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<title>pthread_key_create</title>
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<basefont size="3"> <a name="pthread_key_create"></a> <a name="tag_03_534"></a><!-- pthread_key_create -->
<!--header start-->
<center><font size="2">The Open Group Base Specifications Issue 6<br>
IEEE Std 1003.1-2001<br>
Copyright &copy; 2001 The IEEE and The Open Group, All Rights reserved.</font></center>
<!--header end-->
<hr size="2" noshade>
<h4><a name="tag_03_534_01"></a>NAME</h4>
<blockquote>pthread_key_create - thread-specific data key creation</blockquote>
<h4><a name="tag_03_534_02"></a>SYNOPSIS</h4>
<blockquote class="synopsis">
<div class="box"><code><tt><sup>[<a href="javascript:open_code('THR')">THR</a>]</sup> <img src="../images/opt-start.gif" alt=
"[Option Start]" border="0"> #include &lt;<a href="../basedefs/pthread.h.html">pthread.h</a>&gt;<br>
<br>
int pthread_key_create(pthread_key_t *</tt><i>key</i><tt>, void (*</tt><i>destructor</i><tt>)(void*)); <img src=
"../images/opt-end.gif" alt="[Option End]" border="0"></tt></code></div>
<tt><br>
</tt></blockquote>
<h4><a name="tag_03_534_03"></a>DESCRIPTION</h4>
<blockquote>
<p>The <i>pthread_key_create</i>() function shall create a thread-specific data key visible to all threads in the process. Key
values provided by <i>pthread_key_create</i>() 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 <a href=
"../functions/pthread_setspecific.html"><i>pthread_setspecific</i>()</a> are maintained on a per-thread basis and persist for the
life of the calling thread.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
</blockquote>
<h4><a name="tag_03_534_04"></a>RETURN VALUE</h4>
<blockquote>
<p>If successful, the <i>pthread_key_create</i>() function shall store the newly created key value at *<i>key</i> and shall return
zero. Otherwise, an error number shall be returned to indicate the error.</p>
</blockquote>
<h4><a name="tag_03_534_05"></a>ERRORS</h4>
<blockquote>
<p>The <i>pthread_key_create</i>() function shall fail if:</p>
<dl compact>
<dt>[EAGAIN]</dt>
<dd>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.</dd>
<dt>[ENOMEM]</dt>
<dd>Insufficient memory exists to create the key.</dd>
</dl>
<p>The <i>pthread_key_create</i>() function shall not return an error code of [EINTR].</p>
</blockquote>
<hr>
<div class="box"><em>The following sections are informative.</em></div>
<h4><a name="tag_03_534_06"></a>EXAMPLES</h4>
<blockquote>
<p>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.</p>
<pre>
<tt>static pthread_key_t key;
static pthread_once_t key_once = PTHREAD_ONCE_INIT;
<br>
static void
make_key()
{
(void) pthread_key_create(&amp;key, NULL);
}
<br>
func()
{
void *ptr;
<br>
(void) pthread_once(&amp;key_once, make_key);
if ((ptr = pthread_getspecific(key)) == NULL) {
ptr = malloc(OBJECT_SIZE);
...
(void) pthread_setspecific(key, ptr);
}
...
}
</tt>
</pre>
<p>Note that the key has to be initialized before <a href="../functions/pthread_getspecific.html"><i>pthread_getspecific</i>()</a>
or <a href="../functions/pthread_setspecific.html"><i>pthread_setspecific</i>()</a> can be used. The <i>pthread_key_create</i>()
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.</p>
<pre>
<tt>static pthread_key_t key;
<br>
func()
{
void *ptr;
<br>
/* KEY NOT INITIALIZED!!! THIS WON'T WORK!!! */
if ((ptr = pthread_getspecific(key)) == NULL &amp;&amp;
pthread_setspecific(key, NULL) != 0) {
pthread_key_create(&amp;key, NULL);
...
