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)

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PTHREAD_RWLOCK_RDLOCK(3)				   BSD Library Functions Manual 				  PTHREAD_RWLOCK_RDLOCK(3)

NAME

     pthread_rwlock_rdlock, pthread_rwlock_tryrdlock -- acquire a read/write lock for reading

SYNOPSIS

     #include <pthread.h>

     int
     pthread_rwlock_rdlock(pthread_rwlock_t *lock);

     int
     pthread_rwlock_tryrdlock(pthread_rwlock_t *lock);

DESCRIPTION

     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_tryrdlock() function performs the same action, 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.

IMPLEMENTATION NOTES

     To prevent writer starvation, writers are favored over readers.

RETURN VALUES

     If successful, the pthread_rwlock_rdlock() and pthread_rwlock_tryrdlock() functions will return zero.  Otherwise an error number will be
     returned to indicate the error.

ERRORS

     The pthread_rwlock_tryrdlock() function will fail if:

     [EBUSY]		The lock could not be acquired because a writer holds the lock or was blocked on it.

     The pthread_rwlock_rdlock() 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.

     [ENOMEM]		Insufficient memory exists to initialize the lock (applies to statically initialized locks only).

SEE ALSO

     pthread_rwlock_init(3), pthread_rwlock_trywrlock(3), pthread_rwlock_unlock(3), pthread_rwlock_wrlock(3)

STANDARDS

     The pthread_rwlock_rdlock() and pthread_rwlock_tryrdlock() functions are expected to conform to Version 2 of the Single UNIX Specification
     (``SUSv2'').

HISTORY

     The pthread_rwlock_rdlock() function first appeared in FreeBSD 3.0.

BSD
								  August 4, 1998							       BSD

Linux and UNIX Man Pages

pthread_rwlockattr_setkind_np(3) [linux man page]

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