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Bazel Tutorial: Configure C++ Toolchains

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This tutorial uses an example scenario to describe how to configure C++ toolchains for a project.

What you'll learn

In this tutorial you learn how to:

Set up the build environment
Use --toolchain_resolution_debug to debug toolchain resolution
Configure the C++ toolchain
Create a Starlark rule that provides additional configuration for the cc_toolchain so that Bazel can build the application with clang
Build the C++ binary for by running bazel build //main:hello-world on a Linux machine
Cross-compile the binary for android by running bazel build //main:hello-world --platforms=//:android_x86_64

Before you begin

This tutorial assumes you are on Linux and have successfully built C++ applications and installed the appropriate tooling and libraries. The tutorial uses clang version 16, which you can install on your system.

Set up the build environment

Set up your build environment as follows:

If you have not already done so, download and install Bazel 7.0.2 or later.
Add an empty WORKSPACE file at the root folder.
Add the following cc_binary target to the main/BUILD file:
```
load("@rules_cc//cc:defs.bzl", "cc_binary")

cc_binary(
    name = "hello-world",
    srcs = ["hello-world.cc"],
)
```
Because Bazel uses many internal tools written in C++ during the build, such as process-wrapper, the pre-existing default C++ toolchain is specified for the host platform. This enables these internal tools to build using that toolchain of the one created in this tutorial. Hence, the cc_binary target is also built with the default toolchain.

Run the build with the following command:

bazel build //main:hello-world

The build succeeds without any toolchain registered in the WORKSPACE.

To further see what's under the hood, run:

bazel build //main:hello-world --toolchain_resolution_debug='@bazel_tools//tools/cpp:toolchain_type'

INFO: ToolchainResolution: Target platform @@local_config_platform//:host: Selected execution platform @@local_config_platform//:host, type @@bazel_tools//tools/cpp:toolchain_type -> toolchain @@bazel_tools~cc_configure_extension~local_config_cc//:cc-compiler-k8

Without specifying --platforms, Bazel builds the target for @local_config_platform//:host using @bazel_tools//cc_configure_extension/local_config_cc//:cc-compiler-k8

Configure the C++ toolchain

To configure the C++ toolchain, repeatedly build the application and eliminate each error one by one as described as following.

It also assumes clang version 9.0.1, although the details should only change slightly between different versions of clang.

Add toolchain/BUILD with

filegroup(name = "empty")

cc_toolchain(
    name = "linux_x86_64_toolchain",
    toolchain_identifier = "linux_x86_64-toolchain",
    toolchain_config = ":linux_x86_64_toolchain_config",
    all_files = ":empty",
    compiler_files = ":empty",
    dwp_files = ":empty",
    linker_files = ":empty",
    objcopy_files = ":empty",
    strip_files = ":empty",
    supports_param_files = 0,
)

toolchain(
    name = "cc_toolchain_for_linux_x86_64",
    toolchain = ":linux_x86_64_toolchain",
    toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
    exec_compatible_with = [
        "@platforms//cpu:x86_64",
        "@platforms//os:linux",
    ],
    target_compatible_with = [
        "@platforms//cpu:x86_64",
        "@platforms//os:linux",
    ],
)

Then register the toolchain with the WORKSPACE with

register_toolchains(
    "//toolchain:cc_toolchain_for_linux_x86_64"
)

This step defines a cc_toolchain and binds it to a toolchain target for the host configuration.

Run the build again. Because the toolchain package doesn't yet define the linux_x86_64_toolchain_config target, Bazel throws the following error:

ERROR: toolchain/BUILD:4:13: in toolchain_config attribute of cc_toolchain rule //toolchain:linux_x86_64_toolchain: rule '//toolchain:linux_x86_64_toolchain_config' does not exist.

In the toolchain/BUILD file, define an empty filegroup as follows:

package(default_visibility = ["//visibility:public"])

filegroup(name = "linux_x86_64_toolchain_config")

Run the build again. Bazel throws the following error:

'//toolchain:linux_x86_64_toolchain_config' does not have mandatory providers: 'CcToolchainConfigInfo'.

