Section: POSIX Programmer's Manual (3P)
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.
--- thread-specific data key deletion
int pthread_key_delete(pthread_key_t key);
function shall delete a thread-specific data key previously returned by
The thread-specific data values associated with
need not be NULL at the time
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
is called. Any attempt to use
following the call to
results in undefined behavior.
function shall be callable from within destructor functions. No
destructor functions shall be invoked by
Any destructor function that may have been associated with
shall no longer be called upon thread exit.
If successful, the
function shall return zero; otherwise, an error number shall be
returned to indicate the error.
function shall not return an error code of
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
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
In particular, destructor functions are not called. There are several
reasons for this division of responsibility:
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.
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
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.
If an implementation detects that the value specified by the
does not refer to a a key value obtained from
or refers to a key that has been deleted with
it is recommended that the function should fail and report an
The Base Definitions volume of POSIX.1-2017,
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 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
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