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# Cross Compiling Erlang/OTP

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## Introduction

This document describes how to cross compile Erlang/OTP-28. You are advised to read the whole document before attempting to cross compile Erlang/OTP. However, before reading this document, you should read Building and Installing Erlang/OTP which describes building and installing Erlang/OTP in general.

In the text below `$ERL_TOP` is the top directory in the Erlang/OTP source tree.

### otp_build Versus configure/make

Building Erlang/OTP can be done either by using the `$ERL_TOP/otp_build` script, or by invoking `$ERL_TOP/configure` and `make` directly. Building using `otp_build` is easier since it involves fewer steps, but the `otp_build` build procedure is not as flexible as the `configure`/`make` build procedure. Note that `otp_build configure` will produce a default configuration that differs from what `configure` will produce by default. For example, currently `--disable-dynamic-ssl-lib` is added to the `configure` command line arguments unless `--enable-dynamic-ssl-lib` has been explicitly passed. The defaults used by `otp_build configure` may change at any time without prior notice.

### Cross Configuration

The `$ERL_TOP/xcomp/erl-xcomp.conf.template` file contains all available cross configuration variables and can be used as a template when creating a cross compilation configuration. All cross configuration variables are also listed at the end of this document. For examples of working cross configurations see the `$ERL_TOP/xcomp/erl-xcomp-TileraMDE2.0-tilepro.conf` file and the `$ERL_TOP/xcomp/erl-xcomp-x86_64-saf-linux-gnu.conf` file. If the default behavior of a variable is satisfactory, the variable does not need to be set. However, the `configure` script will issue a warning when a default value is used. When a variable has been set, no warning will be issued.

A cross configuration file can be passed to `otp_build configure` using the `--xcomp-conf` command line argument. Note that `configure` does not accept this command line argument. When using the `configure` script directly, pass the configuration variables as arguments to `configure` using a `<VARIABLE>=<VALUE>` syntax. Variables can also be passed as environment variables to `configure`. However, if you pass the configuration in the environment, make sure to unset all of these environment variables before invoking `make`; otherwise, the environment variables might set make variables in some applications, or parts of some applications, and you may end up with an erroneously configured build.

### What can be Cross Compiled?

All Erlang/OTP applications except the `wx` application can be cross compiled. The build of the `wx` driver will currently be automatically disabled when cross compiling.

### Compatibility

The build system, including cross compilation configuration variables used, may be subject to non backward compatible changes without prior notice. Current cross build system has been tested when cross compiling some Linux/GNU systems, but has only been partly tested on other platforms.

### Patches

Please submit any patches for cross compiling in a way consistent with this system. All input is welcome as we have a very limited set of cross compiling environments to test with. If a new configuration variable is needed, add it to `$ERL_TOP/xcomp/erl-xcomp.conf.template`, and use it in `configure.in`. Other files that might need to be updated are:

- `$ERL_TOP/xcomp/erl-xcomp-vars.sh`
- `$ERL_TOP/erl-build-tool-vars.sh`
- `$ERL_TOP/erts/aclocal.m4`
- `$ERL_TOP/xcomp/README.md`
- `$ERL_TOP/xcomp/erl-xcomp-*.conf`

Note that this might be an incomplete list of files that need to be updated.

General information on how to submit patches can be found at: http://wiki.github.com/erlang/otp/submitting-patches

## Build and Install Procedure

We will first go through the `configure`/`make` build procedure which people probably are most familiar with.

### Building With configure/make Directly

Change directory into the top directory of the Erlang/OTP source tree.

    $ cd $ERL_TOP

In order to compile Erlang code, a small Erlang bootstrap system has to be built, or an Erlang/OTP system of the same release as the one being built has to be provided in the `$PATH`. The Erlang/OTP for the target system will be built using this Erlang system, together with the cross compilation tools provided.

If you want to build using a compatible Erlang/OTP system in the `$PATH`, jump to Cross Building the System.

#### Building a Bootstrap System

    $ ./configure --enable-bootstrap-only
    $ make

The `--enable-bootstrap-only` argument to `configure` isn't strictly necessary, but will speed things up. It will only run `configure` in applications necessary for the bootstrap, and will disable a lot of things not needed by the bootstrap system. If you run `configure` without `--enable-boostrap-only` you also have to run make as `make bootstrap`; otherwise, the whole system will be built.

#### Cross Building the System

    $ ./configure --host=<HOST> --build=<BUILD> [Other Config Args]
    $ make

`<HOST>` is the host/target system that you build for. It does not have to be a full `CPU-VENDOR-OS` triplet, but can be. The full canonicalized `CPU-VENDOR-OS` triplet will be created by executing `$ERL_TOP/make/autoconf/config.sub <HOST>`. If `config.sub` fails, you need to be more specific.

