c++ - 子进程中的数组分配无效

Question

我有一个定制的“迷你 shell ”代码，我在其中将用户提供的字符串解析为子进程中终端中的单个单词。

子代码如下:

child_pid = fork();

            if (child_pid == 0)
            {
                char* args[100];
                int   prev_pos = 0;
                int   pos;
                int   i = 0;

                cout << "Inside loop" << endl;
                while((pos = command.find(" ", prev_pos)) != string::npos)
                {
                    //cout << command.substr(prev_pos, pos) << endl;
                    if (i >= 1)
                    {
                        cout << endl << "Before Assignment: " << i - 1 << "   " << args[i - 1] <<"kwe"<< endl;
                    }

                    args[i] = const_cast<char*>((command.substr(prev_pos, (pos - prev_pos))).c_str());
                    cout << i << "   " << args[i] <<"befre cndition aftr ass"<< endl;
                    //cout << command.substr(prev_pos, pos) << endl;
                    if (i >= 1)
                    {
                        int j=i-1;
                        cout << "After Assignment: "<< i - 1 << "   " << args[j] <<"eefe"<< endl;
                    }
                    cout << i << "   " << args[i] << endl;

                    if (i >= 1)
                    {
                        for (int j = 0; j <= i; j++)
                        {
                            cout << "\t" << j << "   " << args[j] << endl;
                        }
                    }

                    prev_pos = (pos + 1);
                    i++;

                }// 'while' loop


                args[i] = const_cast<char*>((command.substr(prev_pos)).c_str());
                cout << i << "   " << args[i] << endl;
                i++;

                cout << "Outside loop" << endl << endl;
                cout << *(args + 0) << endl << endl;
                for (int j = 0; j < i; j++)
                {
                    cout << j << "   " << args[j] << endl;
                }

                args[i] = NULL;

                cout << endl << endl << args[0] << endl << endl;
                //execvpe(args[0], args, envp);

                cout << "Unknown command\n";
                exit(0);

            }// 'if'

问题 是，每当我将输入字符串的一个词分配给 args[i] 时，所有从索引 0 到 (i - 1) 也被赋予相同的值。 So，如果用户输入，比方说，ls asd dajd adakjs，那么我最终(在 while 循环结束时)得到所有 args[i] 的值为“adakjs”，如以下输出所示(cout 语句仅用于调试目的):

Inside loop
0   lsbefre cndition aftr ass
0   ls

Before Assignment: 0   lskwe
1   asdbefre cndition aftr ass
After Assignment: 0   asdeefe
1   asd
  0   asd
  1   asd

Before Assignment: 1   asdkwe
2   dajdbefre cndition aftr ass
After Assignment: 1   dajdeefe
2   dajd
  0   dajd
  1   dajd
  2   dajd
3   adakjs
Outside loop

adakjs

0   adakjs
1   adakjs
2   adakjs
3   adakjs


adakjs

Unknown command

为什么会这样？我如何修改我的代码，以便在 while 循环结束时，每个单词都存储到 arg[i]s 中？提前致谢!

Answer 1

有问题的行是这样的:

args[i] = const_cast<char*>((command.substr(prev_pos, (pos - prev_pos))).c_str());

substr 函数返回一个字符串对象，您从中获得一个指针，然后该字符串对象被破坏，留下一个指向数据的杂散指针不再存在。这将导致 undefined behavior .

您需要为字符串分配内存以使其永久化。