Build System Changes for Android.mk Writers

USER deprecation {#USER}

USER will soon be nobody in many cases due to the addition of a sandbox around the Android build. Most of the time you shouldn't need to know the identity of the user running the build, but if you do, it's available in the make variable BUILD_USERNAME for now.

Similarly, the hostname tool will also be returning a more consistent value of android-build. The real value is available as BUILD_HOSTNAME.

BUILD_NUMBER removal from Android.mk {#BUILD_NUMBER}

BUILD_NUMBER should not be used directly in Android.mk files, as it would trigger them to be re-read every time the BUILD_NUMBER changes (which it does on every build server build). If possible, just remove the use so that your builds are more reproducible. If you do need it, use BUILD_NUMBER_FROM_FILE:

    mytool --build_number $(BUILD_NUMBER_FROM_FILE) -o $@

That will expand out to a subshell that will read the current BUILD_NUMBER whenever it's run. It will not re-run your command if the build number has changed, so incremental builds will have the build number from the last time the particular output was rebuilt.

DIST_DIR, dist_goal, and dist-for-goals {#dist}

DIST_DIR and dist_goal are no longer available when reading Android.mk files (or other build tasks). Always use dist-for-goals instead, which takes a PHONY goal, and a list of files to copy to $DIST_DIR. Whenever dist is specified, and the goal would be built (either explicitly on the command line, or as a dependency of something on the command line), that file will be copied into $DIST_DIR. For example,

$(call dist-for-goals,foo,bar/baz)

will copy bar/baz into $DIST_DIR/baz when m foo dist is run.

Renames during copy

Instead of specifying just a file, a destination name can be specified, including subdirectories:

$(call dist-for-goals,foo,bar/baz:logs/foo.log)

will copy bar/baz into $DIST_DIR/logs/foo.log when m foo dist is run.

.PHONY rule enforcement {#phony_targets}

There are several new warnings/errors meant to ensure the proper use of .PHONY targets in order to improve the speed and reliability of incremental builds.

.PHONY-marked targets are often used as shortcuts to provide "friendly" names for real files to be built, but any target marked with .PHONY is also always considered dirty, needing to be rebuilt every build. This isn't a problem for aliases or one-off user-requested operations, but if real builds steps depend on a .PHONY target, it can get quite expensive for what should be a tiny build.

...mk:42: warning: PHONY target "out/.../foo" looks like a real file (contains a "/")

Between this warning and the next, we're requiring that .PHONY targets do not have "/" in them, and real file targets do have a "/". This makes it more obvious when reading makefiles what is happening, and will help the build system differentiate these in the future too.

...mk:42: warning: writing to readonly directory: "kernel-modules"

This warning will show up for one of two reasons:

  1. The target isn't intended to be a real file, and should be marked with .PHONY. This would be the case for this example.
  2. The target is a real file, but it's outside the output directories. All outputs from the build system should be within the output directory, otherwise m clean is unable to clean the build, and future builds may not work properly.
...mk:42: warning: real file "out/.../foo" depends on PHONY target "buildbins"

If the first target isn't intended to be a real file, then it should be marked with .PHONY, which will satisfy this warning. This isn't the case for this example, as we require .PHONY targets not to have '/' in them.

If the second (PHONY) target is a real file, it may unnecessarily be marked with .PHONY.

.PHONY and calling other build systems

One common pattern (mostly outside AOSP) that we've seen hit these warning is when building with external build systems (firmware, bootloader, kernel, etc). Those are often marked as .PHONY because the Android build system doesn't have enough dependencies to know when to run the other build system again during an incremental build.

We recommend to build these outside of Android, and deliver prebuilts into the Android tree instead of decreasing the speed and reliability of the incremental Android build.

In cases where that's not desired, to preserve the speed of Android incrementals, over-specifying dependencies is likely a better option than marking it with .PHONY:

out/target/.../zImage: $(sort $(shell find -L $(KERNEL_SRCDIR)))

For reliability, many of these other build systems do not guarantee the same level of incremental build assurances as the Android Build is attempting to do -- without custom checks, Make doesn't rebuild objects when CFLAGS change, etc. In order to fix this, our recommendation is to do clean builds for each of these external build systems every time anything they rely on changes. For relatively smaller builds (like the kernel), this may be reasonable as long as you're not trying to actively debug the kernel.

export and unexport deprecation {#export_keyword}

The export and unexport keywords have been deprecated, and will throw warnings or errors depending on where they are used.