CcToolchainConfigInfo is a provider that you use to configure your C++ toolchains. To fix this error, create a Starlark rule that provides CcToolchainConfigInfo to Bazel by making a toolchain/cc_toolchain_config.bzl file with the following content:

def _impl(ctx):
    return cc_common.create_cc_toolchain_config_info(
        ctx = ctx,
        toolchain_identifier = "k8-toolchain",
        host_system_name = "local",
        target_system_name = "local",
        target_cpu = "k8",
        target_libc = "unknown",
        compiler = "clang",
        abi_version = "unknown",
        abi_libc_version = "unknown",
    )

cc_toolchain_config = rule(
    implementation = _impl,
    attrs = {},
    provides = [CcToolchainConfigInfo],
)

cc_common.create_cc_toolchain_config_info() creates the needed provider CcToolchainConfigInfo. To use the cc_toolchain_config rule, add a load statement to toolchain/BUILD right below the package statement:

load(":cc_toolchain_config.bzl", "cc_toolchain_config")

And replace the "linux_x86_64_toolchain_config" filegroup with a declaration of a cc_toolchain_config rule:

cc_toolchain_config(name = "linux_x86_64_toolchain_config")

Run the build again. Bazel throws the following error:

.../BUILD:1:1: C++ compilation of rule '//:hello-world' failed (Exit 1)
src/main/tools/linux-sandbox-pid1.cc:421:
"execvp(toolchain/DUMMY_GCC_TOOL, 0x11f20e0)": No such file or directory
Target //:hello-world failed to build`

At this point, Bazel has enough information to attempt building the code but it still does not know what tools to use to complete the required build actions. You will modify the Starlark rule implementation to tell Bazel what tools to use. For that, you need the tool_path() constructor from @bazel_tools//tools/cpp:cc_toolchain_config_lib.bzl:

# toolchain/cc_toolchain_config.bzl:
# NEW
load("@bazel_tools//tools/cpp:cc_toolchain_config_lib.bzl", "tool_path")

def _impl(ctx):
    tool_paths = [ # NEW
        tool_path(
            name = "gcc",
            path = "/usr/bin/clang",
        ),
        tool_path(
            name = "ld",
            path = "/usr/bin/ld",
        ),
        tool_path(
            name = "ar",
            path = "/usr/bin/ar",
        ),
        tool_path(
            name = "cpp",
            path = "/bin/false",
        ),
        tool_path(
            name = "gcov",
            path = "/bin/false",
        ),
        tool_path(
            name = "nm",
            path = "/bin/false",
        ),
        tool_path(
            name = "objdump",
            path = "/bin/false",
        ),
        tool_path(
            name = "strip",
            path = "/bin/false",
        ),
    ]

    return cc_common.create_cc_toolchain_config_info(
        ctx = ctx,
        toolchain_identifier = "local",
        host_system_name = "local",
        target_system_name = "local",
        target_cpu = "k8",
        target_libc = "unknown",
        compiler = "clang",
        abi_version = "unknown",
        abi_libc_version = "unknown",
        tool_paths = tool_paths, # NEW
    )

Make sure that /usr/bin/clang and /usr/bin/ld are the correct paths for your system.

Run the build again. Bazel throws the following error:

ERROR: main/BUILD:3:10: Compiling main/hello-world.cc failed: absolute path inclusion(s) found in rule '//main:hello-world':
the source file 'main/hello-world.cc' includes the following non-builtin files with absolute paths (if these are builtin files, make sure these paths are in your toolchain):
  '/usr/include/c++/13/ctime'
  '/usr/include/x86_64-linux-gnu/c++/13/bits/c++config.h'
  '/usr/include/x86_64-linux-gnu/c++/13/bits/os_defines.h'
  ...

Bazel needs to know where to search for included headers. There are multiple ways to solve this, such as using the includes attribute of cc_binary, but here this is solved at the toolchain level with the cxx_builtin_include_directories parameter of cc_common.create_cc_toolchain_config_info. Beware that if you are using a different version of clang, the include path will be different. These paths may also be different depending on the distribution.

Modify the return value in toolchain/cc_toolchain_config.bzl to look like this:

return cc_common.create_cc_toolchain_config_info(
    ctx = ctx,
    cxx_builtin_include_directories = [ # NEW
        "/usr/lib/llvm-16/lib/clang/16/include",
        "/usr/include",
    ],
    toolchain_identifier = "local",
    host_system_name = "local",
    target_system_name = "local",
    target_cpu = "k8",
    target_libc = "unknown",
    compiler = "clang",
    abi_version = "unknown",
    abi_libc_version = "unknown",
    tool_paths = tool_paths,
)

Run the build command again, you will see an error like:

/usr/bin/ld: bazel-out/k8-fastbuild/bin/main/_objs/hello-world/hello-world.o: in function `print_localtime()':
hello-world.cc:(.text+0x68): undefined reference to `std::cout'

The reason for this is because the linker is missing the C++ standard library and it can't find its symbols. There are many ways to solve this, such as using the linkopts attribute of cc_binary. Here it is solved by making sure that any target using the toolchain doesn't have to specify this flag.

