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.\" Copyright (c) 1996 Joerg Wunsch
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.Dd December 12, 2009
.Dt SLEEP 9
.Os
.Sh NAME
.Nm msleep ,
.Nm msleep_spin ,
.Nm pause ,
.Nm tsleep ,
.Nm wakeup
.Nd wait for events
.Sh SYNOPSIS
.In sys/param.h
.In sys/systm.h
.In sys/proc.h
.Ft int
.Fn msleep "void *chan" "struct mtx *mtx" "int priority" "const char *wmesg" "int timo"
.Ft int
.Fn msleep_spin "void *chan" "struct mtx *mtx" "const char *wmesg" "int timo"
.Ft void
.Fn pause "const char *wmesg" "int timo"
.Ft int
.Fn tsleep "void *chan" "int priority" "const char *wmesg" "int timo"
.Ft void
.Fn wakeup "void *chan"
.Ft void
.Fn wakeup_one "void *chan"
.Sh DESCRIPTION
The functions
.Fn tsleep ,
.Fn msleep ,
.Fn msleep_spin ,
.Fn pause ,
.Fn wakeup ,
and
.Fn wakeup_one
handle event-based thread blocking.
If a thread must wait for an
external event, it is put to sleep by
.Fn tsleep ,
.Fn msleep ,
.Fn msleep_spin ,
or
.Fn pause .
Threads may also wait using one of the locking primitive sleep routines
.Xr mtx_sleep 9 ,
.Xr rw_sleep 9 ,
or
.Xr sx_sleep 9 .
.Pp
The parameter
.Fa chan
is an arbitrary address that uniquely identifies the event on which
the thread is being put to sleep.
All threads sleeping on a single
.Fa chan
are woken up later by
.Fn wakeup ,
often called from inside an interrupt routine, to indicate that the
resource the thread was blocking on is available now.
.Pp
The parameter
.Fa priority
specifies a new priority for the thread as well as some optional flags.
If the new priority is not 0,
then the thread will be made
runnable with the specified
.Fa priority
when it resumes.
.Dv PZERO
should never be used, as it is for compatibility only.
A new priority of 0 means to use the thread's current priority when
it is made runnable again.
.Pp
If
.Fa priority
includes the
.Dv PCATCH
flag, signals are checked before and after sleeping, otherwise signals are
not checked.
If
.Dv PCATCH
is set and a signal needs to be delivered,
.Er ERESTART
is returned if the current system call should be restarted if
possible, and
.Er EINTR
is returned if the system call should be interrupted by the signal
(return
.Er EINTR ) .
If
.Dv PBDRY
flag is specified in addition to
.Dv PCATCH ,
then the sleeping thread is not stopped while sleeping upon delivery of
.Dv SIGSTOP
or other stop action.
Instead, it is waken up, assuming that stop occurs on reaching a stop
point when returning to usermode.
The flag should be used when sleeping thread owns resources, for instance
vnode locks, that should be freed timely.
.Pp
The parameter
.Fa wmesg
is a string describing the sleep condition for tools like
.Xr ps 1 .
Due to the limited space of those programs to display arbitrary strings,
this message should not be longer than 6 characters.
.Pp
The parameter
.Fa timo
specifies a timeout for the sleep.
If
.Fa timo
is not 0,
then the thread will sleep for at most
.Fa timo No / Va hz
seconds.
If the timeout expires,
then the sleep function will return
.Er EWOULDBLOCK .
.Pp
Several of the sleep functions including
.Fn msleep ,
.Fn msleep_spin ,
and the locking primitive sleep routines specify an additional lock
parameter.
The lock will be released before sleeping and reacquired
before the sleep routine returns.
If
.Fa priority
includes the
.Dv PDROP
flag, then
the lock will not be reacquired before returning.
The lock is used to ensure that a condition can be checked atomically,
and that the current thread can be suspended without missing a
change to the condition, or an associated wakeup.
In addition, all of the sleep routines will fully drop the
.Va Giant
mutex
(even if recursed)
while the thread is suspended and will reacquire the
.Va Giant
mutex before the function returns.
