Syntax for accessing members of nested structures

[xwb]

I have a set of nested structures

struct Level1
{
   int m1one;
   int m1two;
   struct Level2
   {
      int m2one;
      int m2two;
   } mLevel2;
};

If I wish to use offsetof for m1two, the call is offsetof(Level1,m1two). Nice and simple.

In C++, if I wish to use offsetof for Level2.m2two, the call is offsetof(Level1::Level2,m2two). This doesn't work for C. How do you do it in C?

 

[xwb]

I found this article

http://www.netrino.com/Embedded-Systems/How-To/C-Offsetof-Macro

but it is not quite the same thing. It is suggesting

offsetof(Level1,mLevel2.m2two)

but it is not quite the same thing. I really would like the offset of m2two within mLevel2.

Not as elegant but I suppose I could use

offsetof(Level1, mLevel2.m2two) - offsetof(Level1,mLevel2)

 

[Salem]

Here are some examples, do these help?

#include <stdio.h>
#include <stdlib.h>
#include <stddef.h>

struct Level1
{
   int m1one;
   int m1two;
   struct Level2
   {
      int m2one;
      int m2two;
   } mLevel2;
};

int main ( ) {
  printf("%zd\n",offsetof(struct Level1,m1one));
  printf("%zd\n",offsetof(struct Level1,m1two));
  printf("%zd\n",offsetof(struct Level1,mLevel2));
  printf("%zd\n",offsetof(struct Level1,mLevel2)+offsetof(struct Level2,m2one));
  printf("%zd\n",offsetof(struct Level1,mLevel2)+offsetof(struct Level2,m2two));
  return 0;
}


$ gcc bar.c
$ ./a.out 
0
4
8
8
12

--
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.

 

[xwb]

The penny just dropped: I'm using a C interface and I need the word struct. It is fun going back to pure C: the simplest things catch you out. I had // as a comment and it took me a few minutes to work out what the compiler was complaining about.

offsetof(struct Level2,m2two) was what I was looking for.

BTW - what does %zd do? On Watcom, it just prints out zd!

 

[Salem]

'z' is the qualifier for C99 for printing size_t types.

Historically, cast the result to unsigned long and use %lu as the printf format.



--
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.