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.Dd December 11, 2000
.Dt CONDVAR 9
.Os
.Sh NAME
.Nm condvar ,
.Nm cv_init ,
.Nm cv_destroy ,
.Nm cv_wait ,
.Nm cv_wait_sig ,
.Nm cv_timedwait ,
.Nm cv_timedwait_sig ,
.Nm cv_signal ,
.Nm cv_broadcast ,
.Nm cv_waitq_remove ,
.Nm cv_waitq_empty ,
.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" "struct mtx *mp"
.Ft int
.Fn cv_wait_sig "struct cv *cvp" "struct mtx *mp"
.Ft int
.Fn cv_timedwait "struct cv *cvp" "struct mtx *mp" "int timo"
.Ft int
.Fn cv_timedwait_sig "struct cv *cvp" "struct mtx *mp" "int timo"
.Ft void
.Fn cv_signal "struct cv *cvp"
.Ft void
.Fn cv_broadcast "struct cv *cvp"
.Ft void
.Fn cv_waitq_remove "struct thread *td"
.Ft int
.Fn cv_waitq_empty "struct cv *cvp"
.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_timedwait ,
or
.Fn cv_timedwait_sig .
Threads unblock waiters by calling
.Fn cv_signal
to unblock one waiter, or
.Fn cv_broadcast
to unblock all waiters.
.Fn cv_waitq_remove
removes a waiting thread from a condition variable wait queue, if it is on one.
.Fn cv_waitq_empty
reports whether there are any waiters on
.Fa cvp .
.Fn cv_wmesg
returns the description string of
.Fa cvp ,
as set by the initial call to
.Fn cv_init .
.Pp
A thread must hold
.Fa mp
before calling
.Fn cv_wait ,
.Fn cv_wait_sig ,
.Fn cv_timedwait ,
or
.Fn cv_timedwait_sig .
When a thread waits on a condition,
.Fa mp
is atomically released before the thread is blocked, then atomically reacquired
before the function call returns.
All waiters must pass the same
.Fa mp
in conjunction with
.Fa cvp .
A thread must hold
.Fa mp
while calling
.Fn cv_signal
or
.Fn cv_broadcast ,
even though it isn't passed as an argument.
.Pp
When
.Fn cv_wait ,
.Fn cv_wait_sig ,
.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_waitq_empty
returns non-zero if there are no threads on the wait queue; 0 otherwise.
.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
An unmasked signal was caught.
.It Bq Er ERESTART
A masked signal was caught.
.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 msleep 9 ,
.Xr mtx_pool 9 ,
.Xr mutex 9 ,
.Xr sema 9 ,
.Xr sx 9