PTHREAD_KEY_CREATE(3P)	   POSIX Programmer's Manual	PTHREAD_KEY_CREATE(3P)



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the	 corresponding
       Linux  manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.


NAME
       pthread_key_create -- thread-specific data key creation

SYNOPSIS
       #include <pthread.h>

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

DESCRIPTION
       The pthread_key_create() function shall create a	 thread-specific  data
       key  visible  to	 all  threads  in  the process. Key values provided by
       pthread_key_create() are opaque objects used to locate  thread-specific
       data. Although the same key value may be used by different threads, the
       values bound to the key by pthread_setspecific() are  maintained	 on  a
       per-thread basis and persist for the life of the calling thread.

       Upon  key creation, the value NULL shall be associated with the new key
       in all active threads. Upon thread creation, the value  NULL  shall  be
       associated with all defined keys in the new thread.

       An  optional destructor function may be associated with each key value.
       At thread exit, if a key value has a non-NULL destructor	 pointer,  and
       the  thread has a non-NULL value associated with that key, the value of
       the key is set to NULL, and then the function pointed to is called with
       the  previously	associated  value  as  its sole argument. The order of
       destructor calls is unspecified if more than one destructor exists  for
       a thread when it exits.

       If,  after all the destructors have been called for all non-NULL values
       with associated destructors, there are still some non-NULL values  with
       associated  destructors,	 then  the  process  is repeated. If, after at
       least {PTHREAD_DESTRUCTOR_ITERATIONS} iterations	 of  destructor	 calls
       for  outstanding	 non-NULL values, there are still some non-NULL values
       with associated destructors, implementations may stop calling  destruc-
       tors, or they may continue calling destructors until no non-NULL values
       with associated destructors exist, even though this might result in  an
       infinite loop.

RETURN VALUE
       If  successful, the pthread_key_create() function shall store the newly
       created key value at *key and shall return zero.	 Otherwise,  an	 error
       number shall be returned to indicate the error.

ERRORS
       The pthread_key_create() function shall fail if:

       EAGAIN The  system  lacked  the	necessary  resources to create another
	      thread-specific data key, or the	system-imposed	limit  on  the
	      total  number  of	 keys  per process {PTHREAD_KEYS_MAX} has been
	      exceeded.

       ENOMEM Insufficient memory exists to create the key.

       The pthread_key_create() function shall not return  an  error  code  of
       [EINTR].

       The following sections are informative.

EXAMPLES
       The  following  example	demonstrates  a	 function  that	 initializes a
       thread-specific data key when it is  first  called,  and	 associates  a
       thread-specific	object	with  each  calling  thread, initializing this
       object when necessary.

	   static pthread_key_t key;
	   static pthread_once_t key_once = PTHREAD_ONCE_INIT;

	   static void
	   make_key()
	   {
	       (void) pthread_key_create(&key, NULL);
	   }

	   func()
	   {
	       void *ptr;

	       (void) pthread_once(&key_once, make_key);
	       if ((ptr = pthread_getspecific(key)) == NULL) {
		   ptr = malloc(OBJECT_SIZE);
		   ...
		   (void) pthread_setspecific(key, ptr);
	       }
	       ...
	   }

       Note that the key has to be initialized before pthread_getspecific() or
       pthread_setspecific()  can be used. The pthread_key_create() call could
       either be explicitly made in a module initialization routine, or it can
       be  done	 implicitly  by the first call to a module as in this example.
       Any attempt to use the key before it is initialized  is	a  programming
       error, making the code below incorrect.

	   static pthread_key_t key;

	   func()
	   {
	       void *ptr;

	      /* KEY NOT INITIALIZED!!!	 THIS WON'T WORK!!! */
	       if ((ptr = pthread_getspecific(key)) == NULL &&
		   pthread_setspecific(key, NULL) != 0) {
		   pthread_key_create(&key, NULL);
		   ...
	       }
	   }

APPLICATION USAGE
       None.

RATIONALE
   Destructor Functions
       Normally,  the value bound to a key on behalf of a particular thread is
       a pointer to storage allocated dynamically on  behalf  of  the  calling
       thread.	The  destructor	 functions specified with pthread_key_create()
       are intended to be used to free this storage  when  the	thread	exits.
       Thread  cancellation  cleanup  handlers cannot be used for this purpose
       because thread-specific data may persist outside the lexical  scope  in
       which the cancellation cleanup handlers operate.

       If the value associated with a key needs to be updated during the life-
       time of the thread, it may be necessary to release the storage  associ-
       ated  with  the	old  value before the new value is bound. Although the
       pthread_setspecific() function could do this automatically,  this  fea-
       ture  is	 not  needed  often  enough  to	 justify the added complexity.
       Instead, the programmer is responsible for freeing the stale storage:

	   pthread_getspecific(key, &old);
	   new = allocate();
	   destructor(old);
	   pthread_setspecific(key, new);

       Note:	 The above example could leak storage if run with asynchronous
		 cancellation  enabled.	 No such problems occur in the default
		 cancellation state if no cancellation	points	occur  between
		 the get and set.

       There  is  no  notion  of a destructor-safe function. If an application
       does not call pthread_exit() from a signal handler, or if it blocks any
       signal whose handler may call pthread_exit() while calling async-unsafe
       functions, all functions may be safely called from destructors.

   Non-Idempotent Data Key Creation
       There  were  requests  to  make	pthread_key_create()  idempotent  with
       respect to a given key address parameter. This would allow applications
       to call pthread_key_create() multiple times for a given key address and
       be  guaranteed  that  only  one	key  would  be created. Doing so would
       require the key value to be previously initialized (possibly at compile
       time)  to  a  known  null value and would require that implicit mutual-
       exclusion be performed based on the address and	contents  of  the  key
       parameter in order to guarantee that exactly one key would be created.

       Unfortunately,  the  implicit  mutual-exclusion would not be limited to
       only pthread_key_create().  On many implementations,  implicit  mutual-
       exclusion  would also have to be performed by pthread_getspecific() and
       pthread_setspecific() in order  to  guard  against  using  incompletely
       stored or not-yet-visible key values. This could significantly increase
       the cost of important operations, particularly pthread_getspecific().

       Thus, this proposal was	rejected.  The	pthread_key_create()  function
       performs	 no  implicit synchronization. It is the responsibility of the
       programmer to ensure that it is called exactly once per key before  use
       of  the	key. Several straightforward mechanisms can already be used to
       accomplish this, including calling explicit module initialization func-
       tions, using mutexes, and using pthread_once().	This places no signif-
       icant burden on the programmer, introduces no possibly confusing ad hoc
       implicit	 synchronization  mechanism,  and  potentially allows commonly
       used thread-specific data operations to be more efficient.

FUTURE DIRECTIONS
       None.

SEE ALSO
       pthread_getspecific(), pthread_key_delete()

       The Base Definitions volume of POSIX.1-2008, <pthread.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),	The  Open  Group  Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
       cal and Electronics Engineers,  Inc  and	 The  Open  Group.   (This  is
       POSIX.1-2008  with  the	2013  Technical Corrigendum 1 applied.) In the
       event of any discrepancy between this version and the original IEEE and
       The  Open Group Standard, the original IEEE and The Open Group Standard
       is the referee document. The original Standard can be  obtained	online
       at http://www.unix.org/online.html .

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IEEE/The Open Group		     2013		PTHREAD_KEY_CREATE(3P)