[Linux/C++] 多線程實例：十字路口車輛調度

實例要求：

有兩條道路雙向兩個車道，即每條路每一個方向只有一個車道，兩條道路十字交叉。假設車輛只能向前直行，而不容許轉彎和後退。若是有4輛車幾乎同時到達這個十字路口，如圖（a）所示；相互交叉地停下來，如圖（b），此時4輛車都將不能繼續向前，這是一個典型的死鎖問題。從操做系統原理的資源分配觀點，若是4輛車都想駛過十字路口，那麼對資源的要求以下：

• 向北行駛的車1須要象限a和b；

• 向西行駛的車2須要象限b和c；

• 向南行駛的車3須要象限c和d；

• 向東行駛的車4須要象限d和a。
  

咱們要實現十字路口交通的車輛同步問題，防止汽車在通過十字路口時產生死鎖和飢餓。在咱們的系統中，東西南北各個方向不斷地有車輛通過十字路口（注意：不僅有4輛），同一個方向的車輛依次排隊經過十字路口。按照交通規則是右邊車輛優先通行，如圖(a)中，若只有car一、car二、car3，那麼車輛經過十字路口的順序是car3->car2->car1。車輛通行總的規則：
1)來自同一個方向多個車輛到達十字路口時，車輛靠右行駛，依次順序經過；
2)有多個方向的車輛同時到達十字路口時，按照右邊車輛優先通行規則，除非該車在十字路口等待時收到一個當即通行的信號；
3)避免產生死鎖；
4)避免產生飢餓；
5)任何一個線程（車輛）不得采用單點調度策略；
6)因爲使用AND型信號量機制會使線程（車輛）併發度下降且引發不公平（部分線程飢餓），本題不得使用AND型信號量機制，即在上圖中車輛不能要求同時知足兩個象限才能順利經過，如南方車輛不能同時判斷a和b是否有空。

個人解決方案（可能存在一些不足，但願你們指出）：

/**
 *方法：使用四種線程表明四個方向的車，經過互斥鎖和信號量實現題目裏的要求。
 */
#include "../lib/myhead.h"
#include <pthread.h>
#include <queue>

#define SLEEP_MS(ms) usleep((ms)*1000) 

using std::queue;
enum Direction{
    NORTH = 1,
    EAST = 2,
    SOUTH = 3,
    WEST = 4
};

/* generate mutex and cond that are required */
template<typename T> struct NormalMutex{
    T val; //store some value that you can use
    bool flag; //control wether using cond
    pthread_mutex_t mutex;
    pthread_cond_t cond;
    NormalMutex():flag(false){
        int err = pthread_mutex_init(&mutex,nullptr);
        if(err!=0) 
            err_exit(err,"mutex init failed");
    }
    /* if you need cond, please set flag true */
    NormalMutex(bool flag):NormalMutex(){
        this->flag = flag;
        if(flag){
            int err = pthread_cond_init(&cond,nullptr);
            if(err!=0)
                err_exit(err,"cond init failed");
        }
    }
    ~NormalMutex(){
        pthread_mutex_destroy(&mutex);
        if(flag)
            pthread_cond_destroy(&cond);
    }
};

/* define the struct containing mutex and cond */
NormalMutex< queue<int> > q_north(true), q_south(true), q_west(true), q_east(true);
NormalMutex<bool> f_north(true), f_south(true), f_west(true), f_east(true);
NormalMutex<int> r_a, r_b, r_c, r_d;

/* define the integer to store the current car in four direction */
NormalMutex<long long> cur_n, cur_s, cur_e, cur_w;

/* define bool to make sure wether a direction has car */
NormalMutex<bool> isin_n, isin_s, isin_e, isin_w;

/* mark the remaining road*/
NormalMutex<int> resource;

/* mark the end of deadlock*/
NormalMutex<bool> dl_over(true);

/* signal four of few val to go firstly */
void init_car(){
    if(q_north.val.size()>0){ //if there are val waiting in the queue, pop one and let it go
        cur_n.val = q_north.val.front(); //pop
        q_north.val.pop();
        pthread_cond_broadcast(&q_north.cond); //let it go
    }
    if(q_south.val.size()>0){
        cur_s.val = q_south.val.front();
        q_south.val.pop();
        pthread_cond_broadcast(&q_south.cond);
    }
    if(q_west.val.size()>0){
        cur_w.val = q_west.val.front();
        q_west.val.pop();
        pthread_cond_broadcast(&q_west.cond);
    }
    if(q_east.val.size()>0){
        cur_e.val = q_east.val.front();
        q_east.val.pop();
        pthread_cond_broadcast(&q_east.cond);
    }
}

int enterTheCrossing(Direction dir,const long long &car_no){
    NormalMutex<int> *road;
    NormalMutex<bool> *isin;
    string direction;
    switch(dir){
        case NORTH:
            road = &r_c; isin = &isin_n; direction = "North"; break;
        case EAST:
            road = &r_b; isin = &isin_e; direction = "East"; break;
        case SOUTH:
              road = &r_a; isin = &isin_s; direction = "South"; break;
        case WEST:
            road = &r_d; isin = &isin_w; direction = "West"; break;
    }
    /* enter the crossing */
    pthread_mutex_lock(&(road->mutex));
    printf("car %lld from %s arrives at crossing\n",car_no,direction.c_str());

