PTHREAD_KEY_CREATE(3)      POSIX Programmer's Manual     PTHREAD_KEY_CREATE(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       pthread_key_create -- thread-specific data key creation

SYNOPSIS
       #include <pthread.h>

       int pthread_key_create(pthread_key_t *key, void (*destructor)(void*));

DESCRIPTION
       The pthread__key__create() function shall create a thread-specific data
       key visible to all threads in the process. Key values provided by
       pthread__key__create() are opaque objects used to locate thread-specific
       data. Although the same key value may be used by different threads, the
       values bound to the key by pthread__setspecific() are maintained on a
       per-thread basis and persist for the life of the calling thread.

       Upon key creation, the value NULL shall be associated with the new key
       in all active threads. Upon thread creation, the value NULL shall be
       associated with all defined keys in the new thread.

       An optional destructor function may be associated with each key value.
       At thread exit, if a key value has a non-NULL destructor pointer, and
       the thread has a non-NULL value associated with that key, the value of
       the key is set to NULL, and then the function pointed to is called with
       the previously associated value as its sole argument. The order of
       destructor calls is unspecified if more than one destructor exists for
       a thread when it exits.

       If, after all the destructors have been called for all non-NULL values
       with associated destructors, there are still some non-NULL values with
       associated destructors, then the process is repeated. If, after at
       least {PTHREAD_DESTRUCTOR_ITERATIONS} iterations of destructor calls
       for outstanding non-NULL values, there are still some non-NULL values
       with associated destructors, implementations may stop calling
       destructors, or they may continue calling destructors until no non-NULL
       values with associated destructors exist, even though this might result
       in an infinite loop.

RETURN VALUE
       If successful, the pthread__key__create() function shall store the newly
       created key value at *key and shall return zero. Otherwise, an error
       number shall be returned to indicate the error.

ERRORS
       The pthread__key__create() function shall fail if:

       EAGAIN The system lacked the necessary resources to create another
              thread-specific data key, or the system-imposed limit on the
              total number of keys per process {PTHREAD_KEYS_MAX} has been
              exceeded.

       ENOMEM Insufficient memory exists to create the key.

       The pthread__key__create() function shall not return an error code of
       [EINTR].

       The following sections are informative.

EXAMPLES
       The following example demonstrates a function that initializes a
       thread-specific data key when it is first called, and associates a
       thread-specific object with each calling thread, initializing this
       object when necessary.


           static pthread_key_t key;
           static pthread_once_t key_once = PTHREAD_ONCE_INIT;

           static void
           make_key()
           {
               (void) pthread_key_create(&key, NULL);
           }

           func()
           {
               void *ptr;

               (void) pthread_once(&key_once, make_key);
               if ((ptr = pthread_getspecific(key)) == NULL) {
                   ptr = malloc(OBJECT_SIZE);
                   ...
                   (void) pthread_setspecific(key, ptr);
               }
               ...
           }

       Note that the key has to be initialized before pthread__getspecific() or
       pthread__setspecific() can be used. The pthread__key__create() call could
       either be explicitly made in a module initialization routine, or it can
       be done implicitly by the first call to a module as in this example.
       Any attempt to use the key before it is initialized is a programming
       error, making the code below incorrect.


           static pthread_key_t key;

           func()
           {
               void *ptr;

              /* KEY NOT INITIALIZED!!!  THIS WILL NOT WORK!!! */
               if ((ptr = pthread_getspecific(key)) == NULL &&
                   pthread_setspecific(key, NULL) != 0) {
                   pthread_key_create(&key, NULL);
                   ...
               }
           }

APPLICATION USAGE
       None.

RATIONALE
   Destructor Functions
       Normally, the value bound to a key on behalf of a particular thread is
       a pointer to storage allocated dynamically on behalf of the calling
       thread. The destructor functions specified with pthread__key__create()
       are intended to be used to free this storage when the thread exits.
       Thread cancellation cleanup handlers cannot be used for this purpose
       because thread-specific data may persist outside the lexical scope in
       which the cancellation cleanup handlers operate.

       If the value associated with a key needs to be updated during the
       lifetime of the thread, it may be necessary to release the storage
       associated with the old value before the new value is bound. Although
       the pthread__setspecific() function could do this automatically, this
       feature is not needed often enough to justify the added complexity.
       Instead, the programmer is responsible for freeing the stale storage:


           pthread_getspecific(key, &old);
           new = allocate();
           destructor(old);
           pthread_setspecific(key, new);

       Note:     The above example could leak storage if run with asynchronous
                 cancellation enabled. No such problems occur in the default
                 cancellation state if no cancellation points occur between
                 the get and set.

       There is no notion of a destructor-safe function. If an application
       does not call pthread__exit() from a signal handler, or if it blocks any
       signal whose handler may call pthread__exit() while calling async-unsafe
       functions, all functions may be safely called from destructors.

   Non-Idempotent Data Key Creation
       There were requests to make pthread__key__create() idempotent with
       respect to a given key address parameter. This would allow applications
       to call pthread__key__create() multiple times for a given key address and
       be guaranteed that only one key would be created. Doing so would
       require the key value to be previously initialized (possibly at compile
       time) to a known null value and would require that implicit mutual-
       exclusion be performed based on the address and contents of the key
       parameter in order to guarantee that exactly one key would be created.

       Unfortunately, the implicit mutual-exclusion would not be limited to
       only pthread__key__create().  On many implementations, implicit mutual-
       exclusion would also have to be performed by pthread__getspecific() and
       pthread__setspecific() in order to guard against using incompletely
       stored or not-yet-visible key values. This could significantly increase
       the cost of important operations, particularly pthread__getspecific().

       Thus, this proposal was rejected. The pthread__key__create() function
       performs no implicit synchronization. It is the responsibility of the
       programmer to ensure that it is called exactly once per key before use
       of the key. Several straightforward mechanisms can already be used to
       accomplish this, including calling explicit module initialization
       functions, using mutexes, and using pthread__once().  This places no
       significant burden on the programmer, introduces no possibly confusing
       ad hoc implicit synchronization mechanism, and potentially allows
       commonly used thread-specific data operations to be more efficient.

FUTURE DIRECTIONS
       None.

SEE ALSO
       pthread__getspecific(), pthread__key__delete()

       The Base Definitions volume of POSIX.1-2017, <pthread.h>

COPYRIGHT
       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 http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                PTHREAD_KEY_CREATE(3)
