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PTHREAD_MUTEXATTR_DESTROY(3P)IX Programmer's ManualEAD_MUTEXATTR_DESTROY(3P)
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.
pthread_mutexattr_destroy, pthread_mutexattr_init — destroy and
initialize the mutex attributes object
#include <pthread.h>
int pthread_mutexattr_destroy(pthread_mutexattr_t *attr);
int pthread_mutexattr_init(pthread_mutexattr_t *attr);
The pthread_mutexattr_destroy() function shall destroy a mutex
attributes object; the object becomes, in effect, uninitialized. An
implementation may cause pthread_mutexattr_destroy() to set the
object referenced by attr to an invalid value.
A destroyed attr attributes object can be reinitialized using
pthread_mutexattr_init(); the results of otherwise referencing the
object after it has been destroyed are undefined.
The pthread_mutexattr_init() function shall initialize a mutex
attributes object attr with the default value for all of the
attributes defined by the implementation.
Results are undefined if pthread_mutexattr_init() is called
specifying an already initialized attr attributes object.
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.
The behavior is undefined if the value specified by the attr argument
to pthread_mutexattr_destroy() does not refer to an initialized mutex
attributes object.
Upon successful completion, pthread_mutexattr_destroy() and
pthread_mutexattr_init() shall return zero; otherwise, an error
number shall be returned to indicate the error.
The pthread_mutexattr_init() function shall fail if:
ENOMEM Insufficient memory exists to initialize the mutex attributes
object.
These functions shall not return an error code of [EINTR].
The following sections are informative.
None.
None.
If an implementation detects that the value specified by the attr
argument to pthread_mutexattr_destroy() does not refer to an
initialized mutex attributes object, it is recommended that the
function should fail and report an [EINVAL] error.
See pthread_attr_destroy(3p) for a general explanation of attributes.
Attributes objects allow implementations to experiment with useful
extensions and permit extension of this volume of POSIX.1‐2008
without changing the existing functions. Thus, they provide for
future extensibility of this volume of POSIX.1‐2008 and reduce the
temptation to standardize prematurely on semantics that are not yet
widely implemented or understood.
Examples of possible additional mutex attributes that have been
discussed are spin_only, limited_spin, no_spin, recursive, and
metered. (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
POSIX.1‐2008. Mutex attributes objects, however, make it possible to
test out these concepts for possible standardization at a later time.
Mutex Attributes and Performance
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).
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 ``slow path'' as
if the mutex were unavailable; then, on the slow path, the
implementation can do the ``real work'' 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 ``most frequently used'' can be
processed most efficiently.
Process Shared Memory and Synchronization
The existence of memory mapping functions in this volume of
POSIX.1‐2008 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).
In order to permit such usage, while at the same time keeping the
usual case (that is, usage within a single process) efficient, a
process-shared option has been defined.
If an implementation supports the _POSIX_THREAD_PROCESS_SHARED
option, then the process-shared attribute can be used to indicate
that mutexes or condition variables may be accessed by threads of
multiple processes.
The default setting of PTHREAD_PROCESS_PRIVATE has been chosen for
the process-shared attribute so that the most efficient forms of
these synchronization objects are created by default.
Synchronization variables that are initialized with the
PTHREAD_PROCESS_PRIVATE process-shared attribute may only be operated
on by threads in the process that initialized them. Synchronization
variables that are initialized with the PTHREAD_PROCESS_SHARED
process-shared 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.
/* sem.h */
struct semaphore {
pthread_mutex_t lock;
pthread_cond_t nonzero;
unsigned count;
};
typedef struct semaphore semaphore_t;
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);
/* sem.c */
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <pthread.h>
#include "sem.h"
semaphore_t *
semaphore_create(char *semaphore_name)
{
int fd;
semaphore_t *semap;
pthread_mutexattr_t psharedm;
pthread_condattr_t psharedc;
fd = open(semaphore_name, O_RDWR | O_CREAT | O_EXCL, 0666);
if (fd < 0)
return (NULL);
(void) ftruncate(fd, sizeof(semaphore_t));
(void) pthread_mutexattr_init(&psharedm);
(void) pthread_mutexattr_setpshared(&psharedm,
PTHREAD_PROCESS_SHARED);
(void) pthread_condattr_init(&psharedc);
(void) pthread_condattr_setpshared(&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(&semap->lock, &psharedm);
(void) pthread_cond_init(&semap->nonzero, &psharedc);
semap->count = 0;
return (semap);
}
semaphore_t *
semaphore_open(char *semaphore_name)
{
int fd;
semaphore_t *semap;
fd = open(semaphore_name, O_RDWR, 0666);
if (fd < 0)
return (NULL);
semap = (semaphore_t *) mmap(NULL, sizeof(semaphore_t),
PROT_READ | PROT_WRITE, MAP_SHARED,
fd, 0);
close (fd);
return (semap);
}
void
semaphore_post(semaphore_t *semap)
{
pthread_mutex_lock(&semap->lock);
if (semap->count == 0)
pthread_cond_signal(&semapx->nonzero);
semap->count++;
pthread_mutex_unlock(&semap->lock);
}
void
semaphore_wait(semaphore_t *semap)
{
pthread_mutex_lock(&semap->lock);
while (semap->count == 0)
pthread_cond_wait(&semap->nonzero, &semap->lock);
semap->count--;
pthread_mutex_unlock(&semap->lock);
}
void
semaphore_close(semaphore_t *semap)
{
munmap((void *) semap, sizeof(semaphore_t));
}
The following code is for three separate processes that create, post,
and wait on a semaphore in the file /tmp/semaphore. 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.
/* create.c */
#include "pthread.h"
#include "sem.h"
int
main()
{
semaphore_t *semap;
semap = semaphore_create("/tmp/semaphore");
if (semap == NULL)
exit(1);
semaphore_close(semap);
return (0);
}
/* post */
#include "pthread.h"
#include "sem.h"
int
main()
{
semaphore_t *semap;
semap = semaphore_open("/tmp/semaphore");
if (semap == NULL)
exit(1);
semaphore_post(semap);
semaphore_close(semap);
return (0);
}
/* wait */
#include "pthread.h"
#include "sem.h"
int
main()
{
semaphore_t *semap;
semap = semaphore_open("/tmp/semaphore");
if (semap == NULL)
exit(1);
semaphore_wait(semap);
semaphore_close(semap);
return (0);
}
None.
pthread_cond_destroy(3p), pthread_create(3p),
pthread_mutex_destroy(3p)
The Base Definitions volume of POSIX.1‐2008, pthread.h(0p)
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 http://www.unix.org/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 2013 PTHREAD_MUTEXATTR_DESTROY(3P)
Pages that refer to this page: pthread.h(0p), pthread_mutexattr_getpshared(3p), pthread_mutexattr_init(3p)
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