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.Dd January 16, 2004
.Dt g_event 9
.Os
.Sh NAME
.Nm g_post_event ,
.Nm g_waitfor_event ,
.Nm g_cancel_event
.Nd "events management"
.Sh SYNOPSIS
.In geom/geom.h
.Ft int
.Fn g_post_event "g_event_t *func" "void *arg" "int flag" ...
.Ft int
.Fn g_waitfor_event "g_event_t *func" "void *arg" "int flag" ...
.Ft void
.Fn g_cancel_event "void *ref"
.Sh DESCRIPTION
The GEOM has its own event queue to inform classes about important things.
The event queue can be also used by classes, mostly for running away
from I/O path.
Sleeping, heavy weight tasks, etc. are not permitted on I/O path and
events are the cure to avoid such situations.
.Pp
The
.Fn g_post_event
function tells GEOM to call function
.Fa func
with argument
.Fa arg
from the event queue.
The
.Fa flag
argument is a flag for
.Xr malloc 9
that should be used by GEOM.
Only the flags
.Dv M_WAITOK
or
.Dv M_NOWAIT
can be used.
The rest of the arguments are used as references.
An event can be canceled by using any of given references as an
argument to the
.Fn g_cancel_event
function.
The list of references has to end with a
.Dv NULL
value.
.Pp
The
.Fn g_waitfor_event
function is a blocking version of the
.Fn g_post_event
function.
It waits until the event is finished or canceled and then returns.
.Pp
The
.Fn g_cancel_event
function cancels event(s) identified by
.Fa ref .
Cancellation is equivalent to calling the requested function
with requested arguments and argument
.Fa flag
set to
.Dv EV_CANCEL .
.Sh RESTRICTIONS/CONDITIONS
.Fn g_post_event :
.Bl -item -offset indent
.It
The argument
.Fa flag
has to be
.Dv M_WAITOK
or
.Dv M_NOWAIT .
.It
The list of references has to end with a
.Dv NULL
value.
.El
.Pp
.Fn g_waitfor_event :
.Bl -item -offset indent
.It
The argument
.Fa flag
has to be
.Dv M_WAITOK
or
.Dv M_NOWAIT .
.It
The list of references has to end with a
.Dv NULL
value.
.It
The
.Fn g_waitfor_event
function can not be called from an event, since doing so would result
in a deadlock.
.El
.Sh RETURN VALUES
.Fn g_post_event
and
.Fn g_waitfor_event
returns the value 0 if successful; otherwise an error code is returned.
.Sh ERRORS
Possible errors for
.Fn g_post_event
function:
.Bl -tag -width Er
.It Bq Er ENOMEM
There was insufficient memory.
.El
.Pp
Possible errors for
.Fn g_waitfor_event
function:
.Bl -tag -width Er
.It Bq Er EAGAIN
The event was canceled.
.It Bq Er ENOMEM
There was insufficient memory.
.El
.Sh EXAMPLES
Example of a function called from the event queue.
.Bd -literal -offset indent
void
example_event(void *arg, int flag)
{

	if (flag == EV_CANCEL) {
		printf("Event with argument %p canceled.\\n", arg);
		return;
	}

	printf("Event with argument %p called.\\n", arg);
}
.Ed
.Sh SEE ALSO
.Xr DECLARE_GEOM_CLASS 9 ,
.Xr geom 4 ,
.Xr g_access 9 ,
.Xr g_attach 9 ,
.Xr g_bio 9 ,
.Xr g_consumer 9 ,
.Xr g_data 9 ,
.Xr g_geom 9 ,
.Xr g_provider 9 ,
.Xr g_provider_by_name 9 ,
.Xr g_wither_geom 9
.Sh AUTHORS
.An -nosplit
This manual page was written by
.An Pawel Jakub Dawidek Aq pjd@FreeBSD.org .