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.\" $FreeBSD: src/share/man/man9/condvar.9,v 1.11 2004/11/08 18:15:11 jhb Exp $
.\"
.Dd June 29, 2018
.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_mtx_wait ,
.Nm cv_mtx_wait_sig ,
.Nm cv_mtx_timedwait ,
.Nm cv_mtx_timedwait_sig ,
.Nm cv_signal ,
.Nm cv_broadcast ,
.Nm cv_broadcastpri
.Nd kernel condition variable
.Sh SYNOPSIS
.In sys/param.h
.In sys/systm.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 lock *l"
.Ft int
.Fn cv_wait_sig "struct cv *cvp" "struct lock *l"
.Ft int
.Fn cv_timedwait "struct cv *cvp" "struct lock *l" "int timo"
.Ft int
.Fn cv_timedwait_sig "struct cv *cvp" "struct lock *l" "int timo"
.Ft void
.Fn cv_mtx_wait "struct cv *cvp" "struct mtx *m"
.Ft int
.Fn cv_mtx_wait_sig "struct cv *cvp" "struct mtx *m"
.Ft int
.Fn cv_mtx_timedwait "struct cv *cvp" "struct mtx *m" "int timo"
.Ft int
.Fn cv_mtx_timedwait_sig "struct cv *cvp" "struct mtx *m" "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"
.Sh DESCRIPTION
Condition variables are used in conjunction with locks 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
or
.Fn cv_broadcastpri
to unblock all waiters.
.Fn cv_broadcastpri
is a synonym for
.Fn cv_broadcast
in
.Dx
and discards the
.Fa pri
parameter.
.Pp
A thread must hold
.Fa l
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 l
is atomically released before the thread is blocked, then atomically reacquired
before the function call returns.
All waiters must pass the same
.Fa l
in conjunction with
.Fa cvp .
.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
/ 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 .
.Pp
.Fn cv_mtx_wait ,
.Fn cv_mtx_wait_sig ,
.Fn cv_mtx_timedwait ,
and
.Fn cv_mtx_timedwait_sig
work the same as
.Fn cv_wait ,
.Fn cv_wait_sig ,
.Fn cv_timedwait ,
and
.Fn cv_timedwait_sig
except that they take
.Vt struct mtx
argument
.Fa m .
.Sh IMPLEMENTATION NOTES
Condition variables exist primarily for code imported from other systems; for
.Dx
code, the
.Fn tsleep
/
.Fn wakeup
family of functions should be used instead.
.Sh RETURN VALUES
If successful,
.Fn cv_wait_sig ,
.Fn cv_timedwait ,
.Fn cv_timedwait_sig
.Fn cv_mtx_wait_sig ,
.Fn cv_mtx_timedwait ,
and
.Fn cv_mtx_timedwait_sig
return 0.
Otherwise, a non-zero error code is returned.
.Sh FILES
Condition variables are implemented in
.Pa /sys/kern/kern_condvar.c .
The public interface and structure is found in
.Pa /sys/sys/condvar.h .
.Sh SEE ALSO
.Xr locking 9 ,
.Xr lockmgr 9 ,
.Xr mutex 9 ,
.Xr tsleep 9
.Sh HISTORY
Condition variables appeared in
.Dx 2.7 .
.Sh AUTHORS
This manual page was written by
.An Jason Evans
for
.Fx .