PTHREAD_SPIN_INIT(3)

NAME
pthread_spin_init, pthread_spin_destroy - initialize or destroy a spin lock

SYNOPSIS
#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

DESCRIPTION
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() on copies of the object referred to by lock is undefined.

RETURN VALUE
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.

ERRORS
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.

VERSIONS
These functions first appeared in glibc in version 2.2.

CONFORMING TO
POSIX.1-2001.

Support for process-shared spin locks is a POSIX option. The option is supported in the glibc implementation.

NOTES
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.

SEE ALSO
pthread_mutex_init(3), pthread_mutex_lock(3), pthread_spin_lock(3), pthread_spin_unlock(3), pthreads(7)

COLOPHON
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