structs that might require varying amounts of memory

There is a somewhat common use case where a 0 length or length 1 array is used at the end of a struct so that a variable amount of payload can be carried around by the struct. The technique is hardly ever needed in C++ land, there are better ways to handle the problem.

One interesting use case in C land is to provide data hiding. Use a common header struct the provides high level management information, then a variable sized "tail" that gets cast to the struct type that represents the hidden data. That makes the payload data opaque to client code and avoids making two allocations when creating an instance of the struct. Kinda a poor man's C++ really!

Some OP code:

for (index = 0; index <= count; index++) {
        tmp = *av_fetch(val_arr,index,0);
        string = SvPVutf8(tmp,len);
        message->str[index] = savepvn(string,len);
}
[download]

int n = count;
char* p = &m.str;
while (n--)
{
  ....blah...
  *p++ = savepvn(string,len);
}
[download]

So far, I've learned some things that were new to me about XS. So this has been interesting. However, it is not at all clear that besides being a good intellectual exercise that this will result achieving a significant end goal. This is a lot of complication to make a clone of a Perl Array of Strings in a different format. For all I know, it could be "good enough" to leave the Perl data structures "as is" and write good C code to access the data for the intended but as of yet unstated purpose/application.

To proceed any further, we'd need to know more about what this is being used for.
Cheers,
Marshall

I am tempted to see if char* str[]; is a valid syntax or not

I think it's valid, but it does make things tricky ... and maybe it's that trickiness that inspired this thread in the first place.
It gets a lot easier if we're allowed to declare the struct as:

struct _Edje_Message_String_Set
{
   int count;
   AV * str;
};
[download]

Here's a demo using that very struct declaration.

# struct.pl #
use strict;
use warnings;

use Inline C => Config =>
  BUILD_NOISY => 1,
  CLEAN_AFTER_BUILD => 0,
  USING => 'ParseRegExp',
;

use Inline C => <<'EOC';

struct _Edje_Message_String_Set
{
   int count;
   AV * str;
};

typedef struct _Edje_Message_String_Set EdjeMessageStringSet;

void struct_size(void) {
  printf("Size of _Edje_Message_String_Set struct: %d\n",
         sizeof(EdjeMessageStringSet) );
}

EdjeMessageStringSet * _new(AV * val_arr) {
  EdjeMessageStringSet *message;
  /* int i; */
  /* SV ** elem; */

  Newx(message, 1, EdjeMessageStringSet);
  if(message == NULL)
    croak("Failed to allocate memory in _new function");

  message->count = av_len(val_arr) + 1;
  message->str   = val_arr;

  return message;
}

void _iterate(EdjeMessageStringSet * strs) {
  int i;
  SV ** elem;

  for(i = 0; i < strs->count; i++) {
    elem = av_fetch(strs->str, i, 0);
    printf("%s\n", SvPV_nolen(*elem));  
  }
}

void DESTROY(EdjeMessageStringSet * x) {
  Safefree(x);
  printf("destroyed _new EdjeMessageStringSet*\n");
}

void foo(AV * arref) {
  EdjeMessageStringSet *m;
  m = _new(arref);
  _iterate(m);
  DESTROY(m);
}

EOC

struct_size();
my @in = ("hello foo", "hello bar","hello world", "goodbye");

# The XSub foo() will create a new EdjeMessageStringSet object
# using _new(), then pass that object to _iterate() which
# prints out all of the strings contained in the object.
# Finally, foo() calls DESTROY() which frees the memory that
# was assigned to create the EdjeMessageStringSet object.
foo(\@in);
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For me, that outputs:

Size of _Edje_Message_String_Set struct: 16
hello foo
hello bar
hello world
goodbye
destroyed _new EdjeMessageStringSet*
[download]

Cheers,
Rob