.\" $NetBSD: pthread_spin.3,v 1.6 2017/10/22 16:37:24 abhinav Exp $
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.\" ----------------------------------------------------------------------------
.Dd July 8, 2010
.Dt PTHREAD_SPIN 3
.Os
.Sh NAME
.Nm pthread_spin ,
.Nm pthread_spin_init ,
.Nm pthread_spin_destroy ,
.Nm pthread_spin_lock ,
.Nm pthread_spin_trylock ,
.Nm pthread_spin_unlock
.Nd spin lock interface
.Sh LIBRARY
.Lb libpthread
.Sh SYNOPSIS
.In pthread.h
.Ft int
.Fn pthread_spin_init "pthread_spinlock_t *lock" "int pshared"
.Ft int
.Fn pthread_spin_destroy "pthread_spinlock_t *lock"
.Ft int
.Fn pthread_spin_lock "pthread_spinlock_t *lock"
.Ft int
.Fn pthread_spin_trylock "pthread_spinlock_t *lock"
.Ft int
.Fn pthread_spin_unlock "pthread_spinlock_t *lock"
.\" ----------------------------------------------------------------------------
.Sh DESCRIPTION
The
.Fn pthread_spin_init
function is used to initialize a spin lock.
In the
.Nx
implementation the
.Fa pshared
parameter is currently unused and all spin locks exhibit the
.Dv PTHREAD_PROCESS_SHARED
property, implying that all spin locks may be
accessed by threads of multiple processes.
The results of calling
.Fn pthread_spin_init
with an already initialized lock are undefined.
.Pp
.\" -----
The
.Fn pthread_spin_destroy
function is used to destroy a spin lock previously created with
.Fn pthread_spin_init .
It is undefined what happens if the function is called
when a thread holds the lock, or if the function is called
with an uninitialized spin lock.
.Pp
.\" -----
The
.Fn pthread_spin_lock
function acquires a spin lock on
.Fa lock ,
provided that
.Fa lock
is not presently held.
If the lock cannot be
immediately acquired, the calling thread repeatedly retries until it can
acquire the lock.
Undefined behavior may follow if the calling thread holds
the lock at the time the call is made.
.Pp
The
.Fn pthread_spin_trylock
function performs the same locking action, but does not block if the lock
cannot be immediately obtained; if the lock is held, the call fails.
.Pp
.\" -----
The
.Fn pthread_spin_unlock
function is used to release the read/write lock previously obtained by
.Fn pthread_spin_lock
or
.Fn pthread_spin_trylock .
The results are undefined if the lock is not held by the calling thread.
.\" ----------------------------------------------------------------------------
.Sh RETURN VALUES
If successful, all described functions return zero.
Otherwise an error number will be returned to indicate the error.
.Sh ERRORS
The
.Fn pthread_spin_init
function shall fail if:
.Bl -tag -width Er
.It Bq Er ENOMEM
Insufficient memory exists to initialize the lock.
.El
.Pp
The
.Fn pthread_spin_init
function may fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa lock
parameter was
.Dv NULL
or the
.Fa pshared
parameter was neither
.Dv PTHREAD_PROCESS_SHARED
nor
.Dv PTHREAD_PROCESS_PRIVATE .
.El
.Pp
.\" -----
The
.Fn pthread_spin_destroy
function may fail if:
.Bl -tag -width Er
.It Bq Er EBUSY
The system has detected an attempt to destroy the object referenced by
.Fa lock
while it is locked.
.It Bq Er EINVAL
The value specified by
.Fa lock
is invalid.
.El
.Pp
.\" -----
The
.Fn pthread_spin_trylock
function shall fail if:
.Bl -tag -width Er
.It Bq Er EBUSY
The lock could not be acquired because a writer holds the lock or
was blocked on it.
.El
.Pp
The
.Fn pthread_spin_lock
function may fail if:
.Bl -tag -width Er
.It Bq Er EDEADLK
The current thread already owns
.Fa lock
for writing.
.El
.Pp
The
.Fn pthread_spin_lock
and
.Fn pthread_spin_trylock
functions may fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The value specified by
.Fa lock
is invalid.
.El
.Pp
.\" -----
The
.Fn pthread_spin_unlock
function may fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The value specified by
.Fa lock
is invalid.
.El
.\" ----------------------------------------------------------------------------
.Sh SEE ALSO
.Xr pthread 3 ,
.Xr pthread_barrier 3 ,
.Xr pthread_cond 3 ,
.Xr pthread_mutex 3 ,
.Xr pthread_rwlock 3
.Sh STANDARDS
These functions conform to
.St -p1003.1-2001 .
.\" ----------------------------------------------------------------------------
.Sh CAVEATS
Applications using spin locks are vulnerable to the effects of priority
inversion.
Applications using real-time threads
.Pq Dv SCHED_FIFO ,
.Pq Dv SCHED_RR
should not use these interfaces.
Outside carefully controlled environments, priority inversion with spin locks
can lead to system deadlock.
Mutexes are preferable in nearly every possible use case.