NAME
pthread_rwlock —
read/write lock
interface
LIBRARY
POSIX Threads Library (libpthread, -lpthread)
SYNOPSIS
#include <pthread.h>
int
pthread_rwlock_init(
pthread_rwlock_t
* restrict lock,
const
pthread_rwlockattr_t * restrict attr);
pthread_rwlock_t lock =
PTHREAD_RWLOCK_INITIALIZER
;
int
pthread_rwlock_destroy(
pthread_rwlock_t
*lock);
int
pthread_rwlock_rdlock(
pthread_rwlock_t
*lock);
int
pthread_rwlock_timedrdlock(
pthread_rwlock_t
* restrict lock,
const
struct timespec * restrict abstime);
int
pthread_rwlock_tryrdlock(
pthread_rwlock_t
*lock);
int
pthread_rwlock_wrlock(
pthread_rwlock_t
*lock);
int
pthread_rwlock_timedwrlock(
pthread_rwlock_t
* restrict lock,
const
struct timespec * restrict abstime);
int
pthread_rwlock_trywrlock(
pthread_rwlock_t
*lock);
int
pthread_rwlock_unlock(
pthread_rwlock_t
*lock);
DESCRIPTION
The
pthread_rwlock_init() function is used to initialize a
read/write lock, with attributes specified by
attr. If
attr is
NULL
, the default
read/write lock attributes are used.
The results of calling
pthread_rwlock_init() with an already
initialized lock are undefined.
The macro
PTHREAD_RWLOCK_INITIALIZER
can be used to
initialize a read/write lock when the allocation can be done statically, no
error checking is required, and the default attributes are appropriate. The
behavior is similar to calling
pthread_rwlock_init() with
attr specified as
NULL
.
The
pthread_rwlock_destroy() function is used to destroy a
read/write lock previously created with
pthread_rwlock_init().
The
pthread_rwlock_rdlock() function acquires a read lock on
lock provided that
lock is not
presently held for writing and no writer threads are presently blocked on the
lock. If the read lock cannot be immediately acquired, the calling thread
blocks until it can acquire the lock.
The
pthread_rwlock_timedrdlock() performs the same action, but
will not wait beyond
abstime to obtain the lock before
returning.
The
pthread_rwlock_tryrdlock() function performs the same
action as
pthread_rwlock_rdlock(), but does not block if the
lock cannot be immediately obtained (i.e., the lock is held for writing or
there are waiting writers).
A thread may hold multiple concurrent read locks. If so,
pthread_rwlock_unlock() must be called once for each lock
obtained.
The results of acquiring a read lock while the calling thread holds a write lock
are undefined.
The
pthread_rwlock_wrlock() function blocks until a write lock
can be acquired against
lock.
The
pthread_rwlock_timedwrlock() performs the same action, but
will not wait beyond
abstime to obtain the lock before
returning.
The
pthread_rwlock_trywrlock() function performs the same
action as
pthread_rwlock_wrlock(), but does not block if the
lock cannot be immediately obtained.
The results are undefined if the calling thread already holds the lock at the
time the call is made.
The
pthread_rwlock_unlock() function is used to release the
read/write lock previously obtained by
pthread_rwlock_rdlock(),
pthread_rwlock_wrlock(),
pthread_rwlock_tryrdlock(), or
pthread_rwlock_trywrlock().
RETURN VALUES
If successful, the
pthread_rwlock_init() function will return
zero. Otherwise an error number will be returned to indicate the error.
If successful, the
pthread_rwlock_destroy(),
pthread_rwlock_rdlock(),
pthread_rwlock_timedrdlock(),
pthread_rwlock_tryrdlock(),
pthread_rwlock_wrlock(),
pthread_rwlock_timedwrlock(),
pthread_rwlock_trywrlock(), and
pthread_rwlock_unlock() will return zero. Otherwise an error
number will be returned to indicate the error.
The results are undefined if
lock is not held by the
calling thread.
ERRORS
The
pthread_rwlock_init() function may fail if:
-
-
- [
EAGAIN
]
- The system lacks the resources to initialize another
read-write lock.
-
-
- [
EBUSY
]
- The system has detected an attempt to re-initialize the
object referenced by lock, a previously initialized
but not yet destroyed read/write lock.
-
-
- [
EINVAL
]
- The value specified by attr is
invalid.
-
-
- [
ENOMEM
]
- Insufficient memory exists to initialize the read-write
lock.
The
pthread_rwlock_destroy() function may fail if:
-
-
- [
EBUSY
]
- The system has detected an attempt to destroy the object
referenced by lock while it is locked.
-
-
- [
EINVAL
]
- The value specified by lock is
invalid.
The
pthread_rwlock_tryrdlock() function shall fail if:
-
-
- [
EBUSY
]
- The lock could not be acquired because a writer holds the
lock or was blocked on it.
The
pthread_rwlock_timedrdlock() function shall fail if:
-
-
- [
ETIMEDOUT
]
- The time specified by abstime was
reached before the lock could be obtained.
The
pthread_rwlock_rdlock(),
pthread_rwlock_timedrdlock(), and
pthread_rwlock_tryrdlock() functions may fail if:
-
-
- [
EAGAIN
]
- The lock could not be acquired because the maximum number
of read locks against lock has been exceeded.
-
-
- [
EDEADLK
]
- The current thread already owns lock
for writing.
-
-
- [
EINVAL
]
- The value specified by lock is
invalid.
The
pthread_rwlock_trywrlock() function shall fail if:
-
-
- [
EBUSY
]
- The calling thread is not able to acquire the lock without
blocking.
The
pthread_rwlock_timedrdlock() function shall fail if:
-
-
- [
ETIMEDOUT
]
- The time specified by abstime was
reached before the lock could be obtained.
The
pthread_rwlock_wrlock(),
pthread_rwlock_timedwrlock(), and
pthread_rwlock_trywrlock() functions may fail if:
-
-
- [
EDEADLK
]
- The calling thread already owns the read/write lock (for
reading or writing).
-
-
- [
EINVAL
]
- The value specified by lock is
invalid.
The
pthread_rwlock_unlock() function may fail if:
-
-
- [
EINVAL
]
- The value specified by lock is
invalid.
-
-
- [
EPERM
]
- The current thread does not own the read/write lock.
SEE ALSO
pthread(3),
pthread_barrier(3),
pthread_cond(3),
pthread_mutex(3),
pthread_rwlockattr(3),
pthread_spin(3)
STANDARDS
These functions conform to
IEEE Std 1003.1-2001
(“POSIX.1”).
BUGS
The
PTHREAD_PROCESS_SHARED
attribute is not
supported.