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<title>pthread_mutexattr_destroy</title>
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<basefont size="3"> <a name="pthread_mutexattr_destroy"></a> <a name="tag_03_544"></a><!-- pthread_mutexattr_destroy -->
<!--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_544_01"></a>NAME</h4>
<blockquote>pthread_mutexattr_destroy, pthread_mutexattr_init - destroy and initialize the mutex attributes object</blockquote>
<h4><a name="tag_03_544_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_mutexattr_destroy(pthread_mutexattr_t *</tt><i>attr</i><tt>);<br>
int pthread_mutexattr_init(pthread_mutexattr_t *</tt><i>attr</i><tt>); <img src="../images/opt-end.gif" alt="[Option End]" border=
"0"></tt></code></div>
<tt><br>
</tt></blockquote>
<h4><a name="tag_03_544_03"></a>DESCRIPTION</h4>
<blockquote>
<p>The <i>pthread_mutexattr_destroy</i>() function shall destroy a mutex attributes object; the object becomes, in effect,
uninitialized. An implementation may cause <i>pthread_mutexattr_destroy</i>() to set the object referenced by <i>attr</i> to an
invalid value. A destroyed <i>attr</i> attributes object can be reinitialized using <i>pthread_mutexattr_init</i>(); the results of
otherwise referencing the object after it has been destroyed are undefined.</p>
<p>The <i>pthread_mutexattr_init</i>() function shall initialize a mutex attributes object <i>attr</i> with the default value for
all of the attributes defined by the implementation.</p>
<p>Results are undefined if <i>pthread_mutexattr_init</i>() is called specifying an already initialized <i>attr</i> attributes
object.</p>
<p>After a mutex attributes object has been used to initialize one or more mutexes, any function affecting the attributes object
(including destruction) shall not affect any previously initialized mutexes.</p>
</blockquote>
<h4><a name="tag_03_544_04"></a>RETURN VALUE</h4>
<blockquote>
<p>Upon successful completion, <i>pthread_mutexattr_destroy</i>() and <i>pthread_mutexattr_init</i>() shall return zero; otherwise,
an error number shall be returned to indicate the error.</p>
</blockquote>
<h4><a name="tag_03_544_05"></a>ERRORS</h4>
<blockquote>
<p>The <i>pthread_mutexattr_destroy</i>() function may fail if:</p>
<dl compact>
<dt>[EINVAL]</dt>
<dd>The value specified by <i>attr</i> is invalid.</dd>
</dl>
<p>The <i>pthread_mutexattr_init</i>() function shall fail if:</p>
<dl compact>
<dt>[ENOMEM]</dt>
<dd>Insufficient memory exists to initialize the mutex attributes object.</dd>
</dl>
<p>These functions 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_544_06"></a>EXAMPLES</h4>
<blockquote>
<p>None.</p>
</blockquote>
<h4><a name="tag_03_544_07"></a>APPLICATION USAGE</h4>
<blockquote>
<p>None.</p>
</blockquote>
<h4><a name="tag_03_544_08"></a>RATIONALE</h4>
<blockquote>
<p>See <a href="pthread_attr_init.html"><i>pthread_attr_init</i>()</a> for a general explanation of attributes. Attributes objects
allow implementations to experiment with useful extensions and permit extension of this volume of IEEE&nbsp;Std&nbsp;1003.1-2001
without changing the existing functions. Thus, they provide for future extensibility of this volume of
IEEE&nbsp;Std&nbsp;1003.1-2001 and reduce the temptation to standardize prematurely on semantics that are not yet widely
implemented or understood.</p>
<p>Examples of possible additional mutex attributes that have been discussed are <i>spin_only</i>, <i>limited_spin</i>,
<i>no_spin</i>, <i>recursive</i>, and <i>metered</i>. (To explain what the latter attributes might mean: recursive mutexes would
allow for multiple re-locking by the current owner; metered mutexes would transparently keep records of queue length, wait time,
