Question

我正在使用pthread进行编程。我需要一个全局变量，它对不同的线程有不同的值。并且线程将使用相同的函数来处理此变量，例如更改其值。如果一个线程更改其值，则不会更改其他线程中的值。所以我尝试使用特定于线程的数据，并写了一个例子。我需要在函数中包装pthread操作。对于exapmle：setspecific（），changedata，printdata（），create_key（），delete_key（）等。

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

pthread_key_t key;
pthread_key_t key2;

struct test_struct {
    int i;
    float k;
}struct_data;

int temp;

int setspecificvar () { /* Set specific data for threads */

    pthread_setspecific (key, &struct_data);
    pthread_setspecific (key2, &temp);

    return 0;
}
int changedata (int i, float k, int tempvar) { /* Change specific data for threads */

    temp = tempvar;
    struct_data.i = i;
    struct_data.k = k;

    return 0;
}

int printdata (int t) {  /* print specific data for threads */

    printf ("The addres in child%d returned from pthread_getspecific(key):0x%p\n",                           \
            t, (struct test_struct *)pthread_getspecific(key));

    printf ("The value of members in structure bound to \"key\" in  child%d:\nstruct_data.i:%d\nstruct_data.k: %f\n", \
            t, ((struct test_struct *)pthread_getspecific (key))->i,                            \
            ((struct test_struct *)pthread_getspecific(key))->k);

    printf ("------------------------------------------------------\n");

    printf ("The addres in child%d returned from pthread_getspecific(key2):0x%p\n",                          \
            t, (int *)pthread_getspecific(key2));
    printf ("The value of \"temp\" bound to \"key2\" in child%d:%d\n", \
            t, *((int *)pthread_getspecific(key2)));

    return 0;
}

void *child1 (void *arg)
{
    setspecificvar ();
    changedata(10, 3.141500, 110); /* Should not change the data in child2 */
    printdata(1);
}

void *child2 (void *arg)
{
    /* sleep (2); */
    setspecificvar ();

    changedata(12, 2.141500, 120); /* Should not change the data in child1 */
    printdata(2);

    changedata (122, 22.141500, 1220); /* Should not change the data in child1 */
    printdata (2);
}

int create_key () {
    pthread_key_create (&key, NULL);
    pthread_key_create (&key2, NULL);
    return 0;
}

int delete_key () {

    pthread_key_delete (key);
    pthread_key_delete (key2);
    return 0;
}

int main (void)
{
    pthread_t tid1, tid2;

    create_key ();
    pthread_create (&tid1, NULL, (void *)child1, NULL);
    pthread_create (&tid2, NULL, (void *)child2, NULL);
    pthread_join (tid1, NULL);
    pthread_join (tid2, NULL);

    delete_key ();

    return 0;
}

我创建了两个线程。当我让一个线程睡2秒。我得到了正确答案。

The addres in child1 returned from pthread_getspecific(key):0x0x8049c98
The value of members in structure bound to *"key" in  child1*:
*struct_data.i:10
struct_data.k: 3.141500*
------------------------------------------------------
The addres in child1 returned from pthread_getspecific(key2):0x0x8049ca0
The value of "temp" bound to *"key2" in child1*:110
The addres in child2 returned from pthread_getspecific(key):0x0x8049c98
The value of members in structure bound to "key" in  child2:
struct_data.i:12
struct_data.k: 2.141500
------------------------------------------------------
The addres in child2 returned from pthread_getspecific(key2):0x0x8049ca0
The value of "temp" bound to "key2" in child2:120
The addres in child2 returned from pthread_getspecific(key):0x0x8049c98
The value of members in structure bound to "key" in  child2:
struct_data.i:122
struct_data.k: 22.141500
------------------------------------------------------
The addres in child2 returned from pthread_getspecific(key2):0x0x8049ca0
The value of "temp" bound to "key2" in child2:1220

当我发表评论/ * sleep（2）; * /，我得到了不正确的答案。

The addres in child1 returned from pthread_getspecific(key):0x0x8049c54
The addres in child2 returned from pthread_getspecific(key):0x0x8049c54
The value of members in structure bound to "key" in  child2:
*struct_data.i:12
struct_data.k: 2.141500*
The value of members in structure bound to *"key" in  child1*:
struct_data.i:12
struct_data.k: 2.141500
------------------------------------------------------
The addres in child1 returned from pthread_getspecific(key2):0x0x8049c5c
The value of "temp" bound to *"key2" in child1*:120
------------------------------------------------------
The addres in child2 returned from pthread_getspecific(key2):0x0x8049c5c
The value of "temp" bound to "key2" in child2:120
The addres in child2 returned from pthread_getspecific(key):0x0x8049c54
The value of members in structure bound to "key" in  child2:
struct_data.i:122
struct_data.k: 22.141500
------------------------------------------------------
The addres in child2 returned from pthread_getspecific(key2):0x0x8049c5c
The value of "temp" bound to "key2" in child2:1220

我希望在没有睡眠的情况下获得正确的结果。一个线程不应该等待另一个线程完成调用pthread_setspecific（），对吗？我该怎么办？谢谢你的考虑。我有权将struct_data定义为全局变量吗？有人可以帮帮我吗？

Answer 1

int setspecificvar () { /* Set specific data for threads */

    pthread_setspecific (key, &struct_data);
    pthread_setspecific (key2, &temp);

    return 0;
}

在这里，您明确地将key和key2都设置为每个线程中的相同值，因此在每个线程中它具有相同的值并不奇怪。尝试在每个线程中将其设置为不同的值，然后在每个线程中都有不同的值。

一个常见的模式是：

致电pthread_getspecific。如果它返回非NULL，请使用该指针。
如果返回NULL，则使用动态分配特定于线程的对象的新实例。调用pthread_setspecific以确保从该线程中对pthread_getspecific的下一次调用返回同一个对象。
在pthread_key_create调用中，请务必注册一个析构函数，以便在线程消失时释放特定于线程的对象的实例。

这将为每个线程提供自己的结构实例。

例如：

int setspecificvar () { /* Set specific data for threads */

    struct test_struct *s = malloc(sizeof(struct test_struct));
    int *i = malloc(sizeof(int *));
    memset(s, 0, sizeof(s));
    memset(i, 0, sizeof(i));

    pthread_setspecific (key, s);
    pthread_setspecific (key2, i);

    return 0;
}

这实际上在每个线程中设置了不同的值。这个：

int changedata (int i, float k, int tempvar) { /* Change specific data for threads */

    struct test_struct *struct_data = pthread_getspecific(key);
    int *temp = pthread_getspecific(key2);

    *temp = tempvar;
    struct_data->i = i;
    struct_data->k = k;

    return 0;
}

实际上使用特定于线程的数据。

如何正确使用特定于线程的数据

1 个答案: