PTHREAD_RWLOCK(3) | Library Functions Manual | PTHREAD_RWLOCK(3)

# NAME

`pthread_rwlock`, `pthread_rwlock_init`, `pthread_rwlock_destroy`, `pthread_rwlock_rdlock`, `pthread_rwlock_timedrdlock`, `pthread_rwlock_tryrdlock`, `pthread_rwlock_wrlock`, `pthread_rwlock_timedwrlock`, `pthread_rwlock_trywrlock`, `pthread_rwlock_unlock` —
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.

July 8, 2010 | NetBSD 9.2