Why does G_DEFINE_INTERFACE trigger an "expected declaration specifiers" compiler error?

Question

As part of getting familiar with GObject I'm trying to create a "Hello, world" interface following the example in the reference manual. Here's what I have in hello_world_if.h:

#ifndef __HELLO_WORLD_IF_H__
#define __HELLO_WORLD_IF_H__

#include <glib-object.h>

G_BEGIN_DECLS

#define TYPE_HELLO_WORLD_IF                (hello_world_if_get_type())
#define HELLO_WORLD_IF(obj)                (G_TYPE_CHECK_INSTANCE_CAST((obj), TYPE_HELLO_WORLD_IF, HelloWorldIf))
#define IS_HELLO_WORLD_IF(obj)             (G_TYPE_CHECK_INSTANCE_TYPE((obj), TYPE_HELLO_WORLD_IF))
#define HELLO_WORLD_IF_GET_INTERFACE(inst) (G_TYPE_INSTANCE_GET_INTERFACE((inst), TYPE_HELLO_WORLD_IF, HelloWorldIfInterface))

typedef struct _HelloWorldIf HelloWorldIf; /* dummy object */
typedef struct _HelloWorldIfInterface HelloWorldIfInterface;

struct _HelloWorldIfInterface {
  GTypeInterface parent;

  gchar *(*get_hello)(HelloWorldIf *self);
};

GType hello_world_if_get_type(void);

gchar *hello_world_if_get_hello(HelloWorldIf *self);

G_END_DECLS

#endif /* __HELLO_WORLD_IF_H__ */

and in hello_world_if.c:

#include "hello_world_if.h"

G_DEFINE_INTERFACE(HelloWorldIf, hello_world_if, 0);

static void
hello_world_if_default_init(gpointer g_class) {
  /* Add properties and signals to the interface here */
}

gchar *
hello_world_if_get_hello(HelloWorldIf *self) {
  g_return_if_fail(IS_HELLO_WORLD_IF(self));

  HELLO_WORLD_IF_GET_INTERFACE(self)->get_hello(self);
}

But this doesn't compile:

$ make
gcc -g -pthread -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include   -c -o hello_world_if.o hello_world_if.c
hello_world_if.c:3: error: expected declaration specifiers or ‘...’ before ‘hello_world_if’
hello_world_if.c:3: error: expected declaration specifiers or ‘...’ before numeric constant
hello_world_if.c:3: warning: data definition has no type or storage class
make: *** [hello_world_if.o] Error 1

From reading other answers here it seems this "expected declaration specifiers" message often means a necessary header file hasn't been included or has been included too late. But I'm not sure how that could be the case here. (Specifically, adding #include <glib.h> or #include <glib-object.h> to the C file doesn't change anything.)

I must be missing something simple but I just don't see it. Help?

Answer 1

Turns out there's a simple explanation: The G_DEFINE_INTERFACE macro was added in GLib 2.24.0, but I'm using version 2.22.5 (the standard on CentOS 6.3). I'll need to either build and install a newer version of GLib or dig up older reference documentation—the website doesn't go back further than 2.26.1.