pthread_rwlockattr_setkind_np man page
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); Compile and link with -pthread.
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
_XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200809L
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 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" (non-portable) in the names.
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.
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The man page pthread_rwlockattr_getkind_np(3) is an alias of pthread_rwlockattr_setkind_np(3).