Bazel build rules for GNU M4

The m4 macro processing language is commonly used as an intermediate format for text-based Unix development tools such as Bison and Flex.

This Bazel ruleset allows GNU M4 to be integrated into a Bazel build. It can be used to perform macro expansion with the //m4:m4.bzl%m4 build rule, or as a dependency in other rules via the Bazel toolchain system.

Currently, the only implementation of m4 supported by this ruleset is GNU M4.

API reference: docs/rules_m4.md

Setup

As a module dependency (bzlmod)

Add the following to your MODULE.bazel:

bazel_dep(name = "rules_m4", version = "0.2.4")

To specify a version or build with additional C compiler options, use the m4_repository_ext module extension:

m4 = use_extension(
    "@rules_m4//m4/extensions:m4_repository_ext.bzl",
    "m4_repository_ext",
)
m4.repository(
    name = "m4",
    version = "1.4.17",
    extra_copts = ["-O3"],
)
use_repo(m4, "m4")
register_toolchains("@m4//:toolchain")

Note that repository names registered with a given bzlmod module extension must be unique within the scope of that extension. See the Bazel module extensions documentation for more details.

As a workspace dependency

Add the following to your WORKSPACE.bazel:

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

http_archive(
    name = "rules_m4",
    # Obtain the package checksum from the release page:
    # https://github.com/jmillikin/rules_m4/releases/tag/v0.2.4
    sha256 = "",
    urls = ["https://github.com/jmillikin/rules_m4/releases/download/v0.2.4/rules_m4-v0.2.4.tar.xz"],
)

load("@rules_m4//m4:m4.bzl", "m4_register_toolchains")

m4_register_toolchains(version = "1.4.18")

To specify a version or build with additional C compiler options, use the m4_repository and m4_toolchain_repository repository rules:

load("@rules_m4//m4:m4.bzl", "m4_repository", "m4_toolchain_repository")

m4_repository(
    name = "m4_v1.4.17_fast",
    version = "1.4.17",
    extra_copts = ["-O3"],
)

m4_toolchain_repository(
    name = "m4_toolchain_v1.4.17_fast",
    m4_repository = "@m4_v1.4.17_fast",
)

register_toolchains("@m4_toolchain_v1.4.17_fast//:toolchain")

Examples

Macro expansion with the //m4:m4.bzl%m4 build rule:

load("@rules_m4//m4:m4.bzl", "m4")

m4(
    name = "hello_world",
    srcs = ["hello_world.in.txt"],
    output = "hello_world.txt",
)

Macro expansion in a genrule:

genrule(
    name = "hello_world_gen",
    srcs = ["hello_world.in.txt"],
    outs = ["hello_world_gen.txt"],
    cmd = "$(M4) $(SRCS) > $@",
    toolchains = ["@rules_m4//m4:current_m4_toolchain"],
)

Writing a custom rule that depends on m4 as a toolchain:

load("@rules_m4//m4:m4.bzl", "M4_TOOLCHAIN_TYPE", "m4_toolchain")

def _my_rule(ctx):
    m4 = m4_toolchain(ctx)
    ctx.actions.run(
        tools = [m4.m4_tool],
        env = m4.m4_env,
        # ...
    )

my_rule = rule(
    implementation = _my_rule,
    toolchains = [M4_TOOLCHAIN_TYPE],
)