在 C 中同时使用条件变量和互斥量来同步线程

Question

根据评论者的要求进行了编辑。

这个程序创建了两个线程。每个线程从两个特定输入文件中的一个读取，每个文件的每行代码包含一个字母或一个“0”。线程应该将字母读入全局字符数组，然后打印出来。问题是，在达到“0”时，活动线程必须将控制转移到另一个线程，该线程不应在该行上有“0”。（我们确信，如果文件 1 在一行上有一个“0”，那么文件 2 在相应的行上有一个字母。多个零可以一个接一个，多个字母也可以。）

文件一

h
0
h
0
h
0
h
0
h
0

文件二

0
i
0
i
0
i
0
i
0
i

我正在尝试使用 pthread mutex lock/unlock 以及信号和等待来完成这项工作。但是，我一直处于僵局状态。

有两个线程。目前，它们相互镜像，这意味着它们做同样的事情，只是使用不同的文件和相反的条件。

线程示例：

char final[1001];
pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t condition1 = PTHREAD_COND_INITIALIZER;
pthread_cond_t condition2 = PTHREAD_COND_INITIALIZER;

int w = 1;

void *get()
{
//start reading
while (count < //number)
{
    pthread_mutex_lock(&lock);

    //read line

    //if we've reached a zero
    {
        w = 2;             

         while(w == 2)
         {
            pthread_cond_wait(&condition1, &lock);
         }


         pthread_mutex_unlock(&lock);
    }
    else
    {   
       if(w == 1)
       {

            if(strlen(placeHolderChars)>0)
            {
                 placeHolderChars[1] = '[=12=]';
            }

            //copy char to array

            w= 2;
            pthread_cond_signal(&condition2);
            pthread_mutex_unlock(&lock);
        }
    }

    if(feof(file))
    {     
        fclose(file);
        break;
    }
    count++;

 }

return 0;
}

更新：当使用较大的文件时，信号先等待策略并没有真正起作用。仍在努力！

Answer 1

此代码似乎有效。主要的重大变化是在进入 while (who == N) 循环之前在另一个线程的条件上添加 pthread_cond_signal()。

其他更改包括基本调试打印，以便更轻松地查看正在发生的事情以及哪个线程在做什么。请注意，调试消息以换行符结尾。

#include <assert.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

extern void *getMessage1(void *arg);
extern void *getMessage2(void *arg);

static char message[4096];

int main(void)
{
    pthread_t id1;
    pthread_t id2;

    pthread_create((&id1), NULL, getMessage1, NULL);
    pthread_create((&id2), NULL, getMessage2, NULL);

    pthread_join(id1, NULL);
    pthread_join(id2, NULL);

    for (int j = 0; j < 1001 && message[j] != '[=10=]'; j++)
        printf("%c ", message[j]);
    putchar('\n');

    return 0;
}

static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
static pthread_cond_t condition1 = PTHREAD_COND_INITIALIZER;
static pthread_cond_t condition2 = PTHREAD_COND_INITIALIZER;

static int who = 1;

void *getMessage1(void *arg)
{
    assert(arg == NULL);
    const char filename[] = "Student1";
    FILE *studentOne = fopen(filename, "r");
    if (studentOne == NULL)
    {
        fprintf(stderr, "Failed to open file %s for reading\n", filename);
        exit(EXIT_FAILURE);
    }

    size_t howManyChars;
    char *placeHolderChars;
    int count = 1;

    while (count < 501)
    {
        placeHolderChars = NULL;
        if (getline(&placeHolderChars, &howManyChars, studentOne) == -1)
            break;
        printf("M1(%d): [%s]\n", count, placeHolderChars);
        pthread_mutex_lock(&lock);
        if (strcmp(placeHolderChars, "0\n") == 0)
        {
            printf("M1: Two's turn - 1\n");
            pthread_cond_signal(&condition2);
            who = 2;
            while (who == 2)
            {
                pthread_cond_wait(&condition1, &lock);
            }
            free(placeHolderChars);
        }
        else
        {
            if (who == 1)
            {
                if (strlen(placeHolderChars) > 0)
                {
                    placeHolderChars[1] = '[=10=]';
                }
                strcat(message, placeHolderChars);
                free(placeHolderChars);
                who = 2;
                pthread_cond_signal(&condition2);
            }
            else
                printf("M1: Two's turn - 2\n");
        }
        pthread_mutex_unlock(&lock);
        count++;
    }

