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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:

              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


       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)