Bazel cc_toolchain dependency declarations headers

[Carlos Galvez]

Hi,

I'm trying to configure a CC toolchain based on x86 GCC. I want to download a specific version of GCC instead of using whatever the user has installed on their machine.

I'm following this guide:

https://docs.bazel.build/versions/main/tutorial/cc-toolchain-config.html

And get stuck in point 8: Bazel is saying I have not declared dependencies to headers:

this rule is missing dependency declarations for the following files included by 'hello/main.cpp':

  '/home/user/.cache/bazel/_bazel_user/4fd10b035034e53930a3873d63a730b2/external/gcc/usr/include/stdc-predef.h'

  '/home/user/.cache/bazel/_bazel_user/4fd10b035034e53930a3873d63a730b2/external/gcc/usr/include/c++/7/iostream'

I have added the sysroot as per the instructions:

    cxx_builtin_include_directories = [

        "/usr/include",

        "%sysroot%/usr/include/c++/7/",

        "%sysroot%/usr/include/x86_64-linux-gnu",

        "%sysroot%/usr/include",

        "%sysroot%/usr/lib/gcc/x86_64-linux-gnu/7/include",

        "%sysroot%/usr/lib/gcc/x86_64-linux-gnu/7/include-fixed",

    ],

    sysroot="external/gcc_x86",

Why is it not working?

Thanks!

/Carlos

[Austin Schuh]

Check out:

https://github.com/frc971/971-Robot-Code/blob/master/tools/cpp/toolchain_config.bzl#L1599

That is a working toolchain we've got setup to do both cross compilation and to use a compiler that Bazel downloads.

Austin

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[Konstantin]

You need to add " external/gcc/usr/include" to  cxx_builtin_include_directories list.

Konstantin

[Carlos Galvez]

Thanks, I've tried both approaches without success. Also it seems "no-canonical-headers" is important, but it still doesn't work.

It seems the problem is that GCC will always use absolute paths in its default search paths, unlike cross-compilers, which will use relative paths if "no-canonical-headers" is active. Quite a pity...

[Konstantin]

You mean "no-canonical-prefixes", right?

[Carlos Galvez]

Yes, sorry, mixed it up with a similar flag.

[Konstantin]

I see GCC also has "no-canonical-system-headers". Have not tried it though - we use Clang. 

[Carlos Galvez]

 @austin  I looked at your toolchain a bit more in depth and have some follow-up questions:

* What decides where you put the ")%" part? sometimes you put it in the middle of the path, sometimes at the end of it.

* I see you still need to disable the default include and link paths and set them manually via isystem, why is that? 

If I set my include paths manually like you do, then I don't need to write anything in cxx_builtin_directories and Bazel is happy.

[Sumanth Murali]

What was the solution for this?
I have tried using custom_builtin_include_directories and isystem for include paths, and still seem to get errors like
This rule is missing dependency declarations for the following files included 

[Konstantin]

Could you share a repro?