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