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)