Manpages - pthread_rwlockattr_setkind_np.3

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);

  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

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 developer 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.

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.

EINVAL

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.

*pthreads*(7)

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