# rules_foreign_cc
[](https://buildkite.com/bazel/rules-foreign-cc)
Rules for building C/C++ projects using foreign build systems inside Bazel projects.
* **Experimental** - API will most definitely change.
* This is not an officially supported Google product
(meaning, support and/or new releases may be limited.)
## Bazel versions compatibility
**Bazel HEAD after [cae1e816e](https://github.com/bazelbuild/bazel/commit/cae1e816e5e1142fbd4aefdd29bffb2cbad71fa8) or 0.23+:**
No flags are required, works on Windows.
**Bazel 0.22:**
No flags are required, but unfortunately Bazel's Starlark C++ API **is broken on Windows**.
**Bazel 0.20 - 0.21:**
Pass the Bazel the following flag:
```
--experimental_cc_skylark_api_enabled_packages=@rules_foreign_cc//tools/build_defs,tools/build_defs,@foreign_cc_impl
```
Where ```rules_foreign_cc``` is the name of this repository in your WORKSPACE file.
## News
**January 2019:**
- Bazel 0.22.0 is released, no flags are needed for this version, but it does not work on Windows (Bazel C++ API is broken).
- Support for versions earlier then 0.20 was removed.
- [rules_foreign_cc take-aways](https://docs.google.com/document/d/1ZVvzvkUVTkPCzI-2z4S4VrSNu4kdaBknz7UnK8vaoZU/edit?usp=sharing) describing the recent work has been published.
- Examples package became the separate workspace.
This also allows to illustrate how to initialize rules_foreign_cc.
- Native tools (cmake, ninja) toolchains were introduced.
Though the user code does not have to be changed (default toolchains are registered, they call the preinstalled binaries by name.),
you may simplify usage of ninja with the cmake_external rule and call it just by name.
Please see examples/cmake_nghttp2 for ninja usage, and WORKSPACE and BUILD files in examples for the native tools toolchains usage
(the locally preinstalled tools are registered by default, the build as part of the build tools are used in examples).
Also, in examples/with_prebuilt_ninja_artefact you can see how to download and use prebuilt artifact.
- Shell script parts were extracted into a separate toolchain.
Shell script inside framework.bzl is first created with special notations:
- 'export var_name=var_value' for defining the environment variable
- '$$var_name$$' for referencing environment variable
- 'shell_command ' for calling shell fragment
The created script is further processed to get the real shell script with shell parts either
replaced with actual fragments or with shell function calls (functions are added into the beginning of the script).
Extracted shell fragments are described in commands.bzl.
Further planned steps in this direction: testing with RBE, shell script fragments for running on Windows without msys/mingw,
tests for shell fragments.
## Building CMake projects:
- Build libraries/binaries with CMake from sources using cmake_external rule
- Use cmake_external targets in cc_library, cc_binary targets as dependency
- Bazel cc_toolchain parameters are used inside cmake_external build
- See full list of cmake_external arguments below 'example'
- cmake_external is defined in ./tools/build_defs
- Works on Ubuntu, Mac OS and Windows(* see special notes below in Windows section) operating systems
**Example:**
(Please see full examples in ./examples)
The example for **Windows** is below, in the section 'Usage on Windows'.
* In `WORKSPACE`, we use a `http_archive` to download tarballs with the libraries we use.
* In `BUILD`, we instantiate a `cmake_external` rule which behaves similarly to a `cc_library`, which can then be used in a C++ rule (`cc_binary` in this case).
In `WORKSPACE`, put
```python
workspace(name = "rules_foreign_cc_usage_example")
load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
# Group the sources of the library so that CMake rule have access to it
all_content = """filegroup(name = "all", srcs = glob(["**"]), visibility = ["//visibility:public"])"""
# Rule repository
http_archive(
name = "rules_foreign_cc",
strip_prefix = "rules_foreign_cc-master",
url = "https://github.com/bazelbuild/rules_foreign_cc/archive/master.zip",
)
load("@rules_foreign_cc//:workspace_definitions.bzl", "rules_foreign_cc_dependencies")
