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Linux Kernel Makefiles
This document describes the Linux kernel Makefiles.
=== Table of Contents
=== 1 Overview
=== 2 Who does what
=== 3 The kbuild files
--- 3.1 Goal definitions
--- 3.2 Built-in object goals - obj-y
--- 3.3 Loadable module goals - obj-m
--- 3.4 Objects which export symbols
--- 3.5 Library file goals - lib-y
--- 3.6 Descending down in directories
--- 3.7 Compilation flags
--- 3.8 Command line dependency
--- 3.9 Dependency tracking
--- 3.10 Special Rules
--- 3.11 $(CC) support functions
=== 4 Host Program support
--- 4.1 Simple Host Program
--- 4.2 Composite Host Programs
--- 4.3 Defining shared libraries
--- 4.4 Using C++ for host programs
--- 4.5 Controlling compiler options for host programs
--- 4.6 When host programs are actually built
--- 4.7 Using hostprogs-$(CONFIG_FOO)
=== 5 Kbuild clean infrastructure
=== 6 Architecture Makefiles
--- 6.1 Set variables to tweak the build to the architecture
--- 6.2 Add prerequisites to archprepare:
--- 6.3 List directories to visit when descending
--- 6.4 Architecture specific boot images
--- 6.5 Building non-kbuild targets
--- 6.6 Commands useful for building a boot image
--- 6.7 Custom kbuild commands
--- 6.8 Preprocessing linker scripts
=== 7 Kbuild Variables
=== 8 Makefile language
=== 9 Credits
=== 10 TODO
=== 1 Overview
The Makefiles have five parts:
Makefile the top Makefile.
.config the kernel configuration file.
arch/$(ARCH)/Makefile the arch Makefile.
scripts/Makefile.* common rules etc. for all kbuild Makefiles.
kbuild Makefiles there are about 500 of these.
The top Makefile reads the .config file, which comes from the kernel
configuration process.
The top Makefile is responsible for building two major products: vmlinux
(the resident kernel image) and modules (any module files).
It builds these goals by recursively descending into the subdirectories of
the kernel source tree.
The list of subdirectories which are visited depends upon the kernel
configuration. The top Makefile textually includes an arch Makefile
with the name arch/$(ARCH)/Makefile. The arch Makefile supplies
architecture-specific information to the top Makefile.
Each subdirectory has a kbuild Makefile which carries out the commands
passed down from above. The kbuild Makefile uses information from the
.config file to construct various file lists used by kbuild to build
any built-in or modular targets.
scripts/Makefile.* contains all the definitions/rules etc. that
are used to build the kernel based on the kbuild makefiles.
=== 2 Who does what
People have four different relationships with the kernel Makefiles.
*Users* are people who build kernels. These people type commands such as
"make menuconfig" or "make". They usually do not read or edit
any kernel Makefiles (or any other source files).
*Normal developers* are people who work on features such as device
drivers, file systems, and network protocols. These people need to
maintain the kbuild Makefiles for the subsystem that they are
working on. In order to do this effectively, they need some overall
knowledge about the kernel Makefiles, plus detailed knowledge about the
public interface for kbuild.
*Arch developers* are people who work on an entire architecture, such
as sparc or ia64. Arch developers need to know about the arch Makefile
as well as kbuild Makefiles.
*Kbuild developers* are people who work on the kernel build system itself.
These people need to know about all aspects of the kernel Makefiles.
This document is aimed towards normal developers and arch developers.
=== 3 The kbuild files
Most Makefiles within the kernel are kbuild Makefiles that use the
kbuild infrastructure. This chapter introduce the syntax used in the
kbuild makefiles.
The preferred name for the kbuild files are 'Makefile' but 'Kbuild' can
be used and if both a 'Makefile' and a 'Kbuild' file exists then the 'Kbuild'
file will be used.
Section 3.1 "Goal definitions" is a quick intro, further chapters provide
more details, with real examples.
--- 3.1 Goal definitions
Goal definitions are the main part (heart) of the kbuild Makefile.
These lines define the files to be built, any special compilation
options, and any subdirectories to be entered recursively.
The most simple kbuild makefile contains one line:
obj-y += foo.o
This tell kbuild that there is one object in that directory named
foo.o. foo.o will be built from foo.c or foo.S.
If foo.o shall be built as a module, the variable obj-m is used.
Therefore the following pattern is often used:
obj-$(CONFIG_FOO) += foo.o
$(CONFIG_FOO) evaluates to either y (for built-in) or m (for module).
If CONFIG_FOO is neither y nor m, then the file will not be compiled
nor linked.
--- 3.2 Built-in object goals - obj-y
The kbuild Makefile specifies object files for vmlinux
in the lists $(obj-y). These lists depend on the kernel
Kbuild compiles all the $(obj-y) files. It then calls
"$(LD) -r" to merge these files into one built-in.o file.
built-in.o is later linked into vmlinux by the parent Makefile.
