pthread_key_t key;
int pthread_key_create(pthread_key *key,void (*destructor)(void*));
int pthread_key_delete(pthread_key_t key);

該函式從TSD池中分配一項，將其值賦給key供以後訪問使用。如果destr_function不為空，線上程退出（pthread_exit()）時將以key所關聯的資料為引數呼叫destr_function()，以釋放分配的緩衝區。不論哪個執行緒呼叫pthread_key_create()，所建立的key都是所有執行緒可訪問的，但各個執行緒可根據自己的需要往key中填入不同的值，這就相當於提供了一個同名而不同值的全域性變數.

pthread_key_delete這個函式並不檢查當前是否有執行緒正使用該TSD，也不會呼叫清理函式（destr_function），而只是將TSD釋放以供下一次呼叫pthread_key_create()使用。因為只有你肯定沒有執行緒持有該鍵值時，才能刪除執行緒私有資料鍵，故通常的做法是不釋放執行緒私有資料鍵。

2）使用執行緒私有資料

int  pthread_setspecific(pthread_key_t  key,  const   void  *value)
void * pthread_getspecific(pthread_key_t key)

/*
 * tsd_once.c
 *
 * Demonstrate use of pthread_once to initialize something
 * exactly once within a multithreaded program.
 *
 * Note that it is often easier to use a statically initialized
 * mutex to accomplish the same result.
 */
#include <pthread.h>
#include <stdio.h>

/*
 * Structure used as the value for thread-specific data key.
 */
typedef struct tsd_tag {
    pthread_t   thread_id;
    char        *string;
} tsd_t;

pthread_key_t tsd_key;           /* Thread-specific data key */
pthread_once_t key_once = PTHREAD_ONCE_INIT;

/*
 * One-time initialization routine used with the pthread_once
 * control block.
 */
void once_routine (void)
{
    int status;

    printf ("initializing key\n");
    status = pthread_key_create (&tsd_key, NULL);
    if (status != 0)
        printf("error Create key");
}
/*
 * Thread start routine that uses pthread_once to dynamically
 * create a thread-specific data key.
 */
void *thread_routine (void *arg)
{
    tsd_t *value;
    int status;

    status = pthread_once (&key_once, once_routine);//一次性初始化
    if (status != 0)
        printf("error Once init");
    value = (tsd_t*)malloc (sizeof (tsd_t));
    if (value == NULL)
        printf("error Allocate key value");
    status = pthread_setspecific (tsd_key, value);
    if (status != 0)
        printf("error Set tsd");
    printf ("%s set tsd value %p\n", arg, value);
    value->thread_id = pthread_self ();
    value->string = (char*)arg;
    value = (tsd_t*)pthread_getspecific (tsd_key);
    printf ("%s starting...\n", value->string);
    sleep (2);
    value = (tsd_t*)pthread_getspecific (tsd_key);
    printf ("%s done...\n", value->string);
    return NULL;    
}

void main (int argc, char *argv[])
{
    pthread_t thread1, thread2;
    int status;

    status = pthread_create (
        &thread1, NULL, thread_routine, "thread 1");
    if (status != 0)
        printf("error Create thread 1");
    status = pthread_create (
        &thread2, NULL, thread_routine, "thread 2");
    if (status != 0)
        printf("error Create thread 2");
    pthread_exit;
}
 

thread 2 set tsd value 0x9de24f0
thread 2 starting...
thread 1 set tsd value 0x9de2500
thread 1 starting...
thread 2 done...
thread 1 done...