[英]How to use Thread-specific data correctly
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
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
我希望在没有睡觉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?
int setspecificvar () { /* Set specific data for threads */
pthread_setspecific (key, &struct_data);
pthread_setspecific (key2, &temp);
return 0;
}
在这里,你明确地将key
和key2
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应该不足为奇。 尝试在每个线程中将其设置为不同的值,然后在每个线程中将具有不同的值。
一个常见的模式是:
调用pthread_getspecific
。 如果它返回非NULL,请使用该指针。
如果它返回NULL,则使用动态分配特定于线程的对象的新实例。 调用pthread_setspecific
以确保从该线程下一次调用pthread_getspecific
返回同一个对象。
在pthread_key_create
调用中,请务必注册析构函数,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。 和这个:
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;
}
这实际上使用特定于线程的数据。
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