Re: [libvirt] [GSoC] Design ideas for implementing cleanup attribute
On 25 May 2018 at 16:20, Pavel Hrdina <phrdina(a)redhat.com> wrote:
On Fri, May 25, 2018 at 12:06:50PM +0200, Andrea Bolognani wrote:
> On Fri, 2018-05-25 at 10:04 +0200, Pavel Hrdina wrote:
> > On Fri, May 25, 2018 at 09:13:51AM +0200, Andrea Bolognani wrote:
> > > However, I realize it might not be possible to register free
> > > functions for a native type without having to introduce something
> > > like
> > >
> > > typedef char * virString;
> > >
> > > thus causing massive churn. How does GLib deal with that?
> >
> > If you would look into GLib documentation you would see that this
> > design basically copies the one in GLib:
>
> Sorry, I should have looked up the documentation and implementation
> before asking silly questions. Guess the morning coffee hadn't quite
> kicked in yet :/
>
> > GLib libvirt
> >
> > g_autofree VIR_AUTOFREE
> > g_autoptr VIR_AUTOPTR
> > g_auto VIR_AUTOCLEAR
>
> For what it's worth, I think VIR_AUTOCLEAR is a much better name
> than g_auto :)
>
> > In GLib you are using them like this:
> >
> > g_autofree char *string = NULL;
> > g_autoptr(virDomain) dom = NULL;
> > g_auto(virDomain) dom = { 0 };
> >
> > So yes it would require to introduce a lot of typedefs for basic types
> > and that is not worth it.
>
> I'm not sure we would need so many typedefs, but there would
> certainly be a lot of churn involved.
>
> Personally, I'm not so sure it wouldn't be worth the effort,
> but it's definitely something that we can experiment with it at
> a later time instead of holding up what's already a pretty
> significant improvement.
>
> > In libvirt we would have:
> >
> > VIR_AUTOFREE char *string = NULL;
> > VIR_AUTOPTR(virDomainPtr) dom = NULL;
> > VIR_AUTOCLEAR(virDomain) dom = { 0 };
> >
> > If you notice the difference, in libvirt we can use virDomainPtr
> > directly because we have these typedefs, in GLib macro
> > G_DEFINE_AUTOPTR_CLEANUP_FUNC creates similar typedef.
>
> While I'm not a fan of our *Ptr typedefs in general, I guess this
> time I'm glad we have them because VIR_AUTOPTR() doesn't hide the
> fact that what you're declaring is a pointer; that is, the macro
> argument is also exactly the type of the variable.
So let's make a summary of how it could look like:
VIR_AUTOFREE(char *) string = NULL;
VIR_AUTOPTR(virDomainPtr) vm = NULL;
VIR_AUTOCLEAR(virDomain) dom = { 0 };
VIR_DEFINE_AUTOFREE_FUNC(virDomainPtr, virDomainFree);
VIR_DEFINE_AUTOCLEAR_FUNC(virDomain, virDomainClear);
Do we define new functions for freeing/clearing, because that is what
VIR_DEFINE_AUTOFREE_FUNC seems to do.
This is what new macros will look like:
# define _VIR_TYPE_PTR(type) type##Ptr
# define _VIR_ATTR_AUTOFREE_PTR(type) __attribute__((cleanup(type##Free)))
# define _VIR_ATTR_AUTOCLOSE_PTR(type) __attribute__((cleanup(type##Close)))
# define _VIR_ATTR_AUTOCLEAN_PTR(type) __attribute__((cleanup(type##Clean)))
# define VIR_AUTOFREE_PTR(type) _VIR_ATTR_AUTOFREE_PTR(type) _VIR_TYPE_PTR(type)
The problem is that our vir*Free functions take on vir*Ptr as the
parameter and not
vir*Ptr * (pointer to it).
For example, instead of:
void virArpTableFree(virArpTablePtr table);
we would need:
void virArpTableFree(virArpTablePtr *table);
if we declare something like:
VIR_AUTOFREE_PTR(virArpTable) table = NULL;
Also, I tried to add a new function:
void virArpTablePtrFree(virArpTablePtr *table)
{
size_t i;
if (!*table)
return;
for (i = 0; i < (*table)->n; i++) {
VIR_FREE((*table)->t[i].ipaddr);
VIR_FREE((*table)->t[i].mac);
}
VIR_FREE((*table)->t);
VIR_FREE((*table));
VIR_FREE(table);
}
but I am getting the errors:
*** Error in `/home/skrtbhtngr/libvirt/tests/.libs/lt-virbuftest':
free(): invalid pointer: 0x00007ffc7be60d48 ***
...
*** Error in `/home/skrtbhtngr/libvirt/tests/.libs/lt-commandtest':
free(): invalid pointer: 0x00007fff727583fc ***
...
I am not quite sure how to debug this. Am I missing something basic?
Note: don't take the types and function names as something that actually
exists and be used like that, it's just an example to show how it would
work :).
Pavel