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  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.

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" (nonportable) in
       the names.

SEE ALSO

       pthreads(7)

COLOPHON

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