Question

我正在创建火车站的模拟。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。火车站的主要轨道一次只能容纳1列火车。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);
}

火车输入档案：

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

该计划的输出是：

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，0号火车从未进入主赛道。我必须误解线程如何被条件变量唤醒并陷入僵局。我错过了什么？

Answer 1

是的，您对pthread_cond_wait有轻微的误解。 man page说：

pthread_cond_timedwait（）和pthread_cond_wait（）函数应该   阻止条件变量。它们应在锁定互斥锁的情况下调用   通过调用线程或未定义的行为结果。

这些函数以原子方式释放互斥锁并导致调用线程   阻止条件变量cond;

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

/* 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);

在多个线程之间共享变量

1 个答案: