### 34.1 Thread-specific Data

The GNU C Library implements functions to allow users to create and manage data specific to a thread. Such data may be destroyed at thread exit, if a destructor is provided. The following functions are defined:

— Function: int pthread_key_create (pthread_key_t *key, void (*destructor)(void*))

> Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

> Create a thread-specific data key for the calling thread, referenced by key.

> Objects declared with the C++11 `thread_local` keyword are destroyed before thread-specific data, so they should not be used in thread-specific data destructors or even as members of the thread-specific data, since the latter is passed as an argument to the destructor function.

— Function: int pthread_key_delete (pthread_key_t key)

> Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

> Destroy the thread-specific data key in the calling thread. The destructor for the thread-specific data is not called during destruction, nor is it called during thread exit.

— Function: void *pthread_getspecific (pthread_key_t key)

> Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

> Return the thread-specific data associated with key in the calling thread.

— Function: int pthread_setspecific (pthread_key_t key, const void *value)

> Preliminary: | MT-Safe | AS-Unsafe corrupt heap | AC-Unsafe corrupt mem | See POSIX Safety Concepts.

> Associate the thread-specific value with key in the calling thread.