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PTHREAD_KEY_DELETE(3P)     POSIX Programmer's Manual    PTHREAD_KEY_DELETE(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_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(3P)