如何正確使用特定於線程的數據

Question

我正在使用pthread進行編程。 我需要一個全局變量，它對不同的線程有不同的值。 並且線程將使用相同的函數來處理此變量，例如更改其值。 如果一個線程更改其值，則不會更改其他線程中的值。 所以我嘗試使用特定於線程的數據，並寫了一個例子。 我需要在函數中包裝pthread操作。 對於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;
}

我創建了兩個線程。 當我讓一個線程睡2秒。 我得到了正確的答案。

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

當我發表評論/ * sleep（2）; * /，我得到了不正確的答案。

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

我希望在沒有睡覺的情況下獲得正確的結果。 一個線程不應該等待另一個線程完成調用pthread_setspecific（），對嗎？ 我該怎么辦？ 謝謝你的考慮。我有權將struct_data定義為全局變量嗎？ 有人可以幫幫我嗎？

Answer 1

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

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

    return 0;
}

在這里，你明確地將key和key2設置為每個線程中的相同值 ，因此在每個線程中它具有相同的值應該不足為奇。 嘗試在每個線程中將其設置為不同的值，然后在每個線程中將具有不同的值。

一個常見的模式是：

1. 調用pthread_getspecific 。 如果它返回非NULL，請使用該指針。

2. 如果它返回NULL，則使用動態分配特定於線程的對象的新實例。 調用pthread_setspecific以確保從該線程下一次調用pthread_getspecific返回同一個對象。

3. 在pthread_key_create調用中，請務必注冊析構函數，以便在線程消失時釋放特定於線程的對象的實例。

這將為每個線程提供自己的結構實例。

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;
}

這實際上在每個線程中設置了不同的值。 和這個：

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;
}

這實際上使用特定於線程的數據。