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 *attr,
  int *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. But see BUGS.


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 non-zero error number.



pref specifies an unsupported value.


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.


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" (non-portable) in the names.




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