    /* things to do after lock the first road */
    isin->val = true; //mark that there is car in north direction
    pthread_mutex_lock(&resource.mutex);
    int tem_re = --resource.val; //let the resource minus one
    pthread_mutex_unlock(&resource.mutex); 

    return tem_re;
}
void detectDeadlock(Direction dir,int tem_re){
    if(tem_re!=0) return ;
    string direction;
    NormalMutex<int> *road;
    NormalMutex<bool> *first, *isin;
    switch(dir){
        case NORTH:
            direction = "East";  road = &r_c;isin = &isin_n; first = &f_east; break;
        case EAST:
            direction = "South"; road = &r_b;isin = &isin_e; first = &f_south; break;
        case SOUTH:
            direction = "West"; road = &r_a;isin = &isin_s; first = &f_west; break;
        case WEST:
            direction = "North"; road = &r_d;isin = &isin_w first = &f_north; break;
    }
    printf("DEADLOCK car jam detected. signal %s to go\n",direction.c_str());
    dl_over.val = false;
    /* deal with the deadlock by making left car go */
    pthread_mutex_unlock(&(road->mutex)); //release the road 
    isin->val = false; //let left car go first
    pthread_cond_signal(&(first->cond));// send the signal to left car

    /* wait the end of deadlock */
    pthread_mutex_lock(&dl_over.mutex);
    while(dl_over.val==false)
        pthread_cond_wait(&dl_over.cond,&dl_over.mutex);
    pthread_mutex_unlock(&dl_over.mutex);

    /* recover from deadlock */   
    pthread_mutex_lock(&(road->mutex)); 
    isin->val = true;
}

void judgeRight(Direction dir){
    NormalMutex<bool> *isin;
    NormalMutex<bool> *first;
    switch(dir){
        case NORTH:
            isin = &isin_w; first = &f_north; break;
        case EAST:
            isin = &isin_n; first = &f_east; break;
        case SOUTH:
            isin = &isin_e; first = &f_south; break;
        case WEST: 
            isin = &isin_s; first = &f_west; break;
    }
    /* juage that if car can go first */
    pthread_mutex_lock(&(first->mutex));
    while(isin->val)
        pthread_cond_wait(&(first->cond),&(first->mutex));
    pthread_mutex_unlock(&(first->mutex));
}

void gotoNextRoad(Direction dir,const long long & car_no){
    string direction;
    NormalMutex<int> *r1,*r2;
    NormalMutex<bool> *isin,*lisin,*first;
    switch(dir){
        case NORTH:
            r1 = &r_c; r2 = &r_d; isin = &isin_n;lisin = &isin_e; first = &f_east; direction = "North";      
            break; 
        case EAST:
            r1 = &r_b; r2 = &r_c; isin = &isin_e;lisin = &isin_s; first = &f_south; direction = "East";
            break;
        case SOUTH:
            r1 = &r_a; r2 = &r_b; isin = &isin_s;lisin = &isin_w; first = &f_west; direction = "South";
            break;
        case WEST:
            r1 = &r_d; r2 = &r_a; isin = &isin_w;lisin = &isin_n; first = &f_north; direction = "West";
            break;
    }
    /* go to next road */
    pthread_mutex_lock(&(r2->mutex));
    /* unlock the first road */    
    pthread_mutex_unlock(&(r1->mutex));
    printf("car %lld from %s leaving crossing\n",car_no,direction.c_str());
    
    /* things to do after unlocking the first road */    
    pthread_mutex_lock(&resource.mutex);
    resource.val++; //resource plus one
    pthread_mutex_unlock(&resource.mutex);
   
       /* out of the deadlock */
    dl_over.val = true;
    pthread_cond_signal(&dl_over.cond);

    /* unlock the second road */
    pthread_mutex_unlock(&(r2->mutex));
   
    isin->val = false; //the road don't have car
    /* if left direction has waiting car,let it go first*/
    pthread_mutex_lock(&(first->mutex));
    first->val = true; //let left car go first, if exist
    pthread_mutex_unlock(&(first->mutex));
    pthread_cond_signal(&first->cond); //send signal to left car
}
void doAfterGo(Direction dir){
    NormalMutex<queue<int> > *qu;
    NormalMutex<long long> *cur;
    switch(dir){
        case NORTH:
            qu = &q_north; cur = &cur_n; break;
        case EAST:
            qu = &q_east; cur = &cur_e; break;
        case SOUTH:
            qu = &q_south; cur = &cur_s; break;
        case WEST:
            qu = &q_west;  cur = &cur_w; break;
    }

    /* let the next car in the same direction to go */
    pthread_mutex_lock(&(qu->mutex));
    pthread_mutex_lock(&(cur->mutex)); 
    cur->val = qu->val.front(); //set next car to go
    qu->val.pop(); //leave the queue
    pthread_mutex_unlock(&(qu->mutex));
    pthread_mutex_unlock(&(cur->mutex));
    pthread_cond_broadcast(&(qu->cond));     
}

void * n_car(void *arg){
    /* get the car_no from arg*/
    long long car_no = reinterpret_cast<long long>(arg);

