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


       #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


       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:

              This is the default.  A thread may hold multiple read locks; that  is,  read  locks
              are  recursive.   According  to  The  Single  Unix  Specification,  the behavior is
              unspecified when a reader tries to place a lock, and there is  no  write  lock  but
              writers   are   waiting.    Giving   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.

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

              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.


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


       EINVAL pref specifies an unsupported value.


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


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


       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 recursively.




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