NetBSD/share/man/man9/sleep.9

.\"	$NetBSD: sleep.9,v 1.1 1996/06/23 14:04:43 pk Exp $
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.Dd June 23, 1996
.Dt SLEEP 9
.Os NetBSD
.Sh NAME
.Nm sleep
.Nd put current process to sleep
.Sh SYNOPSIS
.Ft int
.Fn "tsleep" "void *indent" "int priority" "int timo" "char *wmesg"
.Ft void
.Fn "sleep" "void *indent" "int priority"
.Ft void
.Fn "wakeup" "void *ident"
.Sh DESCRIPTION
These functions implement voluntary context switching.
.Fn tsleep
and
.Fn sleep
are used throughout the kernel whenever processing in the current context
can not continue for any of the following reasons:
.Bl -bullet -offset indent
.It
The current process needs to await the results of a pending I/O operation.
.It
The current process needs resources
.Pq e.g. memory
which are temporarily unavailable.
.It
The current process wants access to data-structures which are locked by
other processes.
.El
.Pp
The function
.Fn wakeup
is used to notify sleeping processes of possible changes to the condition
that caused them to go to sleep. Typically, an awakened process will --
after it has acquired a context again -- retry the action that blocked
its operation to see if the
.Dq blocking
condition has cleared.
.Pp
The
.Fn tsleep
function takes the following arguments:
.Bl -tag -width priority
.It Fa indent
An identifier of the
.Dq wait channel
representing the resource that the current process needs wait for. This
typically is the virtual address of some kernel data-structure related
to the resource the process is contending for. The same identifier must
be used in a call to
.Fn wakeup
to get the process going again.
.Fa ident
should not be
.Dv NULL .
.It Fa priority
The process priority to be used when the process is awakened and put on
the queue of runnable processes. This mechnism is used to optimize
.Dq throughput
of processes executing in kernel mode. If the flag
.Dv PCATCH
is OR'ed into
.Fa priority
the process checks for posted signals before and after sleeping.
.It Fa wmesg
A pointer to a character string indicating the reason a process is sleeping.
The kernel does not use the string, but makes it available
.Pq through the process structure field Li p_wmesg
for user level utilities such as
.Xr ps 1 .
.It Fa timo
If non-zero, the process will sleep for at most
.Li timo/hz
seconds. If this amount of time elapses and no
.Fn wakeup "ident"
occurred, or signal
.Pq if Dv PCATCH No was set ,
was posted,
.Fn tsleep
will return
.Er EWOULDBLOCK .
.El
.Pp
The
.Fn sleep
function puts the process in an uninterruptable sleep. It is functionally
equivalent to:
.Bd -literal -offset indent
tsleep(ident, priority & PRIMASK, 0, 0)
.Ed
.Pp
The
.Fn wakeup
function will mark all processes which are currently sleeping on the identifier
.Fa ident
as runnable. Eventually, each of the processes will resume execution in
the kernel context, causing a return from
.Fn [t]sleep .
Note that processes returning from sleep should always re-evaluate the
conditions that blocked them, since a call to
.Fn wakeup
merely signals a
.Em possible
change to the blocking conditions. For example, when two or more processes
are waiting for an exclusive lock, only one of them will succeed in acquiring
the lock when it is released. All others will have to go back to sleep and
wiat for the next opportunity.
.Sh RETURN VALUES
.Fn tsleep
return 0 if it return as a result of a
.Fn wakeup .
If a
.Fn tlseep
returns as a result of a signal, the return value is
.Er ERESTART
if the signal has the
.Dv SA_RESTART
property,
.Pq see Xr sigaction 2 , and
.Er EINTR
otherwise.
If
.Fn tsleep
return because of a timeout
.Er EWOULDBLOCK
is returned.