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PTHREAD_KEY_DELETE(P)	   POSIX Programmer's Manual	 PTHREAD_KEY_DELETE(P)



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_delete - thread-specific data key deletion

SYNOPSIS
       #include <pthread.h>

       int pthread_key_delete(pthread_key_t key);


DESCRIPTION
       The pthread_key_delete() function shall delete a	 thread-specific  data
       key  previously	returned  by pthread_key_create(). The thread-specific
       data  values  associated	 with  key  need  not  be  NULL	 at  the  time
       pthread_key_delete() is called.	It is the responsibility of the appli-
       cation to free any application storage or perform any  cleanup  actions
       for  data  structures  related to the deleted key or associated thread-
       specific data in any threads; this cleanup can be done either before or
       after  pthread_key_delete() is called. Any attempt to use key following
       the call to pthread_key_delete() results in undefined behavior.

       The  pthread_key_delete()  function  shall  be  callable	 from	within
       destructor  functions.  No  destructor  functions  shall	 be invoked by
       pthread_key_delete(). Any destructor function that may have been	 asso-
       ciated with key shall no longer be called upon thread exit.

RETURN VALUE
       If  successful,	the  pthread_key_delete()  function shall return zero;
       otherwise, an error number shall be returned to indicate the error.

ERRORS
       The pthread_key_delete() function may fail if:

       EINVAL The key value is invalid.


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

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       None.

RATIONALE
       A thread-specific data key deletion function has been included in order
       to allow the resources associated with an unused	 thread-specific  data
       key  to	be  freed.  Unused  thread-specific data keys can arise, among
       other scenarios, when a dynamically loaded module that allocated a  key
       is unloaded.

       Conforming  applications	 are  responsible  for	performing any cleanup
       actions needed for data	structures  associated	with  the  key	to  be
       deleted,	 including data referenced by thread-specific data values.  No
       such cleanup is done by pthread_key_delete(). In particular, destructor
       functions  are  not called. There are several reasons for this division
       of responsibility:

	1. The associated destructor functions used  to	 free  thread-specific
	   data at thread exit time are only guaranteed to work correctly when
	   called in the  thread  that	allocated  the	thread-specific	 data.
	   (Destructors	 themselves  may  utilize thread-specific data.) Thus,
	   they cannot be used to free thread-specific data in	other  threads
	   at  key  deletion  time.  Attempting	 to  have them called by other
	   threads at key deletion time would  require	other  threads	to  be
	   asynchronously  interrupted. But since interrupted threads could be
	   in an arbitrary state, including holding locks  necessary  for  the
	   destructor  to  run, this approach would fail. In general, there is
	   no safe mechanism whereby an implementation could free  thread-spe-
	   cific data at key deletion time.


	2. Even	 if  there were a means of safely freeing thread-specific data
	   associated with keys to be deleted, doing  so  would	 require  that
	   implementations be able to enumerate the threads with non-NULL data
	   and potentially keep them from creating more	 thread-specific  data
	   while  the key deletion is occurring. This special case could cause
	   extra synchronization in the normal case, which would otherwise  be
	   unnecessary.


       For  an	application to know that it is safe to delete a key, it has to
       know that all the threads that might potentially ever use  the  key  do
       not attempt to use it again. For example, it could know this if all the
       client threads have called a cleanup procedure declaring that they  are
       through	with  the module that is being shut down, perhaps by setting a
       reference count to zero.

FUTURE DIRECTIONS
       None.

SEE ALSO
       pthread_key_create()    ,    the	   Base	   Definitions	  volume    of
       IEEE Std 1003.1-2001, <pthread.h>

COPYRIGHT
       Portions	 of  this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
       --  Portable  Operating	System	Interface (POSIX), The Open Group Base
       Specifications Issue 6, Copyright (C) 2001-2003	by  the	 Institute  of
       Electrical  and	Electronics  Engineers, Inc and The Open Group. 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.opengroup.org/unix/online.html .



IEEE/The Open Group		     2003		 PTHREAD_KEY_DELETE(P)
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