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Configurable Build Attributes

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Configurable attributes, commonly known as select(), is a Bazel feature that lets users toggle the values of BUILD rule attributes at the command line.

This can be used, for example, to write multiplatform libraries that automatically choose the right implementation or feature-configurable binaries that can be custom assembled at build time.

Example

//myapp/BUILD:
cc_binary(
    name = "mybinary",
    srcs = ["main.cc"],
    deps = select({
        ":arm_build": [":arm_lib"],
        ":x86_debug_build": [":x86_dev_lib"],
        "//conditions:default": [":generic_lib"]
    })
)

config_setting(
    name = "arm_build",
    values = { "cpu": "arm" }
)

config_setting(
    name = "x86_debug_build",
    values = {
      "cpu": "x86",
      "compilation_mode": "dbg"
    }
)

This declares a cc_binary that "chooses" its deps based on the flags at the command line. Specficially, deps becomes:

Command deps =
bazel build //myapp:mybinary --cpu=arm [":arm_lib"]
bazel build //myapp:mybinary --c dbg --cpu=x86 [":x86_dev_lib"]
bazel build //myapp:mybinary --cpu=ppc [":generic_lib"]
bazel build //myapp:mybinary -c dbg --cpu=ppc [":generic_lib"]

 

select() turns any attribute into a dictionary that maps configuration conditions to desired values. Configuration conditions are build labels that reference config_setting rules. Values are any value the attribute can normally take.

Matches must be unambiguous: either exactly one condition must match or, if multiple conditions match, one's values must be a strict superset of all others' (for example, values = {"cpu": "x86", "compilation_mode": "dbg"} is an unambiguous specialization of values = {"cpu": "x86"}). The built-in condition //conditions:default automatically matches when nothing else does.

This example uses deps. But select() works just as well on srcs, resources, cmd, or practically any other attribute. Only a small number of attributes are non-configurable, and those are clearly annotated.

Configuration Conditions

Each key in a configurable attribute is a label reference to a config_setting rule. This is just a collection of expected command line flag settings. By encapsulating these in a rule, it's easy to maintain "standard" conditions that can be referenced across rules and BUILD files.

The core config_setting syntax is:

config_setting(
    name = "meaningful_condition_name",
    values = {
        "flag1": "expected_value1",
        "flag2": "expected_value2",
        ...
    }
)

flagN is an arbitrary Bazel command line flag. value is the expected value for that flag. A config_setting matches when all of its flags match.

values entries use the same parsing logic as at the actual command line. This means:

  • values = { "compilation_mode": "opt" } matches bazel build -c opt ...
  • values = { "java_header_compilation": "true" } matches bazel build --java_header_compilation=1 ...
  • values = { "java_header_compilation": "0" } matches bazel build --nojava_header_compilation ...

config_setting only works with flags that affect build rule output. For example, --show_progress isn't allowed because this only affects how Bazel reports progress to the user.

config_setting semantics are intentionally simple. For example, there's no direct support for OR chaining (although a Skylark convenience function provides this). Consider writing Skylark macros for complicated flag logic.

Defaults

The built-in condition //conditions:default matches when no other condition matches.

Because of the "exactly one match" rule, a configurable attribute with no match and no default condition triggers a "no matching conditions" error. This can protect against silent failures from unexpected build flags:

//foo:
config_setting(
    name = "foobar",
    values = { "define": "foo=bar" }
)

cc_library(
    name = "my_lib",
    srcs = select({
        ":foobar": ["foobar_lib.cc"],
    })
)
$ bazel build //foo:my_lib --define foo=baz
ERROR: Configurable attribute "srcs" doesn't match this configuration (would
a default condition help?).
Conditions checked:
  //foo:foobar

select() can include a no_match_error for custom failure messages.

Custom Keys

Since config_setting currently only supports built-in Bazel flags, the level of custom conditioning it can support is limited. For example, there's no Bazel flag for IncludeSpecialProjectFeatureX.

