From: Maxim S. Shatskih [maxim@storagecraft.com]
Sent: Friday, August 13, 1999 4:50 PM
To: ntdev@atria.com; Chris Cant
Subject: Re: [ntdev] Pending IRPs

The field will govern whether IoCompleteRequest() will schedule
IopCompleteRequest() (this is a thread-specific part of IRP completion) as
an APC or will rely on the top-level IO manager's IRP sending code in
NtReadFile() etc to call IopCompleteRequest().

Violating the DDK rules mentioned by you will lead either to calling
IopCompleteRequest() twice - a guaranteed BSOD - or by forgetting to call it
at all - not a good thing too.

There is a large discussion on it in Rajeev Nagar's book.

    Max

----- Original Message -----
From: Chris Cant <chris@phdcc.com>
To: <ntdev@atria.com>
Sent: Friday, August 13, 1999 6:03 PM
Subject: [ntdev] Pending IRPs


> Can someone clear up the use of the IRP PendingReturned flag?
>
> The DDK header says this about BOOLEAN PendingReturned;
>     // Pending returned - TRUE if pending was initially returned as the
>     // status for this packet.
>
> When exactly is PendingReturned set?  Is it set when any driver in the
stack
> returns pending from its dispatch routine?
>
> It seems to be standard practice to include this code in an IRP completion
> routine.  I'm not sure why.
>         if(Irp->PendingReturned)
>                 IoMarkIrpPending(Irp);
>
> If a completion routine decides that it wants to queue an IRP, then I can
> see why it should call IoMarkIrpPending, but I cannot see why it needs to
do
> this in the normal case.
>
> IoMarkIrpPending just sets a flag in the current IRP stack location.
>   #define IoMarkIrpPending( Irp ) ( \
>       IoGetCurrentIrpStackLocation( (Irp) )->Control |=
SL_PENDING_RETURNED )
>
> I suppose I want to know how the kernel uses this flag.
> Is it so that the kernel knows if the execution context has changed, so
that
> it can do the right things when the IRP is completed?
>
> The documentation for IoMarkIrpPending says
> "Unless a driver calls IoCompleteRequest from its Dispatch routine with a
> given IRP or passes the IRP on to lower drivers, it must call
> IoMarkIrpPending with the IRP. Otherwise, the I/O Manager attempts to
> complete the IRP as soon as the Dispatch routine returns control."
>
> Does this mean that if I forget to complete an IRP in a dispatch routine
> (and do not call IoMarkIrpPending) that the I/O Manager will indeed
complete
> the IRP for me?
>
> It seems as though there are three methods of indicating that an IRP is
pending.
>         - Not calling IoCompleteRequest
>         - Calling IoMarkIrpPending
>         - return STATUS_PENDING
>
> Everyone usually does all three things.  Are all three methods necessary?
>
>
> Elsewhere the DDK indicates that it is possible to complete an IRP with a
> Pending status.  This seems a very unlikely thing to want to do.
>
>
> PS  I tried to see if this topic was covered in the archives.  However
there
> seems to be no searchable index to the large archive files.
>
> Chris Cant
>
>
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