уничтожить и инициализировать объект атрибутов мьютекса (destroy and initialize the mutex attributes object)
Пролог (Prolog)
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.
Имя (Name)
pthread_mutexattr_destroy, pthread_mutexattr_init — destroy and
initialize the mutex attributes object
Синопсис (Synopsis)
#include <pthread.h>
int pthread_mutexattr_destroy(pthread_mutexattr_t *attr);
int pthread_mutexattr_init(pthread_mutexattr_t *attr);
Описание (Description)
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.
Возвращаемое значение (Return value)
Upon successful completion, pthread_mutexattr_destroy() and
pthread_mutexattr_init() shall return zero; otherwise, an error
number shall be returned to indicate the error.
Ошибки (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.
Примеры (Examples)
None.
Использование в приложениях (Application usage)
None.
Обоснование (Rationale)
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.1‐2017 without changing the existing functions.
Thus, they provide for future extensibility of this volume of
POSIX.1‐2017 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.1‐2017. 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.1‐2017 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);
}
Будущие направления (Future directions)
None.
Смотри также (See also)
pthread_cond_destroy(3p), pthread_create(3p),
pthread_mutex_destroy(3p)
The Base Definitions volume of POSIX.1‐2017, pthread.h(0p)