    /* block and wait the signal from init_car() or val over it */
    pthread_mutex_lock(&q_north.mutex); 
    while(cur_n.val != car_no){
        pthread_cond_wait(&q_north.cond,&q_north.mutex);
    }
    pthread_mutex_unlock(&q_north.mutex);

    int tem_re = enterTheCrossing(NORTH,car_no);
    detectDeadlock(NORTH,tem_re);
    judgeRight(NORTH);
    gotoNextRoad(NORTH,car_no);
    doAfterGo(NORTH);
    return nullptr;
}
/* the thread representing the car coming from east */
void * e_car(void *arg){
    /* get the car_no from arg*/
    long long car_no = reinterpret_cast<long long>(arg);

    pthread_mutex_lock(&q_east.mutex);
    while(cur_e.val != car_no){
        pthread_cond_wait(&q_east.cond,&q_east.mutex);
    }
    pthread_mutex_unlock(&q_east.mutex);

    int tem_re = enterTheCrossing(EAST,car_no);
    detectDeadlock(EAST,tem_re);
    judgeRight(EAST);
    gotoNextRoad(EAST,car_no);
    doAfterGo(EAST);
    return nullptr;
}

/* the thread representing the car from south */
void * s_car(void *arg){
    /* get the car_no from arg*/
    long long car_no = reinterpret_cast<long long>(arg);

    pthread_mutex_lock(&q_south.mutex);
    while(cur_s.val != car_no){
        pthread_cond_wait(&q_south.cond,&q_south.mutex);
    }
    pthread_mutex_unlock(&q_south.mutex);

    int tem_re = enterTheCrossing(SOUTH,car_no);
    detectDeadlock(SOUTH,tem_re);
    judgeRight(SOUTH);
    gotoNextRoad(SOUTH,car_no);
    doAfterGo(SOUTH);
    return nullptr;
}

/* the thread representing the car from west */
void * w_car(void *arg){
    /* get the car_no from arg*/
    long long car_no = reinterpret_cast<long long>(arg);

    pthread_mutex_lock(&q_west.mutex);
    while(cur_w.val != car_no){
        pthread_cond_wait(&q_west.cond,&q_west.mutex);
    }
    pthread_mutex_unlock(&q_west.mutex);

    int tem_re = enterTheCrossing(WEST,car_no);
    detectDeadlock(WEST,tem_re);
    judgeRight(WEST);
    gotoNextRoad(WEST,car_no);
    doAfterGo(WEST);
    return nullptr;
}
int main(int argc,char *argv[]){
    
    /* check the argv */
    if(argc!=2){
        cout<<"Please input the car stream."<<endl;
        exit(0);
    }

    /* initialize the variable */
    cur_n.val = cur_s.val = cur_e.val = cur_w.val = 0;
    isin_n.val = isin_s.val = isin_e.val = isin_w.val = false;
    resource.val = 4;
    int err = 0;
    int carNumber = strlen(argv[1]);
    pthread_t tids[carNumber+1];
    
    /* create all cars and push them into queue */
    for(int i=1;i<=carNumber;++i){
        switch(argv[1][i-1]){
            case 'n':
                q_north.val.push(i);
                err = pthread_create(&tids[i],nullptr,n_car,reinterpret_cast<void *>(i));
                if(err!=0)
                    err_exit(err,"can't create thread");
                break;
            case 'w':
                q_west.val.push(i);
                err = pthread_create(&tids[i],nullptr,w_car,reinterpret_cast<void *>(i));
                if(err!=0)
                    err_exit(err,"can't create thread");
                break;
            case 's':
                q_south.val.push(i);
                err = pthread_create(&tids[i],nullptr,s_car,reinterpret_cast<void *>(i));
                if(err!=0)
                    err_exit(err,"can't create thread");
                break;
            case 'e':
                q_east.val.push(i);
                err = pthread_create(&tids[i],nullptr,e_car,reinterpret_cast<void *>(i));
                if(err!=0)
                    err_exit(err,"can't create thread");
                break;
        }
    }
    /* wake up the car in front of queue */
    init_car();

    /* join threads */
    for(int i=1;i<=carNumber;++i){
        err = pthread_join(tids[i],nullptr);
        if(err!=0)
            err_exit(err,"can't join thread %d",i);
    }
    exit(0);
}

代碼中使用到的error handler函數：

static void
err_doit(bool, int, const char *, va_list);

void
err_exit(int error, const char *fmt,...){
    va_list ap;
    va_start(ap,fmt);
    err_doit(true,error,fmt,ap);
    va_end(ap);
    exit(1);
}

static void
err_doit(bool errnoflag, int error, const char *fmt, va_list ap){
    char buf[MAXLINE];
    vsnprintf(buf,MAXLINE-1,fmt,ap);
    if(errnoflag)
        snprintf(buf+strlen(buf),MAXLINE-strlen(buf)-1,": %s",strerror(error));
    cerr<<buf<<endl;
}