bionic (3) pthread_rwlockattr_setkind_np.3.gz

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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

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