This function creates a thread-specific data key visible to all threads in the process. Key values provided by pthread_key_create() are opaque objects used to locate thread-specific data. Although the same key value may be used by different threads, the values bound to the key by pthread_setspecific() are maintained on a per-thread basis and persist for the life of the calling thread.

Upon key creation, the value NULL is associated with the new key in all active threads. Upon thread creation, the value NULL is associated with all defined keys in the new thread.

An optional destructor function may be associated with each key value. At thread exit, if a key value has a non-NULL destructor pointer, and the thread has a non-NULL value associated with that key, the function pointed to is called with the current associated value as its sole argument. Destructors can be called in any order.

If, after all the destructors have been called for all keys with non-NULL values, there are still some keys with non-NULL values, the process will be repeated. If, after at least PTHREAD_DESTRUCTOR_ITERATIONS iterations of destructor calls for outstanding non-NULL values, there are still some keys with non-NULL values, the process is continued, even though this might result in an infinite loop.

If successful, the pthread_key_create() function stores the newly created key value at *key and returns 0. Otherwise, an error number is returned to indicate the error.

The pthread_key_create() function will fail if:

EAGAIN

The system lacked the necessary resources to create another thread-specific data key, or the system-imposed limit on the total number of keys per process PTHREAD_KEYS_MAX has been exceeded.

ENOMEM

Insufficient memory exists to create the key.

The pthread_key_create() function will not return an error code of EINTR.

See attributes(5) for descriptions of the following attributes:

pthread_getspecific(3THR), pthread_setspecific(3THR), pthread_key_delete(3THR), attributes(5), standards(5)