Status: Draft (as of May 18, 2006)
Contents
The primary goals of reworking the build system are (1) to make dependencies work more reliably, so that when files need to rebuilt, they are, and (2) to improve performance of the build system so that unnecessary modules are not rebuilt, and so doing a top-level build when little or nothing needs to be done for a build takes as little time as possible.
Given the above objective, these are the overall principles and use cases that we will support. This is not an exhaustive list.
It needs to be possible to build the Android platform for multiple targets. This means:
To achieve the objectives, the build system will be rewritten to use make non-recursively. For more background on this, read Recursive Make Considered Harmful. For those that don't want PDF, here is the Google translated version.
When developing a component, for example a C++ shared library, it must be possible to easily rebuild just that component, and not have to wait more than a couple seconds for dependency checks, and not have to wait for unneeded components to be built.
To set the target, and other options, some people on the team like to have a configuration file in a directory so they do not have an environment setup script to run, and others want an environment setup script to run so they can run builds in different terminals on the same tree, or switch back and forth in one terminal. We will support both.
Object files and other intermediate files will be generated into a directory
that is separate from the source tree. The goal is to have make clean be
"rm -rf
The SDK will be a tarball that will allow non-OS-developers to write apps. The apps will actually be built by first building the SDK, and then building the apps against that SDK. This will hopefully (1) make writing apps easier for us, because we won't have to rebuild the OS as much, and we can use the standard java-app development tools, and (2) allow us to dog-food the SDK, to help ensure its quality. Cedric has suggested (and I agree) that apps built from the SDK should be built with ant. Stay tuned for more details as we figure out exactly how this will work.
Dependencies should all be automatic. Unless there is a custom tool involved (e.g. the webkit has several), the dependencies for shared and static libraries, .c, .cpp, .h, .java, java libraries, etc., should all work without intervention in the Android.mk file.
Wildcarding source file will be discouraged. It may be useful in some
scenarios. The default $(wildcard *) will not work due to the
current directory being set to the root of the build tree.
It will be possible to generate more than one target from a given subdirectory. For example, libutils generates a shared library for the target and a static library for the host.
Android.mk is the standard name for the makefile fragments that control the building of a given module. Only the top directory should have a file named "Makefile".
Currently, the simulator is not built to use shared libraries. This should be fixed, and now is a good time to do it. This implies getting shared libraries to work on Mac OS.
These things would be nice to have, and this is a good place to record them, however these are not promises.
The hope is to be able to do two builds for different combos in the same tree at the same time, but this is a stretch goal, not a requirement. Doing two builds in the same tree, not at the same time must work. (update: it's looking like we'll get the two builds at the same time working)
Problems can arise if you delete a header file that is referenced in ".d" files. The easy way to deal with this is "make clean". There should be a better way to handle it. (from fadden)
One way of solving this is introducing a dependency on the directory. The problem is that this can create extra dependecies and slow down the build. It's a tradeoff.
General way to perform builds across the set of known platforms. This would make it easy to perform multiple platform builds when testing a change, and allow a wide-scale "make clean". Right now the buildspec.mk or environment variables need to be updated before each build. (from fadden)
We will eventually need to add support for creating locales and carrier customizations to the SDK, but that will not be addressed right now.
You've read (or scrolled past) all of the motivations for this build system, and you want to know how to use it. This is the place.
The Building document describes how do do builds.
. build/envsetup.shyou'll get a few helpful shell functions:
make from the top of the tree. This is
useful because you can run make from within subdirectories. If you have the
TOP environment variable set, it uses that. If you don't, it looks
up the tree from the current directory, trying to find the top of the tree.cd to the top of the tree.When building for a particular product, it's often useful to have minor variations on what is ultimately the final release build. These are the currently-defined "flavors" or "types" (we need to settle on a real name for these).
eng |
This is the default flavor. A plain "make" is the
same as "make eng". droid is an alias
for eng.
|
user |
"make user"
This is the flavor intended to be the final release bits.
|
userdebug |
"make userdebug"
The same as
|
If you build one flavor and then want to build another, you should run
"make installclean" between the two makes to guarantee that
you don't pick up files installed by the previous flavor. "make
clean" will also suffice, but it takes a lot longer.
