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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(3) 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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