pthread_key_create(3p) — Linux manual page



PROLOG         top

       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         top

       pthread_key_create — thread-specific data key creation

SYNOPSIS         top

       #include <pthread.h>

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

DESCRIPTION         top

       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         top

       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         top

       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         top

       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
               (void) pthread_key_create(&key, NULL);

               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;

               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);



RATIONALE         top

   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();
           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.



SEE ALSO         top

       pthread_getspecific(3p), pthread_key_delete(3p)

       The Base Definitions volume of POSIX.1‐2017, pthread.h(0p)

COPYRIGHT         top

       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 .

       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 .

IEEE/The Open Group               2017            PTHREAD_KEY_CREATE(3P)

Pages that refer to this page: pthread.h(0p)pthread_getspecific(3p)pthread_key_delete(3p)