Plans for truly custom flags are underway. In the meantime, --define is the best approach for these purposes. --define is a bit awkward to use and wasn't originally designed for this purpose. We recommend using it sparingly until true custom flags are available. For example, don't use --define to specify multiple variants of top-level binary. Just use multiple rules instead.

To trigger an arbitrary condition with --define, write

config_setting(
    name = "bar",
    values = { "define": "foo=bar" }
)

config_setting(
    name = "baz",
    values = { "define": "foo=baz" }
)

and run $ bazel build //my:target --define foo=baz.

The values attribute can't contain multiple defines. This is because each instance has the same dictionary key. To solve this, use define_values:

config_setting(
    name = "bar_and_baz",
    define_values = {
        "foo": "bar", # matches --define foo=bar
        "baz": "bat", # matches --define baz=bat
    }
)

When defines appear in both values and define_values, all must match for the config_setting to match.

Platforms

While the ability to specify multiple flags on the command line provides flexibility, it can also be burdensome to individually set each --cpu, -crosstool_top, etc. flag every time you want to build a target. Platforms allow you to consolidate these into simple bundles.

sh_binary(
    name = "my_rocks_rule",
    srcs = select({
        ":basalt" : ["pyroxene.sh"],
        ":marble" : ["calcite.sh"],
        "//conditions:default": ["feldspar.sh"]
    })
)

config_setting(
    name = "basalt",
    constraint_values = [
        ":igneous",
        ":black"
    ]
)

config_setting(
    name = "marble",
    constraint_values = [
        ":white",
        ":metamorphic"
        ":smooth"
    ]
)

constraint_setting(name = "color")
constraint_value(name = "black", constraint_setting = "color")
constraint_value(name = "white", constraint_setting = "color")
constraint_setting(name = "texture")
constraint_value(name = "smooth", constraint_setting = "texture")
constraint_setting(name = "type")
constraint_value(name = "igneous", constraint_setting = "type")
constraint_value(name = "metamorphic", constraint_setting = "type")

platform(
    name = "basalt_platform",
    constraint_values = [
        ":black",
        ":igneous"]
)

platform(
    name = "marble_platform",
    constraint_values = [
        ":white",
        ":smooth"
        ":metamorphic"
    ]
)

The platform specified on the command line matches a config_setting that contains the same set (or a superset) of constraint_values and triggers that config_setting as a match in the select() statement.

For example, in order to set the srcs attribute of my_rocks_rule to calcite.sh, simply run

bazel build my_app:my_rocks_rule --platforms=marble_platform

Without platforms, this might look something like

bazel build my_app:my_rocks_rule --define color=light --define texture=smooth --define type=metamorphic

Platforms are still under development. See the documentation and roadmap for details.

Short Keys

Since configuration keys are rule labels, their names can get long and unwieldy. This can be mitigated with local variable definitions:

Before:

sh_binary(
    name = "my_rule",
    srcs = select({
        "//my/project/my/team/configs:config1": ["my_rule_1.sh"],
        "//my/project/my/team/configs:config2": ["my_rule_2.sh"],
    })
)

After:

CONFIG1="//my/project/my/team/configs:config1"
CONFIG2="//my/project/my/team/configs:config2"

sh_binary(
    name = "my_rule",
    srcs = select({
        CONFIG1: ["my_rule_1.sh"],
        CONFIG2: ["my_rule_2.sh"],
    })
)

For more complex expressions, use Skylark macros:

Before:

//foo/BUILD
genrule(
    name = "my_rule",
    srcs = [],
    outs = ["my_rule.out"],
    cmd = select({
        "//my/project/my/team/configs/config1": "echo custom val: this > $@",
        "//my/project/my/team/configs/config2": "echo custom val: that > $@",
        "//conditions:default": "echo default output > $@"
    })
)

After:

//foo/genrule_select.bzl:
def select_echo(input_dict):
  echo_cmd = "echo %s > $@"
  out_dict = {"//conditions:default": echo_cmd % "default output" }
  for (key, val) in input_dict.items():
    cmd = echo_cmd % ("custom val: " + val)
    out_dict["//my/project/my/team/configs/config" + key] = cmd
  return select(out_dict)
//foo/BUILD:
load("//foo:genrule_select.bzl", "select_echo")
genrule(
    name = "my_rule",
    srcs = [],
    outs = ["my_rule.out"],
    cmd = select_echo({
        "1": "this",
        "2": "that",
    })
)

