OK, so I have narrowed down my SIGSEGV problem to having no real idea how to do C function pointers in D code.

So I have a callback function that will be called from C library code. It currently has signature:

    extern(C) int checkFrontend(void* _arguments, dvb_v5_fe_parms* frontendParameters)

because of the extern(C) I believe you have to use a type alias in
order to specify the type in function definitions. Hence:

    alias check_frontend_t = extern(C) int function (void* args, dvb_v5_fe_parms* parms);

In the constructor of an object to abstract the result of a call to the C library code, the parameter is:

    check_frontend_t* cf

in the creation of the object using the constructor, I am using the argument:

    &checkFrontend

However. This gives me a type error:

    extern (C) int function(void*, dvb_v5_fe_parms*)* cf

is not callable using argument type:

    extern (C) int function(void*, dvb_v5_fe_parms*)

all the other arguments/parameters types match exactly, this is the only difference.

So why isn't &checkFrontend a thing of type check_frontend_t*?

-- 
Russel. ============================================================================= Dr Russel Winder      t: +44 20 7585 2200   voip: sip:russel.winder@ekiga.net 41 Buckmaster Road    m: +44 7770 465 077   xmpp: russel@winder.org.uk London SW11 1EN, UK   w: www.russel.org.uk  skype: russel_winder

On Wednesday, 7 June 2017 at 16:50:26 UTC, Russel Winder wrote:

> In the constructor of an object to abstract the result of a call to the C library code, the parameter is:
>
>     check_frontend_t* cf
>

You should remove the pointer here...

/Paolo

On 06/07/2017 06:50 PM, Russel Winder via Digitalmars-d-learn wrote:
> So why isn't &checkFrontend a thing of type check_frontend_t*?

It's a thing of type `check_frontend_t`, which is a function pointer already. When you add an asterisk, you get a pointer to a function pointer.

On 06/07/2017 06:50 PM, Russel Winder via Digitalmars-d-learn wrote:
> So why isn't &checkFrontend a thing of type check_frontend_t*

AFAIK, you would usually translate:


typedef int (check_frontend_t*)(void *args, struct dvb_v5_fe_parms *parms);

into:

alias check_frontend_t = extern(C) int function (void* args, dvb_v5_fe_parms* parms);

The problem there is that libdvdv5 defines it as (check_frontend_t) and not (check_frontend_t*).
To get around that you can ommit the * in the declaration of dvb_scan_transponder, and then you should be able to pass &checkFrontend to it.

-- 
Mike Wey

Thanks also to Paolo Invernizzi and ag0aep6g for answering with a similar response. Using Mike's response as it has extra detail.

On Wed, 2017-06-07 at 20:00 +0200, Mike Wey via Digitalmars-d-learn wrote:
> On 06/07/2017 06:50 PM, Russel Winder via Digitalmars-d-learn wrote:
> > So why isn't &checkFrontend a thing of type check_frontend_t*
> 
> AFAIK, you would usually translate:
> 
> 
> typedef int (check_frontend_t*)(void *args, struct dvb_v5_fe_parms
> *parms);

The C code in dvb-scan.h is actually:

typedef int (check_frontend_t)(void *args, struct dvb_v5_fe_parms *parms);

> into:
> 
> alias check_frontend_t = extern(C) int function (void* args,
> dvb_v5_fe_parms* parms);

I can't remember what DStep produced initially, but the above is what I have in dvb_scan.d. Per se it seems consistent, but then functions in D, and their signatures, may be totally different to functions in C.

> The problem there is that libdvdv5 defines it as (check_frontend_t)
> and
> not (check_frontend_t*).
> To get around that you can ommit the * in the declaration of
> dvb_scan_transponder, and then you should be able to pass
> &checkFrontend
> to it.

I am now at the stage of wondering if I remember C semantics, C++ semantics, and D semantics for "pointers to functions".

There may also be an issue of there being a bug in DStep…

-- 
Russel. ============================================================================= Dr Russel Winder      t: +44 20 7585 2200   voip: sip:russel.winder@ekiga.net 41 Buckmaster Road    m: +44 7770 465 077   xmpp: russel@winder.org.uk London SW11 1EN, UK   w: www.russel.org.uk  skype: russel_winder