Question

我有一个基于网络（TCP-IP）的自定义IPC系统。

考虑代码（以及下面的解释）：

#include "boost/shared_ptr.hpp"
#include <string>

using namespace std;

class TCommand {
public:
  typedef boost::shared_ptr<TCommand> Ptr;

  TCommand() {
      cout << "    Creating TCommand..." << endl;
  }

  virtual ~TCommand() {
      cout << "    Destroying TCommand..." << endl;
  }

  static TCommand * factory(int classID);

  virtual void parse(const char *data, int dataSize) = 0;
  virtual void print() = 0;
  virtual std::string getType() = 0;

};


class TPingCommand : public TCommand {
public:
  static const int classID = 1;
  int value;

  TPingCommand() : TCommand() {
      cout << "    Creating TPingCommand..." << endl;
  }

  virtual ~TPingCommand() {
      cout << "    Destroying TPingCommand..." << endl;
  }

  virtual void parse(const char *data, int dataSize) {
    if (dataSize < 4) throw 1;

    this->value = data[0] << 24 | data[1] << 16 | data[2] << 8 | data[3];
  }

  virtual void print() {
      cout << "  TPingCommand:" << endl;
      cout << "    value = " << dec << this->value << " (0x" << hex << this->value << ")" << endl;
  }

  virtual std::string getType() {
      return "TPingCommand";
  }
};

class TOtherCommand : public TCommand {
public:
  static const int classID = 2;
  int value;
  char value2;
  short int value3;

  TOtherCommand() : TCommand() {
      cout << "    Creating TOtherCommand..." << endl;
  }

  virtual ~TOtherCommand() {
      cout << "    Destroying TOtherCommand..." << endl;
  }

  virtual void parse(const char *data, int dataSize) {
    if (dataSize < 7) throw 1;

    this->value = data[0] << 24 | data[1] << 16 | data[2] << 8 | data[3];
    this->value2 = data[4];
    this->value3 = data[5] << 8 | data[6];
  }

  virtual void print() {
      cout << "  TOtherCommand:" << endl;
      cout << "    value  = " << dec << this->value << " (0x" << hex << this->value << ")" << endl;
      cout << "    value2 = " << dec << this->value2 << " (0x" << hex << (int)this->value2 << ")" << endl;
      cout << "    value3 = " << dec << this->value3 << " (0x" << hex << this->value3 << ")" << endl;
  }

  virtual std::string getType() {
      return "TOtherCommand";
  }
};


TCommand * TCommand::factory(int classID) {
    cout << "  Factory for classID = " << dec << classID << " (0x" << hex << classID << ")" << endl;
    switch (classID) {
    case TPingCommand::classID: return new TPingCommand(); break;
    case TOtherCommand::classID: return new TOtherCommand(); break;
    default: throw 1;
    }
  }


TCommand::Ptr receiveFromNetwork(int test, TCommand::Ptr knownCommand)
{
    // Receive command header from network.
    // int classID is the command class internal ID.
    // int dataSize is the command's body size in bytes.
    // For instance:
    //   int classId = 2;
    //   int datasize = 7;

    int classId = 1;
    int dataSize = 4;
    char data[10];

    if (test == 0) {
        cout << "  Using test data 0..." << endl;
        classId = 1;
        dataSize = 4;
        data[0] = 0x01; data[1] = 0x02; data[2] = 0x03; data[3] = 0x04;
    } else if (test == 1) {
        cout << "  Using test data 1..." << endl;
        classId = 2;
        dataSize = 7;
        data[0] = 0x11; data[1] = 0x12; data[2] = 0x13; data[3] = 0x14; data[4] = 0x41; data[5] = 0x16; data[6] = 0x17;
    }

    TCommand::Ptr cmd;
    if (knownCommand == 0) {
        cout << "  No command provided." << endl;
        cmd.reset(TCommand::factory(classId));
        cout << "  Command created from factory: " << cmd->getType() << endl;
    } else {
        cmd = knownCommand;
        cout << "  Command provided: " << cmd->getType() << endl;
    }

    cout << "  Parsing data..." << endl;
    cmd->parse(data, dataSize);

    // The command was identified as TOtherCommand (classID = 2).
    // The factory returned a TOtherCommand instance.
    // The TOtherCommand's parse method will check the dataSize is suitable (7 bytes are necessary).
    // The parse method will unserialize data to their fields.
    // This way, the fields would be:
    //    data = 0x11121314;
    //    data2 = 0x42; // 'A' as char.
    //    data3 = 0x1213;

  return cmd;
}

void caller() {
    // Case 1 (ok):
    // I know I'm going to receive a TPingCommand.
    cout << "Test case 1:" << endl;
    TCommand::Ptr known(new TPingCommand());
    TCommand::Ptr cmd1 = receiveFromNetwork(0, known);
    cmd1->print();

    // Case 2 (problems):
    cout << "Test case 2:" << endl;
    TCommand::Ptr dummy;
    TCommand::Ptr cmd2 = receiveFromNetwork(1, dummy);
    cmd2->print();

    cout << "Teardown..." << endl;
}

int main() {
    caller();
}

receiveFromNetwork是一种修改后的工厂方法，用于从网络接收命令，但是，在少数情况下，我先前知道我将接收哪种类型的命令，因此我创建了实例它的传递给函数（如已知的命令）。命令类派生自TCommand类。函数返回knownCommand（没有必要，因为你把它作为参数传递，但它对其他情况很有用。）

所有其他情况，从网络接收的前几个字节描述命令classID，我用它在此函数内创建合适的TCommand实例。然后，该命令将从网络数据中解析，并在函数末尾返回。 knownCommand只是一个虚拟的TCommand实例。

当我通过knownCommand时，效果很好。当我将伪命令作为参数传递时，它会崩溃（据我所知，双重自由）。

我考虑过对命令使用TCommand引用，但是，我不能这样做，因为我必须返回一个共享指针，它会导致相同的原始指针由两个不同的共享指针实例管理（一个来自调用者方法，另一个来自receiveFromNetwork方法。）

任何人都知道如何解决这个问题？

以下是有问题的场景的部分valgrind输出：

==31859== Thread 2:
==31859== Invalid read of size 4
==31859==    at 0x805D7B0: boost::detail::sp_counted_impl_p<TVideoGetSourceSizeCommand>::dispose() (checked_delete.hpp:34)
==31859==    by 0x407FF42: Server::receiveFromNetwork() (sp_counted_base_gcc_x86.hpp:145)
==31859==    by 0x40800AF: Server::serverThread(void*) (Server.cpp:107)
==31859==    by 0x434496D: start_thread (pthread_create.c:300)
==31859==    by 0x42B398D: clone (clone.S:130)

非常感谢。

Answer 1

发现问题。

在代码的调用部分中，有一个shared_ptr以错误的方式“重复”。由此产生的错误是这样的：

boost::shared_ptr<DerivedType> ptr1(new DerivedType(...));
boost::shared_ptr<BaseType> ptr2(ptr1.get());

当然它会导致重复的FREE和程序崩溃。

代码并不是那么愚蠢，但最终是简化的错误场景。尽管如此，这是一个微不足道的错误。

感谢所有帮助人员。

在修改后的工厂方法中boost :: shared_ptr

1 个答案: