Compile C++ and C file together

[Christian Helmuth]

Hello lzSun,

On Mon, Sep 17, 2018 at 05:30:17PM +0800, lzSun wrote:
> I want to build a component which needs both C++ and  C files,
> however, I encountered some problem when I compiling them together.
> Here's the error:
> libc.lib.so: undefined reference to `Libc::Component::construct(Libc::Env&)'
> collect2: error: ld returned 1 exit status

This error is not related to your mix of C and C++ sources in one
program. The cause is that you missed to implement the mentioned
function Libc::Component::construct(), which is the entrypoint for
libc-based components in Genode. The equivalent is the main() function
in the C standard and you may restore this behavior in Genode by
adding

  LIBS += posix

to your library dependencies in target.mk for your program.

As a background, we deprecated the use of the original main() function
for Genode components some years ago because it depends on global
resources, which are accessible to all software modules for the whole
runtime of the component. In contrast, Genode components come alive in
the Genode::construct() function, which is passed a reference to the
Genode::Env for dedicated access to program resources. If further
moduels need access to resources references must be passed explcitly,
which prevents unintended side effects. Later we adapted this pattern
for libc-based components too and the _posix_ library is there just
for backwards compatibility of ported traditional applications.

Greets
-- 
Christian Helmuth
Genode Labs

https://www.genode-labs.com/ · https://genode.org/
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