一个bazel使用示例

# Cross Compilation For cross compilation, you have to specify a custom platform to let Bazel know that you are compiling for a different platform than the default host platform. The example code is setup to cross compile from the following hosts to the the following targets: * {linux, x86_64} -> {linux, aarch64} * {linux, aarch64} -> {linux, x86_64} * {darwin, x86_64} -> {linux, x86_64} * {darwin, x86_64} -> {linux, aarch64} * {darwin, aarch64 (Apple Silicon)} -> {linux, x86_64} * {darwin, aarch64 (Apple Silicon)} -> {linux, aarch64} You cross-compile by calling the target. `bazel build //:hello_world_x86_64` or `bazel build //:hello_world_aarch64` Notice, the default target `//...` does not know about the many different target platform and will report an error. Instead, if you want to build for all platforms at once, call the filegroup target: `bazel build //:all` ## Setup The setup requires three steps, first declare dependencies and toolchains in your MODULE.bazel, second configure LLVM and Rust for cross compilation, and third the configuration of the cross compilation platforms so you can use it binary targets. ### Dependencies Configuration You add the required rules for cross compilation to your MODULE.bazel as shown below. ```Starlark # Rules for cross compilation # https://github.com/bazelbuild/platforms/releases bazel_dep(name = "platforms", version = "0.0.10") # https://github.com/bazel-contrib/toolchains_llvm bazel_dep(name = "toolchains_llvm", version = "1.0.0") ``` ## LLVM Configuration Next, you have to configure the LLVM toolchain because rules_rust still needs a cpp toolchain for cross compilation and you have to add the specific platform triplets to the Rust toolchain. Suppose you want to compile a Rust binary that supports linux on both, X86 and ARM. In that case, you have to setup three LLVM toolchains: 1) LLVM for the host 2) LLVM for X86 3) LLVM for ARM (aarch64) For the host LLVM, you just specify a LLVM version and then register the toolchain as usual. The target LLVM toolchains, however, have dependencies on system libraries for the target platform. Therefore, it is required to download a so- called sysroot that contains a root file system with all those system libraries for the specific target platform. To do so, please add the following to your MODULE.bazel ```Starlark # https://github.com/bazelbuild/bazel/blob/master/tools/build_defs/repo/http.bzl http_archive = use_repo_rule("@bazel_tools//:http.bzl", "http_archive") # Both, cross compilation and MUSL still need a C/C++ toolchain with sysroot. _BUILD_FILE_CONTENT = """ filegroup( name = "{name}", srcs = glob(["*/**"]), visibility = ["//visibility:public"], ) """ # Download sysroot # https://commondatastorage.googleapis.com/chrome-linux-sysroot/ http_archive( name = "org_chromium_sysroot_linux_x64", build_file_content = _BUILD_FILE_CONTENT.format(name = "sysroot"), sha256 = "f6b758d880a6df264e2581788741623320d548508f07ffc2ae6a29d0c13d647d", urls = ["https://commondatastorage.googleapis.com/chrome-linux-sysroot/toolchain/2e7ada854015a4cc60fc812112d261af44213ed0/debian_bullseye_amd64_sysroot.tar.xz"], ) http_archive( name = "org_chromium_sysroot_linux_aarch64", build_file_content = _BUILD_FILE_CONTENT.format(name = "sysroot"), sha256 = "902d1a40a5fd8c3764a36c8d377af5945a92e3d264c6252855bda4d7ef81d3df", urls = ["https://commondatastorage.googleapis.com/chrome-linux-sysroot/toolchain/41a6c8dec4c4304d6509e30cbaf9218dffb4438e/debian_bullseye_arm64_sysroot.tar.xz"], ) ``` Here, we declare to new http downloads that retrieve the sysroot for linux_x64 (Intel/AMD) and linux_aarch64 (ARM/Apple Silicon). Note, these are only sysroots, that means you have to configure LLVM next to use these files. As mentioned earlier, three LLVM toolchains needs to be configured and to do that, please add the following to your MODULE.bazel ```Starlark LLVM_VERSIONS = { "": "16.0.0", } # Host