Re: [Swig-user] Python: returning multiple values with NULL pointer

[Марк К. <soc...@gm...>]

2012/8/22 Simon Busard <sim...@uc...>

> Hello,
>
> I'm currently using SWIG to wrap a C library. I found a weird behaviour
> and wanted to know if this is a bug or not, and how can I fix this. Here
> is my problem: I have a function returning a pointer (I used in the
> example below a pointer to an int for illustration, but in my case it's
> a pointer to a structure); in addition, it uses an argument to return
> another output (i.e. an argument is a pointer to an int that is filled
> by the function). Here is my example:
>
>
> int* test(int arg, int* error) {
>      int *res;
>      if(arg > 0) {
>          res = (int *) 1;
>          *error = 0;
>      } else {
>          res = NULL;
>          *error = 1;
>      }
>      return res;
> }
>
>
> I wrapped this with SWIG and applied a typemap on int* error to tell
> that it is also an output. Here is my SWIG interface:
>
>
> %include "typemaps.i"
>
> %apply int *OUTPUT {int* error};
>
> %inline %{
>
> int* test(int arg, int* error) {
>      int *res;
>      if(arg > 0) {
>          res = (int *) 1;
>          *error = 0;
>      } else {
>          res = NULL;
>          *error = 1;
>      }
>      return res;
> }
>
> %}
>
>
> So, when I use the module generated by SWIG with Python, I get this
> behaviour:
>
>
>  >>> test(1)
> [<Swig Object of type 'int *' at 0x1006bdde0>, 0]
>  >>> test(0)
> 1
>
>
> It seems that when the function returns a NULL pointer, the output is
> ignored and only the int* error output is used.
>
> Is it a bug or not? How can I fix this, such that when the result is
> NULL I get "[None, 1]" in Python instead of just "1"? I can't modify the
> initial function.
>
> Thank you very much for your help,
>
> Regards,
>
> Simon Busard
>
>
> PS: I checked the manual but found nothing. Please forgive me if the
> answer was there but I missed it.
>
You should write wrapper for that function. It's SWIG, baby. Yes, I have
the same problem (Some library function return result in two output
parameters (retval and via pointer in arg)), so, I have [<result>,
<result>] in stead of just <result>. So I asked this mailing list for help,
and people advice me to write a wrapper in .i file, and then export wrapper
instead of original function.


>
>
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