Multiple Selects

select can appear multiple times in the same attribute:

sh_binary(
    name = "my_rule",
    srcs = ["always_include.sh"]
    + select({
          ":armeabi_mode": ["armeabi_src.sh"],
          ":x86_mode": ["x86_src.sh"],
      })
    + select({
          ":opt_mode": ["opt_extras.sh"],
          ":dbg_mode": ["dbg_extras.sh"],
      })
)

select cannot appear inside another select (i.e. AND chaining). If you need to AND selects together, either use an intermediate rule:

sh_binary
    name = "my_rule",
    srcs = ["always_include.sh"],
    deps = select({
        ":armeabi_mode": [":armeabi_lib"],
        ...
    })
)

sh_library(
    name = "armeabi_lib",
    srcs = select({
        ":opt_mode":  ["armeabi_with_opt.sh"],
        ...
    })
)

or write a Skylark macro to do the same thing automatically.

This approach doesn't work for non-deps attributes (like genrule:cmd). For these, extra config_settings may be necessary:

config_setting(
    name = "armeabi_and_opt",
    values = {
        "cpu": "armeabi",
        "compilation_mode": "opt"
    }
)

OR Chaining

Consider the following:

sh_binary
    name = "my_rule",
    srcs = ["always_include.sh"],
    deps = select({
        ":config1": [":standard_lib"],
        ":config2": [":standard_lib"],
        ":config3": [":standard_lib"],
        ":config4": [":special_lib"],
    })
)

Most conditions evaluate to the same dep. But this syntax is verbose, hard to maintain, and refactoring-unfriendly. It would be nice to not have to repeat [":standard_lib"] over and over.

One option is to predefine the declaration as a BUILD variable:

STANDARD_DEP = [":standard_lib"]

sh_binary
    name = "my_rule",
    srcs = ["always_include.sh"],
    deps = select({
        ":config1": STANDARD_DEP,
        ":config2": STANDARD_DEP,
        ":config3": STANDARD_DEP,
        ":config4": [":special_lib"],
    })
)

This makes it easier to manage the dependency. But it still adds unnecessary duplication.

select() doesn't support native syntax for ORed conditions. For this, use the Skylib utility selects.

load("@bazel_skylib//:lib.bzl", "selects")
sh_binary
    name = "my_rule",
    srcs = ["always_include.sh"],
    deps = selects.with_or({
        (":config1", ":config2", ":config3"): [":standard_lib"],
        ":config4": [":special_lib"],
    })
)

This automatically expands the select to the original syntax above.

For AND chaining, see here.

Custom Error Messages

By default, when no condition matches, the owning rule fails with the error:

ERROR: Configurable attribute "deps" doesn't match this configuration (would
a default condition help?).
Conditions checked:
  //tools/cc_target_os:darwin
  //tools/cc_target_os:android

This can be customized with no_match_error:

cc_library(
    name = "my_lib",
    deps = select({
        "//tools/cc_target_os:android": [":android_deps"],
        "//tools/cc_target_os:windows": [":windows_deps"],
    }, no_match_error = "Please build with an Android or Windows toolchain"
    )
)
$ bazel build //foo:my_lib
ERROR: Configurable attribute "deps" doesn't match this configuration: Please
build with an Android or Windows toolchain

Skylark Compatibility

Skylark is compatible with configurable attributes in limited form.

Skylark rule implementations receive the resolved values of configurable attributes. For example, given:

//myproject/BUILD:
some_skylark_rule(
    name = "my_rule",
    some_attr = select({
        ":foo_mode": [":foo"],
        ":bar_mode": [":bar"],
    })
)
$ bazel build //myproject/my_rule --define mode=foo

Skylark rule implementation code sees ctx.attr.some_attr as [":foo"].

