Cross compilation support for ARM Linux

[hsjoberg]

I've been trying to get cross compilation to work for Linux.
The dependencies (sqlite3 and libgmp) compiles fine using the arm-linux-gnueabi toolchain.

My issue so far has been to understand how CCAN could work together with cross compilation.
A binary ("configurator") seems to be compiled and later executed in the Makefile, which causes conflicts if you would naively just try to compile using the ARM compiler (CC=linux-arm-gnueabi-cc) when running the makefile.

Any advice on how I should reason around this issue and/or how CCAN should operate with cross compilation?

If you need any more information about the specifics regarding the issues/errors I can provide them. :)

[cdecker]

Does this work:

make CC=clang clean ccan/tools/configurator/configurator
make CC=linux-arm-gnueabi-cc default

That'd compile the configurator for the current arch, and then uses it when cross-compiling.

[hsjoberg]

@cdecker Thanks! Yes it moves forward at least.

Libs using autotools/autoconfig will fail:

checking whether we are cross compiling... configure: error: in `/home/coco/Projects/C/lightning-arm-linux/external/libwally-core':
configure: error: cannot run C compiled programs.
If you meant to cross compile, use `--host'.
See `config.log' for more details
external/Makefile:40: recipe for target 'external/libwally-core/src/secp256k1/libsecp256k1.la' failed
make: *** [external/libwally-core/src/secp256k1/libsecp256k1.la] Error 1

I'll try to manually edit the Makefile(s) to make sure configure runs with the host flag and see if it's possible to progress further.

[cdecker]

Perfect, looking forward to this, will be really helpful for deterministic builds as well #423.

[icota]

@hsjoberg This PR will help you overcome that hurdle. Now I have the build failing due to ARM binaries like ccan/ccan/cdump/tools/cdump-enumstr trying to run on x86. I guess it's a step in the right direction 😄

[hsjoberg]

Very nice @icota!
That's as far as I got as well, but good that you've started a PR about this.

I'll try to pick up this again soon and perhaps ping rusty on IRC. :)

[icota]

As at d4d1c4a you can cross compile but make sure to install qemu-user. Run make CC=clang clean ccan/tools/configurator/configurator and in case you get stuck at the cdump-enumstr step do a make clean -C ccan/ccan/cdump/tools && make CC=clang -C ccan/ccan/cdump/tools. I'm not sure how to tweak Makefile not to cross-compile these two bits.

On Android I need to drop the -Werror flag but on a more vanilla variant of Linux I'm sure the whole process is smooth sailing.

[cdecker]

With #1035 merged this should work now. Can you confirm this @hsjoberg?

[hsjoberg]

@cdecker @icota I can confirm that this compiles and seems to be producing working binaries (only tested lightning-cli so far), if "machine-native" cc is being used instead of the $(CC) variable (which would be set to ARM gcc) in ccan/config.h: ccan/tools/configurator/configurator:

# make clean ccan/tools/configurator/configurator # Clean ccan configurator

make CC=arm-linux-gnueabi-gcc HOST=arm-linux-gnueabi LDFLAGS="-L/<arm lib path>" default

# When cdump-enumstr fails:
make clean -C ccan/ccan/cdump/tools && make CC=clang -C ccan/ccan/cdump/tools

# And then make default again:
make CC=arm-linux-gnueabi-gcc HOST=arm-linux-gnueabi LDFLAGS="-L/<arm lib path>" default

[cdecker]

Thanks @hsjoberg, any way we can make it foolproof, e.g., by not using $(CC) for the configurator? Or can we disentangle CC and HOST?

[cdecker]

This was addressed in 229f288

[droark]

Hi. Came across this while looking into some CCAN stuff. Just curious, would anybody mind posting the directions they're using to cross-compile now that everything is (supposedly) good to go? I'm attempting to cross-compile for Raspberry Pi on Ubuntu 17.10 and am having issues.

Thanks!