矩阵中的多线程搜索，行索引未正确显示

Question

这是我在 StackOverflow 上的第一个问题。我正在尝试对 C 中的互斥体进行练习，要求我在矩阵 N x N 中找到一个元素。每个线程都必须检查每一行中的元素。当一个线程找到该元素时，它应该告诉其他人以便他们可以退出而无需继续搜索。我已经使用标志编写了一个解决方案来检查是否找到了元素，但无法弄清楚为什么行索引没有正确显示。我认为矩阵中的内存地址是连续的，所以我认为使用 % 操作数可以帮助我。我也不确定线程​​函数是否正确编写。你能帮我弄清楚为什么行索引没有正确显示吗？如果还有其他优雅的解决方案？

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

#define MAXNTHREADS 1000000
#define MIN(a,b) (a < b ? a : b)

void * search (void *);
int nthreads;
char tosearch;
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
int flag;

int main (int argc, char **argv)
{
    pthread_t * threads;
    char * matrix;

    if (argc != 3)
    {
        fprintf (stderr, "usage: matrix <char> <#threads>\n");
        exit (-1);
    }

    nthreads = MIN (atoi(argv[2]), MAXNTHREADS);
    tosearch = argv[1][0];

    matrix = malloc (nthreads * nthreads);
    threads = malloc (sizeof (pthread_t) * nthreads);

    printf ("Insert %d elements\n", nthreads * nthreads);

    for (int i = 0; i < nthreads; i ++)
    {
        for (int j = 0; j < nthreads; j ++)
        {
            scanf ("%c", matrix + i * nthreads + j);
            getchar();  
        }
    }

    printf ("MATRIX:\n");
    for (int i = 0; i < nthreads; i ++)
    {
        for (int j = 0; j < nthreads; j ++)
        {
            printf ("%c\t", *(matrix + i * nthreads + j));  
        }

        printf ("\n");
    }



    for (int i = 0; i < nthreads; i ++)
    {
        pthread_create (threads + i, NULL, search, matrix + i * nthreads);
    }

    for (int i = 0; i < nthreads; i ++)
    {
        pthread_join (* (threads + i), NULL);
    }

    free (matrix);
    free (threads);

    exit (0);
}

void * search (void *arg)
{
    printf ("Process %ld started\n", pthread_self());

    for (int i = 0; i < nthreads; i ++)
    {

        pthread_mutex_lock (&mutex);
        if (flag == 0)
        {
            if (*((char *)arg + i) == tosearch)
            {
                printf ("Element found by thread %ld in position %ld %d\n", pthread_self(),  (((unsigned long)arg + nthreads) % nthreads), i);
                flag  = 1;
                pthread_mutex_unlock (&mutex);
                return NULL;
            }

            pthread_mutex_unlock (&mutex);
        }
        else
        {
            pthread_mutex_unlock (&mutex);
            printf ("Process %ld exits\n", pthread_self());
            pthread_exit (NULL);
        }
    }

    printf ("Process %ld, found nothing\n", pthread_self());
    pthread_exit (NULL);
}

Answer 1

我通过将指向矩阵的指针存储为全局变量来解决索引问题。然后我将矩阵起始地址减去元素地址并将结果除以 nthreads。这样我就有了元素的正确行索引。无论如何，我不确定这是解决这个问题的一个优雅的解决方案。

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

#define MAXNTHREADS 1000000
#define MIN(a,b) (a < b ? a : b)

void * search (void *);
int nthreads;
char tosearch;
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
int flag;
char *matrix;

然后在函数搜索中：

void * search (void *arg)
{
    printf ("Process %ld started\n", pthread_self());

    for (int i = 0; i < nthreads; i ++)
    {

        pthread_mutex_lock (&mutex);
        if (flag == 0)
        {
            if (*((char *)arg + i) == tosearch)
            {
                printf ("Element found by thread %ld in position %ld %d\n", pthread_self(), (((unsigned long)arg + i) - (unsigned long)matrix) / nthreads, i);
                flag  = 1;
                pthread_mutex_unlock (&mutex);
                return NULL;
            }

            pthread_mutex_unlock (&mutex);
        }
        else
        {
            pthread_mutex_unlock (&mutex);
            printf ("Process %ld exits\n", pthread_self());
            pthread_exit (NULL);
        }
    }

    printf ("Process %ld, found nothing\n", pthread_self());
    pthread_exit (NULL);
}