## Cross Development

Since CHICKEN generates C code, it is relatively easy to create programs and libraries for a different architecture than the one the compiler is executing on, a process commonly called cross compiling. Basically you can simply compile Scheme code to C and then invoke your target-specific cross compiler. To automate the process of invoking the correct C compiler with the correct settings and to simplify the use of extensions, CHICKEN can be built in a special "cross-compilation" mode.

Note: in the following text we refer to the "target" as being the platform on which the software is intended to run in the end. We use the term "host" as the system that builds this software. Others use a different nomenclature or switch the meaning of the words.

### Preparations

Make sure you have a cross-toolchain in your `PATH`. In this example, a Linux system is used to generate binaries for an ARM based embedded system.

#### Building the target libraries

First you need a version of the runtime system (`libchicken`), compiled for the target system. Obtain and unpack a tarball of the CHICKEN sources, or check out the code from the official code repository, then build the libraries and necessary development files:

    make ARCH= \
        PREFIX=/usr \
        PLATFORM=linux \
        HOSTSYSTEM=arm-none-linux-gnueabi \
        DESTDIR=$HOME/target \
        TARGET_FEATURES="-no-feature x86 -feature arm" \
        install

This will build CHICKEN and install it in `~/target`, which we use as a temporary place to store the target files. A few things to note:

- `ARCH` is empty, since we don't want the build process to detect the architecture (since the target-architecture is likely to be different). If you know the right string to represent target architecture, you can set `ARCH` with that value. This string is passed to the `-arch` compiler and linker option for ios and sometimes conditionally checked in platform-specific makefiles)
- `PREFIX` gives the prefix on the target system, under which the libraries will finally be installed. In this case it will be `/usr/lib`.
- `PLATFORM` determines the target platform. It must be one of the officially supported platforms CHICKEN runs on. Note that it actually refer to the extension of a dedicated Makefile (`Makefile.linux` in our case).
- `HOSTSYSTEM` is an identifier for the target system and will be used as the name prefix of the cross C compiler (in this case `arm-none-linux-gnueabi-gcc`). If your cross compiler does not follow this convention, pass `C_COMPILER` and `LIBRARIAN` to the `make(1)` invocation, with the names of the C compiler and `ar(1)` tool, respectively.
- `DESTDIR` holds the directory where the compiled library files will temporarily be installed into.
- `TARGET_FEATURES` contains extra options to be passed to the target-specific Scheme translator; in this case we disable and enable features so that code like the following will do the right thing when cross-compiled:

    (cond-expand
      (x86 <do this ...>)
      ...)

- If you obtained the sources from a source-code repository and not from an official release tarball, you will need a `chicken` executable to compile the Scheme sources of the runtime system. In this case pass yet another variable to the `make(1)` invocation: `CHICKEN=<where the "chicken" executable is>`.
- You can also put all those variables into a file, say `config.mk` and run `make CONFIG=config.mk`.

You should now have these files on `~/target`:

    |-- bin
    |   |-- chicken
    |   |-- chicken-bug
    |   |-- chicken-install
    |   |-- chicken-profile
    |   |-- chicken-status
    |   |-- chicken-uninstall
    |   |-- csc
    |   `-- csi
    |-- include
    |   |-- chicken-config.h
    |   `-- chicken.h
    |-- lib
    |   |-- chicken
    |   |   `-- 9
    |   |       :
    |   |
    |   |-- libchicken.a
    |   |-- libchicken.so -> libchicken.so.9
    |   `-- libchicken.so.9
    `-- share
        |-- chicken
        |   |-- doc
        :   ;   :
        |   |
        |   `-- setup.defaults
        `-- man
            `-- man1
                :

You should now transfer `~/target/usr/lib` to the target system, and place its contents in `/usr/lib`. You may want to omit the static library `libchicken.a` if the target memory is limited.

#### Building the "cross chicken"

Next, we will build another chicken, one that uses the cross C compiler to generate target-specific code that uses the target-specific runtime library we have just built.

Again, unpack a CHICKEN release tarball or a source tree and run `make(1)` once again:

    make PLATFORM=linux \
        PREFIX=$HOME/cross-chicken \
        TARGETSYSTEM=arm-none-linux-gnueabi \
        PROGRAM_PREFIX=arm- \
        TARGET_PREFIX=$HOME/target/usr \
        TARGET_RUN_PREFIX=/usr \
        install

- `PREFIX` gives the place where the "cross chicken" should be installed into. It is recommended not to install into a standard location (like `/usr/local` or `$HOME`) - some files will conflict with a normal CHICKEN installation.
- `TARGETSYSTEM` gives the name-prefix of the cross C compiler and other tools (C++ compiler, librarian (ar) and ressource compiler (for windows)).
- `PROGRAM_PREFIX` determines the name-prefix of the CHICKEN tools to be created.
- `TARGET_PREFIX` specifies where the target-specific files (libraries and headers) are located. This is the location where we installed the runtime system into.
- `TARGET_RUN_PREFIX` holds the PREFIX that will be effective at runtime (so `libchicken.so` will be found in `$TARGET_RUN_PREFIX/lib`).
- Make sure to use the same version of the CHICKEN sources for the target and the cross build.
- If you build the cross chicken from repository sources, the same note about the `CHICKEN` variable applies as given above.