# Call this function from the WORKSPACE file to initialize rules_foreign_cc
# dependencies and let neccesary code generation happen
# (Code generation is needed to support different variants of the C++ Starlark API.).
#
# Args:
# native_tools_toolchains: pass the toolchains for toolchain types
# '@rules_foreign_cc//tools/build_defs:cmake_toolchain' and
# '@rules_foreign_cc//tools/build_defs:ninja_toolchain' with the needed platform constraints.
# If you do not pass anything, registered default toolchains will be selected (see below).
#
# register_default_tools: if True, the cmake and ninja toolchains, calling corresponding
# preinstalled binaries by name (cmake, ninja) will be registered after
# 'native_tools_toolchains' without any platform constraints.
# The default is True.
rules_foreign_cc_dependencies(["//:my_cmake_toolchain", "//:my_ninja_toolchain"])
# OpenBLAS source code repository
http_archive(
name = "openblas",
build_file_content = all_content,
strip_prefix = "OpenBLAS-0.3.2",
urls = ["https://github.com/xianyi/OpenBLAS/archive/v0.3.2.tar.gz"],
)
# Eigen source code repository
http_archive(
name = "eigen",
build_file_content = all_content,
strip_prefix = "eigen-git-mirror-3.3.5",
urls = ["https://github.com/eigenteam/eigen-git-mirror/archive/3.3.5.tar.gz"],
)
```
and in `BUILD`, put
```python
load("@rules_foreign_cc//tools/build_defs:cmake.bzl", "cmake_external")
cmake_external(
name = "openblas",
# Values to be passed as -Dkey=value on the CMake command line;
# here are serving to provide some CMake script configuration options
cache_entries = {
"NOFORTRAN": "on",
"BUILD_WITHOUT_LAPACK": "no",
},
lib_source = "@openblas//:all",
# We are selecting the resulting static library to be passed in C/C++ provider
# as the result of the build;
# However, the cmake_external dependants could use other artefacts provided by the build,
# according to their CMake script
static_libraries = ["libopenblas.a"],
)
cmake_external(
name = "eigen",
# These options help CMake to find prebuilt OpenBLAS, which will be copied into
# $EXT_BUILD_DEPS/openblas by the cmake_external script
cache_entries = {
"BLA_VENDOR": "OpenBLAS",
"BLAS_LIBRARIES": "$EXT_BUILD_DEPS/openblas/lib/libopenblas.a",
},
headers_only = True,
lib_source = "@eigen//:all",
# Dependency on other cmake_external rule; can also depend on cc_import, cc_library rules
deps = [":openblas"],
)
```
then build as usual:
```bash
$ devbazel build //examples/cmake_pcl:eigen
```
**Usage on Windows**
When using on Windows, you should start Bazel in MSYS2 shell, as the shell script inside cmake_external assumes this.
Also, you should explicitly specify **make commands and option to generate CMake crosstool file**.
The default generator for CMake will be detected automatically, or you can specify it explicitly.
**The tested generators:** Visual Studio 15, Ninja and NMake.
The extension '.lib' is assumed for the static libraries by default.
Example usage (see full example in ./examples/cmake_hello_world_lib):
Example assumes that MS Visual Studio and Ninja are installed on the host machine, and Ninja bin directory is added to PATH.
```python
cmake_external(
# expect to find ./lib/hello.lib as the result of the build
name = "hello",
# This option can be omitted
cmake_options = ["-G \"Visual Studio 15 2017 Win64\""],
generate_crosstool_file = True,
lib_source = ":srcs",
# .vcxproj or .sln file must be specified argument, as multiple files are generated by CMake
make_commands = ["MSBuild.exe INSTALL.vcxproj"],
)
cmake_external(
name = "hello_ninja",
# expect to find ./lib/hello.lib as the result of the build
lib_name = "hello",
# explicitly specify the generator
cmake_options = ["-GNinja"],
generate_crosstool_file = True,
lib_source = ":srcs",
# specify to call ninja after configuring
make_commands = [
"ninja",
"ninja install",
],
)
cmake_external(
name = "hello_nmake",
# explicitly specify the generator
cmake_options = ["-G \"NMake Makefiles\""],
generate_crosstool_file = True,
lib_source = ":srcs",
# specify to call nmake after configuring
make_commands = [
"nmake",
"nmake install",
],
# expect to find ./lib/hello.lib as the result of the build
static_libraries = ["hello.lib"]
)
```
**cmake_external arguments:**
Mandatory arguments:
