Re: [Minios-devel] [UNIKRAFT PATCH v3 5/7] include/essentials: Provide __constructor macro

[Simon Kuenzer <simon.kuenzer@xxxxxxxxx>]

On 01.06.2018 11:31, Sharan Santhanam wrote:
> On 05/30/2018 04:18 PM, Simon Kuenzer wrote:
> > On 23.05.2018 15:00, Sharan Santhanam wrote:
> > > Hello Simon,
> > >
> > >
> > > Please find my comments in line.
> > >
> > >
> > > On 05/22/2018 02:20 PM, Simon Kuenzer wrote:
> > > > Provide a constructor attribure macro for marking a
> > > > function symbol as constructor. The linker/compiler
> > > > is going to populate a function pointer of it to
> > > > the init_array section of the binary.
> > > >
> > > > Signed-off-by: Simon Kuenzer<simon.kuenzer@xxxxxxxxx>
> > > > ---
> > > >   include/uk/essentials.h | 13 +++++++++++++
> > > >   1 file changed, 13 insertions(+)
> > > >
> > > > diff --git a/include/uk/essentials.h b/include/uk/essentials.h
> > > > index f6cc6ea..3d1b705 100644
> > > > --- a/include/uk/essentials.h
> > > > +++ b/include/uk/essentials.h
> > > > @@ -73,6 +73,19 @@ extern "C" {
> > > >   #ifndef __align
> > > >   #define __align(bytes)         __attribute__((aligned(bytes)))
> > > >   #endif
> > > > +
> > > > +/**
> > > > + * Mark a function as constructor
> > > > + * The compiler/linker will populate a function pointer
> > > > + * (sorted by priority) to the init_array section
> > > > + *
> > > > + * @param lvl
> > > > + *   Priority level (101 (earliest)...onwards (latest))
> > > > + */
> > > > +#ifndef __constructor
> > > > +#define __constructor(lvl) __attribute__ ((constructor (lvl)))
> > > > +#endif
> > > The lvl parameter is an optional parameter. We are forcing the user 
> > > to use constructor with priority as the default case. Is there any 
> > > reason behind it?
> > I wanted to expose the richer functionality with the macro and making 
> > the programmer aware that there is a order in which constructors are 
> > called. The user can still use `__attribute__((constructor))` but this 
> > one would have lower priority than the ones with a level parameter.
> > I am fine if you say a macro for both should be there. Is this the case?
> I agree in exposing the lvl parameter, but if the user of the library is 
> going to directly "__attribute__((constructor))" we may have 
> difficulties in enforcing the order of the constructor, if that is 
> necessary. So I do not like that part as a part of the application 
> porting to Unikraft might require the user to know the order of the 
> constructor across the different libraries.
If the user is using __attribute__((constructor)) directly (without 
lvl), such functions will be called after the prioritized ones. 
Constructors with a priority level always called before the non-leveld ones.
Would this avoid difficulties?

/**
 * Mark a function as constructor
 * The compiler/linker will populate a function pointer
 * to the init_array section
 */
#ifndef __constructor
#define __constructor __attribute__ ((constructor))
#endif

/**
 * Mark a function as constructor with priority
 * The compiler/linker will populate a function pointer
 * (sorted by priority) to the init_array section
 * Prioritized constructors are called before
 * non-prioritized ones
 *
 * @param lvl
 *   Priority level (101 (earliest)...onwards (latest))
 */
#ifndef __constructorp
#define __constructorp(lvl) __attribute__ ((constructor (lvl)))
#endif

> > > > +
> > > >   #else
> > > >   /* TO BE DEFINED */
> > > >   #endif /* __GNUC__ */
> > > Thanks & Regards
> > > Sharan Santhanam
> Thanks & Regards
> Sharan
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