Skylark macros can accept select() clauses and pass them through to native rules. But they cannot directly manipulate them. For example, there's no way for a Skylark macro to convert

`select({"foo": "val"}, ...)`

to

`select({"foo": "val_with_suffix"}, ...)`.

This is for two reasons.

First, macros that need to know which path a select will choose cannot work because macros are evaluated in Bazel's loading phase, which occurs before flag values are known. This is a core Bazel design restriction that's unlikely to change any time soon.

Second, macros that just need to iterate over all select paths, while technically feasible, lack a coherent UI. Further design is necessary to change this.

Bazel Query and Cquery

Bazel query operates over Bazel's loading phase. This means it doesn't know what command line flags will be applied to a rule since those flags aren't evaluated until later in the build (during the analysis phase). So the query command can't accurately determine which path a configurable attribute will follow.

Bazel cquery has the advantage of being able to parse build flags and operating post-analysis phase so it correctly resolves configurable attributes. It doesn't have full feature parity with query but supports most major functionality and is actively being worked on. Querying the following build file...

//myproject/BUILD:
cc_library(
    name = "my_lib",
    deps = select({
        ":long": [":foo_dep"],
        ":short": [":bar_dep"],
    })
)
config_setting(
    name = 'long',
    values = { "define": "dog=dachshund" }
)
config_setting(
    name = 'short',
    values = { "define": "dog=pug" }
)

...would return the following results.

$ bazel query 'deps(//myproject:my_lib)'
//myproject:my_lib
//myproject:foo_dep
//myproject:bar_dep

$ bazel cquery 'deps(//myproject:my_lib)' --define dog=pug
//myproject:my_lib
//myproject:bar_dep

FAQ

Why doesn't select() work in Skylark?

select() does work in Skylark! See Skylark compatibility for details.

The key issue this question usually means is that select() doesn't work in Skylark macros. These are different than Skylark rules. See the Skylark documentation on rules and macros to understand the difference. Here's an end-to-end example:

Define a Skylark rule and macro:

# myproject/defs.bzl:

# Rule implementation: when an attribute is read, all select()s have already
# been resolved. So it looks like a plain old attribute just like any other.
def _impl(ctx):
  name = ctx.attr.name
  allcaps = ctx.attr.my_config_string.upper()  # This works fine on all values.
  print("My name is " + name + " with custom message: " + allcaps)

# Skylark rule declaration:
my_custom_bazel_rule = rule(
    implementation = _impl,
    attrs = {"my_config_string": attr.string()}
)

# Skylark macro declaration:
def my_custom_bazel_macro(name, my_config_string):
  allcaps = my_config_string.upper() # This line won't work with select(s).
  print("My name is " + name + " with custom message: " + allcaps)

Instantiate the rule and macro:

# myproject/BUILD:
load("//myproject:defx.bzl", "my_custom_bazel_rule")
load("//myproject:defs.bzl", "my_custom_bazel_macro")

my_custom_bazel_rule(
    name = "happy_rule",
    my_config_string = select({
        "//tools/target_cpu:x86": "first string",
        "//tools/target_cpu:ppc": "second string",
    }),
)

my_custom_bazel_macro(
    name = "happy_macro",
    my_config_string = "fixed string",
)

my_custom_bazel_macro(
    name = "sad_macro",
    my_config_string = select({
        "//tools/target_cpu:x86": "first string",
        "//tools/target_cpu:ppc": "other string",
    }),
)

Building fails because sad_macro can't process the select():

$ bazel build //myproject:all
ERROR: /myworkspace/myproject/BUILD:17:1: Traceback
  (most recent call last):
File "/myworkspace/myproject/BUILD", line 17
my_custom_bazel_macro(name = "sad_macro", my_config_stri..."}))
File "/myworkspace/myproject/defs.bzl", line 4, in
  my_custom_bazel_macro
my_config_string.upper()
type 'select' has no method upper().
ERROR: error loading package 'myproject': Package 'myproject' contains errors.

