pthread_spin_init, pthread_spin_destroy - initialize or destroy a spin lock
#include <pthread.h>
int pthread_spin_init(pthread_spinlock_t *lock, int pshared);
int pthread_spin_destroy(pthread_spinlock_t *lock);
Compile and link with
-pthread.
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
pthread_spin_init(),
pthread_spin_destroy():
_POSIX_C_SOURCE >= 200112L
General note: Most programs should use mutexes instead of spin locks.
Spin locks are primarily useful in conjunction with real-time scheduling
policies. See NOTES.
The
pthread_spin_init() function allocates any resources required for the
use of the spin lock referred to by
lock and initializes the lock to be
in the unlocked state. The
pshared argument must have one of the
following values:
- PTHREAD_PROCESS_PRIVATE
- The spin lock is to be operated on only by threads in the same process as
the thread that calls pthread_spin_init(). (Attempting to share the
spin lock between processes results in undefined behavior.)
- PTHREAD_PROCESS_SHARED
- The spin lock may be operated on by any thread in any process that has
access to the memory containing the lock (i.e., the lock may be in a
shared memory object that is shared among multiple processes).
Calling
pthread_spin_init() on a spin lock that has already been
initialized results in undefined behavior.
The
pthread_spin_destroy() function destroys a previously initialized
spin lock, freeing any resources that were allocated for that lock. Destroying
a spin lock that has not been previously been initialized or destroying a spin
lock while another thread holds the lock results in undefined behavior.
Once a spin lock has been destroyed, performing any operation on the lock other
than once more initializing it with
pthread_spin_init() results in
undefined behavior.
The result of performing operations such as
pthread_spin_lock(3),
pthread_spin_unlock(3), and
pthread_spin_destroy(3) on
copies of the object referred to by
lock is undefined.
On success, there functions return zero. On failure, they return an error
number. In the event that
pthread_spin_init() fails, the lock is not
initialized.
pthread_spin_init() may fail with the following errors:
- EAGAIN
- The system has insufficient resources to initialize a new spin lock.
- ENOMEM
- Insufficient memory to initialize the spin lock.
These functions first appeared in glibc in version 2.2.
POSIX.1-2001.
Support for process-shared spin locks is a POSIX option. The option is supported
in the glibc implementation.
Spin locks should be employed in conjunction with real-time scheduling policies
(
SCHED_FIFO, or possibly
SCHED_RR). Use of spin locks with
nondeterministic scheduling policies such as
SCHED_OTHER probably
indicates a design mistake. The problem is that if a thread operating under
such a policy is scheduled off the CPU while it holds a spin lock, then other
threads will waste time spinning on the lock until the lock holder is once
more rescheduled and releases the lock.
If threads create a deadlock situation while employing spin locks, those threads
will spin forever consuming CPU time.
User-space spin locks are
not applicable as a general locking solution.
They are, by definition, prone to priority inversion and unbounded spin times.
A programmer using spin locks must be exceptionally careful not only in the
code, but also in terms of system configuration, thread placement, and
priority assignment.
pthread_mutex_init(3),
pthread_mutex_lock(3),
pthread_spin_lock(3),
pthread_spin_unlock(3),
pthreads(7)