace std;

int main(){
    string s;
    cin >> s; 
    if (s == "almafa")
        cout << "ok";
}

In gdb I disassembled main and started the program, entered 'testing' as input, and stopped at the comparison 0x0000000000400bb7:

   0x0000000000400bab <+53>:    lea    -0x40(%rbp),%rax
   0x0000000000400baf <+57>:    mov    $0x400d24,%esi
   0x0000000000400bb4 <+62>:    mov    %rax,%rdi
   0x0000000000400bb7 <+65>:    callq  0x400c6a <bool std::operator==<char, std::char_traits<char>, std::allocator<char> >(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, char const*)>

I inspected the esi and rdi registers:

(gdb) x/s $esi
0x400d24:   "almafa"
(gdb) x/s $rdi
0x7fffffffddc0: "\320\335\377\377\377\177"

What is in the rdi register? I expected it will contain my input.

Answer 1

rdi is the pointer to your std::string object.

Note that the function call is to an operator== that takes a const char * parameter.

The std::string class defines an operator== that takes a const char * parameter, and this is what's being invoked here, with rdi pointing to an in-memory representation of the std::string object, and with the parameter in esi.

What is in the rdi register? I expected it will contain my input.

It is not your input. Your input was stored in the std::string object, and that's what's being pointed to, here.