and so on.) Since there is not yet wide agreement on the usefulness of these resulting from shared implementation and usage
experience, they are not yet specified in this volume of IEEE&nbsp;Std&nbsp;1003.1-2001. Mutex attributes objects, however, make it
possible to test out these concepts for possible standardization at a later time.</p>
<h5><a name="tag_03_544_08_01"></a>Mutex Attributes and Performance</h5>
<p>Care has been taken to ensure that the default values of the mutex attributes have been defined such that mutexes initialized
with the defaults have simple enough semantics so that the locking and unlocking can be done with the equivalent of a test-and-set
instruction (plus possibly a few other basic instructions).</p>
<p>There is at least one implementation method that can be used to reduce the cost of testing at lock-time if a mutex has
non-default attributes. One such method that an implementation can employ (and this can be made fully transparent to fully
conforming POSIX applications) is to secretly pre-lock any mutexes that are initialized to non-default attributes. Any later
attempt to lock such a mutex causes the implementation to branch to the &quot;slow path&quot; as if the mutex were unavailable; then, on
the slow path, the implementation can do the &quot;real work&quot; to lock a non-default mutex. The underlying unlock operation is more
complicated since the implementation never really wants to release the pre-lock on this kind of mutex. This illustrates that,
depending on the hardware, there may be certain optimizations that can be used so that whatever mutex attributes are considered
&quot;most frequently used&quot; can be processed most efficiently.</p>
<h5><a name="tag_03_544_08_02"></a>Process Shared Memory and Synchronization</h5>
<p>The existence of memory mapping functions in this volume of IEEE&nbsp;Std&nbsp;1003.1-2001 leads to the possibility that an
application may allocate the synchronization objects from this section in memory that is accessed by multiple processes (and
therefore, by threads of multiple processes).</p>
<p>In order to permit such usage, while at the same time keeping the usual case (that is, usage within a single process) efficient,
a <i>process-shared</i> option has been defined.</p>
<p>If an implementation supports the _POSIX_THREAD_PROCESS_SHARED option, then the <i>process-shared</i> attribute can be used to
indicate that mutexes or condition variables may be accessed by threads of multiple processes.</p>
<p>The default setting of PTHREAD_PROCESS_PRIVATE has been chosen for the <i>process-shared</i> attribute so that the most
efficient forms of these synchronization objects are created by default.</p>
<p>Synchronization variables that are initialized with the PTHREAD_PROCESS_PRIVATE <i>process-shared</i> attribute may only be
operated on by threads in the process that initialized them. Synchronization variables that are initialized with the
PTHREAD_PROCESS_SHARED <i>process-shared</i> attribute may be operated on by any thread in any process that has access to it. In
particular, these processes may exist beyond the lifetime of the initializing process. For example, the following code implements a
simple counting semaphore in a mapped file that may be used by many processes.</p>
<pre>
<tt>/* sem.h */
struct semaphore {
pthread_mutex_t lock;
pthread_cond_t nonzero;
unsigned count;
};
typedef struct semaphore semaphore_t;
<br>
semaphore_t *semaphore_create(char *semaphore_name);
semaphore_t *semaphore_open(char *semaphore_name);
void semaphore_post(semaphore_t *semap);
void semaphore_wait(semaphore_t *semap);
void semaphore_close(semaphore_t *semap);
<br>
/* sem.c */
#include &lt;sys/types.h&gt;