Copy the following code to toolchain/cc_toolchain_config.bzl:

# NEW
load("@bazel_tools//tools/build_defs/cc:action_names.bzl", "ACTION_NAMES")
# NEW
load(
    "@bazel_tools//tools/cpp:cc_toolchain_config_lib.bzl",
    "feature",    # NEW
    "flag_group", # NEW
    "flag_set",   # NEW
    "tool_path",
)

all_link_actions = [ # NEW
    ACTION_NAMES.cpp_link_executable,
    ACTION_NAMES.cpp_link_dynamic_library,
    ACTION_NAMES.cpp_link_nodeps_dynamic_library,
]

def _impl(ctx):
    tool_paths = [
        tool_path(
            name = "gcc",
            path = "/usr/bin/clang",
        ),
        tool_path(
            name = "ld",
            path = "/usr/bin/ld",
        ),
        tool_path(
            name = "ar",
            path = "/bin/false",
        ),
        tool_path(
            name = "cpp",
            path = "/bin/false",
        ),
        tool_path(
            name = "gcov",
            path = "/bin/false",
        ),
        tool_path(
            name = "nm",
            path = "/bin/false",
        ),
        tool_path(
            name = "objdump",
            path = "/bin/false",
        ),
        tool_path(
            name = "strip",
            path = "/bin/false",
        ),
    ]

    features = [ # NEW
        feature(
            name = "default_linker_flags",
            enabled = True,
            flag_sets = [
                flag_set(
                    actions = all_link_actions,
                    flag_groups = ([
                        flag_group(
                            flags = [
                                "-lstdc++",
                            ],
                        ),
                    ]),
                ),
            ],
        ),
    ]

    return cc_common.create_cc_toolchain_config_info(
        ctx = ctx,
        features = features, # NEW
        cxx_builtin_include_directories = [
            "/usr/lib/llvm-9/lib/clang/9.0.1/include",
            "/usr/include",
        ],
        toolchain_identifier = "local",
        host_system_name = "local",
        target_system_name = "local",
        target_cpu = "k8",
        target_libc = "unknown",
        compiler = "clang",
        abi_version = "unknown",
        abi_libc_version = "unknown",
        tool_paths = tool_paths,
    )

cc_toolchain_config = rule(
    implementation = _impl,
    attrs = {},
    provides = [CcToolchainConfigInfo],
)

Running bazel build //main:hello-world, it should finally build the binary successfully for host.
In toolchain/BUILD, copy the cc_toolchain_config, cc_toolchain, and toolchain targets and replace linux_x86_64 with android_x86_64in target names.

In WORKSPACE, register the toolchain for android
```
register_toolchains(
    "//toolchain:cc_toolchain_for_linux_x86_64",
    "//toolchain:cc_toolchain_for_android_x86_64"
)
```
Run bazel build //main:hello-world --android_platforms=//toolchain:android_x86_64 to build the binary for Android.

In practice, Linux and Android should have different C++ toolchain configs. You can either modify the existing cc_toolchain_config for the differences or create a separate rules (i.e. CcToolchainConfigInfo provider) for separate platforms.

Review your work

In this tutorial you learned how to configure a basic C++ toolchain, but toolchains are more powerful than this simple example.

The key take-aways are:

You need to specify a matching platforms flag in the command line for Bazel to resolve to the toolchain for the same constraint values on the platform. The documentation holds more information about language specific configuration flags.
You have to let the toolchain know where the tools live. In this tutorial there is a simplified version where you access the tools from the system. If you are interested in a more self-contained approach, you can read about workspaces. Your tools could come from a different workspace and you would have to make their files available to the cc_toolchain with target dependencies on attributes, such as compiler_files. The tool_paths would need to be changed as well.
You can create features to customize which flags should be passed to different actions, be it linking or any other type of action.

Bazel Tutorial: Configure C++ Toolchains

What you'll learn

Before you begin

Set up the build environment

Configure the C++ toolchain

Review your work

Further reading