freebsd-skq/share/man/man9/condvar.9

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.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.
The
.Fn cv_wait_unlock
function does not reacquire the lock before returning.
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