Sometimes you want to just build one thing. The following pseudotargets are there for your convenience:
make droid is the normal build. This target
is here because the default target has to have a name.make all builds everything make
droid does, plus everything whose LOCAL_MODULE_TAGS do not
include the "droid" tag. The build server runs this to make sure
that everything that is in the tree and has an Android.mk builds.make clean-libutils and it will delete libutils.so and all of the
intermediate files, or you can type make clean-Home and it will
clean just the Home app.make clean deletes all of the output and
intermediate files for this configuration. This is the same as rm -rf
out/<configuration>/make clobber deletes all of the output
and intermediate files for all configurations. This is the same as
rm -rf out/.make dataclean deletes contents of the data
directory inside the current combo directory. This is especially useful on the
simulator and emulator, where the persistent data remains present between
builds.LOCAL_MODULE
in an Android.mk is made into a pseudotarget. For example, make
runtime might be shorthand for make
out/linux-x86-debug/system/bin/runtime (which would work), and
make libkjs might be shorthand for make
out/linux-x86-debug/system/lib/libkjs.so (which would also work).make targets will print a list of all of
the LOCAL_MODULE names you can make.You have a new library, a new app, or a new executable. For each of the common types of modules, there is a corresponding file in the templates directory. It will usually be enough to copy one of these, and fill in your own values. Some of the more esoteric values are not included in the templates, but are instead just documented here, as is the documentation on using custom tools to generate files.
Mostly, you can just look for the TODO comments in the templates and do what it says. Please remember to delete the TODO comments when you're done to keep the files clean. The templates have minimal documentation in them, because they're going to be copied, and when that gets stale, the copies just won't get updated. So read on...
Use the templates/apps file.
This template is pretty self-explanitory. See the variables below for more details.
Use the templates/java_library file.
The interesting thing here is the value of LOCAL_MODULE, which becomes the name of the jar file. (Actually right now, we're not making jar files yet, just directories of .class files, but the directory is named according to what you put in LOCAL_MODULE). This name will be what goes in the LOCAL_JAVA_LIBRARIES variable in modules that depend on your java library.
Use the templates/executable file, or the
templates/executable_host file.
This template has a couple extra options that you usually don't need. Please delete the ones you don't need, and remove the TODO comments. It makes the rest of them easier to read, and you can always refer back to the templates if you need them again later.
By default, on the target these are built into /system/bin, and on the
host, they're built into LOCAL_MODULE_PATH or LOCAL_MODULE_RELATIVE_PATH. See
Putting targets elsewhere
for more.
Use the templates/shared_library file, or the
templates/shared_library_host file.
Remember that on the target, we use shared libraries, and on the host, we use static libraries, since executable size isn't as big an issue, and it simplifies distribution in the SDK.
Use the templates/static_library file, or the
templates/static_library_host file.
Remember that on the target, we use shared libraries, and on the host, we use static libraries, since executable size isn't as big an issue, and it simplifies distribution in the SDK.
If you have a tool that generates source files for you, it's possible
to have the build system get the dependencies correct for it. Here are
a couple of examples. $@ is the make built-in variable for
"the current target." The red parts are the parts you'll
need to change.
You need to put this after you have declared LOCAL_PATH and
LOCAL_MODULE, because the $(local-generated-sources-dir)
and $(local-host-generated-sources-dir) macros use these variables
to determine where to put the files.
Here, there is one generated file, called chartables.c, which doesn't depend on anything. And is built by the tool built to $(HOST_OUT_EXECUTABLES)/dftables. Note on the second to last line that a dependency is created on the tool.
intermediates:= $(local-generated-sources-dir) GEN := $(intermediates)/chartables.c $(GEN): PRIVATE_CUSTOM_TOOL = $(HOST_OUT_EXECUTABLES)/dftables $@ $(GEN): $(HOST_OUT_EXECUTABLES)/dftables $(transform-generated-source) LOCAL_GENERATED_SOURCES += $(GEN)
Here as a hypothetical example, we use use cat as if it were to transform a file. Pretend that it does something useful. Note how we use a target-specific variable called PRIVATE_INPUT_FILE to store the name of the input file.
