# pthread_key_delete

## 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.
    


## NAME



           pthread_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 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 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.
    
           If an implementation detects that the value specified by the _key_
           argument to _pthread_key_delete_() does not refer to a a key value
           obtained from _pthread_key_create_() or refers to a key that has
           been deleted with _pthread_key_delete_(), it is recommended that the
           function should fail and report an **[EINVAL]** error.
    


## FUTURE DIRECTIONS



           None.
    


* * *