Pthread Posix素因子化得到奇怪的结果？

Question

我有这个项目，我从命令行接收输入，如“54 342 12”，它假设为每个输入创建一个线程，并让线程返回一个整数数组，然后主线程支持打印输出不同的主要因素分解。但我收到奇怪的输出，如一堆零。我不知道为什么，任何帮助将不胜感激。

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


typedef struct _thread_data_t {
        int tid;
        } thread_data_t;

void *runner(void *param);

int main(int argc, char *argv[]) {

        pthread_t thr[argc];
        pthread_attr_t attr;
        int i, rc;
        //int *primeFactor;
        //primeFactor = (int *)malloc(sizeof(int)*argc);
        //thread_data_t thr_data[argc];
        printf("Prime Numbers: ");

        //Get the default attributes
        pthread_attr_init(&attr);
        //creat the thread
        for(i = 1; i < argc; ++i){
        //thr_data[i].tid = i;
        if ((rc = pthread_create(&thr[i],&attr,runner,argv[i]))){
                fprintf(stderr, "error: pthread_create, rc: %d\n", rc);
                return EXIT_FAILURE;
                }
        }

        //Wait for the thread to exit
        for(i = 1; i < argc; ++i){
        void *returnValue;
        int r = 0;
        int x = (sizeof(returnValue) / sizeof(returnValue[0])) - 1;
        pthread_join(thr[i], &returnValue);
                for(r = 0; r < x; r++){
                //int c = (int *)returnValue[r];
                printf("%d ",  ((int *)returnValue)[r]);
                }
        }
        printf("\nComplete\n");

}

//The Thread will begin control in this function
void *runner(void *param) {
        int *primeFactors;
        int num = atoi(param);
        primeFactors = (int *)malloc(sizeof(int)*num);
        int i, j, isPrime;
        int k = 0;
        for(i=2; i<=num; i++)
        {
                if(num%i==0)
                {
                        isPrime=1;
                        for(j=2; j<=i/2; j++)
                        {
                                if(i%j==0)
                                {
                                        isPrime = 0;
                                        break;
                                }
                        }

                        if(isPrime==1)
                        {
                                primeFactors[k] = i;
                                k++;
                        }
                }
        }


        //Exit the thread
//      pthread_exit(0);

//      pthread_exit((void *)primeFactors);
        pthread_exit(primeFactors);
}

Answer 1

这行是个问题：

int x = (sizeof(returnValue) / sizeof(returnValue[0])) - 1;

sizeof array / sizeof array[0]仅适用于纯数组，而不适用于指针。请注意，returnValue是一个指针，因此sizeof(returnValue)不会返回线程创建的整数序列的字节大小，它会为您提供指针需要存储在内存中的字节数。在x86_64架构上，它很可能是8，所以x在大多数情况下将大于素数因子的实际数量，你可以访问指针超出界限，这就是你看到垃圾值的原因。

因为要返回指向malloced位置的指针，所以还需要返回长度。最好的方法是为返回值创建一个结构并将信息存储在那里。

创建一个结构

struct thread_result
{
    int *factors;
    size_t len;
};

并使用以下信息返回指向此结构的指针：

void *runner(void *param) {
        int *primeFactors;
        int num = atoi(param);
        if(num == 0)
            pthread_exit(NULL);

        struct thread_result *res = calloc(1, sizeof *res);

        res->factors = NULL;
        res->len = 0;

        int *tmp;

        if(res == NULL)
            pthread_exit(NULL);

        int i, j, isPrime;
        int k = 0;
        for(i=2; i<=num; i++)
        {
            if(num%i==0)
            {
                isPrime=1;
                for(j=2; j<=i/2; j++)
                {
                    if(i%j==0)
                    {
                            isPrime = 0;
                            break;
                    }
                }

