線程不適當地工作

我在C線程不適當地工作

#include <stdlib.h> 
#include <stdio.h> 
#include <pthread.h> 
#include <semaphore.h> 
#include <unistd.h> 

typedef struct 
{ 
    int customerID; 
    int ticketNumber; 
    sem_t * freeCashiers; 
    sem_t Cashiers[2]; 
}Customer; 

int firstIsFree = 1; 
int secondIsFree = 1; 

void * buyTheTickets(void * data) 
{ 
    Customer * currentCustomer = (Customer *)data; 
    int ID = currentCustomer->customerID; 

    currentCustomer->ticketNumber = rand()%15+1; 

    sem_wait(currentCustomer->freeCashiers);  

    sem_wait(&(currentCustomer->Cashiers[0])); 

    if(firstIsFree) 
    { 
     firstIsFree = 0; 
     printf("First Cashier Sold %d Ticket To %d Customer\n",currentCustomer->ticketNumber,ID); 

     usleep(1000000); 
     firstIsFree = 1; 

     sem_post(&(currentCustomer->Cashiers[0])); 
     sem_post(currentCustomer->freeCashiers); 

     return 0; 
    } 

    sem_post(&(currentCustomer->Cashiers[0])); 

    sem_wait(&(currentCustomer->Cashiers[1])); 

    if(secondIsFree) 
    { 
     secondIsFree = 0; 
     printf("Second Cashier Sold %d Ticket To %d Customer\n",currentCustomer->ticketNumber,ID); 

     usleep(1000000); 
     secondIsFree = 1; 

     sem_post(&(currentCustomer->Cashiers[1])); 
     sem_post(currentCustomer->freeCashiers); 

     return 0; 
    } 
    sem_post(&(currentCustomer->Cashiers[1])); 
} 

int main() 
{ 
    int numberOfCustomers = 15; 
    sem_t * numberOfCashiers = (sem_t*)malloc(sizeof(sem_t)); 
    sem_init(numberOfCashiers,0,2); 
    sem_t first; 
    sem_t second; 
    sem_init(&first,0,1); 
    sem_init(&second,0,1); 

    for(int i=1;i<=numberOfCustomers;i++) 
    { 
     pthread_t * curTh = (pthread_t*)malloc(sizeof(pthread_t)); 
     Customer * curCu = (Customer *)malloc(sizeof(Customer)); 
     curCu->customerID = i; 
     curCu->freeCashiers = numberOfCashiers; 
     curCu->Cashiers[0] = first; 
     curCu->Cashiers[1] = second; 
     pthread_create(curTh,NULL,buyTheTickets,curCu);  
    } 

    pthread_exit(0); 
    return 0; 
}

編寫這些代碼，它給輸出：

First Cashier Sold 14 Ticket To 1 Customer 

Second Cashier Sold 2 Ticket To 2 Customer 

First Cashier Sold 13 Ticket To 3 Customer 

Second Cashier Sold 11 Ticket To 4 Customer 

First Cashier Sold 9 Ticket To 5 Customer 

Second Cashier Sold 11 Ticket To 6 Customer 

First Cashier Sold 2 Ticket To 7 Customer 

Second Cashier Sold 13 Ticket To 8 Customer 

First Cashier Sold 10 Ticket To 9 Customer 

Second Cashier Sold 2 Ticket To 10 Customer 

First Cashier Sold 3 Ticket To 11 Customer 

Second Cashier Sold 8 Ticket To 12 Customer 

First Cashier Sold 6 Ticket To 13 Customer 

Second Cashier Sold 5 Ticket To 14 Customer 

First Cashier Sold 9 Ticket To 15 Customer

的問題是，如果sem_wait(&(currentCustomer->Cashiers[0]));後的第二個線程去這意味着sem_post(&(currentCustomer->Cashiers[0]));這已經發生了所以firstIsFree已經是1，爲什麼它會打印第二個printf？

來源

2016-11-22 Giorgi Cercvadze

哪個場合想要打扮呢？ ...;） – LPs

@LP沒有得到這個問題，在這個領域新的，你可以問另一種方式？ –

好吧。這是一個笑話：[不適當]（http://dictionary.cambridge.org/it/dizionario/inglese/inappropriate）。我在看你的代碼。如果我看到什麼，我會回到你身邊。 – LPs

下面的問題出現在您的代碼中，這可能會導致未定義的行爲。請糾正它並再次嘗試執行：

在您的主函數中，您使用局部變量來獲取信號量first和second，並且您的主函數似乎不等待線程退出而退出。您的主要功能需要呼叫pthread_join。

來源

2016-11-22 14:48:38 Jay

究竟應該在哪裏調用pthread_join？ –

在主函數中添加一個循環，緊接在pthread_create循環之後，併爲所有創建的線程調用pthread_join。 – Jay

你完全正確，但並不存在風險：這是一個具體的行爲。 – LPs

線程不適當地工作

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