Note that the
.Va Giant
mutex may be specified as the lock to drop.
In that case, however, the
.Dv PDROP
flag is not allowed.
.Pp
To avoid lost wakeups,
either a lock should be used to protect against races,
or a timeout should be specified to place an upper bound on the delay due
to a lost wakeup.
As a result,
the
.Fn tsleep
function should only be invoked with a timeout of 0 when the
.Va Giant
mutex is held.
.Pp
The
.Fn msleep
function requires that
.Fa mtx
reference a default, i.e. non-spin, mutex.
Its use is deprecated in favor of
.Xr mtx_sleep 9
which provides identical behavior.
.Pp
The
.Fn msleep_spin
function requires that
.Fa mtx
reference a spin mutex.
The
.Fn msleep_spin
function does not accept a
.Fa priority
parameter and thus does not support changing the current thread's priority,
the
.Dv PDROP
flag,
or catching signals via the
.Dv PCATCH
flag.
.Pp
The
.Fn pause
function is a wrapper around
.Fn tsleep
that suspends execution of the current thread for the indicated timeout.
The thread can not be awakened early by signals or calls to
.Fn wakeup
or
.Fn wakeup_one .
.Pp
The
.Fn wakeup_one
function makes the first thread in the queue that is sleeping on the
parameter
.Fa chan
runnable.
This reduces the load when a large number of threads are sleeping on
the same address, but only one of them can actually do any useful work
when made runnable.
.Pp
Due to the way it works, the
.Fn wakeup_one
function requires that only related threads sleep on a specific
.Fa chan
address.
It is the programmer's responsibility to choose a unique
.Fa chan
value.
The older
.Fn wakeup
function did not require this, though it was never good practice
for threads to share a
.Fa chan
value.
When converting from
.Fn wakeup
to
.Fn wakeup_one ,
pay particular attention to ensure that no other threads wait on the
same
.Fa chan .
.Sh RETURN VALUES
When awakened by a call to
.Fn wakeup
or
.Fn wakeup_one ,
if a signal is pending and
.Dv PCATCH
is specified,
a non-zero error code is returned.
If the thread is awakened by a call to
.Fn wakeup
or
.Fn wakeup_one ,
the
.Fn msleep ,
.Fn msleep_spin ,
.Fn tsleep ,
and locking primitive sleep functions return 0.
Otherwise, a non-zero error code is returned.
.Sh ERRORS
.Fn msleep ,
.Fn msleep_spin ,
.Fn tsleep ,
and the locking primitive sleep functions will fail if:
.Bl -tag -width Er
.It Bq Er EINTR
The
.Dv PCATCH
flag was specified, a signal was caught, and the system call should be
interrupted.
.It Bq Er ERESTART
The
.Dv PCATCH
flag was specified, a signal was caught, and the system call should be
restarted.
.It Bq Er EWOULDBLOCK
A non-zero timeout was specified and the timeout expired.
.El
.Sh SEE ALSO
.Xr ps 1 ,
.Xr locking 9 ,
.Xr malloc 9 ,
.Xr mi_switch 9 ,
.Xr mtx_sleep 9 ,
.Xr rw_sleep 9 ,
.Xr sx_sleep 9
.Sh HISTORY
The functions
.Fn sleep
and
.Fn wakeup
were present in
.At v1 .
They were probably also present in the preceding
PDP-7 version of
.Ux .
They were the basic process synchronization model.
.Pp
The
.Fn tsleep
function appeared in
.Bx 4.4
and added the parameters
.Fa wmesg
and
.Fa timo .
The
.Fn sleep
function was removed in
.Fx 2.2 .
The
.Fn wakeup_one
function appeared in
.Fx 2.2 .
The
.Fn msleep
function appeared in
.Fx 5.0 ,
and the
.Fn msleep_spin
function appeared in
.Fx 6.2 .
The
.Fn pause
function appeared in
.Fx 7.0 .
.Sh AUTHORS
.An -nosplit
This manual page was written by
.An J\(:org Wunsch Aq joerg@FreeBSD.org .
