Using the toolchain is as simple as adding the toolchain’s bin directory in your PATH, such as:

export PATH="${PATH}:/your/toolchain/path/bin"

Depending on the project being compiled, there may be different ways to specify the toolchain.

If the software uses GNU autotools or a similar configure script, you shoul use the --host tuple to tell the build system to use your toolchain (if the software package uses the autotools system you should also pass --build, for completeness):

./configure --host=your-host-tuple --build=your-build-tuple

Other systems may need, for example:

make CC=your-host-tuple-gcc
make CROSS_COMPILE=your-host-tuple-
make CHOST=your-host-tuple

and so on. Please read the documentation of the package being compiled as to how it can be cross-compiled.


in the above example, host refers to the host of your program (i.e. the target of the toolchain), not the host of the toolchain; and build refers to the machine where you build your program, that is the host of the toolchain.

Assembling a root filesystem

Assembling a root filesystem for a target device requires the successive building of a set of software packages for the target architecture. Building a package potentially requires artifacts which were generated as part of an earlier build. Note that not all artifacts which are installed as part of a package are desirable on a target’s root filesystem (e.g., man/info files, include files, etc.). Therefore we must distinguish between a staging directory and a rootfs directory.

A staging directory is a location into which we install all the build artifacts. We can then point future builds to this location so they can find the appropriate header and library files. A rootfs directory is a location into which we place only the files we want to have on our target.

There are four schools of thought here:

  • Option 1: Install directly into the sysroot of the toolchain.

    By default (i.e., if you don’t pass any arguments to the tools which would change this behaviour) the toolchain that is built by crosstool-NG will only look in its toolchain directories for system header and library files:

    #include "..." search starts here:
    #include <...> search starts here:
    <ct-ng install path>/lib/gcc/<host tuple>/4.5.2/include
    <ct-ng install path>/lib/gcc/<host tuple>/4.5.2/include-fixed
    <ct-ng install path>/lib/gcc/<host tuple>/4.5.2/../../../../<host tuple>/include
    <ct-ng install path>/<host tuple>/sysroot/usr/include

    In other words, the compiler will automagically find headers and libraries without extra flags if they are installed under the toolchain’s sysroot directory.

    However, this is bad because the toolchain gets poluted, and can not be re-used.

    $ ./configure --build=<build tuple> --host=<host tuple> \
          --prefix=/usr --enable-foo-bar...
    $ make
    $ make DESTDIR=/<ct-ng install path>/<host tuple>/sysroot install
  • Option 2: Copy the toolchain’s sysroot to the staging area.

    If you start off by copying the toolchain’s sysroot directory to your staging area, you can simply proceed to install all your packages’ artifacts to the same staging area. You then only need to specify a --sysroot=<staging area> option to the compiler of any subsequent builds and all your required header and library files will be found/used.

    This is a viable option, but requires the user to always specify CFLAGS in order to include --sysroot=<staging area>, or requires the use of a wrapper to a few select tools (gcc, ld…) to pass this flag.

    Instead of polluting the toolchain’s sysroot you are copying its contents to a new location and polluting the contents in that new location. By specifying the --sysroot option you’re effectively abandoning the default sysroot in favour of your own.

    Incidentally this is what buildroot does using a wrapper, when using an external toolchain.

    $ cp -a $(<host tuple>-gcc --your-cflags-except-sysroot -print-sysroot) \
    $ ./configure --build=<build tuple> --host=<host tuple>          \
                  --prefix=/usr --enable-foo-bar...                  \
                  CC="<host tuple>-gcc --syroot=/path/to/staging"    \
                  CXX="<host tuple>-g++ --sysroot=/path/to/staging"  \
                  LD="<host tuple>-ld --sysroot=/path/to/staging"    \
                  AND_SO_ON="tuple-andsoon --sysroot=/path/to/staging"
    $ make
    $ make DESTDIR=/path/to/staging install
  • Option 3: Use separate staging and sysroot directories.

    In this scenario you use a staging area to install programs, but you do not pre-fill that staging area with the toolchain’s sysroot. In this case the compiler will find the system includes and libraries in its sysroot area but you have to pass appropriate CPPFLAGS and LDFLAGS to tell it where to find your headers and libraries from your staging area (or use a wrapper).

    $ ./configure --build=<build tuple> --host=<host tuple>          \
                  --prefix=/usr --enable-foo-bar...                  \
                  CPPFLAGS="-I/path/to/staging/usr/include"          \
                  LDFLAGS="-L/path/to/staging/lib -L/path/to/staging/usr/lib"
    $ make
    $ make DESTDIR=/path/to/staging install
  • Option 4: A mix of options 2 and 3, using carefully crafted union mounts.