#include &lt;sys/stat.h&gt;
#include &lt;sys/mman.h&gt;
#include &lt;fcntl.h&gt;
#include &lt;pthread.h&gt;
#include "sem.h"
<br>
semaphore_t *
semaphore_create(char *semaphore_name)
{
int fd;
semaphore_t *semap;
pthread_mutexattr_t psharedm;
pthread_condattr_t psharedc;
<br>
fd = open(semaphore_name, O_RDWR | O_CREAT | O_EXCL, 0666);
if (fd &lt; 0)
return (NULL);
(void) ftruncate(fd, sizeof(semaphore_t));
(void) pthread_mutexattr_init(&amp;psharedm);
(void) pthread_mutexattr_setpshared(&amp;psharedm,
PTHREAD_PROCESS_SHARED);
(void) pthread_condattr_init(&amp;psharedc);
(void) pthread_condattr_setpshared(&amp;psharedc,
PTHREAD_PROCESS_SHARED);
semap = (semaphore_t *) mmap(NULL, sizeof(semaphore_t),
PROT_READ | PROT_WRITE, MAP_SHARED,
fd, 0);
close (fd);
(void) pthread_mutex_init(&amp;semap-&gt;lock, &amp;psharedm);
(void) pthread_cond_init(&amp;semap-&gt;nonzero, &amp;psharedc);
semap-&gt;count = 0;
return (semap);
}
<br>
semaphore_t *
semaphore_open(char *semaphore_name)
{
int fd;
semaphore_t *semap;
<br>
fd = open(semaphore_name, O_RDWR, 0666);
if (fd &lt; 0)
return (NULL);
semap = (semaphore_t *) mmap(NULL, sizeof(semaphore_t),
PROT_READ | PROT_WRITE, MAP_SHARED,
fd, 0);
close (fd);
return (semap);
}
<br>
void
semaphore_post(semaphore_t *semap)
{
pthread_mutex_lock(&amp;semap-&gt;lock);
if (semap-&gt;count == 0)
pthread_cond_signal(&amp;semapx-&gt;nonzero);
semap-&gt;count++;
pthread_mutex_unlock(&amp;semap-&gt;lock);
}
<br>
void
semaphore_wait(semaphore_t *semap)
{
pthread_mutex_lock(&amp;semap-&gt;lock);
while (semap-&gt;count == 0)
pthread_cond_wait(&amp;semap-&gt;nonzero, &amp;semap-&gt;lock);
semap-&gt;count--;
pthread_mutex_unlock(&amp;semap-&gt;lock);
}
<br>
void
semaphore_close(semaphore_t *semap)
{
munmap((void *) semap, sizeof(semaphore_t));
}
</tt>
</pre>
<p>The following code is for three separate processes that create, post, and wait on a semaphore in the file <b>/tmp/semaphore</b>.
Once the file is created, the post and wait programs increment and decrement the counting semaphore (waiting and waking as
required) even though they did not initialize the semaphore.</p>
<pre>
<tt>/* create.c */
#include "pthread.h"
#include "sem.h"
<br>
int
main()
{
semaphore_t *semap;
<br>
semap = semaphore_create("/tmp/semaphore");
if (semap == NULL)
exit(1);
semaphore_close(semap);
return (0);
}
<br>
/* post */
#include "pthread.h"
#include "sem.h"
<br>
int
main()
{
semaphore_t *semap;
<br>
semap = semaphore_open("/tmp/semaphore");
if (semap == NULL)
exit(1);
semaphore_post(semap);
semaphore_close(semap);
return (0);
}
<br>
/* wait */
#include "pthread.h"
#include "sem.h"
<br>
int
main()
{
semaphore_t *semap;
<br>
semap = semaphore_open("/tmp/semaphore");
if (semap == NULL)
exit(1);
semaphore_wait(semap);
semaphore_close(semap);
return (0);
}
</tt>
</pre>
</blockquote>
<h4><a name="tag_03_544_09"></a>FUTURE DIRECTIONS</h4>
<blockquote>
<p>None.</p>
</blockquote>
<h4><a name="tag_03_544_10"></a>SEE ALSO</h4>
<blockquote>
<p><a href="pthread_cond_destroy.html"><i>pthread_cond_destroy</i>()</a> , <a href=
"pthread_create.html"><i>pthread_create</i>()</a> , <a href="pthread_mutex_destroy.html"><i>pthread_mutex_destroy</i>()</a> , <a
href="#">pthread_mutexattr_destroy</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_544_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_544_12"></a>Issue 6</h4>
<blockquote>
<p>The <i>pthread_mutexattr_destroy</i>() and <i>pthread_mutexattr_init</i>() functions are marked as part of the Threads
option.</p>
<p>IEEE PASC Interpretation 1003.1c #27 is applied, updating the ERRORS section.</p>
</blockquote>
<div class="box"><em>End of informative text.</em></div>
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