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.  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 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" (non-portable) in the names.

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 recursively.

SEE ALSO

       pthreads(7)

COLOPHON

       This  page  is  part  of  release  4.15  of  the  Linux man-pages project.  A description of the project,
       information  about  reporting  bugs,  and  the  latest  version  of  this   page,   can   be   found   at
       https://www.kernel.org/doc/man-pages/.

Linux Programmer's Manual                          2018-02-02                   PTHREAD_RWLOCKATTR_SETKIND_NP(3)