rules_flex

April 18, 2026 · View on GitHub

Bazel rules for Flex, the Fast Lexical Analyzer.

flex

load("@rules_flex//flex:flex.bzl", "flex")

flex(name, src, flex_options, language, output_src_ext)

Generate C/C++ source code for a Flex lexical analyzer.

This rule exists for special cases where the build needs to perform further modification of the generated .c / .h before compilation. Most users will find the flex_cc_library rule more convenient.

The output groups cc_srcs and cc_hdrs provide access to the generated {name}.c / {name}.cc sources and (if available) the {name}.h header.

Example

load("@rules_flex//flex:flex.bzl", "flex")

flex(
    name = "hello",
    src = "hello.l",
)

ATTRIBUTES

Name	Description	Type	Mandatory	Default
name	A unique name for this target.	Name	required
src	A Flex source file. Unless `language` is set, the source's file extension will determine whether Flex operates in C or C++ mode: Inputs with file extension `.l` generate outputs `{name}.c` and `{name}.h`. Inputs with file extension `.ll`, `.l++`, `.lxx`, or `.lpp` generate output `{name}.cc`. This is equivalent to invoking Flex as `flex++`. The C++ output depends on `FlexLexer.h`, which is part of the Flex source distribution and may be obtained from the Flex toolchain.	Label	required
flex_options	Additional options to pass to the `flex` command. These will be added to the command args immediately before the source file.	List of strings	optional	`[]`
language	Which language to generate the lexer in.	String	optional	`""`
output_src_ext	Override the file extension used for the generated source file. By default the extension is auto-detected according to the `language` attribute, which is itself auto-detected according to the input file extension.	String	optional	`""`

flex_cc_library

load("@rules_flex//flex:flex.bzl", "flex_cc_library")

flex_cc_library(name, deps, src, defines, flex_options, include_prefix, language, linkstatic,
                local_defines, output_src_ext, strip_include_prefix)

Generate a C/C++ library for a Flex lexical analyzer.

Example

load("@rules_flex//flex:flex.bzl", "flex_cc_library")

flex_cc_library(
    name = "hello_lib",
    src = "hello.l",
)

cc_binary(
    name = "hello",
    srcs = ["hello_main.c"],
    deps = [":hello_lib"],
)

ATTRIBUTES

Name	Description	Type	Mandatory	Default
name	A unique name for this target.	Name	required
deps	A list of other C/C++ libraries that the library depends on. See `cc_library.deps` for more details.	List of labels	optional	`[]`
src	A Flex source file. Unless `language` is set, the source's file extension will determine whether Flex operates in C or C++ mode: Inputs with file extension `.l` generate outputs `{name}.c` and `{name}.h`. Inputs with file extension `.ll`, `.l++`, `.lxx`, or `.lpp` generate output `{name}.cc`. This is equivalent to invoking Flex as `flex++`. The C++ output depends on `FlexLexer.h`, which is part of the Flex source distribution and may be obtained from the Flex toolchain.	Label	required
defines	List of defines to add to the compile line of this and all dependent targets. See `cc_library.defines` for more details.	List of strings	optional	`[]`
flex_options	Additional options to pass to the `flex` command. These will be added to the command args immediately before the source file.	List of strings	optional	`[]`
include_prefix	A prefix to add to the path of the generated header. See `cc_library.include_prefix` for more details.	String	optional	`""`
language	Which language to generate the lexer in.	String	optional	`""`
linkstatic	Disable creation of a shared library output. See `cc_library.linkstatic` for more details.	Boolean	optional	`False`
local_defines	List of defines to add to the compile line of this target. See `cc_library.local_defines` for more details.	List of strings	optional	`[]`
output_src_ext	Override the file extension used for the generated source file. By default the extension is auto-detected according to the `language` attribute, which is itself auto-detected according to the input file extension.	String	optional	`""`
strip_include_prefix	A prefix to strip from the path of the generated header. See `cc_library.strip_include_prefix` for more details.	String	optional	`""`

flex_toolchain_info

load("@rules_flex//flex:flex.bzl", "flex_toolchain_info")

flex_toolchain_info(name, flex_env, flex_lexer_h, flex_tool)

Provides ToolchainInfo and TemplateVariableInfo for the Flex toolchain.

ATTRIBUTES

Name	Description	Type	Mandatory	Default
name	A unique name for this target.	Name	required
flex_env	Additional environment variables to set when running `flex_tool`.	Dictionary: String -> String	optional	`{}`
flex_lexer_h	Label of `FlexLexer.h`, used for generated C++ lexers.	Label	required
flex_tool	A `FilesToRunProvider` for the `flex` binary.	Label	required

FlexToolchainInfo

load("@rules_flex//flex:flex.bzl", "FlexToolchainInfo")

FlexToolchainInfo(all_files, flex_tool, flex_env, flex_lexer_h)

Provider for a Flex toolchain.

FIELDS

Name	Description
all_files	A `depset` containing all files comprising this Flex toolchain.
flex_tool	A `FilesToRunProvider` for the `flex` binary.
flex_env	Additional environment variables to set when running `flex_tool`.
flex_lexer_h	A `File` for the `FlexLexer.h` header.

flex_register_toolchains

load("@rules_flex//flex:flex.bzl", "flex_register_toolchains")

flex_register_toolchains(version, extra_copts)

A helper function for Flex toolchains registration.

