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 correspond-
       ing 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 application
       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 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-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-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-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 Technol-
       ogy -- 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)