Early in the make system, during product configuration and BoardConfig.mk reading: these will throw a warnings, and will be an error in the future. Device specific configuration should not be able to affect common core build steps -- we're looking at triggering build steps to be invalidated if the set of environment variables they can access changes. If device specific configuration is allowed to change those, switching devices with the same output directory could become significantly more expensive than it already can be.

Later, during Android.mk files, and later tasks: these will throw errors, since it is increasingly likely that they are being used incorrectly, attempting to change the environment for a single build step, and instead setting it for hundreds of thousands.

It is not recommended to just move the environment variable setting outside of the build (in vendorsetup.sh, or some other configuration script or wrapper). We expect to limit the environment variables that the build respects in the future, others will be cleared. (There will be methods to get custom variables into the build, just not to every build step)

Instead, write the export commands into the rule command lines themselves:

    rm -rf $@
    export MY_ENV_A="$(MY_A)"; make ...

If you want to set many environment variables, and/or use them many times, write them out to a script and source the script:

envsh := $(intermediates)/env.sh
    rm -rf $@
    echo 'export MY_ENV_A="$(MY_A)"' >$@
    echo 'export MY_ENV_B="$(MY_B)"' >>$@

$(intermediates)/generated_output.img: PRIVATE_ENV := $(envsh)
$(intermediates)/generated_output.img: $(envsh) a/b/c/package.sh
    rm -rf $@
    source $(PRIVATE_ENV); make ...
    source $(PRIVATE_ENV); a/b/c/package.sh ...

Implicit make rules are obsolete {#implicit_rules}

Implicit rules look something like the following:


%.o : %.foo

These can have wide ranging effects across unrelated modules, so they're now obsolete. Instead, use static pattern rules, which are similar, but explicitly match the specified outputs:

libs := $(foreach lib,libfoo libbar,$(TARGET_OUT_SHARED_LIBRARIES)/$(lib)_vendor.so)
$(libs): %_vendor.so: %.so

files := $(wildcard $(LOCAL_PATH)/*.foo)
gen := $(patsubst $(LOCAL_PATH)/%.foo,$(intermediates)/%.o,$(files))
$(gen): %.o : %.foo

Removing '/' from Valid Module Names {#name_slash}

The build system uses module names in path names in many places. Having an extra '/' or '../' being inserted can cause problems -- and not just build breaks, but stranger invalid behavior.

In every case we've seen, the fix is relatively simple: move the directory into LOCAL_MODULE_RELATIVE_PATH (or LOCAL_MODULE_PATH if you're still using it). If this causes multiple modules to be named the same, use unique module names and LOCAL_MODULE_STEM to change the installed file name:

include $(CLEAR_VARS)
LOCAL_MODULE := ver1/code.bin

include $(CLEAR_VARS)
LOCAL_MODULE := ver2/code.bin

Can be rewritten as:

include $(CLEAR_VARS)
LOCAL_MODULE := ver1_code.bin

include $(CLEAR_VARS)
LOCAL_MODULE := ver2_code.bin

You just need to make sure that any other references (PRODUCT_PACKAGES, LOCAL_REQUIRED_MODULES, etc) are converted to the new names.

Valid Module Names {#name}

We've adopted lexical requirements very similar to Bazel's requirements for target names. Valid characters are a-z, A-Z, 0-9, and the special characters _.+-=,@~. This currently applies to LOCAL_PACKAGE_NAME, LOCAL_MODULE, and LOCAL_MODULE_SUFFIX, and LOCAL_MODULE_STEM*.

Many other characters already caused problems if you used them, so we don't expect this to have a large effect.

PATH Tools {#PATH_Tools}

The build has started restricting the external host tools usable inside the build. This will help ensure that build results are reproducible across different machines, and catch mistakes before they become larger issues.

