PTHREAD_SPECIFIC(3)					     Library Functions Manual					       PTHREAD_SPECIFIC(3)

NAME

       pthread_key_create, pthread_key_delete, pthread_setspecific, pthread_getspecific - management of thread-specific data

SYNOPSIS

       #include <pthread.h>

       int pthread_key_create(pthread_key_t *key, void (*destr_function) (void *));

       int pthread_key_delete(pthread_key_t key);

       int pthread_setspecific(pthread_key_t key, const void *pointer);

       void * pthread_getspecific(pthread_key_t key);

DESCRIPTION

       Programs  often need global or static variables that have different values in different threads. Since threads share one memory space, this
       cannot be achieved with regular variables. Thread-specific data is the POSIX threads answer to this need.

       Each thread possesses a private memory block, the thread-specific data area, or TSD area for short. This area is indexed by TSD	keys.  The
       TSD  area  associates  values of type void * to TSD keys. TSD keys are common to all threads, but the value associated with a given TSD key
       can be different in each thread.

       For concreteness, the TSD areas can be viewed as arrays of void * pointers, TSD keys as integer indices into these arrays, and the value of
       a TSD key as the value of the corresponding array element in the calling thread.

       When a thread is created, its TSD area initially associates NULL with all keys.

       pthread_key_create  allocates  a  new TSD key. The key is stored in the location pointed to by key. There is a limit of PTHREAD_KEYS_MAX on
       the number of keys allocated at a given time. The value initially associated with the returned key  is  NULL  in  all  currently  executing
       threads.

       The  destr_function  argument,  if  not	NULL,  specifies  a  destructor  function  associated  with  the key. When a thread terminates via
       pthread_exit or by cancellation, destr_function is called with arguments the value associated with the key in that thread. The  destr_func-
       tion is not called if that value is NULL. The order in which destructor functions are called at thread termination time is unspecified.

       Before  the  destructor	function is called, the NULL value is associated with the key in the current thread.  A destructor function might,
       however, re-associate non-NULL values to that key or some other key.  To deal with this, if after all the destructors have been called  for
       all  non-NULL  values,  there  are  still some non-NULL values with associated destructors, then the process is repeated.  The LinuxThreads
       implementation stops the process after PTHREAD_DESTRUCTOR_ITERATIONS iterations, even if some non-NULL values with  associated  descriptors
       remain.	Other implementations may loop indefinitely.

       pthread_key_delete deallocates a TSD key. It does not check whether non-NULL values are associated with that key in the currently executing
       threads, nor call the destructor function associated with the key.

       pthread_setspecific changes the value associated with key in the calling thread, storing the given pointer instead.

       pthread_getspecific returns the value currently associated with key in the calling thread.

RETURN VALUE

       pthread_key_create, pthread_key_delete, and pthread_setspecific return 0 on success and a non-zero error code on  failure.  If  successful,
       pthread_key_create stores the newly allocated key in the location pointed to by its key argument.

       pthread_getspecific returns the value associated with key on success, and NULL on error.

ERRORS

       pthread_key_create returns the following error code on error:

	      EAGAIN PTHREAD_KEYS_MAX keys are already allocated

       pthread_key_delete and pthread_setspecific return the following error code on error:

	      EINVAL key is not a valid, allocated TSD key

       pthread_getspecific returns NULL if key is not a valid, allocated TSD key.

AUTHOR

       Xavier Leroy <Xavier.Leroy@inria.fr>

SEE ALSO

       pthread_create(3), pthread_exit(3), pthread_testcancel(3).

EXAMPLE

       The following code fragment allocates a thread-specific array of 100 characters, with automatic reclaimation at thread exit:

	      /* Key for the thread-specific buffer */
	      static pthread_key_t buffer_key;

	      /* Once-only initialisation of the key */
	      static pthread_once_t buffer_key_once = PTHREAD_ONCE_INIT;

	      /* Allocate the thread-specific buffer */
	      void buffer_alloc(void)
	      {
		pthread_once(&buffer_key_once, buffer_key_alloc);
		pthread_setspecific(buffer_key, malloc(100));
	      }

	      /* Return the thread-specific buffer */
	      char * get_buffer(void)
	      {
		return (char *) pthread_getspecific(buffer_key);
	      }

	      /* Allocate the key */
	      static void buffer_key_alloc()
	      {
		pthread_key_create(&buffer_key, buffer_destroy);
	      }

	      /* Free the thread-specific buffer */
	      static void buffer_destroy(void * buf)
	      {
		free(buf);
	      }

								   LinuxThreads 					       PTHREAD_SPECIFIC(3)