`<BUILD>` should equal the `CPU-VENDOR-OS` triplet of the system that you build on. If you execute `$ERL_TOP/make/autoconf/config.guess`, it will in most cases print the triplet you want to use for this.

The use of `<HOST>` and `<BUILD>` values that differ will trigger cross compilation. Note that if `<HOST>` and `<BUILD>` differ, the canonicalized values of `<HOST>` and `<BUILD>` must also differ. If they do not, the configuration will fail.

Pass the cross compilation variables as command line arguments to `configure` using a `<VARIABLE>=<VALUE>` syntax.

#### Note

You can not pass a configuration file using the `--xcomp-conf` argument when you invoke `configure` directly. The `--xcomp-conf` argument can only be passed to `otp_build configure`.

`make` will verify that the Erlang/OTP system used when building is of the same release as the system being built, and will fail if this is not the case. It is possible, however not recommended, to force the cross compilation even though the wrong Erlang/OTP system is used. This by invoking `make` like this: `make ERL_XCOMP_FORCE_DIFFERENT_OTP=yes`.

#### Warning

Invoking `make ERL_XCOMP_FORCE_DIFFERENT_OTP=yes` might fail, silently produce suboptimal code, or silently produce erroneous code.

#### Installing

You can either install using the Installing Using Paths Determined by configure, or install manually.

##### Installing Using Paths Determined by configure

    $ make install DESTDIR=<TEMPORARY_PREFIX>

`make install` will install at a location specified when doing `configure`. `configure` arguments specifying where the installation should reside are for example: `--prefix`, `--exec-prefix`, `--libdir`, `--bindir`, etc. By default it will install under `/usr/local`. You typically do not want to install your cross build under `/usr/local` on your build machine. Using DESTDIR will cause the installation paths to be prefixed by `$DESTDIR`. This makes it possible to install and package the installation on the build machine without having to place the installation in the same directory on the build machine as it should be executed from on the target machine.

When `make install` has finished, change directory into `$DESTDIR`, package the system, move it to the target machine, and unpack it. Note that the installation will only be working on the target machine at the location determined by `configure`.

##### Installing Manually

    $ make release RELEASE_ROOT=<RELEASE_DIR>

`make release` will copy what you have built for the target machine to `<RELEASE_DIR>`. The `Install` script will not be run. The content of `<RELEASE_DIR>` is what by default ends up in `/usr/local/lib/erlang`.

The `Install` script used when installing Erlang/OTP requires common Unix tools such as `sed` to be present in your `$PATH`. If your target system does not have such tools, you need to run the `Install` script on your build machine before packaging Erlang/OTP. The `Install` script should currently be invoked as follows in the directory where it resides (the top directory):

    $ ./Install [-cross] [-minimal|-sasl] <ERL_ROOT>

where:

- `-minimal` Creates an installation that starts up a minimal amount of applications, i.e., only `kernel` and `stdlib` are started. The minimal system is normally enough, and is what `make install` uses.
- `-sasl` Creates an installation that also starts up the `sasl` application.
- `-cross` For cross compilation. Informs the install script that it is run on the build machine.
- `<ERL_ROOT>` - The absolute path to the Erlang installation to use at run time. This is often the same as the current working directory, but does not have to be. It can follow any other path through the file system to the same directory.

If neither `-minimal`, nor `-sasl` is passed as argument you will be prompted.

You can now either do:

- Decide where the installation should be located on the target machine, run the `Install` script on the build machine, and package the installed installation. The installation just need to be unpacked at the right location on the target machine:

      $ cd <RELEASE_DIR>
      $ ./Install -cross [-minimal|-sasl] <ABSOLUTE_INSTALL_DIR_ON_TARGET>

or:

- Package the installation in `<RELEASE_DIR>`, place it wherever you want on your target machine, and run the `Install` script on your target machine:

      $ cd <ABSOLUTE_INSTALL_DIR_ON_TARGET>
      $ ./Install [-minimal|-sasl] <ABSOLUTE_INSTALL_DIR_ON_TARGET>

### Building With the otp_build Script

    $ cd $ERL_TOP

    $ ./otp_build configure --xcomp-conf=<FILE> [Other Config Args]

alternatively:

    $ ./otp_build configure --host=<HOST> --build=<BUILD> [Other Config Args]

If you have your cross compilation configuration in a file, pass it using the `--xcomp-conf=<FILE>` command line argument. If not, pass `--host=<HOST>`, `--build=<BUILD>`, and the configuration variables using a `<VARIABLE>=<VALUE>` syntax on the command line (same as in Cross Building the System). Note that `<HOST>` and `<BUILD>` have to be passed one way or the other; either by using `erl_xcomp_host=<HOST>` and `erl_xcomp_build=<BUILD>` in the configuration file, or by using the `--host=<HOST>`, and `--build=<BUILD>` command line arguments.

