Usage demonstration of pthread_once and pthread_key_create

static pthread_key_t key;static pthread_once_t key_once = PTHREAD_ONCE_INIT;void make_key(){    fprintf(stderr, "make_key\n");    pthread_key_create(&key, NULL);}void* thread_routine(void* user_data){    void *ptr;    long num = (long)user_data;    pthread_once(&key_once, make_key);    if ((ptr = pthread_getspecific(key)) == NULL)    {        ptr = malloc(4);        if (num == 1)        {            strcpy((char*)ptr, "abc");        }        else        {            strcpy((char*)ptr, "def");        }        pthread_setspecific(key, ptr);        fprintf(stderr, "thread %ld call pthread_setspecific\n", num);    }    while (1)    {        void* data = pthread_getspecific(key);        if ((data = pthread_getspecific(key)) == NULL)        {            assert(0);        }        fprintf(stderr, "thread %ld running %s\n", num, (char*)data);        sleep(3);    }    return NULL;}int main(){    pthread_t t1;    pthread_t t2;    int r1, r2;    r1 = pthread_create(&t1, NULL, thread_routine, (void*)1);    r2 = pthread_create(&t2, NULL, thread_routine, (void*)2);    assert(r1 == 0 && r2 == 0);    pthread_join(t1, NULL);    pthread_join(t2, NULL);    return 0;}

　　

Usage demonstration of pthread_once and pthread_key_create