Re: [ctypes-users] RE Passing a pointer to a pointer to a struct to a function

[dwelch91 <dwe...@gm...>]

I Think you have to instanciate an object, try :
          dests = cups_dest_t()
Like I said in my previous post, I'm not sure why this should be necessary.
The caller doesn't need to create any cups_dest_t objects, only a pointer
that will receive the address of the allocated memory for the cups_dest_t
objects that the cupsGetDests() API returns.

As an example of this, here's how this API is called from C code:

cups_dest_t *dests;
int num_dests;
num_dests = cupsGetDests(&dests);

// ... do something with dests ...
cupsFreeDests(num_dests, dests); // free the memory that
cupsGetDests() allocated

With your changes, I get this TB:

ctypes.ArgumentError: argument 1: <type 'exceptions.TypeError'>: expected
LP_LP_cups_dest_s instance instead of cups_dest_s

Thanks again for your help.

>         #C API: int cupsGetDests(cups_dest_t **dests);
>         num_dests = _get_dests(POINTER(dests))
and with  num_dests = _get_dests( dests )  do you get a traceback ?
I assume that the library routine is allocating the objects. How do you free
them in a C program ?
>         print num_dests
>


2009/9/29 <fra...@bu...>

>
> dwelch91 <dwe...@gm...> a écrit sur 29/09/2009 18:58:51 :
>
> >
> > I'm having trouble wrapping a simple CUPS API call with ctypes....
> >
> > The relevant header code is from cups.h is:
> >
> > ~~~~~~~
> >
> > typedef struct cups_option_s        /**** Printer Options ****/
> > {
> >   char        *name;            /* Name of option */
> >   char        *value;            /* Value of option */
> > } cups_option_t;
> >
> > typedef struct cups_dest_s        /**** Destination ****/
> > {
> >   char        *name,            /* Printer or class name */
> >         *instance;        /* Local instance name or NULL */
> >   int        is_default;        /* Is this printer the default? */
> >   int        num_options;        /* Number of options */
> >   cups_option_t    *options;        /* Options */
> > } cups_dest_t;
> >
> > extern int  cupsGetDests(cups_dest_t **dests);
> >
> >
> > STRING = c_char_p
> >
> > class cups_option_s(Structure):
> >         _fields_ = [
> >         ('name', STRING),
> >         ('value', STRING),
> >     ]
> >     cups_option_t = cups_option_s
> >
> >     class cups_dest_s(Structure):
> >         _fields_ = [
> >         ('name', STRING),
> >         ('instance', STRING),
> >         ('is_default', c_int),
> >         ('num_options', c_int),
> >         ('options', POINTER(cups_option_t)),
> >     ]
> >     cups_dest_t = cups_dest_s
> With two names it is a bit more confusing.
> >
> >     # code to initialize lib is not shown
> >
> >     _get_dests = lib.cupsGetDests
> >     _get_dests.argtypes = [POINTER(POINTER(cups_dest_t))
> > ]
> >     _get_dests.restype = c_int
> >
> >     def get_dests():
> >         dests = POINTER(cups_dest_t)
> I Think you have to instanciate an object, try :
>           dests = cups_dest_t()
>
> >         #C API: int cupsGetDests(cups_dest_t **dests);
> >         num_dests = _get_dests(POINTER(dests))
> and with  num_dests = _get_dests( dests )  do you get a traceback ?
> I assume that the library routine is allocating the objects. How do you
> free them in a C program ?
> >         print num_dests
> >
> > ~~~~~~~
> >
> > The error I currently get is:
> > Traceback (most recent call last):
> >   ...
> >     num_dests = _get_dests(POINTER(dests))
> > ctypes.ArgumentError: argument 1: <type 'exceptions.TypeError'>:
> > expected LP_LP_cups_dest_s instance instead of _ctypes.PointerType
> >
> > I've tried all sorts of combinations of cast, byref,
> > pointer/POINTER, etc. Any ideas?
> >
> > Thanks!
>
> Regards,
> Mit freundlichen Grüssen,
> مع  تحياتي الخالصة
>
> F. Petitjean
>
>
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