                if(isPrime==1)
                {
                    tmp = realloc(res->factors, (k+1) * sizeof *res->factors);
                    if(tmp == NULL)
                    {
                        free(res->factors);
                        free(res);
                        pthread_exit(NULL);
                    }
                    res->factors = tmp;

                    res->factors[k++] = i;
                    res->len = k;
                }
            }
        }


        pthread_exit(res);
}

现在你可以得到这样的值：

for(i = 1; i < argc; ++i){
    void *data;
    pthread_join(thr[i], &data);

    if(data == NULL)
        continue; // error in thread

    struct thread_result *res = data;

    for(size_t r = 0; r < res->len; r++){
        printf("%d ", res->factors[r]);
    }

    free(res->factors);
    free(res);
}

并且不要忘记用它来破坏线程属性

pthread_attr_destroy(&attr);

在离开main之前。但是您没有为线程设置任何属性，因此您可以使用以下命令创建线程：

pthread_create(&thr[i], NULL, runner, argv[i]);

还有don't cast malloc，你必须检查malloc的返回值。

编辑

OP在评论中写道

我认为我正确地更新了所有代码以匹配您所说的但现在我收到错误“Segmentation dump（core dumped）”任何想法为什么会这样？

其实没有，但你一定错过了某些地方，因为当我编译并运行这段代码时：

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


typedef struct _thread_data_t {
        int tid;
        } thread_data_t;

void *runner(void *param);

struct thread_result
{
    int *factors;
    size_t len;
};

int main(int argc, char *argv[]) {

    pthread_t thr[argc];
    int i, rc;
    printf("Prime Numbers:\n");

    for(i = 1; i < argc; ++i){
        if ((rc = pthread_create(&thr[i],NULL,runner,argv[i]))){
            fprintf(stderr, "error: pthread_create, rc: %d\n", rc);
            return EXIT_FAILURE;
        }
    }


    for(i = 1; i < argc; ++i){
        void *data;
        pthread_join(thr[i], &data);

        if(data == NULL)
            continue; // error in thread

        struct thread_result *res = data;

        for(size_t r = 0; r < res->len; r++){
            printf("%d ", res->factors[r]);
        }

        free(res->factors);
        free(res);

        puts("");
    }


}

void *runner(void *param) {
    int num = atoi(param);
    if(num == 0)
        pthread_exit(NULL);

    struct thread_result *res = calloc(1, sizeof *res);

    res->factors = NULL;
    res->len = 0;

    int *tmp;

    if(res == NULL)
        pthread_exit(NULL);

    int i, j, isPrime;
    int k = 0;
    for(i=2; i<=num; i++)
    {
        if(num%i==0)
        {
            isPrime=1;
            for(j=2; j<=i/2; j++)
            {
                if(i%j==0)
                {
                        isPrime = 0;
                        break;
                }
            }

            if(isPrime==1)
            {
                tmp = realloc(res->factors, (k+1) * sizeof *res->factors);
                if(tmp == NULL)
                {
                    free(res->factors);
                    free(res);
                    pthread_exit(NULL);
                }
                res->factors = tmp;

                res->factors[k++] = i;
                res->len = k;
            }
        }
    }


    pthread_exit(res);
}

我得到这个输出：

$ valgrind ./a 54 342 12
==17697== Memcheck, a memory error detector
==17697== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==17697== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==17697== Command: ./a 54 342 12
==17697== 
Prime Numbers:
2 3 
2 3 19 
2 3 
==17697== 
==17697== HEAP SUMMARY:
==17697==     in use at exit: 0 bytes in 0 blocks
==17697==   total heap usage: 19 allocs, 19 frees, 3,640 bytes allocated
==17697== 
==17697== All heap blocks were freed -- no leaks are possible
==17697== 
==17697== For counts of detected and suppressed errors, rerun with: -v
==17697== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

这告诉我一切正常，所有记忆都被释放了。

因此，当您从我的答案中复制并粘贴代码时，您肯定会犯错。