Relocatable toolchain

[Yann E. MORIN]

Filipp, All,

On 2013-06-17 14:40 +0000, Filipp Andjelo spake thusly:
> Hi,
> 
> I built a toolchain using crosstool-ng and it works fine, as far as I 
> don't move it to another folder. I want to have a toolchain, which I can 
> put everywhere, but how?

As Thomas already pointed out, the toolchains built by crosstool-NG
*are* relocatable (well, should be, otherwise this is a bug, but so far
I never encountered such a condition).

> If I run "gcc --print-sysroot", I get absolute path to the directory, 
> where toolchain was built.

This is expected.

The sysroot handling in gcc is a bit tricky (I don't really know the
exact gory details, but here are an excerpt based on experience):
  - gcc first tries to see if it's configured sysroot exists
    - if so, uses it
  - if not, then gcc tries to see if it is running from the location
    it's been configured for
  - if not, gcc apllies to its configured sysroot the same change, and
    uses the new path for the surrent sysroot

For example, let's say gcc was configured with:
    --prefix=/opt/toolchain
    --sysroot=/opt/toolchain/tuple/sysroot

But you then moved the toolchain:
    /opt/toolchain    -->   /some/place/new/base

gcc will see the relocation; then it constructs a relative path to the
sysroot from its configuration:
    /opt/toolchain/tuple/sysroot
    ^^^^^^^^^^^^^^  <- this is gcc's --prefix, so it gets removed.

    /tuple/sysroot    <- this is the relative path to the sysroot.

And gcc now adds its new location in front:
    /some/place/new/base/tuple/sysroot

Note-1: if the sysroot is not a sub-path of the prefix, this will not
work, and the toolchain will not be relocatable. That's why crosstool-NG
insists on not letting the user specify the path to the sysroot in the
menuconfig, and computes it itself.

Note-2: there can be a case where the toolchain has been relocated, but
the original sysroot already exists (eg. another toolchain was built
with the same prefix). In this case, you'd get in much trouble!
Especially if the sysroot of the new toolchain is read-only, you'd get
weird build errors later on, or if it is read-write, you'd get either
weirder build errors, or worse, run-time errors.. Well, I said sysroot
path computation by gcc was tricky, didn't I? ;-)

> If I check the same for google's Android 
> toolchain, I get relative path
> to the gcc. How did they do that?

I don't know why/how Android's toolchains are built thus, but we can't
do it for crosstool-NG (long story made short: canadian builds).

> Currently, I have a workaround using CFLAGS="--sysroot=path/to/sysroot", 
> but It's not acceptable solution here.

No, and this is wrong. The toolchain should be relocatable. Can you
share the .config that produces this non-relocatable toolchain, please?

Regards,
Yann E. MORIN.

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