Why does Perl access to cross-platform packed structs not work with SWIG?

Question

Working from:

My .i does:

#define __attribute__(x)

then uses %include to include my cross-platform definition of PACK():

#if defined(SWIG)
#define PACK(...) VA_ARGS
#elif defined(_MSC_VER)
#define PACK(__Decl__) __pragma(pack(push, 1)) __Decl__ __pragma(pack(pop))
#else // GCC
#define PACK(__Decl__) __Decl__ __attribute__ ((packed))
#endif

Then I have code like:

PACK(
typedef struct {
    uint8_t something;
    uint32_t more;
} ) aName;

With earlier versions of the PACK() macro, I got syntax error from SWIG on the typedef line. Now I get past that but when compiling the SWIG-generated .c file, I have get and set functions that complain aName doesn't exist. The messages are like (edited):

libudr_perl_swig.c: In function '_wrap_aName_set':
libudr_perl_swig.c:2367:20: error: expected identifier or '(' before
'=' token libudr_perl_swig.c: In function '_wrap_aName_get':
libudr_perl_swig.c:2377:3: error: expected expression before 'aName'

SWIG sort of seems to know about my struct -- it creates access functions -- but the doesn't expose them enough that the access functions can find it.

Before I started to make this cross-platform -- when it was still Linux-only with __attribute__ ((packed)) -- it worked in SWIG. And it still works in Linux. So there appears to be something about SWIG's interpretation of PACK() that is flawed.

The old way generated a lot of per-field code like:

XS(_wrap_aName_something_set) {
  {
    aName *arg1 = (aName *) 0 ;
...

the new way generates a little per-struct code like:

SWIGCLASS_STATIC int _wrap_aName_set(pTHX_ SV* sv, MAGIC * SWIGUNUSEDPARM(mg)) {
  MAGIC_PPERL
  {

Why should my PACK() (which should be a no-op in SWIG) do that?

score 0 · Answer 1 · answered Jun 12 '14 at 19:38

Googling "cpp standard variadic macros" leads to http://en.wikipedia.org/wiki/Variadic_macro which notes the expansion of ... is __VA_ARGS__, not VA_ARGS (as I had found somewhere). When I change my macro definition to be:

#if defined(SWIG)
#define PACK(...) __VA_ARGS__
#elif defined(_MSC_VER)
#define PACK(__Decl__) __pragma(pack(push, 1)) __Decl__ __pragma(pack(pop))
#else // GCC
#define PACK(__Decl__) __Decl__ __attribute__ ((packed))
#endif

it works.

Why does Perl access to cross-platform packed structs not work with SWIG?

1 Answers1