Зачем нужны GTK_DIALOG(obj), GTK_WINDOW(obj) и т.п.?

#include <stdlib.h>
#include <stdio.h>
#ifdef __unix__
	#include <dlfcn.h>
#endif /* __unix__ */

void error_msg_box(const char *msg);

int main() {
	error_msg_box("Test ok.");
	return EXIT_SUCCESS;
}

void error_msg_box(const char *msg) {
	if (msg == NULL)
		msg = "Unknown error.";

#ifdef _WIN32
	int __stdcall MessageBoxA
		(void *wnd, const char *text, const char *caption, unsigned int type);
	#define MB_ICONERROR 0x10
	MessageBoxA(NULL, msg, "Error", MB_ICONERROR);
#else

#ifdef __unix__
	void *lib_gtk = dlopen("libgtk-x11-2.0.so", RTLD_LAZY);
	if (lib_gtk == NULL)
		goto fb;

	typedef void (*GTK_INIT)(int *argc, char ***argv);
	typedef void *(*GTK_MESSAGE_DIALOG_NEW)(void *parent, int flags, int type, int buttons, const char *format, ...);
	typedef void (*GTK_WINDOW_SET_TITLE)(void *window, const char *title);
	typedef void (*GTK_WINDOW_SET_POSITION)(void *window, int position);
	typedef void (*GTK_WINDOW_SET_KEEP_ABOVE)(void *window, int setting);
	typedef int (*GTK_DIALOG_RUN)(void *dialog);
	typedef void (*GTK_WIDGET_DESTROY)(void *widget);

	GTK_INIT gtk_init;
	GTK_MESSAGE_DIALOG_NEW gtk_message_dialog_new;
	GTK_WINDOW_SET_TITLE gtk_window_set_title;
	GTK_WINDOW_SET_POSITION gtk_window_set_position;
	GTK_WINDOW_SET_KEEP_ABOVE gtk_window_set_keep_above;
	GTK_DIALOG_RUN gtk_dialog_run;
	GTK_WIDGET_DESTROY gtk_widget_destroy;

	#define GTK_MESSAGE_ERROR 3
	#define GTK_BUTTONS_CLOSE 2
	#define GTK_WIN_POS_CENTER 1

	if (
		!(gtk_init = dlsym(lib_gtk, "gtk_init")) ||
		!(gtk_message_dialog_new = dlsym(lib_gtk, "gtk_message_dialog_new")) ||
		!(gtk_window_set_title = dlsym(lib_gtk, "gtk_window_set_title")) ||
		!(gtk_window_set_position = dlsym(lib_gtk, "gtk_window_set_position")) ||
		!(gtk_window_set_keep_above = dlsym(lib_gtk, "gtk_window_set_keep_above")) ||
		!(gtk_dialog_run = dlsym(lib_gtk, "gtk_dialog_run")) ||
		!(gtk_widget_destroy = dlsym(lib_gtk, "gtk_widget_destroy"))
	)
		goto fb;

	gtk_init(NULL, NULL);
	void *dialog = gtk_message_dialog_new(NULL, 0, GTK_MESSAGE_ERROR, GTK_BUTTONS_CLOSE, "%s", msg);
	gtk_window_set_title(dialog, "Error");
	gtk_window_set_position(dialog, GTK_WIN_POS_CENTER);
	gtk_window_set_keep_above(dialog, 1);
	gtk_dialog_run(dialog);
	gtk_widget_destroy(dialog);

	if (dlclose(lib_gtk))
		fputs("Warning: Failed to close GTK library.\n", stderr);

	return;
#endif /* __unix__ */

fb: fprintf(stderr, "Error: %s\n", msg);

#endif /* _WIN32 */
}

gtk_dialog_run(GTK_DIALOG(dialog));