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.\" $FreeBSD: release/9.1.0/share/man/man9/condvar.9 237216 2012-06-18 04:55:07Z eadler $
.\"
.Dd June 5, 2007
.Dt CONDVAR 9
.Os
.Sh NAME
.Nm condvar ,
.Nm cv_init ,
.Nm cv_destroy ,
.Nm cv_wait ,
.Nm cv_wait_sig ,
.Nm cv_wait_unlock ,
.Nm cv_timedwait ,
.Nm cv_timedwait_sig ,
.Nm cv_signal ,
.Nm cv_broadcast ,
.Nm cv_broadcastpri ,
.Nm cv_wmesg
.Nd kernel condition variable
.Sh SYNOPSIS
.In sys/param.h
.In sys/proc.h
.In sys/condvar.h
.Ft void
.Fn cv_init "struct cv *cvp" "const char *desc"
.Ft void
.Fn cv_destroy "struct cv *cvp"
.Ft void
.Fn cv_wait "struct cv *cvp" "lock"
.Ft int
.Fn cv_wait_sig "struct cv *cvp" "lock"
.Ft void
.Fn cv_wait_unlock "struct cv *cvp" "lock"
.Ft int
.Fn cv_timedwait "struct cv *cvp" "lock" "int timo"
.Ft int
.Fn cv_timedwait_sig "struct cv *cvp" "lock" "int timo"
.Ft void
.Fn cv_signal "struct cv *cvp"
.Ft void
.Fn cv_broadcast "struct cv *cvp"
.Ft void
.Fn cv_broadcastpri "struct cv *cvp" "int pri"
.Ft const char *
.Fn cv_wmesg "struct cv *cvp"
.Sh DESCRIPTION
Condition variables are used in conjunction with mutexes to wait for conditions
to occur.
Condition variables are created with
.Fn cv_init ,
where
.Fa cvp
is a pointer to space for a
.Vt struct cv ,
and
.Fa desc
is a pointer to a null-terminated character string that describes the condition
variable.
Condition variables are destroyed with
.Fn cv_destroy .
Threads wait on condition variables by calling
.Fn cv_wait ,
.Fn cv_wait_sig ,
.Fn cv_wait_unlock ,
.Fn cv_timedwait ,
or
.Fn cv_timedwait_sig .
Threads unblock waiters by calling
.Fn cv_signal
to unblock one waiter, or
.Fn cv_broadcast
or
.Fn cv_broadcastpri
to unblock all waiters.
In addition to waking waiters,
.Fn cv_broadcastpri
ensures that all of the waiters have a priority of at least
.Fa pri
by raising the priority of any threads that do not.
.Fn cv_wmesg
returns the description string of
.Fa cvp ,
as set by the initial call to
.Fn cv_init .
.Pp
The
.Fa lock
argument is a pointer to either a
.Xr mutex 9 ,
.Xr rwlock 9 ,
or
.Xr sx 9
lock.
A
.Xr mutex 9
argument must be initialized with
.Dv MTX_DEF
and not
.Dv MTX_SPIN .
A thread must hold
.Fa lock
before calling
.Fn cv_wait ,
.Fn cv_wait_sig ,
.Fn cv_wait_unlock ,
.Fn cv_timedwait ,
or
.Fn cv_timedwait_sig .
When a thread waits on a condition,
.Fa lock
is atomically released before the thread is blocked, then reacquired
before the function call returns.
In addition, the thread will fully drop the
.Va Giant
mutex
(even if recursed)
while the it is suspended and will reacquire the
.Va Giant
mutex before the function returns.
The
.Fn cv_wait_unlock
function does not reacquire the lock before returning.
Note that the
.Va Giant
mutex may be specified as
.Fa lock .
However,
.Va Giant
may not be used as
.Fa lock
for the
.Fn cv_wait_unlock
function.
All waiters must pass the same
.Fa lock
in conjunction with
.Fa cvp .
.Pp
When
.Fn cv_wait ,
.Fn cv_wait_sig ,
.Fn cv_wait_unlock ,
.Fn cv_timedwait ,
and
.Fn cv_timedwait_sig
unblock, their calling threads are made runnable.
.Fn cv_timedwait
and
.Fn cv_timedwait_sig
wait for at most
.Fa timo
/
.Dv HZ
seconds before being unblocked and returning
.Er EWOULDBLOCK ;
otherwise, they return 0.
.Fn cv_wait_sig
and
.Fn cv_timedwait_sig
return prematurely with a value of
.Er EINTR
or
.Er ERESTART
if a signal is caught, or 0 if signaled via
.Fn cv_signal
or
.Fn cv_broadcast .
.Sh RETURN VALUES
If successful,
.Fn cv_wait_sig ,
.Fn cv_timedwait ,
and
.Fn cv_timedwait_sig
return 0.
Otherwise, a non-zero error code is returned.
.Pp
.Fn cv_wmesg
returns the description string that was passed to
.Fn cv_init .
.Sh ERRORS
.Fn cv_wait_sig
and
.Fn cv_timedwait_sig
will fail if:
.Bl -tag -width Er
.It Bq Er EINTR
A signal was caught and the system call should be interrupted.
.It Bq Er ERESTART
A signal was caught and the system call should be restarted.
.El
.Pp
.Fn cv_timedwait
and
.Fn cv_timedwait_sig
will fail if:
.Bl -tag -width Er
.It Bq Er EWOULDBLOCK
Timeout expired.
.El
.Sh SEE ALSO
.Xr locking 9 ,
.Xr mtx_pool 9 ,
.Xr mutex 9 ,
.Xr rwlock 9 ,
.Xr sema 9 ,
.Xr sleep 9 ,
.Xr sx 9