To start with, this includes replacing the $PATH with our own directory of tools, mirroring that of the host PATH. The only difference so far is the removal of the host GCC tools. Anything that is not explicitly in the configuration as allowed will continue functioning, but will generate a log message. This is expected to become more restrictive over time.

The configuration is located in build/soong/ui/build/paths/config.go, and contains all the common tools in use in many builds. Anything not in that list will currently print a warning in the $OUT_DIR/soong.log file, including the command and arguments used, and the process tree in order to help locate the usage.

In order to fix any issues brought up by these checks, the best way to fix them is to use tools checked into the tree -- either as prebuilts, or building them as host tools during the build.

As a temporary measure, you can set TEMPORARY_DISABLE_PATH_RESTRICTIONS=true in your environment to temporarily turn off the error checks and allow any tool to be used (with logging). Beware that GCC didn't work well with the interposer used for logging, so this may not help in all cases.

Deprecating / obsoleting envsetup.sh variables in Makefiles

It is not required to source envsetup.sh before running a build. Many scripts, including a majority of our automated build systems, do not do so. Make will transparently make every environment variable available as a make variable. This means that relying on environment variables only set up in envsetup.sh will produce different output for local users and scripted users.

Many of these variables also include absolute path names, which we'd like to keep out of the generated files, so that you don't need to do a full rebuild if you move the source tree.

To fix this, we're marking the variables that are set in envsetup.sh as deprecated in the makefiles. This will trigger a warning every time one is read (or written) inside Kati. Once all the warnings have been removed for a particular variable, we'll switch it to obsolete, and any references will become errors.

envsetup.sh variables with make equivalents

instead of use

All of the make variables may be relative paths from the current directory, or absolute paths if the output directory was specified as an absolute path. If you need an absolute variable, convert it to absolute during a rule, so that it's not expanded into the generated ninja file:

$(PRODUCT_OUT)/gen.img: my/src/path/gen.sh
    export PRODUCT_OUT=$$(cd $(PRODUCT_OUT); pwd); cd my/src/path; ./gen.sh -o $${PRODUCT_OUT}/gen.img


In Android.mk files, you can always assume that the current directory is the root of the source tree, so this can just be replaced with '.' (which is what $TOP is hardcoded to), or removed entirely. If you need an absolute path, see the instructions above.

Stop using PATH directly {#PATH}

This isn't only set by envsetup.sh, but it is modified by it. Due to that it's rather easy for this to change between different shells, and it's not ideal to reread the makefiles every time this changes.

In most cases, you shouldn't need to touch PATH at all. When you need to have a rule reference a particular binary that's part of the source tree or outputs, it's preferrable to just use the path to the file itself (since you should already be adding that as a dependency).

Depending on the rule, passing the file path itself may not be feasible due to layers of unchangable scripts/binaries. In that case, be sure to add the dependency, but modify the PATH within the rule itself:

$(TARGET): myscript my/path/binary
    PATH=my/path:$$PATH myscript -o $@

Stop using PYTHONPATH directly {#PYTHONPATH}

Like PATH, this isn't only set by envsetup.sh, but it is modified by it. Due to that it's rather easy for this to change between different shells, and it's not ideal to reread the makefiles every time.

The best solution here is to start switching to Soong's python building support, which packages the python interpreter, libraries, and script all into one file that no longer needs PYTHONPATH. See fontchain_lint for examples of this:

If you still need to use PYTHONPATH, do so within the rule itself, just like path:

$(TARGET): myscript.py $(sort $(shell find my/python/lib -name '*.py'))
    PYTHONPATH=my/python/lib:$$PYTHONPATH myscript.py -o $@


Specify Framework Compatibility Matrix Version in device manifest by adding a target-level attribute to the root element <manifest>. If PRODUCT_COMPATIBILITY_MATRIX_LEVEL_OVERRIDE is 26 or 27, you can add "target-level"="1" to your device manifest instead.


Clang is the default and only supported Android compiler, so there is no reason for this option to exist.

Other envsetup.sh variables {#other_envsetup_variables}


These are all exported from envsetup.sh, but don't have clear equivalents within the makefile system. If you need one of them, you'll have to set up your own version.