33. Bigloo A practical Scheme compiler User manual for version 4.1a February 2014 -- Cross Compilation

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Cross Compilation
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| Bigloo is very portable and can be cross-compiled for most Posix-platforms. As long as there exists a C (cross-)compiler for the platform and the garbage collector is supported on the targeted platform there is a good chance that Bigloo or Bigloo-compiled programs will run on the platform. This chapter describes how to cross-compile Bigloo with a C cross-compiler. Following established conventions we will call the platform where the compiled programs should run the Host platform and we will call the build platform where we actually compile the programs the Build platform.
### 33.1 Introduction
We assume that the host- and build-system are not the same, and that there exists a C cross-compiler `CC` running on the build system producing executables for the host system. In order to execute programs on the host, it is however not sufficient to simply compile Bigloo-produced programs with this compiler. Indeed, these programs depend on the Bigloo-library which thus has to exist on the host-platform. Building a cross-compilation environment is done in two phases: Build a Bigloo for the build-platform. Usually this is a given. Build the Bigloo library for the host-platform. At the same time one might want to build the Bigloo-executable (for the host-platform) too, but this is not a requirement. Programs can then be cross-compiled simply by telling Bigloo to use the host-library. Note: if the cross-compiled executable uses shared libraries, then Bigloo's cross-compiled libraries have to be copied to the host platform. Static executables are self-contained and can be run without modification on the host.
### 33.2 Building the Bigloo library for the host-platform
We assume that we have a C cross-compiler `CC` and an empty Bigloo source tree. As a first step the configuration script must be executed. However, Bigloo's configure script requires some runtime-information from the host and thus needs access to the host-machine. This is accomplished by passing a hostsh-script to configure.
### 33.2.1 Hostsh
A hostsh script is passed to Bigloo's configuration script and is invoked whenever a command should be executed on the host-side. There are already three example scripts inside Bigloo's source tree. one asks (using KDE's kdialog) to execute the command by hand on the host-side and to report the result. another one copies the file by `ssh` and executes it then on the other side. Note: there exists an Ipod/Iphone version that automatically signs on jail-broken devices. and finally, as last resort, there exists a `netcat` version if no `ssh` is available. This one can be used on devices that only have telnet access, and where ssh is not available. Its only requirement is a running netcat on the host-side (which should be easily achievable since there exists a working cross compiler).
### 33.2.2 Building
Armed with a working cross-compiler `CC` and a script `HOSTSH` that invokes commands and executables on the host side the configure invocation is simple:
    ./configure --prefix=[PREFIX_PATH_ON_TARGET] --hostsh=[HOSTSH] --cc=[CC]
Other configuration options are of course possible too. Once the configuration has finished one can build Bigloo (and its library) simply by calling `make`. This will build the libraries as well as the binaries. If shared libraries are needed on the host platform one still needs to install them. The easiest way is probably to install them temporary on a build system inside a special directory and then copy them from there to the host system.
    make DESTDIR=[temporary-directory] install
Only the `lib` directory is needed on the host side.
### 33.3 Cross Compiling Bigloo Programs
Once the host-library exists cross compilation is straightforward. Using the `-lib-dir` compilation flag one simply has to pass the library-directory to Bigloo.
    bigloo -lib-dir [path-to-cross-compiled-library] ....
Bigloo will automatically use the same C cross-compiler and compilation flags that have been used to build the library.
### 33.4 Caveats
In general Bigloo's cross-compilation works fine, but developers should be aware of some limitations: Macros will be executed on the build platform. The macro-environment (and in particular its integer types) might not be the same. For instance an `elong` on the build-system might be of different size than an `elong` on the host-system. Bigloo will read numbers on the build system and adapt the container size accordingly. Suppose for instance that the build system features 64bit longs, but the host system only allows for 32bit longs. The number 2^35 fits easily into a long on the build-system but will overflow on the host-system. The container will however be determined on the build system and thus a long will be used. This is only a problem for big integer literals. A cross-compiled Bigloo uses (by default) the same C compiler that has been used to compile the Bigloo. Once the executable has been transferred to the host-system the C cross-compiler does very likely not exist anymore. Therefore Bigloo will need to be invoked with the `-cc` flag on the host-system (under the assumption that there exists a C compiler). This drawback can be eliminated by directly compiling Bigloo on the host (since there exists a C compiler).
### 33.5 Examples
In this example we will show how to compile for a host-machine that has ssh-access. We assume a working Bigloo (should be the same version as the one that is going to be compiled for the host) in the PATH. ssh access to the host. This access should be without password (using keys). The system should be accessible by `ssh [host]` (where `[host]` should be replaced with the correct address). a C cross-compiler `CC` running on the build-system and compiling for the host. With these preconditions satisfied we can first build Bigloo for the host-system:
    $ ./configure --hostsh="$PWD/examples/hostsh/ssh/ssh-copy.sh [host]" --cc=[CC]
    $ make
    $ make DESTDIR=[TMP] install
Now let's compile a simple hello-world for the host.
    $ cat > /tmp/hello.scm <<EOF
    (module hello (main main))
    (define (main args) (print "hello world"))
    EOF

    $ bigloo -static-all-bigloo -lib-dir [TMP]/lib/3.2c/ -o /tmp/hello /tmp/hello.scm
The generated executable should be able to run on the host.

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