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_function 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)