`otp_build configure` will configure both for the bootstrap system on the build machine and the cross host system.

    $ ./otp_build boot -a

`otp_build boot -a` will first build a bootstrap system for the build machine and then do the cross build of the system.

    $ ./otp_build release -a <RELEASE_DIR>

`otp_build release -a` will do the same as `make release` in [Installing Manually], and you will after this have to do a manual ./Install on either the host or target.

## Building and Installing the Documentation

After the system has been cross built you can build and install the documentation the same way as after a native build of the system. See the How to Build the Documentation section in the Building and Installing Erlang/OTP document for information on how to build the documentation.

## Testing the cross compiled system

Some of the tests that come with erlang use native code to test. This means that when cross compiling erlang you also have to cross compile test suites in order to run tests on the target host. To do this you first have to release the tests as usual.

    $ make release_tests

or

    $ ./otp_build tests

The tests will be released into `$ERL_TOP/release/tests`. After releasing the tests you have to install the tests on the build machine. You supply the same xcomp file as to `./otp_build` in Building With the otp_build Script.

    $ cd $ERL_TOP/release/tests/test_server/
    $ $ERL_TOP/bootstrap/bin/erl -eval 'ts:install([{xcomp,"<FILE>"}])' -s ts compile_testcases -s init stop

You should get a lot of printouts as the testcases are compiled. Once done you should copy the entire `$ERL_TOP/release/tests` folder to the cross host system.

Then go to the cross host system and setup the erlang installed to be in your `$PATH`. Then go to what previously was `$ERL_TOP/release/tests/test_server` and issue the following command.

    $ erl -s ts install -s ts run all_tests -s init stop

The configure should be skipped and all tests should hopefully pass. For more details about how to use ts run `erl -s ts help -s init stop`

## Currently Used Configuration Variables

Note that you cannot define arbitrary variables in a cross compilation configuration file. Only the ones listed below will be guaranteed to be visible throughout the whole execution of all `configure` scripts. Other variables needs to be defined as arguments to `configure` or exported in the environment.

### Variables for otp_build Only

Variables in this section are only used, when configuring Erlang/OTP for cross compilation using `$ERL_TOP/otp_build configure`.

#### Note

These variables currently have no effect if you configure using the `configure` script directly.

- `erl_xcomp_build` - The build system used. This value will be passed as `--build=$erl_xcomp_build` argument to the `configure` script. It does not have to be a full `CPU-VENDOR-OS` triplet, but can be. The full `CPU-VENDOR-OS` triplet will be created by `$ERL_TOP/make/autoconf/config.sub $erl_xcomp_build`. If set to `guess`, the build system will be guessed using `$ERL_TOP/make/autoconf/config.guess`.

- `erl_xcomp_host` - Cross host/target system to build for. This value will be passed as `--host=$erl_xcomp_host` argument to the `configure` script. It does not have to be a full `CPU-VENDOR-OS` triplet, but can be. The full `CPU-VENDOR-OS` triplet will be created by `$ERL_TOP/make/autoconf/config.sub $erl_xcomp_host`.

- `erl_xcomp_configure_flags` - Extra configure flags to pass to the `configure` script.

### Cross Compiler and Other Tools

If the cross compilation tools are prefixed by `<HOST>-` you probably do not need to set these variables (where `<HOST>` is what has been passed as `--host=<HOST>` argument to `configure`). Compiler and other tools can otherwise be identified via variables passed as arguments on the command line to `configure`, in then xcomp file, or as environment variables. For more information see the Important Variables Inspected by configure section in Building and Installing Erlang/OTP.

### Cross System Root Locations

- `erl_xcomp_sysroot` - The absolute path to the system root of the cross compilation environment. Currently, the `crypto`, `odbc`, `ssh` and `ssl` applications need the system root. These applications will be skipped if the system root has not been set. The system root might be needed for other things too. If this is the case and the system root has not been set, `configure` will fail and request you to set it.

- `erl_xcomp_isysroot` - The absolute path to the system root for includes of the cross compilation environment. If not set, this value defaults to `$erl_xcomp_sysroot`, i.e., only set this value if the include system root path is not the same as the system root path.

### Optional Feature, and Bug Tests

These tests cannot (always) be done automatically when cross compiling. You usually do not need to set these variables.

