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


       #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);


       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

       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

       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

       !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.


       !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.


       !pthread_key_create! returns the following error code on error:

                     !PTHREAD_KEYS_MAX! keys are already allocated

       !pthread_key_delete! and  !pthread_setspecific!  return  the  following
       error code on error:

                     |key| is not a valid, allocated TSD key

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


       Xavier Leroy <>


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


       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)

                                 LinuxThreads              PTHREAD_SPECIFIC(3)