pthread_rwlockattr_setkind_np(3) [linux man page]

PTHREAD_RWLOCKATTR_SETKIND_NP(3)			     Library Functions Manual				  PTHREAD_RWLOCKATTR_SETKIND_NP(3)

NAME

       pthread_rwlockattr_setkind_np,  pthread_rwlockattr_getkind_np  -  set/get  the read-write lock kind of the thread read-write lock attribute
       object

SYNOPSIS

       #include <pthread.h>

       int pthread_rwlockattr_setkind_np(pthread_rwlockattr_t *attr,
					  int pref);
       int pthread_rwlockattr_getkind_np(const pthread_rwlockattr_t *attr,
					  int *pref);

       Compile and link with -pthread.

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       pthread_rwlockattr_setkind_np(), pthread_rwlockattr_getkind_np():
	   _XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200809L

DESCRIPTION

       The pthread_rwlockattr_setkind_np() function sets the "lock kind" attribute of the read-write lock attribute object referred to by attr	to
       the value specified in pref.  The argument pref may be set to one of the following:

       PTHREAD_RWLOCK_PREFER_READER_NP
	      This is the default.  A thread may hold multiple read locks; that is, read locks are recursive.  According to The Single Unix Speci-
	      fication, the behavior is unspecified when a reader tries to place a lock, and there is no write lock but writers are waiting.  Giv-
	      ing preference to the reader, as is set by PTHREAD_RWLOCK_PREFER_READER_NP, implies that the reader will receive the requested lock,
	      even if a writer is waiting.  As long as there are readers, the writer will be starved.

       PTHREAD_RWLOCK_PREFER_WRITER_NP
	      This is intended as the write lock analog of PTHREAD_RWLOCK_PREFER_READER_NP.  But see BUGS.

       PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP
	      Setting the lock kind to this avoids writer starvation as long as any read locking is not done in a recursive fashion.

       The pthread_rwlockattr_getkind_np() function returns the value of the lock kind attribute of the read-write lock attribute object  referred
       to by attr in the pointer pref.

RETURN VALUE

       On  success,  these  functions  return  0.   Given  valid  pointer  arguments,  pthread_rwlockattr_getkind_np() always succeeds.  On error,
       pthread_rwlockattr_setkind_np() returns a non-zero error number.

ERRORS

       EINVAL pref specifies an unsupported value.

BUGS

       Setting the value read-write lock  kind	to  PTHREAD_RWLOCK_PREFER_WRITER_NP  results  in  the  same  behavior  as  setting  the  value	to
       PTHREAD_RWLOCK_PREFER_READER_NP.   As long as a reader thread holds the lock, the thread holding a write lock will be starved.  Setting the
       lock kind to PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP allows writers to run, but, as the name implies a  writer  may  not  lock  recur-
       sively.

VERSIONS

       The pthread_rwlockattr_getkind_np() and pthread_rwlockattr_setkind_np() functions first appeared in glibc 2.1.

CONFORMING TO

       These functions are non-standard GNU extensions; hence the suffix "_np" (non-portable) in the names.

SEE ALSO

       pthreads(7)

Linux Programmer's Manual					    2014-10-15					  PTHREAD_RWLOCKATTR_SETKIND_NP(3)

Check Out this Related Man Page

pthread_rwlock_rdlock(3T)												 pthread_rwlock_rdlock(3T)

NAME

       pthread_rwlock_rdlock(), pthread_rwlock_tryrdlock() - lock or attempt to lock a read-write lock for reading

SYNOPSIS

PARAMETERS

	      rwlock	Pointer to the read-write lock to be locked for reading.

DESCRIPTION

       The  function  applies  a  read lock to the read-write lock object referenced by rwlock.  The calling thread shall acquire a read lock if a
       writer does not hold the lock and there are no writers blocked on the lock.  It is unspecified whether the calling thread acquires the lock
       when  a writer does not hold the lock and there are writers waiting for 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 blocks (that is, it does not return from the  call)  until	it
       can acquire the lock.  Results are undefined if the calling thread currently owns a write lock on rwlock.

       Implementations shall be allowed to favor writers over readers to avoid writer starvation.

       A thread may hold multiple concurrent locks on rwlock (that is, successfully call the function n times). If so, the thread must perform the
       matching unlocks (that is, it must call the function n times).

       The function applies a read lock as in the function with the exception that the function fails if any thread holds a write lock	on  rwlock
       or there are writers blocked on rwlock.

       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, upon return from the signal handler, the thread shall resume waiting
       for the read-write lock as if it was not interrupted.

RETURN VALUE

       Upon successful completion, and return zero. Otherwise, an error number is returned to indicate the error (the variable is not set).

ERRORS

       If any of the following occur, the function returns the corresponding error number:

	      [EBUSY]	     The read-write lock rwlock could not be acquired for reading because a writer holds the lock or was blocked on it.

       For each of the following conditions, if the condition is detected, the and functions return the corresponding error number:

	      [EINVAL]	     The value specified by rwlock does not refer to an initialized read-write lock.

	      [EDEADLK]      The current thread already owns the read-write lock for writing.

	      [EAGAIN]	     The read lock could not be acquired because the maximum number of read locks for rwlock has been exceeded. This error
			     is not detected on HP-UX.

AUTHOR

       and were developed by X/Open.

SEE ALSO

       pthread_rwlock_init(3T), pthread_rwlock_destroy(3T), pthread_rwlock_trywrlock(3T), pthread_rwlock_wrlock(3T), pthread_rwlock_unlock(3T).

STANDARDS CONFORMANCE

								  Pthread Library					 pthread_rwlock_rdlock(3T)

Linux and UNIX Man Pages

pthread_rwlockattr_setkind_np(3) [linux man page]

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