```name, lib_source```
```python
attrs: {
# CMake only:
#
# Relative install prefix to be passed to CMake in -DCMAKE_INSTALL_PREFIX
"install_prefix": attr.string(mandatory = False),
# CMake cache entries to initialize (they will be passed with -Dkey=value)
# Values, defined by the toolchain, will be joined with the values, passed here.
# (Toolchain values come first)
"cache_entries": attr.string_dict(mandatory = False, default = {}),
# CMake environment variable values to join with toolchain-defined.
# For example, additional CXXFLAGS.
"env_vars": attr.string_dict(mandatory = False, default = {}),
# Other CMake options
"cmake_options": attr.string_list(mandatory = False, default = []),
# When True, CMake crosstool file will be generated from the toolchain values,
# provided cache-entries and env_vars (some values will still be passed as -Dkey=value
# and environment variables).
# If CMAKE_TOOLCHAIN_FILE cache entry is passed, specified crosstool file will be used
# When using this option, it makes sense to specify CMAKE_SYSTEM_NAME in the
# cache_entries - the rule makes only a poor guess about the target system,
# it is better to specify it manually.
"generate_crosstool_file": attr.bool(mandatory = False, default = False),
#
# From framework.bzl:
#
# Library name. Defines the name of the install directory and the name of the static library,
# if no output files parameters are defined (any of static_libraries, shared_libraries,
# interface_libraries, binaries_names)
# Optional. If not defined, defaults to the target's name.
"lib_name": attr.string(mandatory = False),
# Label with source code to build. Typically a filegroup for the source of remote repository.
# Mandatory.
"lib_source": attr.label(mandatory = True, allow_files = True),
# Optional compilation definitions to be passed to the dependencies of this library.
# They are NOT passed to the compiler, you should duplicate them in the configuration options.
"defines": attr.string_list(mandatory = False, default = []),
#
# Optional additional inputs to be declared as needed for the shell script action.
# Not used by the shell script part in cc_external_rule_impl.
"additional_inputs": attr.label_list(mandatory = False, allow_files = True, default = []),
# Optional additional tools needed for the building.
# Not used by the shell script part in cc_external_rule_impl.
"additional_tools": attr.label_list(mandatory = False, allow_files = True, default = []),
#
# Optional part of the shell script to be added after the make commands
"postfix_script": attr.string(mandatory = False),
# Optinal make commands, defaults to ["make", "make install"]
"make_commands": attr.string_list(mandatory = False, default = ["make", "make install"]),
#
# Optional dependencies to be copied into the directory structure.
# Typically those directly required for the external building of the library/binaries.
# (i.e. those that the external buidl system will be looking for and paths to which are
# provided by the calling rule)
"deps": attr.label_list(mandatory = False, allow_files = True, default = []),
# Optional tools to be copied into the directory structure.
# Similar to deps, those directly required for the external building of the library/binaries.
"tools_deps": attr.label_list(mandatory = False, allow_files = True, default = []),
#
# Optional name of the output subdirectory with the header files, defaults to 'include'.
"out_include_dir": attr.string(mandatory = False, default = "include"),
# Optional name of the output subdirectory with the library files, defaults to 'lib'.
"out_lib_dir": attr.string(mandatory = False, default = "lib"),
# Optional name of the output subdirectory with the binary files, defaults to 'bin'.
"out_bin_dir": attr.string(mandatory = False, default = "bin"),
#
# Optional. if true, link all the object files from the static library,
# even if they are not used.
"alwayslink": attr.bool(mandatory = False, default = False),
# Optional link options to be passed up to the dependencies of this library
"linkopts": attr.string_list(mandatory = False, default = []),
#
# Output files names parameters. If any of them is defined, only these files are passed to
# Bazel providers.
# if no of them is defined, default lib_name.a/lib_name.lib static library is assumed.
#
# Optional names of the resulting static libraries.
"static_libraries": attr.string_list(mandatory = False),
# Optional names of the resulting shared libraries.
"shared_libraries": attr.string_list(mandatory = False),
# Optional names of the resulting interface libraries.
"interface_libraries": attr.string_list(mandatory = False),
# Optional names of the resulting binaries.
"binaries": attr.string_list(mandatory = False),
# Flag variable to indicate that the library produces only headers
"headers_only": attr.bool(mandatory = False, default = False),
}
```
## Design document:
[External C/C++ libraries rules](https://docs.google.com/document/d/1Gv452Vtki8edo_Dj9VTNJt5DA_lKTcSMwrwjJOkLaoU/edit?usp=sharing)