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


#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);
[Note] Note
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
[Note] Note

Compile and link with −pthread.


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:


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.


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.


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.


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.



pref specifies an unsupported value.


The pthread_rwlockattr_getkind_np() and pthread_rwlockattr_setkind_np() functions first appeared in glibc 2.1.


These functions are non-standard GNU extensions; hence the suffix "_np" (nonportable) in the names.




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