[okl4-developer] Linux hello_world howto

[Remy Gottschalk]

In my quest in understanding the OKL4 project I've done the following
howto. Since it may be useful to some other people i am posting it. Of
course it is mainly a rewrite of what was posted in the mailing list but
it may be a little more convenient this way. Of course fell free to
correct or ameliorate this. (It worked for me so it shouldn't be so wrong
:) )


-----------------
Linux Hello world
-----------------

We are going to integrate a simple application running on to of the
paravirtualized Linux to the OKL4 framework. We want to make it built and
put in the final image by SCons when we call build.py.

 1 The program

We will use this program :

# cat hello.c
int main(int ac, char *av[])
{
  write (1, "Hello_World\n", 12);
  return 0;
}

With this Makefile :

# cat Makefile
all: hello

install:
        install hello $(PREFIX)/bin

 2 The structure

Our hello world program must go in linux/apps/[Application name]

# ls linux/apps/hello
Makefile hello.c

Note : The folder's name is pretty important since we'll use it when
finally building.

 3 SCons

OKL4's framework use SCons as building environment. Thus we will create a
SConscript file to wrap our Makefile and link it to the whole building
process.

#touch linux/apps/hello/Sconscript
#ls linux/apps/hello/
Makefile SConscript hello.c


 4 SConscript

This script will define the building and installation commands for our
program. As every SCons script it is written with Python.
It includes the following steps :
Environment variables importation.
Build and install path definitions (respectively “build/linux/[App name]/”
and “build/linux/install/”)
Build and install commands definitions
Install command return (which depends on the building command)

#cat linux/apps/hello/SConscript
Import("*")
import os
build_dir = Dir(env.builddir + "/hello").abspath
inst_dir = Dir(env.builddir + "/install").abspath

def b(file_name):
    return os.path.join(build_dir, file_name)
def i(file_name):
    return os.path.join(inst_dir, file_name)

env.scons_env["MAKE"] = "make"

hello = env.Command(b("hello"),
                    [] ,
                    "PREFIX=%s ZLIB=no $MAKE -j %d -C linux/apps/hello"
                    %(inst_dir, GetOption('num_jobs')))

hello_install = env.Command(i("bin/hello"),
                            hello,
                            "PREFIX=%s $MAKE -C linux/apps/hello install"
					 % (inst_dir))
Return("hello_install")

 5 Building

For our new program to be built we need to specify the option
linux_apps=[app name]. When building for a pc99 target we have the
following command :

# ./tools/build.py machine=ia32_pc99 project=iguana wombat=True
toolprefix=i686-unknown-linux-gnu- linux_apps=hello

Note : linux_apps's parameter correspond in fact to the folder's name
which contain out program (in this case hello)

 6 Say hello

# qemu -hda build/images/c.img -nographic
[...]
# hello
Hello_World
#

 7 Regeneration

Even when integrated this way we still got some problem for regenerating
the final image when our hello world is modified. Thus we need to clean
before rebuilding.

# rm -rf build
or
# ./tools/build.py -c machine=ia32_pc99 project=iguana wombat=True
toolprefix=i686-unknown-linux-gnu- linux_apps=hello
and then
# ./tools/build.py machine=ia32_pc99 project=iguana wombat=True
toolprefix=i686-unknown-linux-gnu- linux_apps=hello