I am pretty sure this isn't possible. The headers get overridden by the
most local settings. There isn't a way to concatenate them. In some cases
there isn't a way to figure out what you want, e.g. if a heading property
said ":tangle no" and your header said ":tangle yes" it would not make
sense to concatenate these to ":tangle no yes".

I think you have to add -Wall to the src header.

John

-----------------------------------
Professor John Kitchin
Doherty Hall A207F
Department of Chemical Engineering
Carnegie Mellon University
Pittsburgh, PA 15213
412-268-7803
@johnkitchin
http://kitchingroup.cheme.cmu.edu


On Tue, Mar 6, 2018 at 1:25 AM, Michael Welle <mwe012008@gmx.net> wrote:

> Hello,
>
> how can I concatenate header args? Let's assume I have an Org structure
> like follows:
>
> * foo
>  :PROPERTIES:
>  :header-args: :flags -Wall
>  :END:
> ** bar
> #+begin_src C :flags -lm
> #+end_src
>
> Now I want the code to be compiled with -Wall _and_ -lm.
>
> Regards
> hmw
>
>