PTHREAD_KEY_DELETE(P) POSIX Programmer's Manual PTHREAD_KEY_DELETE(P)
pthread_key_delete - thread-specific data key deletion
int pthread_key_delete(pthread_key_t key);
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.
If successful, the pthread_key_delete() function shall return zero; otherwise, an error
number shall be returned to indicate the error.
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.
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
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.
pthread_key_create() , the Base Definitions volume of IEEE Std 1003.1-2001, <pthread.h>
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)