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 cancelation, 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 glibc  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.

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 reclamation 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);
              }