}
}
</tt>
</pre>
</blockquote>
<h4><a name="tag_03_534_07"></a>APPLICATION USAGE</h4>
<blockquote>
<p>None.<br>
</p>
</blockquote>
<h4><a name="tag_03_534_08"></a>RATIONALE</h4>
<blockquote>
<p></p>
<h5><a name="tag_03_534_08_01"></a>Destructor Functions</h5>
<p>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 <i>pthread_key_create</i>() are intended to be used to free this
storage when the thread exits. Thread cancelation cleanup handlers cannot be used for this purpose because thread-specific data may
persist outside the lexical scope in which the cancelation cleanup handlers operate.</p>
<p>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 <a href=
"../functions/pthread_setspecific.html"><i>pthread_setspecific</i>()</a> 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:</p>
<pre>
<tt>pthread_getspecific(key, &amp;old);
new = allocate();
destructor(old);
pthread_setspecific(key, new);
</tt>
</pre>
<basefont size="2">
<dl>
<dt><b>Note:</b></dt>
<dd>The above example could leak storage if run with asynchronous cancelation enabled. No such problems occur in the default
cancelation state if no cancelation points occur between the get and set.</dd>
</dl>
<basefont size="3">
<p>There is no notion of a destructor-safe function. If an application does not call <a href=
"../functions/pthread_exit.html"><i>pthread_exit</i>()</a> from a signal handler, or if it blocks any signal whose handler may call
<a href="../functions/pthread_exit.html"><i>pthread_exit</i>()</a> while calling async-unsafe functions, all functions may be
safely called from destructors.</p>
<h5><a name="tag_03_534_08_02"></a>Non-Idempotent Data Key Creation</h5>
<p>There were requests to make <i>pthread_key_create</i>() idempotent with respect to a given <i>key</i> address parameter. This
would allow applications to call <i>pthread_key_create</i>() multiple times for a given <i>key</i> 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 <i>key</i>
parameter in order to guarantee that exactly one key would be created.</p>
<p>Unfortunately, the implicit mutual-exclusion would not be limited to only <i>pthread_key_create</i>(). On many implementations,
implicit mutual-exclusion would also have to be performed by <a href=
"../functions/pthread_getspecific.html"><i>pthread_getspecific</i>()</a> and <a href=
"../functions/pthread_setspecific.html"><i>pthread_setspecific</i>()</a> in order to guard against using incompletely stored or
not-yet-visible key values. This could significantly increase the cost of important operations, particularly <a href=
"../functions/pthread_getspecific.html"><i>pthread_getspecific</i>()</a>.</p>
<p>Thus, this proposal was rejected. The <i>pthread_key_create</i>() 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 <a href="../functions/pthread_once.html"><i>pthread_once</i>()</a>. This places no significant burden on the programmer,
introduces no possibly confusing <i>ad hoc</i> implicit synchronization mechanism, and potentially allows commonly used
thread-specific data operations to be more efficient.</p>
</blockquote>
<h4><a name="tag_03_534_09"></a>FUTURE DIRECTIONS</h4>
<blockquote>
<p>None.</p>
</blockquote>
<h4><a name="tag_03_534_10"></a>SEE ALSO</h4>
<blockquote>
<p><a href="pthread_getspecific.html"><i>pthread_getspecific</i>()</a> , <a href=
"pthread_key_delete.html"><i>pthread_key_delete</i>()</a> , the Base Definitions volume of IEEE&nbsp;Std&nbsp;1003.1-2001, <a href=
"../basedefs/pthread.h.html"><i>&lt;pthread.h&gt;</i></a></p>
</blockquote>
<h4><a name="tag_03_534_11"></a>CHANGE HISTORY</h4>
<blockquote>
<p>First released in Issue 5. Included for alignment with the POSIX Threads Extension.</p>
</blockquote>
<h4><a name="tag_03_534_12"></a>Issue 6</h4>
<blockquote>
<p>The <i>pthread_key_create</i>() function is marked as part of the Threads option.</p>
<p>IEEE PASC Interpretation 1003.1c #8 is applied, updating the DESCRIPTION.</p>
</blockquote>
<div class="box"><em>End of informative text.</em></div>
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