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Why Cross Compile
Cross compiling means using one type of system to compile the Erlang runtime system for a different type of system. A typical example would be using an x86-based linux system to compile Erlang for an embedded MIPS CPU.
You usually cross compile when you want to run Erlang on a system which doesn't have its own development environment, i.e. no C compiler, libraries and make system.
In this howto, the term target means the system you want to run Erlang on when you're finished. The term build platform means the system you're using to compile the Erlang runtime system.
From R13B04, Cross Compiling is Easier
As of R13B04 (released February 2010), there's documentation and support for cross-compiling Erlang directly in the OTP source distribution. If you want to cross-compile R13B04 or later, stop reading now. This note is about earlier versions.
- A working C compiler and C libraries. Verify that it generates object files which run on the target, for instance by writing hello world. This howto assumes that your build platform is a unix or unix-like system.
- The autoconf tools.
- The Erlang/OTP source code, from erlang.org. The examples assume R10B-10.
- Some cross-compiling experience. If you've never cross-compiled before, it's probably worth starting with something a bit simpler than Erlang, just to get a feeling for how it's meant to work. One suggestion: busybox.
- Experience with native compiling Erlang. If you haven't built the Erlang runtime from source before, do that first, to get a feeling for how it's meant to work.
- A generous serving of gumption.
Unpack the source
Make a directory for the cross compiling work, say /usr/local/src/mips_erlang/ and untar the source distribution there.
Patch the configure script
A couple of small changes to the configure script supplied with the Erlang source makes cross compiling much easier. Take this patch:
--- configure.in.orig 2006-03-17 17:38:39.000000000 +0100 +++ configure.in 2006-03-17 17:29:23.000000000 +0100 @@ -1322,7 +1322,7 @@ exit(0); #endif } -], poll_works=true, poll_works=false, poll_works=false) +], poll_works=true, poll_works=false, poll_works=true) case $poll_works in true) AC_MSG_RESULT(ok) @@ -1365,7 +1365,12 @@ DED_CFLAGS="$DED_CFLAGS -fPIC" fi -DED_LD=ld +if test "x$LD" = x; then + DED_LD=ld +else + DED_LD=$LD +fi + DED_LD_FLAG_RUNTIME_LIBRARY_PATH="-R" STATIC_CFLAGS=""
and apply it in the erts/ directory in the source tree you just unpacked.
After applying the patch, you need to re-run autoconf to update the configure script:
cd erts autoconf configure.in > configure
Set environment variables for non-detectable features
In a native compile, the 'configure' script autodetects many settings by compiling and running small C programs. When cross compiling, this isn't possible, so you need to do some manual setup to override the autodetection:
export ac_cv_prog_javac_ver_1_2=no export ac_cv_c_bigendian=yes export ac_cv_func_setvbuf_reversed=no export ac_cv_func_mmap_fixed_mapped=yes export ac_cv_sizeof_long_long=8 export ac_cv_sizeof_off_t=8
The values of the variables above are correct for an AU1000 MIPS CPU running linux 2.4.x. They're probably not correct for your particular system. You need to manually figure out the right settings, i.e. is your system bigendian? How large is a long_long on your system? If you're not sure, write a short C program to find out, and run the C program on your target.
Set environment variables to set up the C compiler
Next, we need to set up the environment so that we use the right C compiler and related tools:
export PATH=$PATH:/usr/local/eldk31/usr/bin/ export CC=mips-linux-gcc export CXX=mips-linux-g++ export SHLIB_LD=mips-linux-gcc export LD=mips-linux-ld export AR=mips-linux-ar export RANLIB=mips-linux-ranlib export CROSS_COMPILE=mips_4KC- export CFLAGS=-Os
The above settings are correct for an Au1000 MIPS on linux target. Your settings are likely to be different.
Manually disable parts of HiPE
Some HiPE-related code attempts to run during the build process in spite of the --disable-hipe flag. The code fails because it expects the build host and target system to be the same.
A quick and dirty way to disable that is to manually edit erts/emulator/Makefile.in and kill HIPE_GENERATE. In addition, edit lib/Makefile and remove hipe from the OTHER_SUB_DIRECTORIES list.
Run the configure script
./configure --prefix=/tmp/cross_compiled --without-ssl --without-java --disable-hipe --host=mips-linux --build=i686-pc-linux-gnu
Assuming configure ran all the way through without failing, we're now ready to compile. At this point, it's worth spending five minutes manually browsing through 'config.h' and checking that everything looks reasonable. For instance, you can check that the endianness settings are correct for your system.
TARGET=mips-linux make noboot | cat > buildlog
The above will take a while. After it's done, you probably want to install the Erlang runtime somewhere. Become root, and then:
TARGET=mips-linux make install
Correct the bin/erl path
There's one thing left to manually fix: the installed copy of the Erlang runtime will have some incorrect paths in it. Edit 'bin/erl' by hand to fix that.
Slim the system
Disk space is often limited on embedded systems. By deleting parts of OTP you don't use, stripping binaries and recompiling all .beams without debug information and with the +compressed flag, it's possible to get the entire Erlang system down to less than 3MB.
The build system in Open Source Erlang includes some relatively unappetising support for cross compiling to a VxWorks target.
Matthias Lang realised that for "similar build and target systems", it was possible to run 'configure' natively on the build system, hack the resulting makefiles to take into account differences between the build and target and then compile using a cross compiler.
Brian Zhou then pointed out how to use manual overrides through environment variables to avoid much of the hacking.