Building succeeds when we comment out sad_macro:

# Comment out sad_macro so it doesn't mess up the build.
$ bazel build //myproject:all
DEBUG: /myworkspace/myproject/defs.bzl:5:3: My name is happy_macro with custom message: FIXED STRING.
DEBUG: /myworkspace/myproject/hi.bzl:15:3: My name is happy_rule with custom message: FIRST STRING.

This is impossible to change because by definition macros are evaluated before Bazel reads the build's command line flags. That means there isn't enough information to evaluate select()s.

Macros can, however, pass select()s as opaque blobs to rules:

# myproject/defs.bzl:
def my_custom_bazel_macro(name, my_config_string):
  print("Invoking macro " + name)
  my_custom_bazel_rule(
      name = name + "_as_rule",
      my_config_string = my_config_string)
$ bazel build //myproject:sad_macro_less_sad
DEBUG: /myworkspace/myproject/defs.bzl:23:3: Invoking macro sad_macro_less_sad.
DEBUG: /myworkspace/myproject/defs.bzl:15:3: My name is sad_macro_less_sad with custom message: FIRST STRING.

Why does select() always return true in Skylark?

Because Skylark macros (but not rules) by definition can't evaluate select(s), any attempt to do so usually produces a an error:

ERROR: /myworkspace/myproject/BUILD:17:1: Traceback
  (most recent call last):
File "/myworkspace/myproject/BUILD", line 17
my_custom_bazel_macro(name = "sad_macro", my_config_stri..."}))
File "/myworkspace/myproject/defs.bzl", line 4, in
  my_custom_bazel_macro
my_config_string.upper()
type 'select' has no method upper().

Booleans are a special case that fail silently, so you should be particularly vigilant with them:

$ cat myproject/defs.bzl:
def my_boolean_macro(boolval):
  print("TRUE" if boolval else "FALSE")

$ cat myproject/BUILD:
load("//myproject:defx.bzl", "my_boolean_macro")
my_boolean_macro(
    boolval = select({
        "//tools/target_cpu:x86": True,
        "//tools/target_cpu:ppc": False,
    }),
)

$ bazel build //myproject:all --cpu=x86
DEBUG: /myworkspace/myproject/defs.bzl:4:3: TRUE.
$ bazel build //myproject:all --cpu=ppc
DEBUG: /myworkspace/myproject/defs.bzl:4:3: TRUE.

This happens because Skylark macros don't understand the contents of select(). So what they're really evaluting is the select() object itself. According to Pythonic design standards, all objects aside from a very small number of exceptions automatically return true.

Can I read select() like a dict in Skylark?

Fine. Skylark macros can't evaluate select(s) because macros are evaluated before Bazel knows what the command line flags are.

Can macros at least read the select()'s dictionary, say, to add an extra suffix to each branch?

Conceptually this is possible. But this isn't yet implemented and is not currently prioritized. What you can do today is prepare a straight dictionary, then feed it into a select():

$ cat myproject/defs.bzl
def selecty_genrule(name, select_cmd):
  for key in select_cmd.keys():
    select_cmd[key] += " WITH SUFFIX"
  native.genrule(
      name = name,
      outs = [name + ".out"],
      srcs = [],
      cmd = "echo " + select(select_cmd + {"//conditions:default": "default"})
        + " > $@"
  )

$ cat myproject/BUILD
selecty_genrule(
    name = "selecty",
    select_cmd = {
        "//tools/target_cpu:x86": "x86 mode",
    },
)

$ bazel build //testapp:selecty --cpu=x86 && cat bazel-genfiles/testapp/selecty.out
x86 mode WITH SUFFIX

If you'd like to support both select() and native types, you can do this:

$ cat myproject/defs.bzl
def selecty_genrule(name, select_cmd):
  cmd_suffix = ""
  if type(select_cmd) == "string":
    cmd_suffix = select_cmd + " WITH SUFFIX"
  elif type(select_cmd) == "dict":
    for key in select_cmd.keys():
      select_cmd[key] += " WITH SUFFIX"
    cmd_suffix = select(select_cmd + {"//conditions:default": "default"})

  native.genrule(
      name = name,
      outs = [name + ".out"],
      srcs = [],
      cmd = "echo " + cmd_suffix + "> $@")