This workspace macro will create a flex_repository named flex_v{version} and register it as a Bazel toolchain.

PARAMETERS

Name	Description	Default Value
version	A supported version of Flex.	`"2.6.4"`
extra_copts	Additional C compiler options to use when building Flex.	`[]`

flex_toolchain

load("@rules_flex//flex:flex.bzl", "flex_toolchain")

flex_toolchain(ctx)

Returns the current FlexToolchainInfo.

PARAMETERS

Name	Description	Default Value
ctx	A rule context, where the rule has a toolchain dependency on `FLEX_TOOLCHAIN_TYPE`.	none

RETURNS

A FlexToolchainInfo.

flex_repository

load("@rules_flex//flex:flex.bzl", "flex_repository")

flex_repository(name, extra_copts, extra_linkopts, patch_strip, patches, version)

Repository rule for Flex.

The resulting repository will have a //bin:flex executable target.

Example

load("@rules_flex//flex:flex.bzl", "flex_repository")

flex_repository(
    name = "flex_v2.6.4",
    version = "2.6.4",
)

ATTRIBUTES

Name	Description	Type	Mandatory	Default
name	A unique name for this repository.	Name	required
extra_copts	Additional C compiler options to use when building Flex.	List of strings	optional	`[]`
extra_linkopts	Additional linker options to use when building Flex.	List of strings	optional	`[]`
patch_strip	Strip the specified number of leading components from patch file names.	Integer	optional	`0`
patches	A mapping from Flex versions to lists of patch files to apply, relative to the root of the Flex source repository. Each patch should be in standard unified diff format.	Dictionary: String -> List of strings	optional	`{"2.6.4": ["//flex/patches:0001-fix-noline-for-top-directives.patch"]}`
version	A supported version of Flex.	String	required

flex_toolchain_repository

load("@rules_flex//flex:flex.bzl", "flex_toolchain_repository")

flex_toolchain_repository(name, flex_repository)

Toolchain repository rule for Flex toolchains.

Toolchain repositories add a layer of indirection so that Bazel can resolve toolchains without downloading additional dependencies.

The resulting repository will have the following targets:

//bin:flex (an alias into the underlying [flex_repository] (#flex_repository))
//:toolchain, which can be registered with Bazel.

Example

load(
    "@rules_flex//flex:flex.bzl",
    "flex_repository",
    "flex_toolchain_repository",
)

flex_repository(
    name = "flex_v2.6.4",
    version = "2.6.4",
)

flex_toolchain_repository(
    name = "flex",
    flex_repository = "@flex_v2.6.4",
)

register_toolchains("@flex//:toolchain")

ATTRIBUTES

Name	Description	Type	Mandatory	Default
name	A unique name for this repository.	Name	required
flex_repository	The name of a `flex_repository`.	String	required

flex_repository_ext

flex_repository_ext = use_extension("@rules_flex//flex/extensions:flex_repository_ext.bzl", "flex_repository_ext")
flex_repository_ext.repository(name, extra_copts, extra_linkopts, version)

Module extension for declaring dependencies on Flex.

The resulting repository will have the following targets:

//bin:flex (an alias into the underlying [flex_repository] (#flex_repository))
//:toolchain, which can be registered with Bazel.

Example

flex = use_extension(
    "@rules_flex//flex/extensions:flex_repository_ext.bzl",
    "flex_repository_ext",
)

flex.repository(name = "flex", version = "2.6.4")
use_repo(flex, "flex")
register_toolchains("@flex//:toolchain")

TAG CLASSES

repository

Attributes

Name	Description	Type	Mandatory	Default
name	An optional name for the repository. The name must be unique within the set of names registered by this extension. If unset, the repository name will default to `"flex_v{version}"`.	Name	optional	`""`
extra_copts	Additional C compiler options to use when building Flex.	List of strings	optional	`[]`
extra_linkopts	Additional linker options to use when building Flex.	List of strings	optional	`[]`
version	A supported version of Flex.	String	optional	`"2.6.4"`

flex_toolchains_ext

flex_toolchains_ext = use_extension("@rules_flex//flex/extensions:flex_toolchains_ext.bzl", "flex_toolchains_ext")
flex_toolchains_ext.toolchain(name, flex_env, flex_lexer_h, flex_tool)

Module extension for declaring Flex toolchains with custom target binaries.

The resulting repository will have one subdirectory per named module tag, which contains a :toolchain target that can be registered with Bazel.

Example

flex_toolchains = use_extension(
    "@rules_flex//flex/extensions:flex_toolchains_ext.bzl",
    "flex_toolchain_ext",
)

flex_toolchains.toolchain(
    name = "custom",
    flex_tool = "//custom_flex:flex",
    flex_lexer_h = "//custom_flex:flex_lexer_h",
)
use_repo(flex_toolchains, "flex_toolchains")
register_toolchains("@flex_toolchains//custom:toolchain")

TAG CLASSES

toolchain

Attributes

Name	Description	Type	Mandatory	Default
name	The name of the toolchain repository to create.	Name	required
flex_env	Additional environment variables to set when running `flex_tool`.	Dictionary: String -> String	optional	`{}`
flex_lexer_h	Label of `FlexLexer.h`, used for generated C++ lexers.	Label	required
flex_tool	The label of an `flex` executable target.	Label	required