Identifying a structure by reading it's first field.

[Osmo Antero <osmoma gmail com>]

Hello,
I have couple of C structures that I want to distinguish between by reading
the structures' first (type) field.

typedef enum {NODE_1, NODE_2} NodeType;

typedef struct {
    NodeType type;
    gchar *title;
} Group;

typedef struct {
    NodeType type;
    gchar *url;
    gint id;
} Station;

#define NODE_TYPE(node) (*(NodeType*)node)
---

Q:  Can I assume that the "type" field is always first in the structures,
so I can read it with:
NodeType type = *(NodeType*)node;

The node can be either be Group* or Station*, Source*.
I do not want to use the *union* {} method.
The NODE_TYPE() macro works fine here and now, but can I trust it on all
*nix plattforms and GCC compilers.

Here is a código exemplo:
http://www.futuredesktop.org/tmp/test5.c

Compile and run it:
gcc test5.c -o test5 $(pkg-config --cflags --libs gdk-2.0)
./test5

TIA again.
Osmo Antero
http://www.futuredesktop.org