intermediates:= $(local-generated-sources-dir) GEN := $(intermediates)/file.c $(GEN): PRIVATE_INPUT_FILE := $(LOCAL_PATH)/input.file $(GEN): PRIVATE_CUSTOM_TOOL = cat $(PRIVATE_INPUT_FILE) > $@ $(GEN): $(LOCAL_PATH)/input.file $(transform-generated-source) LOCAL_GENERATED_SOURCES += $(GEN)
If you have several files that are all similar in name, and use the same tool, you can combine them. (here the *.lut.h files are the generated ones, and the *.cpp files are the input files)
intermediates:= $(local-generated-sources-dir)
GEN := $(addprefix $(intermediates)/kjs/, \
array_object.lut.h \
bool_object.lut.h \
)
$(GEN): PRIVATE_CUSTOM_TOOL = perl libs/WebKitLib/WebKit/JavaScriptCore/kjs/create_hash_table $< -i > $@
$(GEN): $(intermediates)/%.lut.h : $(LOCAL_PATH)/%.cpp
$(transform-generated-source)
LOCAL_GENERATED_SOURCES += $(GEN)
Unbundled build has several meanings by the context. Let me explain the meaning by the flags related to "unbundled build"
The source tree might not have the full platform sources. It turns on
TARGET_BUILD_USE_PREBUILT_SDKS, unless
UNBUNDLED_BUILD_SDKS_FROM_SOURCE is set. It is always set if
TARGET_BUILD_APPS or TARGET_BUILD_UNBUNDLED_IMAGE is set.
It is an internal flag. If it is set, prebuilt SDKs are used, even if a module's
LOCAL_SDK_VERSION is current (including system_current,
core_current, and so on). If it is unset, build current SDKs,
and use them as usual.
It is an internal flag as well. If it is set, dexpreopt is disabled.
It is always set if TARGET_BUILD_APPS or TARGET_BUILD_UNBUNDLED_IMAGE is set,
because dexpreopt tightly depends on the platform.
Build the apps that can be distributed outside the platform, so it turns on
TARGET_BUILD_UNBUNDLED and DISABLE_PREOPT.
Also, it turns on TARGET_BUILD_USE_PREBUILT_SDKS, unless
UNBUNDLED_BUILD_SDKS_FROM_SOURCE is set.
It is similar to TARGET_BUILD_APPS, but its target is an unbundled partition
(such as the vendor partition). Accordingly, it sets TARGET_BUILD_UNBUNDLED and DISABLE_PREOPT.
We can call the partition unbundled, because the partition can be distributed outside the platform.
And also, it turns on TARGET_BUILD_USE_PREBUILT_SDKS, unless
UNBUNDLED_BUILD_SDKS_FROM_SOURCE is set.
Sometimes you need to set flags specifically for different platforms. Here is a list of which values the different build-system defined variables will be set to and some examples.
|
HOST_OS linux darwin |
HOST_ARCH x86 x86_64 |
HOST_BUILD_TYPE release debug |
|
TARGET_ARCH arm arm64 x86 x86_64 |
TARGET_BUILD_TYPE release debug |
There are also special variables to use instead of conditionals. Many of the normal variables (LOCAL_SRC_FILES, LOCAL_CFLAGS, etc) can be conditionally added to with _{arch} _{32|64}, and for the host, _{os}.
ifeq ($(TARGET_BUILD_TYPE),release) LOCAL_CFLAGS += -DNDEBUG=1 endif LOCAL_CFLAGS_arm += -DTARGET_IS_ARM LOCAL_CFLAGS_64 += -DBIG_POINTER # from libutils # Use the futex based mutex and condition variable # implementation from android-arm because it's shared mem safe LOCAL_SRC_FILES_linux += futex_synchro.c LOCAL_LDLIBS_linux += -lrt -ldl
If you have modules that normally go somewhere, and you need to have them build somewhere else, read this.
If you have modules that need to go in a subdirectory of their normal location, for example HAL modules that need to go in /system/lib/hw or /vendor/lib/hw, set LOCAL_MODULE_RELATIVE_PATH in your Android.mk, for example:
LOCAL_MODULE_RELATIVE_PATH := hw
If you have modules that need to go in an entirely different location, for example the root filesystem instead of in /system, add these lines to your Android.mk:
LOCAL_MODULE_PATH := $(TARGET_ROOT_OUT) LOCAL_UNSTRIPPED_PATH := $(TARGET_ROOT_OUT_UNSTRIPPED)
For executables and libraries, you need to specify a
LOCAL_UNSTRIPPED_PATH location if you specified a
LOCAL_MODULE_PATH, because on target builds, we keep
the unstripped executables so GDB can find the symbols.
LOCAL_UNSTRIPPED_PATH is not necessary if you only specified
LOCAL_MODULE_RELATIVE_PATH.
