c - 在 C 中将不同的线程汇总在一起？

Question

#include<pthread.h>
#include<stdio.h>
#include<stdlib.h>
#define NUM_THREADS 4

int arraySum = 0;
int array[] = {20, 18, 16, 14, 12, 10, 8, 6, 4, 2, -10, -20, -30, -40, 15, 23, 25, 75, 45, 33};
int low = 0;
int high = 4;

// Function that calculates sum of 5 elements of an array
void *ArraySum(void *threadid)
{
    // Iterating from index low to high
    for(int i = low; i < high; i++)
    {
        // Accumulating array sum
        arraySum = arraySum + array[i];
    }
    // Updating lowest and highest index
    low = low + 5;
    high = high + 5;
    // Exiting current thread
    pthread_exit(NULL);
}
// Main function
int main(int argc, char *argv[])
{
    int rc, t;
    // Creating thread array
    pthread_t threads[NUM_THREADS];
    // Iterating over each thread
    for(t = 0; t < NUM_THREADS; t++)
    {
        // Creating a thread
        rc = pthread_create(&threads[t], NULL, ArraySum, &t);
        // Checking for status
        if(rc)
        {
        printf(" Error; return code from pthread_create() is %d \n", rc);
        exit(-1);
        }
    }
     /* Waiting till all threads finish their execution */
     for (t = 0; t < NUM_THREADS; t++)
     {
         pthread_join(threads[t], NULL);
     }
     // Printing Array sum
     printf("\nArray Sum: %d \n", arraySum);
     pthread_exit(NULL);
 }

目标是创建 4 个不同的线程并将数组中的所有数字加在一起。例如，第一个线程将添加前 5 个数字 (20 + 18 + 16 + 14 + 12)。第二个线程将添加下 5 个，依此类推。

当我运行它时，我得到的总数是 164，但我预计是 211。我相信我已经创建了 4 个不同的线程并正确加入它们，但输出是错误的。

Answer 1

这里我写了不同线程求和的解决方案。我基本上使用线程 ID 来确定每个线程的开始/结束索引。因此，每个线程计算一个部分和，因此不需要同步。当一个线程退出时，他的部分和被返回并累积在最终和中。

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

#define NUM_THREADS 4

int array[] = {20, 18, 16, 14, 12, 10, 8, 6, 4, 2, -10, -20, -30, -40, 15, 23, 25, 75, 45, 33};

// Function that calculates sum of 5 elements of an array
void *ArraySum(void *arg)
{
    int id = (int)arg;
    int partial_sum = 0;
    int begin = id * (NUM_THREADS + 1);
    int end = id * (NUM_THREADS + 1) + NUM_THREADS;

    // Iterating from index low to high
    for(int i = begin; i <= end; i++)
    {
        // Accumulating array partial sum
        partial_sum += array[i];
    }
    // Exiting current thread
    pthread_exit((void *)partial_sum);
}
// Main function
int main(int argc, char *argv[])
{
    int rc, t;
    int arraySum = 0;
    void *retvalue;

    // Creating thread array
    pthread_t threads[NUM_THREADS];

    // Iterating over each thread
    for(t = 0; t < NUM_THREADS; t++)
    {
        // Creating a thread
        rc = pthread_create(&threads[t], NULL, ArraySum, (void *)t);
        // Checking for status
        if(rc)
        {
            printf(" Error; return code from pthread_create() is %d \n", rc);
            exit(-1);
        }
    }
    /* Waiting till all threads finish their execution */
    for (t = 0; t < NUM_THREADS; t++)
    {
         pthread_join(threads[t], &retvalue);
         arraySum += (int)retvalue;
    }
    // Printing Array sum
    printf("\nArray Sum: %d \n", arraySum);
    pthread_exit(NULL);
}