stl map同时写示例代码

Question

I want to simulate the abnormal behavior of stl map in simultaneous write condition. Here I am using a single map and simultaneously inserting data from multiple threads. As we are using only one map object, hence it should not allow. Please go through the following sample code

    #include <iostream>
    #include <map>
    #include <iterator>
    #include <algorithm>

    extern "C"
    {
    #include <pthread.h>
    #include <string.h>
    #include <stdlib.h>
    }

    using namespace std;

//functor to print map
    struct PrintMp
    {
       void operator()(pair<int,int> pr)
       {
          cout<<pr.first<<" => "<<pr.second<<endl;
       }
    };
//thread 1
//inserts continuous no from 0 to 4999    
    void* ins_th1(void *arg)
    {
       map<int, int> *mp = static_cast<map<int, int>* >(arg);
       for(int i =0 ; i<5000; i++)
          mp->insert(pair<int,int>(i, i+1000));

       return NULL;
    }

//thread 1
//inserts continuous no from 0 to 4999    
    void* ins_th2(void *arg)
    {
       map<int, int> *mp = static_cast<map<int,int>* >(arg);
       for(int i=5000; i<10000; i++)
          mp->insert(pair<int, int>(i, i+2000));
       return NULL;
    }


    int main()
    {
       typedef map<int, int> IntMapType;

       IntMapType mp;
       PrintMp MpPrintObj;
       int rc;

       pthread_t th1, th2;
    //thread 1 creation
       rc = pthread_create(&th1, NULL, ins_th1, static_cast<void*>(&mp));
       if ( rc != 0)
       {
          cerr<<strerror(rc)<<"in thread1"<<endl;
          exit(EXIT_FAILURE);
       }
    //thread 2 creation
       rc = pthread_create(&th2, NULL, ins_th2, static_cast<void*>(&mp));
       if(rc!=0)
       {
          cerr<<strerror(rc)<<"in thread2"<<endl;
          exit(EXIT_FAILURE);
       }
    //lets wait for the thread to finish
       rc = pthread_join(th1, NULL);
       if ( rc != 0)
       {
          cerr<<strerror(rc)<<"join failure for thread1"<<endl;
          exit(EXIT_FAILURE);
       }

       rc = pthread_join(th2, NULL);
       if ( rc != 0)
       {
          cerr<<strerror(rc)<<"join failure for thread2"<<endl;
          exit(EXIT_FAILURE);
       }

       cout<<"Map data"<<endl;
    //now print it
       for_each(mp.begin(), mp.end(), MpPrintObj);
       cout<<endl;

       return 0;
    }

But it doesn't work. Can anyone suggest me some approach ?

Answer 1

You are only testing insertion, which may or may not be implemented in a thread safe fashion, for all you know. But, your test is not complete. Allow the threads to write the same key into the map , and you will likely experience an error that would not happen without multiple threads.

   // ins_th1
   for(int i =0 ; i<10000; i++)
      mp->insert(pair<int,int>(i, i+1000));

   // ins_th2
   for(int i=0; i<10000; i++)
      mp->insert(pair<int, int>(i, i+2000));

You should test deletion from the map as well. When I modified your program to populate the map , before launching the threads, and only had the threads remove things from the map , the program live-locked.

   // ins_th1
   for(int i =0 ; i<5000; i++)
      mp->erase(i);

   // ins_th2
   for(int i=5000; i<10000; i++)
      mp->erase(i);

   // near top of main
   for(int i =0 ; i<5000; i++)
      mp.insert(pair<int,int>(i, i+1000));
   for(int i=5000; i<10000; i++)
      mp.insert(pair<int, int>(i, i+2000));
   //... launch threads

Answer 2

I tried to implement as you said.

But despite not using any synchronization mechanism, I am getting the perfect result.