在多个线程之间共享变量

Posted 2023-02-22

【中文标题】在多个线程之间共享变量

【英文标题】：Share variable between multiple threads

【发布时间】：2015-06-17 22:09:58

【问题描述】：

我正在创建一个火车站的模拟。从文件中读取列车信息。文件的每一行代表一列火车，每列火车都有自己的线程。火车站的主轨道一次只能停1辆火车。这是我的代码：

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <string.h>
#include <readline/readline.h>
#include <unistd.h>

pthread_mutex_t main_track_mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t  main_track_condition_var = PTHREAD_COND_INITIALIZER;
int main_track_status = 0;

void *train_function(void *args) 
    /* Parse train information */
    int line_number = atoi(strtok(args, ":,"));
    char *priority_direction = strtok(NULL,":,");
    int loading_time = atoi(strtok(NULL, ":,"));
    int crossing_time = atoi(strtok(NULL, ":,"));

    /* Load train */
    sleep(loading_time/10);
    printf("Train %d is ready to go %s\n",line_number,priority_direction);

    /* If the main track is currently in use, wait for it to become available */
    while(main_track_status)
        pthread_cond_wait(&main_track_condition_var, &main_track_mutex);

    /* Use the main track */
    pthread_mutex_lock(&main_track_mutex);
    main_track_status = 1;
    printf("Train %d is ON the main track going %s\n",line_number,priority_direction);
    sleep(crossing_time/10);
    main_track_status = 0;

    /* Notify other trains main track is empty */
    pthread_mutex_unlock(&main_track_mutex);
    pthread_cond_signal(&main_track_condition_var);
    printf("Train %d is OFF the main track after going %s\n",line_number,priority_direction);

    pthread_exit(0);


int main() 
    FILE *ptr_file;
    char buff[10];
    int train_count = 0;
    char *train;
    char line[15];
    pthread_t trains[3];

    ptr_file = fopen("./trains.txt", "r");

    if (!ptr_file) 
    
        perror("fopen for trains.txt failed");
        exit(EXIT_FAILURE);
    

    /* Create train for each line of file */
    while (fgets(buff,10, ptr_file)!=NULL) 
        train = (char*)malloc(10 * sizeof(char));

        /* Include line number from file in train information */
        sprintf(line, "%d:", train_count);  
        strcat(line, buff);
        strcpy(train, line);

        if(pthread_create(&trains[train_count], NULL, &train_function, (void *) train))
        
            perror("pthread create failed");
            exit(EXIT_FAILURE);
        
        train_count++;
    
    fclose(ptr_file);

    /* Wait for all trains to leave the station */
    for (int x = 0; x < train_count; x++) 
        pthread_join(trains[x], NULL);
    

    free(train);
    exit(EXIT_SUCCESS);

火车输入文件：

e:10,6
W:5,7
E:3,10

程序的输出是：

Train 1 is ready to go W
Train 1 is ON the main track going W
Train 1 is OFF the main track after going W
Train 2 is ready to go E
Train 2 is ON the main track going E
Train 0 is ready to go e
Train 2 is OFF the main track after going E

我认为我的错误在于 train_function。如您所见，火车 0 永远无法进入主轨道。我一定是误解了线程是如何被条件变量唤醒的，并且陷入了死锁。我错过了什么？

【问题讨论】：

this 会有帮助吗？正如那里所解释的，我认为你应该先锁定互斥锁，然后等待条件

【参考方案1】：

是的，你对pthread_cond_wait有一点误解。 man page 说：

pthread_cond_timedwait() 和 pthread_cond_wait() 函数应 阻塞条件变量。应在互斥锁锁定的情况下调用它们 由调用线程或未定义的行为结果。

这些函数原子地释放互斥体并导致调用线程 阻塞条件变量 cond;

所以你需要在调用pthread_cond_wait之前锁定。也就是说，你的代码应该是：

/* If the main track is currently in use, wait for it to become available */
pthread_mutex_lock(&main_track_mutex); /* SHOULD LOCK HERE */
while(main_track_status)
    pthread_cond_wait(&main_track_condition_var, &main_track_mutex);

/* Use the main track */
//pthread_mutex_lock(&main_track_mutex); /* INCORRECT */
//<..SNIP..>
pthread_mutex_unlock(&main_track_mutex);
pthread_cond_signal(&main_track_condition_var);

【讨论】：

我会确保更仔细地阅读手册页。谢谢。

您最好从临界区内部发出信号，即在解锁互斥锁之前。