PTHREAD_KEY_DELETE

PROLOG

NAME

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>  

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