> ## Documentation Index
> Fetch the complete documentation index at: https://preview.bazel.build/llms.txt
> Use this file to discover all available pages before exploring further.
# Bazel Tutorial: Configure C++ Toolchains
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 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 19`, which you can install on your system.
Set up the build environment
Set up your build environment as follows:
1. If you have not already done so, [download and install Bazel 7.0.2](https://bazel.build/install) or later.
2. Add an empty `MODULE.bazel` file at the root folder.
3. Add the following `cc_binary` target to the `main/BUILD` file:
```python theme={null}
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.
4. Run the build with the following command:
```bash theme={null}
bazel build //main:hello-world
```
The build succeeds without any toolchain registered in `MODULE.bazel`.
To further see what's under the hood, run:
```bash theme={null}
bazel build //main:hello-world --toolchain_resolution_debug='@bazel_tools//tools/cpp:toolchain_type'
INFO: ToolchainResolution: Target platform @@platforms//host:host: Selected execution platform @@platforms//host: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
`@platforms//host` using
`@bazel_tools+cc_configure_extension+local_config_cc//:cc-compiler-k8`.
To configure the C++ toolchain, repeatedly build the application and eliminate
each error one by one as described as following.
Note: This tutorial assumes you're using Bazel 7.0.2 or later. If you're using
an older release of Bazel, use `--incompatible_enable_cc_toolchain_resolution`
flag to enable C++ toolchain resolution.
It also assumes `clang version 9.0.1`, although the details should only change
slightly between different versions of clang.
1. Add `toolchain/BUILD` with
```python theme={null}
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 add appropriate dependencies and register the toolchain with
`MODULE.bazel` with
```python theme={null}
bazel_dep(name = "platforms", version = "0.0.10")
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.
2. Run the build again. Because the `toolchain` package doesn't yet define the
`linux_x86_64_toolchain_config` target, Bazel throws the following error:
```bash theme={null}
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.
```
3. In the `toolchain/BUILD` file, define an empty filegroup as follows:
```python theme={null}
package(default_visibility = ["//visibility:public"])
filegroup(name = "linux_x86_64_toolchain_config")
```
4. Run the build again. Bazel throws the following error:
```bash theme={null}
'//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:
```python theme={null}
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:
```python theme={null}
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:
```python theme={null}
cc_toolchain_config(name = "linux_x86_64_toolchain_config")
```
5. Run the build again. Bazel throws the following error:
```bash theme={null}
.../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`](https://source.bazel.build/bazel/+/4eea5c62a566d21832c93e4c18ec559e75d5c1ce:tools/cpp/cc_toolchain_config_lib.bzl;l=400):
```python theme={null}
# 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", # Compiler is referenced by the name "gcc" for historic reasons.
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. Note that the compiler is referenced by the name "gcc" for
historic reasons.
6. Run the build again. Bazel throws the following error:
```bash theme={null}
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`](/rules/lib/toplevel/cc_common#create_cc_toolchain_config_info)
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:
```python theme={null}
return cc_common.create_cc_toolchain_config_info(
ctx = ctx,
cxx_builtin_include_directories = [ # NEW
"/usr/lib/llvm-19/lib/clang/19/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,
)
```
7. Run the build command again, you will see an error like:
```bash theme={null}
/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`:
```python theme={null}
# 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", # Compiler is referenced by the name "gcc" for historic reasons.
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-19/lib/clang/19/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],
)
```
Note that this code uses the GNU C++ library libstdc++. If you want to use
the LLVM C++ library, use "-lc++" instead of "-lstdc++".
8. Running `bazel build //main:hello-world`, it should finally build the binary
successfully for host.
9. In `toolchain/BUILD`, copy the `cc_toolchain_config`, `cc_toolchain`, and
`toolchain` targets and replace `linux_x86_64` with `android_x86_64`in
target names.
In `MODULE.bazel`, register the toolchain for android
```python theme={null}
register_toolchains(
"//toolchain:cc_toolchain_for_linux_x86_64",
"//toolchain:cc_toolchain_for_android_x86_64"
)
```
10. 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 example.
The key takeaways 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](/concepts/platforms).
* 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
[external dependencies](/external/overview). Your tools could come from a
different module 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.
Further reading
For more details, see [C++ toolchain configuration](/docs/cc-toolchain-config-reference)