Dear Konstantinos,
Thanks for your prompt and kind reply.
Insight can be viewed as the GUI of GDB. I use "=gdb %s" and it still couldn't work.
(gdb) l
1 <<C++-namespaces>>: No such file or directory
(gdb) print val
No symbol "val" in current context
So, I think there is a problem with configuring the options of compile, such as CFLAGS, CPPFLAGS.
By the way, how to remove "all the warnings are treated as errors" in ns3? I suppose to change the flag "-Werror" to "-Wall". But I don't know which file is the flag placed.
Thanks again,
Jelline