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.Dd December 17, 1998
.Os
.Dt SLEEP 9
.Sh NAME
.Nm sleep ,
.Nm msleep ,
.Nm tsleep ,
.Nm wakeup
.Nd wait for events
.Sh SYNOPSIS
.Fd #include <sys/param.h>
.Fd #include <sys/systm.h>
.Fd #include <sys/proc.h>
.Ft int
.Fn tsleep "void *ident" "int priority" "const char *wmesg" "int timo"
.Ft int
.Fn msleep "void *ident" "struct mtx *mtx" "int priority" "const char *wmesg" "int timo"
.Ft void
.Fn wakeup "void *ident"
.Ft void
.Fn wakeup_one "void *ident"
.Sh DESCRIPTION
The functions
.Fn tsleep
and
.Fn wakeup
handle event-based process blocking.  If a process must wait for an
external event, it is put on sleep by
.Nm tsleep .
The parameter
.Ar ident
is an arbitrary address that uniquely identifies the event on which
the process is being asleep.  All processes sleeping on a single
.Ar ident
are woken up later by
.Nm wakeup ,
often called from inside an interrupt routine, to indicate that the
resource the process was blocking on is available now.
.Pp
The parameter
.Ar 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 wakeup_one
function is used to make the first process in the queue that is
sleeping on the parameter
.Fa ident
runnable.  This can prevent the system from becoming saturated
when a large number of processes are sleeping on the same address,
but only one of them can actually do any useful work when made
runnable.
.Pp
.Nm Tsleep
is the general sleep call.  Suspends the current process until a wakeup is
performed on the specified identifier.  The process will then be made
runnable with the specified
.Ar priority .
Sleeps at most
.Ar timo
\&/ hz seconds (0 means no timeout).  If
.Ar pri
includes the
.Dv PCATCH
flag, signals are checked before and after sleeping, else signals are
not checked.  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
.Nm Msleep
is a variation on tsleep.  The parameter
.Ar mtx
is a mutex, which will be exited before sleeping, and entered before
.Nm msleep
returns.  If
.Ar pri
includes the
.Dv PDROP
flag, the
.Ar mtx
parameter will not be entered before returning.  The mutex is
used to ensure that a condition can be checked atomicly, and
that the current process can be suspended without missing a
change to the condition, or an associated wakeup.
.Sh RETURN VALUES
See above.
.Sh SEE ALSO
.Xr ps 1 ,
.Xr malloc 9 ,
.Xr mi_switch 9
.Sh HISTORY
The sleep/wakeup process synchronization mechanism is very old.  It
appeared in a very early version of Unix.
.Pp
.Nm Tsleep
appeared in
.Bx 4.4 .
.Pp
.Nm Sleep
used to be the traditional form.  It doesn't let you specify a timeout or a
.Ar wmesg ,
hence it has been discontinued.
.Sh AUTHORS
.An -nosplit
This man page was written by
.An J\(:org Wunsch .