.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "PTHREAD_KEY_DELETE" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" pthread_key_delete 
.SH 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.
.SH NAME
pthread_key_delete \- thread-specific data key deletion
.SH SYNOPSIS
.LP
\fB#include <pthread.h>
.br
.sp
int pthread_key_delete(pthread_key_t\fP \fIkey\fP\fB); \fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
The \fIpthread_key_delete\fP() function shall delete a thread-specific
data key previously returned by \fIpthread_key_create\fP(). The thread-specific
data values associated with \fIkey\fP
need not be NULL at the time \fIpthread_key_delete\fP() 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
\fIpthread_key_delete\fP() is called. Any attempt to use
\fIkey\fP following the call to \fIpthread_key_delete\fP() results
in undefined behavior.
.LP
The \fIpthread_key_delete\fP() function shall be callable from within
destructor functions. No destructor functions shall be
invoked by \fIpthread_key_delete\fP(). Any destructor function that
may have been associated with \fIkey\fP shall no longer be
called upon thread exit.
.SH RETURN VALUE
.LP
If successful, the \fIpthread_key_delete\fP() function shall return
zero; otherwise, an error number shall be returned to
indicate the error.
.SH ERRORS
.LP
The \fIpthread_key_delete\fP() function may fail if:
.TP 7
.B EINVAL
The \fIkey\fP value is invalid.
.sp
.LP
The \fIpthread_key_delete\fP() function shall not return an error
code of [EINTR].
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
None.
.SH RATIONALE
.LP
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.
.LP
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 \fIpthread_key_delete\fP(). In
particular, destructor functions are not called. There are several
reasons for this division of responsibility:
.IP " 1." 4
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.
.LP
.IP " 2." 4
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.
.LP
.LP
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.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIpthread_key_create\fP(), the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, \fI<pthread.h>\fP
.SH 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 .