C++：多线程编程学习:利用多线程进行归并排序。

首先学习一下简单的线程知识

：(1)pthread_join函数,用来阻塞当前的线程。直到指定线程结束。

：(2)pthread_create,用来创建线程。

下面的代码开了五个线程用来输出一个函数。

#include <pthread.h>#include <iostream>#include <fstream>#include <string>#include <vector>#include <cstdlib>#include "unistd.h"using namespace std;#define NUM_THREADS 5vector<int> read_file(string filename){    ifstream file(filename);    int each_element;    vector<int> data;    if (file){        while(file>>each_element) {            data.push_back(each_element);        }    }    return data;}void* say(void* thread_id){    auto tid = *((int*)(thread_id));    sleep(2*tid);    cout<<"hello, ID: "<<tid<<endl;}void* merge_sort(void * arg){}int main(){    pthread_t tids[NUM_THREADS];//define the id of thread    int index[NUM_THREADS];    /*for(const auto &x : tids){        cout<<x<<endl;    }*/    pthread_attr_t attr;    void* status;    pthread_attr_init(&attr);// init attr    pthread_attr_setdetachstate(&attr,PTHREAD_CREATE_JOINABLE);//set attr as not joinable    for(size_t i = 0;i!=NUM_THREADS;++i)    {        index[i] = i;        auto ret = pthread_create(&tids[i],NULL,say,(void*)(&index[i]));        if (ret){            cout<<"pthread_create error,error code = "<<ret<<endl;        }    }    pthread_attr_destroy(&attr);//destroy the attr and wait other thread    for(size_t i = 0;i<NUM_THREADS;++i)    {        auto ret = pthread_join(tids[i],&status);        if (ret){            cout<<"Error:unable to join "<<ret<<endl;            exit(-1);        }        cout<<"main: completed thread id "<<i<<endl;    }    pthread_exit(NULL);    //pthread_create(thread,NULL,merge_sort,)    return 0;}

下面的代码则是用多线程进行归并排序,由于使用模板参数,该程序可以比较任意的类型的数据。下面说几个编程时遇到的问题

。

(1):似乎线程的目标函数必须是void* 类型的函数,否则编译不通过。

(2):在传输参数的时候,必须将参数封装在一个结构体里面,然后将结构体的指针转换成void*类型。

(3):因为我们要进行模板编程,所以要将结构体,以及相应的处理函数都写成模板的形式。

(4):将数据用vector表示而不是数组的形式,这样可用不需要知道数据的长度,结构体内部使用该vector的引用。

(5):使用decltype()获取类型,如该使用decltype(a[0])的形式会获得一个引用的类型,这不是需要的,所以要利用decltype(a);

#include <pthread.h>#include <iostream>#include <fstream>#include <string>#include <vector>#include <cstdlib>#include <typeinfo>#include <algorithm>#include "unistd.h"using namespace std;#define NUM_THREADS 5template<typename T>struct Leninfo{       Leninfo(int _l,int _h,T& _arr):l(_l),h(_h),arr(_arr){}        int l;        int h;        T& arr;};vector<int> read_file(string filename){    ifstream file(filename);    int each_element;    vector<int> data;    if (file){        while(file>>each_element) {            data.push_back(each_element);        }    }    return data;}template<typename T>void my_merge(T& arr,int l,int mid,int r){    size_t i = 0,j = 0,k = l;    size_t bound1 = mid-l,bound2 = r-mid;    T first;    T second;    for(size_t i = l;i!=mid;++i){first.push_back(arr[i]);}    for(size_t i = mid;i!=r;++i){second.push_back(arr[i]);}    while(i<bound1 & j<bound2)    {        if (first[i]<second[j]){ arr[k++] = first[i++];}        else{arr[k++] = second[j++];}    }     while (i<bound1){arr[k++] = first[i++]; }     while (j<bound2){arr[k++] = second[j++];}}template<typename T>void* merge_sort(void* info)// merge the arr[l]->arr[h-1]{    auto arg = (Leninfo<T>*)(info);    size_t l = arg->l,h = arg->h;    T& arr = arg->arr;    if (h-l < 5) { sort(arr.begin()+l,arr.begin()+h);    return NULL;}    pthread_t tid1,tid2;    auto mid = (l+h)>>1;    Leninfo<T> info1 = Leninfo<T>(l,mid,arr),info2 = Leninfo<T>(mid,h,arr);    pthread_create(&tid1,NULL,merge_sort<T>,(void*)(&info1));  //merge_sort(l,mid);    pthread_create(&tid2,NULL,merge_sort<T>,(void*)(&info2));//merge_sort(mid,h);    void *status1,*status2;    pthread_join(tid1,&status1);    pthread_join(tid2,&status2);    my_merge(arr,l,mid,h);}int main(){    void* status;    pthread_t tids;//define the id of thread    vector<string> data = {"azcd","afgh","z","hijk","sdf","hlus"};    auto s = Leninfo<vector<string>>(0,data.size(),data);    pthread_create(&tids,NULL,merge_sort<decltype(data)>,(void*)&(s));    pthread_join(tids,&status);    for(const auto &x : data){cout<<x<<endl;}    pthread_exit(NULL);    return 0;}