#### Warning

Setting these variables wrong may cause hard to detect runtime errors. If you need to change these values, really make sure that the values are correct.

#### Note

Some of these values will override results of tests performed by `configure`, and some will not be used until `configure` is sure that it cannot figure the result out.

The `configure` script will issue a warning when a default value is used. When a variable has been set, no warning will be issued.

- `erl_xcomp_after_morecore_hook` - `yes|no`. Defaults to `no`. If `yes`, the target system must have a working `__after_morecore_hook` that can be used for tracking used `malloc()` implementations core memory usage. This is currently only used by unsupported features.

- `erl_xcomp_bigendian` - `yes|no`. No default. If `yes`, the target system must be big endian. If `no`, little endian. This can often be automatically detected, but not always. If not automatically detected, `configure` will fail unless this variable is set. Since no default value is used, `configure` will try to figure this out automatically.

- `erl_xcomp_double_middle` - `yes|no`. Defaults to `no`. If `yes`, the target system must have doubles in "middle-endian" format. If `no`, it has "regular" endianness.

- `erl_xcomp_clock_gettime_cpu_time` - `yes|no`. Defaults to `no`. If `yes`, the target system must have a working `clock_gettime()` implementation that can be used for retrieving process CPU time.

- `erl_xcomp_getaddrinfo` - `yes|no`. Defaults to `no`. If `yes`, the target system must have a working `getaddrinfo()` implementation that can handle both IPv4 and IPv6.

- `erl_xcomp_gethrvtime_procfs_ioctl` - `yes|no`. Defaults to `no`. If `yes`, the target system must have a working `gethrvtime()` implementation and is used with procfs `ioctl()`.

- `erl_xcomp_dlsym_brk_wrappers` - `yes|no`. Defaults to `no`. If `yes`, the target system must have a working `dlsym(RTLD_NEXT, <S>)` implementation that can be used on `brk` and `sbrk` symbols used by the `malloc()` implementation in use, and by this track the `malloc()` implementations core memory usage. This is currently only used by unsupported features.

- `erl_xcomp_kqueue` - `yes|no`. Defaults to `no`. If `yes`, the target system must have a working `kqueue()` implementation that returns a file descriptor which can be used by `poll()` and/or `select()`. If `no` and the target system has not got `epoll()` or `/dev/poll`, the kernel-poll feature will be disabled.

- `erl_xcomp_linux_clock_gettime_correction` - `yes|no`. Defaults to `yes` on Linux; otherwise, `no`. If `yes`, `clock_gettime(CLOCK_MONOTONIC, _)` on the target system must work. This variable is recommended to be set to `no` on Linux systems with kernel versions less than 2.6.

- `erl_xcomp_linux_nptl` - `yes|no`. Defaults to `yes` on Linux; otherwise, `no`. If `yes`, the target system must have NPTL (Native POSIX Thread Library). Older Linux systems have LinuxThreads instead of NPTL (Linux kernel versions typically less than 2.6).

- `erl_xcomp_linux_usable_sigaltstack` - `yes|no`. Defaults to `yes` on Linux; otherwise, `no`. If `yes`, `sigaltstack()` must be usable on the target system. `sigaltstack()` on Linux kernel versions less than 2.4 are broken.

- `erl_xcomp_linux_usable_sigusrx` - `yes|no`. Defaults to `yes`. If `yes`, the `SIGUSR1` and `SIGUSR2` signals must be usable by the ERTS. Old LinuxThreads thread libraries (Linux kernel versions typically less than 2.2) used these signals and made them unusable by the ERTS.

- `erl_xcomp_poll` - `yes|no`. Defaults to `no` on Darwin/MacOSX; otherwise, `yes`. If `yes`, the target system must have a working `poll()` implementation that also can handle devices. If `no`, `select()` will be used instead of `poll()`.

- `erl_xcomp_putenv_copy` - `yes|no`. Defaults to `no`. If `yes`, the target system must have a `putenv()` implementation that stores a copy of the key/value pair.

- `erl_xcomp_reliable_fpe` - `yes|no`. Defaults to `no`. If `yes`, the target system must have reliable floating point exceptions.

- `erl_xcomp_posix_memalign` - `yes|no`. Defaults to `yes` if `posix_memalign` system call exists; otherwise `no`. If `yes`, the target system must have a `posix_memalign` implementation that accepts larger than page size alignment.

- `erl_xcomp_code_model_small` - `yes|no`. Default to `no`. If `yes`, the target system must place the beam.smp executable in the lower 2 GB of memory. That is it should not use position independent executable.

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