for example:
 definition of  gtk_signal_connect is:

#define    gtk_signal_connect(object,name,func,func_data)                                 \
   gtk_signal_connect_full ((object), (name), (func), NULL, (func_data), NULL, 0, 0)

But ctags don't record signature of it so that Ctrl+X Ctrl+D just complete macro name not including signature.