Look in core/envsetup.mk for all of the variables defining
places to build things.
These are the variables that you'll commonly see in Android.mk files, listed alphabetically.
But first, a note on variable naming:
include $(CLEAR_VARS) line, so you can rely on them
being empty after including that file. Most of the variables you'll use
in most modules are LOCAL_ variables.Set this to a list of modules built with BUILD_HOST_JAVA_LIBRARY
to have their jars passed to javac with -processorpath for use as annotation
processors.
Set this to a list of classes to be passed to javac as -processor arguments. This list is would be unnecessary, as javac will autodetect annotation processor classes, except that the Grok tool that is used on the Android source code does not autodetect them and requires listing them manually.
In Android.mk files that include $(BUILD_PACKAGE) set this
to the set of files you want built into your app. Usually:
LOCAL_ASSET_FILES += $(call find-subdir-assets)
This will probably change when we switch to ant for the apps' build system.
If you want to use a different C compiler for this module, set LOCAL_CC to the path to the compiler. If LOCAL_CC is blank, the appropriate default compiler is used.
If you want to use a different C++ compiler for this module, set LOCAL_CXX to the path to the compiler. If LOCAL_CXX is blank, the appropriate default compiler is used.
If you have additional flags to pass into the C or C++ compiler, add them here. For example:
LOCAL_CFLAGS += -DLIBUTILS_NATIVE=1
If you have additional flags to pass into only the C++ compiler, add them here. For example:
LOCAL_CPPFLAGS += -ffriend-injection
LOCAL_CPPFLAGS is guaranteed to be after LOCAL_CFLAGS
on the compile line, so you can use it to override flags listed in
LOCAL_CFLAGS.
If your C++ files end in something other than ".cpp",
you can specify the custom extension here. For example:
LOCAL_CPP_EXTENSION := .cc
Normally, the compile line for C and C++ files includes global include
paths and global cflags. If LOCAL_NO_DEFAULT_COMPILER_FLAGS
is non-empty, none of the default includes or flags will be used when compiling
C and C++ files in this module.
LOCAL_C_INCLUDES, LOCAL_CFLAGS, and
LOCAL_CPPFLAGS will still be used in this case, as will
any DEBUG_CFLAGS that are defined for the module.
This will be going away.
The set of files to copy to the install include tree. You must also
supply LOCAL_COPY_HEADERS_TO.
This is going away because copying headers messes up the error messages, and may lead to people editing those headers instead of the correct ones. It also makes it easier to do bad layering in the system, which we want to avoid. We also aren't doing a C/C++ SDK, so there is no ultimate requirement to copy any headers.
This will be going away.
The directory within "include" to copy the headers listed in
LOCAL_COPY_HEADERS to.
This is going away because copying headers messes up the error messages, and may lead to people editing those headers instead of the correct ones. It also makes it easier to do bad layering in the system, which we want to avoid. We also aren't doing a C/C++ SDK, so there is no ultimate requirement to copy any headers.
Additional directories to instruct the C/C++ compilers to look for header
files in. These paths are rooted at the top of the tree. Use
LOCAL_PATH if you have subdirectories of your own that you
want in the include paths. For example:
LOCAL_C_INCLUDES += extlibs/zlib-1.2.3
LOCAL_C_INCLUDES += $(LOCAL_PATH)/src
You should not add subdirectories of include to
LOCAL_C_INCLUDES, instead you should reference those files
in the #include statement with their subdirectories. For
example:
#include <utils/KeyedVector.h>
not #include <KeyedVector.h>
There are some components that are doing this wrong, and should be cleaned up.
Set LOCAL_MODULE_TAGS to any number of whitespace-separated
tags. If the tag list is empty or contains droid, the module
will get installed as part of a make droid. Otherwise, it will
only get installed by running make <your-module>
or with the make all pseudotarget.
Set LOCAL_REQUIRED_MODULES to any number of whitespace-separated
module names, like "libblah" or "Email". If this module is installed, all
of the modules that it requires will be installed as well. This can be
used to, e.g., ensure that necessary shared libraries or providers are
installed when a given app is installed.
If your executable should be linked statically, set
LOCAL_FORCE_STATIC_EXECUTABLE:=true. There is a very short
list of libraries that we have in static form (currently only libc).
Files that you add to LOCAL_GENERATED_SOURCES will be
automatically generated and then linked in when your module is built.
See the Custom Tools template makefile for an
example.
