NAME

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

LIBRARY

       POSIX threads library (libpthread, -lpthread)

SYNOPSIS

       #include <pthread.h>

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

   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 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.

       PTHREAD_RWLOCK_PREFER_WRITER_NP
              This  is intended as the write lock analog of PTHREAD_RWLOCK_PREFER_READER_NP.  This is ignored by
              glibc because the POSIX requirement to support recursive read locks would  cause  this  option  to
              create  trivial  deadlocks; instead use PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP which ensures
              the application developer will not take recursive read locks thus avoiding deadlocks.

       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 nonzero error number.

ERRORS

       EINVAL pref specifies an unsupported value.

STANDARDS

       GNU; hence the suffix "_np" (nonportable) in the names.

HISTORY

       glibc 2.1.

SEE ALSO

       pthreads(7)