    The staging area is a union mount of:

    • the sysroot as a read-only branch

    • the real staging area as a read-write branch

      This also requires passing --sysroot to point to the union mount, but has other advantages, such as allowing per-package staging, and a few more obscure pros. It also has its disadvantages, as it potentially requires non-root users to create union mounts. Additionally, union mounts are not yet mainstream in the Linux kernel, so it requires patching. There is a FUSE-based unionfs implementation, but development is almost stalled, and there are a few gotchas.

      $ (good luck!)

It is strongly advised not to use the toolchain sysroot directory as an install directory (i.e., option 1) for your programs/packages. If you do so, you will not be able to use your toolchain for another project. It is even strongly advised that your toolchain is chmod-ed to read-only once successfully install, so that you don’t go polluting your toolchain with your programs’/packages’ files. This can be achieved by selecting the “Render the toolchain read-only” from crosstool-NG’s “Paths and misc options” configuration page.

Thus, when you build a program/package, install it in a separate, staging, directory and let the cross-toolchain continue to use its own, pristine, sysroot directory.

When you are done building and want to assemble your rootfs you could simply take the full contents of your staging directory and use the populate script to add in the necessary files from the sysroot. However, the staging area you have created will include lots of build artifacts that you won’t necessarily want/need on your target. For example: static libraries, header files, linking helper files, man/info pages. You’ll also need to add various configuration files, scripts, and directories to the rootfs so it will boot.

Therefore you’ll probably end up creating a separate rootfs directory which you will populate from the staging area, necessary extras, and then use crosstool-NG’s populate script to add the necessary sysroot libraries.

The populate script

When your root directory is ready, it is still missing some important bits: the toolchain’s libraries. To populate your root directory with those libs, just run:

your-target-tuple-populate -s /your/root -d /your/root-populated

This will copy /your/root into /your/root-populated, and put the needed and only the needed libraries there. Thus you don’t pollute /your/root with any cruft that would no longer be needed should you have to remove stuff. /your/root always contains only those things you install in it.

You can then use /your/root-populated to build up your file system image, a tarball, or to NFS-mount it from your target, or whatever you need.

The populate script accepts the following options:

  • -s src_dir: Use src_dir as the un-populated root directory.

  • -d dst_dir: Put the populated root directory in dst_dir.

  • -l lib1 [...]: Always add specified libraries.

  • -L file: Always add libraries listed in file.

  • -f: Remove dst_dir if it previously existed; continue even if any library specified with -l or -L is missing.

  • -v: Be verbose, and tell what’s going on (you can see exactly where libs are coming from).

  • -h: Print the help.

See your-target-tuple-populate -h for more information on the options.

Here is how populate works:

  1. Perform some sanity checks:
    • src_dir and dst_dir are specified,
    • src_dir exists,
    • unless forced, dst_dir does not exist,
    • src_dir != dst_dir.
  2. Copy src_dir to dst_dir.

  3. Add forced libraries to dst_dir:
    • build the list from -l and -L options,
    • get forced libraries from the sysroot (see below for heuristics)
      • abort on the first missing library, unless -f is specified.
  4. Add all missing libraries to dst_dir:
    • scan dst_dir for every ELF files that are executable or shared object
    • list the “NEEDED Shared library” fields
      • check if the library is already in dst_dir/lib or dst_dir/usr/lib
      • if not, get the library from the sysroot
        • if it’s in sysroot/lib, copy it to dst_dir/lib
        • if it’s in sysroot/usr/lib, copy it to dst_dir/usr/lib
        • in both cases, use the SONAME of the library to create the file in dst_dir
        • if it was not found in the sysroot, this is an error.

The cross-ldd script

There is another script provided by crosstool-NG to work with the sysroot on the host. A dynamically linked application will load certain shared libraries at runtime. These libraries in turn may require some more shared libraries as dependencies. The search paths for each of these dynamic objects differ, and finding the shared libraries required for a given application is not always trivial. Crosstool-NG attempts to solve this task by providing a ${CT_TARGET}-ldd script in the generated toolchain (optionally, if the “Install a cross ldd-like helper” option is selected in the configuration).

This script recursively resolves all the dynamic library dependencies and outputs the list of libraries in a format compatible with that of GNU libc’s ldd script. It needs to have the system root specified, using either the --root option or the CT_XLDD_ROOT environment variable.

For example:

powerpc64-multilib-linux-gnu-gcc -o example example.c
powerpc64-multilib-linux-gnu-ldd --root=`powerpc64-multilib-linux-gnu-gcc -print-sysroot` example

produces an output like this: => /lib64/ (0x00000000deadbeef) => /lib64/ (0x00000000deadbeef)

The load addresses are obviously bogus, as this script does not actually load the libraries.