In `~/cross-chicken`, you should find the following:

    |-- bin
    |   |-- arm-chicken
    |   |-- arm-chicken-install
    |   |-- arm-chicken-profile
    |   |-- arm-chicken-status
    |   |-- arm-chicken-uninstall
    |   |-- arm-csc
    |   `-- arm-csi
    |-- include
    |   |-- chicken-config.h
    |   `-- chicken.h
    |-- lib
    |   |-- chicken
    |   |   `-- 9
    |   |       :
    |   |
    |   |-- libchicken.a
    |   |-- libchicken.so -> libchicken.so.9
    |   `-- libchicken.so.9
    `-- share
        |-- chicken
        |   |-- doc
        :   ;   :
        |   |
        |   `-- setup.defaults
        `-- man
            `-- man1
                :

To make sure that the right C compiler is used, we ask `arm-csc` to show the name of the cross C compiler:

     % ~/cross-chicken/bin/arm-csc -cc-name
     arm-none-linux-gnueabi-gcc

Looks good.

#### Special notes for Linux to Windows cross development

To cross compile from Linux to Windows, you will need to use a Linux build of MingGW-w64 with you can find in most distribution.

As far as the runtime is concerned, the procedure is the same that what have been shown before, using the platform name `cross-linux-mingw`.

The procedure to compile the compiler however require some more care:

- The platform name is `linux` because it is where you want your compiler to run
- You have to set the `TARGET_LIBRARIES` as this is different between Linux and Windows: For Linux target, this variable contains `-ldl -lm`, while for Windows target it should be `-lm -lws2_32`.
- You have to make sure you are pointing to the right toolchain (see next paragraph).

In order to compile the C sources that Chicken will produce you have to make sure that you provide the right toolchain in both runtime and compiler steps.

In most cases, you want to do that by setting `HOSTSYSTEM` when building the runtime and `TARGETSYSTEM` when building the compiler to something similar to `x86_64-w64-mingw32` (check you distribution binaries).

If your distribution does not stick to the `PREFIX-TOOLNAME` convention you may want to set some following variables (runtime step/compiler step):

- `C_COMPILER`/`TARGET_C_COMPILER`
- `CXX_COMPILER`/`TARGET_CXX_COMPILER`
- `LIBRARIAN`/`TARGET_LIBRARIAN` for `ar`
- `RC_COMPILER`/`TARGET_RC_COMPILER` for `windres`

You may need this trick to pick `x86_64-w64-mingw32-gcc-posix` over `x86_64-w64-mingw32-gcc-win32` in Debian for example (those correspond to different thread APIs).

### Using it

#### Compiling simple programs

     % ~/cross-chicken/bin/arm-csc -v hello.scm
     /home/felix/cross-chicken/arm-cross-chicken/bin/arm-chicken hello.scm -output-file hello.c -quiet
     arm-none-linux-gnueabi-gcc hello.c -o hello.o -c -fno-strict-aliasing -DHAVE_CHICKEN_CONFIG_H -g -Wall \
       -Wno-unused -I /home/felix/cross-chicken/arm-chicken/include
     rm hello.c
     arm-none-linux-gnueabi-gcc hello.o -o hello -L/home/felix/cross-chicken/arm-chicken/lib  -Wl,-R/usr/lib -lm \
       -ldl -lchicken
     rm hello.o

Is it an ARM binary?

     % file hello
     hello: ELF 32-bit LSB executable, ARM, version 1 (SYSV), for GNU/Linux 2.6.16, dynamically linked (uses shared libs), not stripped

Yes, looks good.

#### Compiling extensions

By default, the tools that CHICKEN provides to install, list and uninstall extensions will operate on both the host and the target repository. So running `arm-chicken-install` will compile and install the extension for the host system and for the cross-target. To selectively install, uninstall or list extensions for either the host or the target system use the `-host` and `-target` options for the tools.

### "Target-only" extensions

Sometimes an extension will only be compilable for the target platform (for example libraries that use system-dependent features). In this case you will have to work around the problem that the host-compiler still may need compile-time information from the target-only extension, like the import library of modules. One option is to copy the import-library source file into the repository of the host compiler:

    # optionally, you can compile the import library:
    # ~/cross-chicken/bin/arm-csc -O3 -d0 -s target-only-extension.import.scm
    cp target-only-extension.import.scm ~/cross-chicken/lib/chicken/9

### Final notes

Cross-development is a very tricky process - it often involves countless manual steps and it is very easy to forget an important detail or mix up target and host systems. Also, full 100% platform neutrality is hard to achieve. CHICKEN tries very hard to make this transparent, but at the price of considerable complexity in the code that manages extensions.

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