Pthreads問題;改變價值等

Question

我知道標題的模糊，我只是想不出還有什麼叫它。

sleeper.h

#ifndef SLEEPER_H 
#define SLEEPER_H 
#include <unistd.h> 

int rideTime(int, int); 
void walkAroundTime(int); 

#endif 

sleeper.c

​​

bumper_cars.c

#include <stdio.h> 
#include <stdlib.h> 
#include <pthread.h> 
#include <time.h> 
#include "sleeper.h" 

void* person(void*); 
int getInLine(int); 
void returnCar(int); 

pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER; 
pthread_cond_t car_availability = PTHREAD_COND_INITIALIZER; 
int available_cars; 
int num_cars; 
int* cars; 

int main(int argc, char* argv[]) { 
    num_cars = atoi(argv[1]); 
    int num_people = atoi(argv[2]); 
    int sim_time = atoi(argv[3]); 

    available_cars = num_cars; 
    cars = calloc(num_cars, sizeof(int)); 
    pthread_t* threads = calloc(num_people, sizeof(pthread_t)); 

    srand((unsigned)time (NULL)); 

    for (int i = 0; i < num_cars; i++) { 
     cars[i] = 1; 
    } 

    for (int i = 0; i < num_people; i++) { 
     int* id = malloc(sizeof(*id)); 
     *id = i + 1; 
     pthread_create(&threads[i], NULL, person, (void*)(id)); 
    } 

    //printf("MAIN: Sleeping for %d seconds\n", sim_time); 
    sleep(sim_time); 
    //printf("Done sleeping.\n"); 

    for (int i = 0; i < num_people; i++) { 
     pthread_cancel(threads[i]); 
    } 

    printf("Simulation complete.\n"); 

    pthread_mutex_destroy(&lock); 
    free(cars); 
    free(threads); 

    exit(EXIT_SUCCESS); 
} 

void* person(void* arg) { 
    int car = 0; 
    int id = *((int *) arg); 
    while (1) { 
     walkAroundTime(id); 
     car = getInLine(id); 
     rideTime(id, car); 
     returnCar(car); 
    } 
} 

int getInLine(int id) { 
    int car = 0; 

    printf("Person %d is waiting for a car.\n", id); 
    pthread_mutex_lock(&lock); 

    if (available_cars == 0) { 
     pthread_cond_wait(&car_availability,&lock); 
    } 

    for (int i = 0; i < num_cars; i++) { 
     if (cars[i] == 1) { 
      car = i + 1; 
      cars[i] = 0; 
      break; 
     } 
    } 
    available_cars--; 

    return car; 
} 

void returnCar(int carID) { 
    available_cars++; 
    cars[carID] = 1; 
    printf("Car %d has been returned.\n", carID); 
    pthread_cond_broadcast(&car_availability); 
    pthread_mutex_unlock(&lock); 
} 

所以...我有兩個問題，我的計劃。最重要的是，它只適用於1輛車。如果有多輛汽車，並且其中一輛已經被佔用，所有其他人仍然必須等待它被退回。

第二個不太重要的問題是，當我用一輛車開始代碼時，汽車的id從0開始，然後在1和0之間交替，當它應該是1時。看着我的代碼，對我來說這些問題的起源並不明顯......而且gdb被證明是相當無益的。

Answer 1

問題在於，一旦您獲取該車以獲得該車，您就會持續太久。我用這種方式修改了功能，在評論中所指出的變化：

int getInLine(int id) { 
    int car = 0; 

    printf("Person %d is waiting for a car.\n", id); 
    pthread_mutex_lock(&lock); 

    // Use a while loop because pthread_cond_wait() is subject to 
    // spurious awakenings. 
    while (available_cars == 0) { 
     pthread_cond_wait(&car_availability,&lock); 
    } 

    for (int i = 0; i < num_cars; i++) { 
     if (cars[i] == 1) { 
      car = i + 1; 
      cars[i] = 0; 
      break; 
     } 
    } 
    available_cars--; 
    // Unlock the mutex here now that we are done acquiring the car. 
    // If we don't unlock the mutex here, no one else can acquire a car. 
    pthread_mutex_unlock(&lock); 

    return car; 
} 

void returnCar(int carID) { 
    // Lock the mutex here before returning the car. 
    pthread_mutex_lock(&lock); 
    available_cars++; 
    // Need -1 here because carID is one bigger than the index. 
    cars[carID-1] = 1; 
    printf("Car %d has been returned.\n", carID); 
    pthread_cond_broadcast(&car_availability); 
    pthread_mutex_unlock(&lock); 
} 

順便說一句，我注意到您使用rand()生成隨機數。但是，如果多個線程同時調用rand()，則會得到相同的隨機數（至少這是我運行程序時發生的情況）。您應該使用rand_r()，併爲每個線程創建不同的種子。