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Linux 2.6 - man page for pthread_key_delete (linux section 3posix)

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

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
       application to free any application storage or perform any cleanup actions for data struc-
       tures  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 associated 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-spe-
       cific  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  dele-
	   tion  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-specific 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-spe-
	   cific 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  exam-
       ple,  it could know this if all the client threads have called a cleanup procedure declar-
       ing 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 orig-
       inal 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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