Advanced Programming in the UNIX® Environment

ptg10805159

448 Thread Control Chapter 12

#include <pthread.h>

int pthread_key_delete(pthread_key_tkey);

Returns: 0 if OK, error number on failure

Note that calling pthread_key_delete will not invoke the destructor function

associated with the key.Tofreeany memory associated with the key’s thread-specific

data values, we need to take additional steps in the application.

We need to ensurethat a key we allocate doesn’t change because of a race during

initialization. Code like the following can result in two threads both calling

pthread_key_create:

void destructor(void *);

pthread_key_t key;

int init_done = 0;

int

threadfunc(void *arg)

{

if (!init_done) {

init_done = 1;

err = pthread_key_create(&key, destructor);

}

..

.

}

Depending on how the system schedules threads, some threads might see one key

value, whereas other threads might see a different value. The way to solve this race is

to usepthread_once.

#include <pthread.h>

pthread_once_tinitflag=PTHREAD_ONCE_INIT;

int pthread_once(pthread_once_t *initflag,void (*initfn)(void));

Returns: 0 if OK, error number on failure

The initflag must be a nonlocal variable (i.e., global or static) and initialized to

PTHREAD_ONCE_INIT.

If each thread callspthread_once,the system guarantees that the initialization

routine,initfn,will be called only once, on the first call topthread_once.The proper

way to create a key without a race is as follows:

void destructor(void *);

pthread_key_t key;

pthread_once_t init_done = PTHREAD_ONCE_INIT;