.\" Copyright (c) 2017, Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH pthread_spin_init 3 2023-03-30 "Linux man-pages 6.05.01"
.SH NAME
pthread_spin_init, pthread_spin_destroy \- initialize or destroy a spin lock
.SH LIBRARY
POSIX threads library
.RI ( libpthread ", " \-lpthread )
.SH SYNOPSIS
.nf
.B #include <pthread.h>
.PP
.BI "int pthread_spin_init(pthread_spinlock_t *" lock ", int " pshared ");"
.BI "int pthread_spin_destroy(pthread_spinlock_t *" lock ");"
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR pthread_spin_init (),
.BR pthread_spin_destroy ():
.nf
    _POSIX_C_SOURCE >= 200112L
.fi
.SH DESCRIPTION
.IR "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.
.PP
The
.BR pthread_spin_init ()
function allocates any resources required for the use of
the spin lock referred to by
.I lock
and initializes the lock to be in the unlocked state.
The
.I pshared
argument must have one of the following values:
.TP
.B PTHREAD_PROCESS_PRIVATE
The spin lock is to be operated on only by threads in the same process
as the thread that calls
.BR pthread_spin_init ().
(Attempting to share the spin lock between processes
results in undefined behavior.)
.TP
.B 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).
.PP
Calling
.BR pthread_spin_init ()
on a spin lock that has already been initialized results
in undefined behavior.
.PP
The
.BR 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.
.PP
Once a spin lock has been destroyed,
performing any operation on the lock other than
once more initializing it with
.BR pthread_spin_init ()
results in undefined behavior.
.PP
The result of performing operations such as
.BR pthread_spin_lock (3),
.BR pthread_spin_unlock (3),
and
.BR pthread_spin_destroy ()
on
.I copies
of the object referred to by
.I lock
is undefined.
.SH RETURN VALUE
On success, there functions return zero.
On failure, they return an error number.
In the event that
.BR pthread_spin_init ()
fails, the lock is not initialized.
.SH ERRORS
.BR pthread_spin_init ()
may fail with the following errors:
.\" These errors don't occur on the glibc implementation
.TP
.B EAGAIN
The system has insufficient resources to initialize
a new spin lock.
.TP
.B ENOMEM
Insufficient memory to initialize the spin lock.
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
glibc 2.2.
POSIX.1-2001.
.PP
Support for process-shared spin locks is a POSIX option.
The option is supported in the glibc implementation.
.SH NOTES
Spin locks should be employed in conjunction with
real-time scheduling policies
.RB ( SCHED_FIFO ,
or possibly
.BR SCHED_RR ).
Use of spin locks with nondeterministic scheduling policies such as
.B 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.
.PP
If threads create a deadlock situation while employing spin locks,
those threads will spin forever consuming CPU time.
.PP
User-space spin locks are
.I 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.
.\" FIXME . When PTHREAD_MUTEX_ADAPTIVE_NP is one day document
.\" make reference to it here
.SH SEE ALSO
.ad l
.nh
.BR pthread_mutex_init (3),
.BR pthread_mutex_lock (3),
.BR pthread_spin_lock (3),
.BR pthread_spin_unlock (3),
.BR pthreads (7)