    fclose(studentOne);
    return 0;
}

void *getMessage2(void *arg)
{
    assert(arg == NULL);
    const char filename[] = "Student2";
    FILE *studentTwo = fopen(filename, "r");
    if (studentTwo == NULL)
    {
        fprintf(stderr, "Failed to open file %s for reading\n", filename);
        exit(EXIT_FAILURE);
    }

    size_t howManyChars;
    char *placeHolderChars;
    int count = 0;

    while (count < 501)
    {
        placeHolderChars = NULL;
        if (getline(&placeHolderChars, &howManyChars, studentTwo) == -1)
            break;
        printf("M2(%d): [%s]\n", count, placeHolderChars);
        pthread_mutex_lock(&lock);
        if (strcmp(placeHolderChars, "0\n") == 0)
        {
            printf("M2: One's turn - 1\n");
            pthread_cond_signal(&condition1);
            who = 1;
            while (who == 1)
            {
                pthread_cond_wait(&condition2, &lock);
            }
            free(placeHolderChars);
        }
        else
        {
            if (who == 2)
            {
                if (strlen(placeHolderChars) > 0)
                {
                    placeHolderChars[1] = '[=10=]';
                }
                strcat(message, placeHolderChars);
                free(placeHolderChars);
                who = 1;
                pthread_cond_signal(&condition1);
            }
            else
                printf("M2: One's turn - 2\n");
        }
        pthread_mutex_unlock(&lock);
        count++;
    }

    fclose(studentTwo);
    return 0;
}

您应该能够改进代码，以便将包含相关的每线程数据（文件名、当前线程条件、其他线程条件，可能是 'thread ID'）的结构传递给单个函数, 所以你只有 getMessage().

输出：

M1(1): [h
]
M2(0): [0
]
M1(2): [0
]
M2: One's turn - 1
M1: Two's turn - 1
M2(1): [i
]
M2(2): [0
]
M2: One's turn - 1
M1(3): [h
]
M1(4): [0
]
M1: Two's turn - 1
M2(3): [i
]
M2(4): [0
]
M2: One's turn - 1
M1(5): [h
]
M1(6): [0
]
M1: Two's turn - 1
M2(5): [i
]
M2(6): [0
]
M2: One's turn - 1
M1(7): [h
]
M1(8): [0
]
M1: Two's turn - 1
M2(7): [i
]
M2(8): [0
]
M2: One's turn - 1
M1(9): [h
]
M1(10): [0
]
M1: Two's turn - 1
M2(9): [i
]
h i h i h i h i h i

我对这段代码不是很满意。正如我暗示的那样，我创建了一个由两个线程使用的单个函数的修改版本，并修改了读取行的打印以避免打印换行符（使输出更紧凑）。有时——并非所有时候——它会在最后陷入僵局。两个示例跟踪，一个工作，一个死锁（程序名称 pth47）：

$ pth47
M2(1): [0]
M2: 1's turn - 1
M1(1): [h]
M1(2): [0]
M1: 2's turn - 1
M2(2): [i]
M2(3): [0]
M2: 1's turn - 1
M1(3): [h]
M1(4): [0]
M1: 2's turn - 1
M2(4): [i]
M2(5): [0]
M2: 1's turn - 1
M1(5): [h]
M1(6): [0]
M1: 2's turn - 1
M2(6): [i]
M2(7): [0]
M2: 1's turn - 1
M1(7): [h]
M1(8): [0]
M1: 2's turn - 1
M2(8): [i]
M2(9): [0]
M2: 1's turn - 1
M1(9): [h]
M1(10): [0]
M1: 2's turn - 1
M2(10): [i]
h i h i h i h i h i 
$ pth47
M1(1): [h]
M2(1): [0]
M1(2): [0]
M2: 1's turn - 1
M1: 2's turn - 1
M2(2): [i]
M2(3): [0]
M2: 1's turn - 1
M1(3): [h]
M1(4): [0]
M1: 2's turn - 1
M2(4): [i]
M2(5): [0]
M2: 1's turn - 1
M1(5): [h]
M1(6): [0]
M1: 2's turn - 1
M2(6): [i]
M1(7): [h]
M1(8): [0]
M2(7): [0]
M1: 2's turn - 1
M2: 1's turn - 1
M1(9): [h]
M1(10): [0]
M2(8): [i]
M1: 2's turn - 1
M2(9): [0]
M2: 1's turn - 1
^C
$

我没有找到异常。它不像'thread one went first'那么简单；有一些例子，第一个线程先运行并且完成得很好。

Answer 2

我觉得提问题的人说了两遍，有点烦人。 FWIW，这是我对重复项的回答： Pthread Synchronization: Back & Forth Reading of Two Text Files

在 C 中同时使用条件变量和互斥量来同步线程

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