LLVM toolchain. llvm.toolchain( name = "llvm_toolchain", llvm_versions = LLVM_VERSIONS, ) use_repo(llvm, "llvm_toolchain", "llvm_toolchain_llvm") # X86 LLVM Toolchain with sysroot. # https://github.com/bazel-contrib/toolchains_llvm/blob/master/tests/WORKSPACE.bzlmod llvm.toolchain( name = "llvm_toolchain_x86_with_sysroot", llvm_versions = LLVM_VERSIONS, ) llvm.sysroot( name = "llvm_toolchain_x86_with_sysroot", label = "@org_chromium_sysroot_linux_x64//:sysroot", targets = ["linux-x86_64"], ) use_repo(llvm, "llvm_toolchain_x86_with_sysroot") # # ARM (aarch64) LLVM Toolchain with sysroot. # https://github.com/bazelbuild/rules_rust/blob/main/examples/bzlmod/cross_compile/WORKSPACE.bzlmod llvm.toolchain( name = "llvm_toolchain_aarch64_with_sysroot", llvm_versions = LLVM_VERSIONS, ) llvm.sysroot( name = "llvm_toolchain_aarch64_with_sysroot", label = "@org_chromium_sysroot_linux_aarch64//:sysroot", targets = ["linux-aarch64"], ) use_repo(llvm, "llvm_toolchain_aarch64_with_sysroot") # Register all LLVM toolchains register_toolchains("@llvm_toolchain//:all") ``` For simplicity, all toolchains are pinned to version LLVM 16 because it is one of the few releases that supports the host (apple-darwin / Ubuntu), and the two targets. For a complete [list off all LLVM releases and supported platforms, see this list.](https://github.com/bazel-contrib/toolchains_llvm/blob/master/toolchain/internal/llvm_distributions.bzl) It is possible to pin different targets to different LLVM versions; [see the documentation for details](https://github.com/bazel-contrib/toolchains_llvm/tree/master?tab=readme-ov-file#per-host-architecture-llvm-version). ### LLVM Troubleshooting On older linux distributions (Ubuntu 16.04) you may encounter an error that C++ versions before C++ 14 are no longer supported. In this case, just install gcc version 7 or newer. This is rare corner case, but there are gcc backports for older distributions, so please upgrade your compiler if you ever see this error. On Ubuntu 20.04 you may see an error that a shared library called libtinfo.so.5 is missing. In that case, just install libtinfo via apt-get since its in the official 20.04 repo. To so, open a terminal and type: ` apt update && apt install -y libtinfo5 ` The libtinfo5 library may have different package names on other distributions, but it is a well known issue. [See this SO discussion](https://stackoverflow.com/questions/48674104/clang-error-while-loading-shared-libraries-libtinfo-so-5-cannot-open-shared-o) for various solutions. On MacOX, it is sufficient to have the Apple Clang compiler installed. I don't recommend installing the full Xcode package unless you're developing software for an Apple device. Instead, the Xcode Command Line Tools provide everything you need at a much smaller download size. In most cases, a simple: `xcode-select --install` From a terminal triggers the installation process. For details and alternative options, [read this article on freebootcamp.](https://www.freecodecamp.org/news/install-xcode-command-line-tools/) Windows is not directly supported, but you can use Linux on Windows with WSL to setup an Ubuntu environment within Windows. Please refer to the [official WSL documentation for details.](https://learn.microsoft.com/en-us/windows/wsl/install) **Rust Toolchain Configuration** The Rust toolchain only need to know the additional platform triplets to download the matching toolchains. To do so, add or or modify your MODULE.bazel with the following entry: ```Starlark # Rust toolchain RUST_EDITION = "2021" RUST_VERSION = "1.79.0" rust = use_extension("@rules_rust//rust:extensions.bzl", "rust") rust.toolchain( edition = RUST_EDITION, versions = [RUST_VERSION], extra_target_triples = [ "aarch64-unknown-linux-gnu", "x86_64-unknown-linux-gnu", ], ) use_repo(rust, "rust_toolchains") register_toolchains("@rust_toolchains//:all") ``` You find the exact platform triplets in t