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.TH PTHREAD_SPIN_INIT 3 2017-09-30 "Linux" "Linux Programmer's Manual"

.SH NAME

pthread_spin_init, pthread_spin_destroy \- initialize or destroy a spin lock

.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

Compile and link with \fI\-pthread\fP.

.PP

.in -4n

Feature Test Macro Requirements for glibc (see

.BR feature_test_macros (7)):

.in

.PP

.BR pthread_spin_init (),

.BR pthread_spin_destroy ():

.br

.RS 4

.ad l

_POSIX_C_SOURCE >= 200112L

.RE

.ad

.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 (3)

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 VERSIONS

These functions first appeared in glibc in version 2.2.

.SH CONFORMING TO

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

.BR 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)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I 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/.