PTHREAD_RWLOCKATTR_SETKIND_NP(3)                                        Library Functions Manual                                        PTHREAD_RWLOCKATTR_SETKIND_NP(3)

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 *restrict attr,
                              int *restrict 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 de‐
              veloper 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)

Linux Programmer's Manual                                                      2021-03-22                                               PTHREAD_RWLOCKATTR_SETKIND_NP(3)