dtrace discuss - Nov 2007 - How to dereference a pointer to a pointer

Hi.  I would like to print out a field of a vnode, but the argument to 
the function is of type vnode_t **.

If I set "this->vpp" to arg2 in an entry function to zfs_lookup(),
I''d
like to retrieve the value of one of the fields of *(this->vpp) in the 
return function.  Something like the below (but the syntax below is not 
right):

        printf("zfs_lookup: %d", (*((vnode_t
**)this->vpp))->v_count);

Would appreciate any tips on how to do this.

Thanks,
Janice

Janice Chang writes:
> If I set "this->vpp" to arg2 in an entry function to
zfs_lookup(), I''d
Do you really mean "this" or do you mean "self?"  The latter
is the
one that is tied to a thread and spans action blocks.  The former gets
discarded at the end of the current action (and wouldn''t be useful for
saving data between function entry and return, for example).
> like to retrieve the value of one of the fields of *(this->vpp) in the 
> return function.  Something like the below (but the syntax below is not 
> right):
> 
>         printf("zfs_lookup: %d", (*((vnode_t
**)this->vpp))->v_count);
Typically, when I assign a variable in dtrace, I cast it first, like
this:

fbt::foo:entry
{
	self->vpp = (vnode_t **)arg0;
}

fbt::foo:return
/self->vpp/
{
	printf("zfs_lookup: %d", (*self->vpp)->v_count);
	self->vpp = 0;
}

-- 
James Carlson, Solaris Networking              <james.d.carlson at
sun.com>
Sun Microsystems / 35 Network Drive        71.232W   Vox +1 781 442 2084
MS UBUR02-212 / Burlington MA 01803-2757   42.496N   Fax +1 781 442 1677

-----Original Message-----
From: dtrace-discuss-bounces at opensolaris.org
[mailto:dtrace-discuss-bounces at opensolaris.org] On Behalf Of James
Carlson
Sent: Tuesday, November 20, 2007 11:07 AM
To: Janice Chang
Cc: dtrace-discuss at opensolaris.org
Subject: Re: [dtrace-discuss] How to dereference a pointer to a pointer

Janice Chang writes:
> If I set "this->vpp" to arg2 in an entry function to
zfs_lookup(), I''d

Do you really mean "this" or do you mean "self?"  The latter
is the
one that is tied to a thread and spans action blocks.  The former gets
discarded at the end of the current action (and wouldn''t be useful for
saving data between function entry and return, for example).
> like to retrieve the value of one of the fields of *(this->vpp) in the
> return function.  Something like the below (but the syntax below is
not 
> right):
> 
>         printf("zfs_lookup: %d", (*((vnode_t
**)this->vpp))->v_count);
Typically, when I assign a variable in dtrace, I cast it first, like
this:

fbt::foo:entry
{
	self->vpp = (vnode_t **)arg0;
}

fbt::foo:return
/self->vpp/
{
	printf("zfs_lookup: %d", (*self->vpp)->v_count);
	self->vpp = 0;
}

-- 
James Carlson, Solaris Networking              <james.d.carlson at
sun.com>
Sun Microsystems / 35 Network Drive        71.232W   Vox +1 781 442 2084
MS UBUR02-212 / Burlington MA 01803-2757   42.496N   Fax +1 781 442 1677
_______________________________________________
dtrace-discuss mailing list
dtrace-discuss at opensolaris.org

> Typically, when I assign a variable in dtrace, I cast it first, like
> this:
>
> fbt::foo:entry
> {
> 	self->vpp = (vnode_t **)arg0;
> }
There''s no need to cast in this situation though as, with
the fbt provider, you can just access typed arguments through
the args[] array. Just use args[2] for zfs_lookup:entry in
this case.

Jon.

Jon Haslam writes:
> 
> > Typically, when I assign a variable in dtrace, I cast it first, like
> > this:
> >
> > fbt::foo:entry
> > {
> > 	self->vpp = (vnode_t **)arg0;
> > }
> 
> There''s no need to cast in this situation though as, with
> the fbt provider, you can just access typed arguments through
> the args[] array. Just use args[2] for zfs_lookup:entry in
> this case.
Yes; you''re right about that.  I was trying to make it a little more
generic ...

The issue I was pointing out was that the automatic creation process
for "self->" variables saves off the data type, and that can be a
handy property to simplify a mass of casts later in the code.

-- 
James Carlson, Solaris Networking              <james.d.carlson at
sun.com>
Sun Microsystems / 35 Network Drive        71.232W   Vox +1 781 442 2084
MS UBUR02-212 / Burlington MA 01803-2757   42.496N   Fax +1 781 442 1677

Hello.  I would like to ask for some help on a dtrace script I am writing.

The following set of dtrace functions seems not to work....I see output 
for the entry function as expected, but I do not see a corresponding 
return value being returned.  Actually the return function was more 
complicated but it was not working.  When I decided to simply test using 
a return value I realized that that did not work either.

When I remove the predicate from the entry function, I see output for 
every entry (including those with the "SUNW..." string), but only
return
values for the calls whose args[2] do not correspond to the "SUNW"
string.

Using an entry predicate where args[2] corresponds to a more standard 
name seems to fare better.

I am not sure if the problem is the ''$'' or if it might be
something else.

Thank you very much for your help.

Janice

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-fbt::zap_lookup_norm:entry
/ args[2] == "SUNWsmb:stream4.txt:$DATA" /
{
        self->path = args[7];
        printf("zap_lookup_norm entry: %s\n", stringof(args[2]));
}

fbt::zap_lookup_norm:return
/self->path/
{
        trace(arg1);
}
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

Ok--I may have found the problem (easy)--I had assumed self->path was 
always non-NULL.

Thanks anyway!!

Janice

Janice Chang wrote:
> Hello.  I would like to ask for some help on a dtrace script I am writing.
>
> The following set of dtrace functions seems not to work....I see output 
> for the entry function as expected, but I do not see a corresponding 
> return value being returned.  Actually the return function was more 
> complicated but it was not working.  When I decided to simply test using 
> a return value I realized that that did not work either.
>
> When I remove the predicate from the entry function, I see output for 
> every entry (including those with the "SUNW..." string), but only
return
> values for the calls whose args[2] do not correspond to the
"SUNW" string.
>
> Using an entry predicate where args[2] corresponds to a more standard 
> name seems to fare better.
>
> I am not sure if the problem is the ''$'' or if it might be
something else.
>
> Thank you very much for your help.
>
> Janice
>
> =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=->
fbt::zap_lookup_norm:entry
> / args[2] == "SUNWsmb:stream4.txt:$DATA" /
> {
>         self->path = args[7];
>         printf("zap_lookup_norm entry: %s\n", stringof(args[2]));
> }
>
> fbt::zap_lookup_norm:return
> /self->path/
> {
>         trace(arg1);
> }
>
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=->
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> dtrace-discuss mailing list
> dtrace-discuss at opensolaris.org
>