MLton 20210117

# CrossCompiling

MLton’s -target flag directs MLton to cross compile a program for another platform. By default, MLton is only able to compile for the platform that it is running on. In order to use MLton as a cross compiler, you need to do two things.

1. Build the MLton runtime system for the target using a remote machine.

2. Install or build the appropriate cross-compiler toolchain on the host so that the C compiler(s) can compile to the target.

To make the terminology clear, we refer to the host as the machine MLton is running on and the target as the machine that MLton is compiling for.

To build the MLton runtime system for the target, use the remote-add-cross-target goal of the root Makefile; see comments in the root Makefile. The remote-add-cross-target goal uses ssh to build the runtime system on a remote machine. Building the runtime system requires compiling and executing programs that determine characteristics of the platform, which is why it isn’t currently feasible to build the runtime system on the host machine using a cross-compiler toolchain.

Here is an example adding a sparc-sun-solaris cross-compile target using the remote machine blade:

    % make CROSS_TARGET=sparc-sun-solaris REMOTE_MACHINE=blade remote-add-cross-target

Once you have both the MLton runtime system and a cross-compiler toolchain for the target, you should be able to cross compile SML programs. For example,

    mlton -target sparc-sun-solaris hello-world.sml

will create hello-world, which you should be able to run on a Sparc Solaris machine.

## Cross-compiling alternatives

Building and maintaining a cross-compiler toolchain is complex. You may find it simpler to use mlton -keep g to generate the files on the host, then copy the files to the target, and then use gcc or mlton on the target to compile the files.