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

       This page is part of release 5.05 of the Linux man-pages project.  A description of the project,
       information about reporting bugs, and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.