объект блокировки записи для чтения (write lock object for reading)
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_rwlock_rdlock, pthread_rwlock_tryrdlock — lock a read-
write lock object for reading
Синопсис (Synopsis)
#include <pthread.h>
int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);
int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock);
Описание (Description)
The pthread_rwlock_rdlock() function shall apply a read lock to
the read-write lock referenced by rwlock. The calling thread
acquires the read lock if a writer does not hold the lock and
there are no writers blocked on the lock.
If the Thread Execution Scheduling option is supported, and the
threads involved in the lock are executing with the scheduling
policies SCHED_FIFO or SCHED_RR, the calling thread shall not
acquire the lock if a writer holds the lock or if writers of
higher or equal priority are blocked on the lock; otherwise, the
calling thread shall acquire the lock.
If the Thread Execution Scheduling option is supported, and the
threads involved in the lock are executing with the
SCHED_SPORADIC scheduling policy, the calling thread shall not
acquire the lock if a writer holds the lock or if writers of
higher or equal priority are blocked on the lock; otherwise, the
calling thread shall acquire the lock.
If the Thread Execution Scheduling option is not supported, it is
implementation-defined whether the calling thread acquires the
lock when a writer does not hold the lock and there are writers
blocked on the lock. If a writer holds the lock, the calling
thread shall not acquire the read lock. If the read lock is not
acquired, the calling thread shall block until it can acquire the
lock. The calling thread may deadlock if at the time the call is
made it holds a write lock.
A thread may hold multiple concurrent read locks on rwlock (that
is, successfully call the pthread_rwlock_rdlock() function n
times). If so, the application shall ensure that the thread
performs matching unlocks (that is, it calls the
pthread_rwlock_unlock() function n times).
The maximum number of simultaneous read locks that an
implementation guarantees can be applied to a read-write lock
shall be implementation-defined. The pthread_rwlock_rdlock()
function may fail if this maximum would be exceeded.
The pthread_rwlock_tryrdlock() function shall apply a read lock
as in the pthread_rwlock_rdlock() function, with the exception
that the function shall fail if the equivalent
pthread_rwlock_rdlock() call would have blocked the calling
thread. In no case shall the pthread_rwlock_tryrdlock() function
ever block; it always either acquires the lock or fails and
returns immediately.
Results are undefined if any of these functions are called with
an uninitialized read-write lock.
If a signal is delivered to a thread waiting for a read-write
lock for reading, upon return from the signal handler the thread
resumes waiting for the read-write lock for reading as if it was
not interrupted.
Возвращаемое значение (Return value)
If successful, the pthread_rwlock_rdlock() function shall return
zero; otherwise, an error number shall be returned to indicate
the error.
The pthread_rwlock_tryrdlock() function shall return zero if the
lock for reading on the read-write lock object referenced by
rwlock is acquired. Otherwise, an error number shall be returned
to indicate the error.
Ошибки (Error)
The pthread_rwlock_tryrdlock() function shall fail if:
EBUSY The read-write lock could not be acquired for reading
because a writer holds the lock or a writer with the
appropriate priority was blocked on it.
The pthread_rwlock_rdlock() and pthread_rwlock_tryrdlock()
functions may fail if:
EAGAIN The read lock could not be acquired because the maximum
number of read locks for rwlock has been exceeded.
The pthread_rwlock_rdlock() function may fail if:
EDEADLK
A deadlock condition was detected or the current thread
already owns the read-write lock for writing.
These functions shall not return an error code of [EINTR].
The following sections are informative.
Примеры (Examples)
None.
Использование в приложениях (Application usage)
Applications using these functions may be subject to priority
inversion, as discussed in the Base Definitions volume of
POSIX.1‐2017, Section 3.291, Priority Inversion.
Обоснование (Rationale)
If an implementation detects that the value specified by the
rwlock argument to pthread_rwlock_rdlock() or
pthread_rwlock_tryrdlock() does not refer to an initialized read-
write lock object, it is recommended that the function should
fail and report an [EINVAL] error.
Будущие направления (Future directions)
None.
Смотри также (See also)
pthread_rwlock_destroy(3p), pthread_rwlock_timedrdlock(3p),
pthread_rwlock_timedwrlock(3p), pthread_rwlock_trywrlock(3p),
pthread_rwlock_unlock(3p)
The Base Definitions volume of POSIX.1‐2017, Section 3.291,
Priority Inversion, Section 4.12, Memory Synchronization,
pthread.h(0p)