If you have additional flags to pass into the javac compiler, add them here. For example:
LOCAL_JAVACFLAGS += -Xlint:deprecation
If you have additional flags to pass into the error prone compiler, add them here. For example:
LOCAL_ERROR_PRONE_FLAGS += -Xep:ClassCanBeStatic:ERROR
When linking Java apps and libraries, LOCAL_JAVA_LIBRARIES
specifies which sets of java classes to include. Currently there are
two of these: core and framework.
In most cases, it will look like this:
LOCAL_JAVA_LIBRARIES := core framework
Note that setting LOCAL_JAVA_LIBRARIES is not necessary
(and is not allowed) when building an APK with
"include $(BUILD_PACKAGE)". The appropriate libraries
will be included automatically.
You can pass additional flags to the linker by setting
LOCAL_LDFLAGS. Keep in mind that the order of parameters is
very important to ld, so test whatever you do on all platforms.
LOCAL_LDLIBS allows you to specify additional libraries
that are not part of the build for your executable or library. Specify
the libraries you want in -lxxx format; they're passed directly to the
link line. However, keep in mind that there will be no dependency generated
for these libraries. It's most useful in simulator builds where you want
to use a library preinstalled on the host. The linker (ld) is a particularly
fussy beast, so it's sometimes necessary to pass other flags here if you're
doing something sneaky. Some examples:
LOCAL_LDLIBS += -lcurses -lpthread
LOCAL_LDLIBS += -Wl,-z,origin
If your package doesn't have a manifest (AndroidManifest.xml), then
set LOCAL_NO_MANIFEST:=true. The common resources package
does this.
LOCAL_PACKAGE_NAME is the name of an app. For example,
Dialer, Contacts, etc. This will probably change or go away when we switch
to an ant-based build system for the apps.
As of January 2018, you almost certainly don't need this option, so please ask and only use it if you understand what you're doing. This feature is experimental and may go away in future.
When compiling language level 9+ .java code in packages that are part of a
a system module, LOCAL_PATCH_MODULE names the module that your
sources and dependencies should be patched into. The Android runtime currently
(Jan 2018) doesn't implement the JEP 261 module system so this option is only
supported at compile time. It should only be needed to compile tests in packages
that exist in libcore and which are inconvenient to move elsewhere.
The directory your Android.mk file is in. You can set it by putting the following as the first line in your Android.mk:
LOCAL_PATH := $(my-dir)
The my-dir macro uses the
MAKEFILE_LIST
variable, so you must call it before you include any other makefiles. Also,
consider that any subdirectories you inlcude might reset LOCAL_PATH, so do your
own stuff before you include them. This also means that if you try to write
several include lines that reference LOCAL_PATH,
it won't work, because those included makefiles might reset LOCAL_PATH.
For host executables, you can specify a command to run on the module after it's been linked. You might have to go through some contortions to get variables right because of early or late variable evaluation:
module := $(HOST_OUT_EXECUTABLES)/$(LOCAL_MODULE)
LOCAL_POST_PROCESS_COMMAND := /Developer/Tools/Rez -d __DARWIN__ -t APPL\
-d __WXMAC__ -o $(module) Carbon.r
When including $(BUILD_MULTI_PREBUILT) or $(BUILD_HOST_PREBUILT), set these to executables that you want copied. They're located automatically into the right bin directory.
When including $(BUILD_MULTI_PREBUILT) or $(BUILD_HOST_PREBUILT), set these to libraries that you want copied. They're located automatically into the right lib directory.
These are the libraries you directly link against. You don't need to pass transitively included libraries. Specify the name without the suffix:
LOCAL_SHARED_LIBRARIES := \
libutils \
libui \
libaudio \
libexpat \
libsgl
The build system looks at LOCAL_SRC_FILES to know what source
files to compile -- .cpp .c .y .l .java. For lex and yacc files, it knows
how to correctly do the intermediate .h and .c/.cpp files automatically. If
the files are in a subdirectory of the one containing the Android.mk, prefix
them with the directory name:
LOCAL_SRC_FILES := \
file1.cpp \
dir/file2.cpp
These are the static libraries that you want to include in your module. Mostly, we use shared libraries, but there are a couple of places, like host executables where we use static libraries instead.
LOCAL_STATIC_LIBRARIES := \
libutils \
libtinyxml
LOCAL_MODULE is the name of what's supposed to be generated
from your Android.mk. For exmample, for libkjs, the LOCAL_MODULE
is "libkjs" (the build system adds the appropriate suffix -- .so .dylib .dll).
