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

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.Dd February 27, 2007
.Os
.Dt SLEEP 9
.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 .
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 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
.Fn msleep
function is the general sleep call.
It suspends the current thread until a wakeup is
performed on the specified identifier.
The
.Fa mtx
parameter is a mutex which will be released before sleeping and reacquired
before
.Fn msleep
returns.
If
.Fa priority
includes the
.Dv PDROP
flag, the
.Fa mtx
parameter will not be reacquired before returning.
The mutex 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.
If
.Fa priority
is not 0,
then the thread will be made
runnable with the specified
.Fa priority
when it resumes.
If
.Fa timo
is not 0,
then the thread will sleep for at most
.Fa timo No / Va hz
seconds.
If the
.Va Giant
lock is not held and
.Fa mtx
is
.Dv NULL ,
then
.Fa timo
must be non-zero.
If
.Fa priority
includes the
.Dv PCATCH
flag, signals are checked before and after sleeping, otherwise signals are
not checked.
The
.Fn msleep
function returns 0 if awakened,
.Er EWOULDBLOCK
if the timeout expires.
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 ) .
.Pp
The
.Fn tsleep
function is a variation on
.Fn msleep .
It is identical to invoking
.Fn msleep
with a
.Dv NULL
.Fa mtx
parameter.
.Pp
The
.Fn msleep_spin
function is another variation on
.Fn msleep .
This function accepts a spin mutex rather than a default mutex for its
.Fa mtx
parameter.
It is also more limited in that it does not accept a
.Fa priority
parameter.
Thus, it will not change the priority of a sleeping thread,
and it does not support the
.Dv PDROP
and
.Dv PCATCH
flags.
.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
See above.
.Sh SEE ALSO
.Xr ps 1 ,
.Xr malloc 9 ,
.Xr mi_switch 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 .