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

       ENOMEM Insufficient memory exists to create the key.

       The pthread_key_create() function shall not return an error code of

       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 WON'T 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

       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

   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

       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.1‐2008, pthread.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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                 2013              PTHREAD_KEY_CREATE(3P)

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