For app modules, use LOCAL_PACKAGE_NAME instead of
LOCAL_MODULE. We're planning on switching to ant for the apps,
so this might become moot.
Instructs the build system to put the module somewhere other than what's
normal for its type. If you override this, make sure you also set
LOCAL_UNSTRIPPED_PATH if it's an executable or a shared library
so the unstripped binary has somewhere to go. An error will occur if you forget
to.
See Putting modules elsewhere for more.
Instructs the build system to put the module in a subdirectory under the
directory that is normal for its type. If you set this you do not need to
set LOCAL_UNSTRIPPED_PATH, the unstripped binaries will also use
the relative path.
See Putting modules elsewhere for more.
This specifies which OSes are supported by this host module. It is not used
for target builds. The accepted values here are combinations of
linux, darwin, and windows. By default,
linux and darwin(MacOS) are considered to be supported. If a module should
build under windows, you must specify windows, and any others to be supported.
Some examples:
LOCAL_MODULE_HOST_OS := linux
LOCAL_MODULE_HOST_OS := darwin linux windows
Instructs the build system to put the unstripped version of the module
somewhere other than what's normal for its type. Usually, you override this
because you overrode LOCAL_MODULE_PATH for an executable or a
shared library. If you overrode LOCAL_MODULE_PATH, but not
LOCAL_UNSTRIPPED_PATH, an error will occur.
See Putting modules elsewhere for more.
These are the static libraries that you want to include in your module without allowing the linker to remove dead code from them. This is mostly useful if you want to add a static library to a shared library and have the static library's content exposed from the shared library.
LOCAL_WHOLE_STATIC_LIBRARIES := \
libsqlite3_android
Any flags to pass to invocations of yacc for your module. A known limitation here is that the flags will be the same for all invocations of YACC for your module. This can be fixed. If you ever need it to be, just ask.
LOCAL_YACCFLAGS := -p kjsyy
You should never have to touch anything in the config directory unless you're adding a new platform, new tools, or adding new features to the build system. In general, please consult with the build system owner(s) (android-build-team) before you go mucking around in here. That said, here are some notes on what's going on under the hood.
In order to make easier for people when the build system changes, when it is necessary to make changes to buildspec.mk or to rerun the environment setup scripts, they contain a version number in the variable BUILD_ENV_SEQUENCE_NUMBER. If this variable does not match what the build system expects, it fails printing an error message explaining what happened. If you make a change that requires an update, you need to update two places so this message will be printed.
You probably shouldn't use these variables. Please consult android-build-team before using them. These are mostly there for workarounds for other issues, or things that aren't completely done right.
If your module needs to depend on anything else that
isn't actually built in to it, you can add those make targets to
LOCAL_ADDITIONAL_DEPENDENCIES. Usually this is a workaround
for some other dependency that isn't created automatically.
This should not be used, since multiple binaries are now created from a single module defintiion.
When a module is built, the module is created in an intermediate directory then copied to its final location. LOCAL_BUILT_MODULE is the full path to the intermediate file. See LOCAL_INSTALLED_MODULE for the path to the final installed location of the module.
Set by the host_xxx.mk includes to tell base_rules.mk and the other includes that we're building for the host.
This should not be used, since multiple binaries are now created from a single module defintiion.
The fully qualified path name of the final location of the module. See LOCAL_BUILT_MODULE for the location of the intermediate file that the make rules should actually be constructing.
Which kind of module this is. This variable is used to construct other variable names used to locate the modules. See base_rules.mk and envsetup.mk.
The suffix that will be appended to LOCAL_MODULE to form
LOCAL_MODULE_NAME. For example, .so, .a, .dylib.
If set to true (the default), the binary will be stripped and a debug link will be set up so that GDB will still work. If set to no_debuglink, the binary will be stripped, but no debug link will be added. If set to keep_symbols, it will strip the debug information, but keep the symbol table. Any other value will prevent stripping.
Used while building the base libraries: libc, libm, libdl. Usually it should be set to "none," as it is in $(CLEAR_VARS). When building these libraries, it's set to the ones they link against. For example, libc, libstdc++ and libdl don't link against anything, and libm links against libc. Normally, when the value is none, these libraries are automatically linked in to executables and libraries, so you don't need to specify them manually.