The order of files in $(obj-y) is significant. Duplicates in
the lists are allowed: the first instance will be linked into
built-in.o and succeeding instances will be ignored.
Link order is significant, because certain functions
(module_init() / __initcall) will be called during boot in the
order they appear. So keep in mind that changing the link
order may e.g. change the order in which your SCSI
controllers are detected, and thus you disks are renumbered.
# Makefile for the kernel ISDN subsystem and device drivers.
# Each configuration option enables a list of files.
obj-$(CONFIG_ISDN) += isdn.o
obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
--- 3.3 Loadable module goals - obj-m
$(obj-m) specify object files which are built as loadable
kernel modules.
A module may be built from one source file or several source
files. In the case of one source file, the kbuild makefile
simply adds the file to $(obj-m).
obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm'
If a kernel module is built from several source files, you specify
that you want to build a module in the same way as above.
Kbuild needs to know which the parts that you want to build your
module from, so you have to tell it by setting an
$(<module_name>-objs) variable.
obj-$(CONFIG_ISDN) += isdn.o
isdn-objs := isdn_net_lib.o isdn_v110.o isdn_common.o
In this example, the module name will be isdn.o. Kbuild will
compile the objects listed in $(isdn-objs) and then run
"$(LD) -r" on the list of these files to generate isdn.o.
Kbuild recognises objects used for composite objects by the suffix
-objs, and the suffix -y. This allows the Makefiles to use
the value of a CONFIG_ symbol to determine if an object is part
of a composite object.
obj-$(CONFIG_EXT2_FS) += ext2.o
ext2-y := balloc.o bitmap.o
ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o
In this example xattr.o is only part of the composite object
ext2.o, if $(CONFIG_EXT2_FS_XATTR) evaluates to 'y'.
Note: Of course, when you are building objects into the kernel,
the syntax above will also work. So, if you have CONFIG_EXT2_FS=y,
kbuild will build an ext2.o file for you out of the individual
parts and then link this into built-in.o, as you would expect.
--- 3.4 Objects which export symbols
No special notation is required in the makefiles for
modules exporting symbols.
--- 3.5 Library file goals - lib-y
Objects listed with obj-* are used for modules or
combined in a built-in.o for that specific directory.
There is also the possibility to list objects that will
be included in a library, lib.a.
All objects listed with lib-y are combined in a single
library for that directory.
Objects that are listed in obj-y and additional listed in
lib-y will not be included in the library, since they will anyway
be accessible.
For consistency objects listed in lib-m will be included in lib.a.
Note that the same kbuild makefile may list files to be built-in
and to be part of a library. Therefore the same directory
may contain both a built-in.o and a lib.a file.
lib-y := checksum.o delay.o
This will create a library lib.a based on checksum.o and delay.o.
For kbuild to actually recognize that there is a lib.a being build
the directory shall be listed in libs-y.
See also "6.3 List directories to visit when descending".
Usage of lib-y is normally restricted to lib/ and arch/*/lib.
--- 3.6 Descending down in directories
A Makefile is only responsible for building objects in its own
directory. Files in subdirectories should be taken care of by
Makefiles in these subdirs. The build system will automatically
invoke make recursively in subdirectories, provided you let it know of
To do so obj-y and obj-m are used.
ext2 lives in a separate directory, and the Makefile present in fs/
tells kbuild to descend down using the following assignment.
obj-$(CONFIG_EXT2_FS) += ext2/
If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular)
the corresponding obj- variable will be set, and kbuild will descend
down in the ext2 directory.
Kbuild only uses this information to decide that it needs to visit
the directory, it is the Makefile in the subdirectory that
specifies what is modules and what is built-in.
It is good practice to use a CONFIG_ variable when assigning directory
names. This allows kbuild to totally skip the directory if the
corresponding CONFIG_ option is neither 'y' nor 'm'.
--- 3.7 Compilation flags
All the EXTRA_ variables apply only to the kbuild makefile
where they are assigned. The EXTRA_ variables apply to all
commands executed in the kbuild makefile.
$(EXTRA_CFLAGS) specifies options for compiling C files with
# drivers/sound/emu10k1/Makefile
EXTRA_CFLAGS += -I$(obj)
ifdef DEBUG
This variable is necessary because the top Makefile owns the
variable $(CFLAGS) and uses it for compilation flags for the
entire tree.
$(EXTRA_AFLAGS) is a similar string for per-directory options
when compiling assembly language source.
EXTRA_AFLAGS := -traditional
$(EXTRA_LDFLAGS) and $(EXTRA_ARFLAGS) are similar strings for
per-directory options to $(LD) and $(AR).
CFLAGS_$@ and AFLAGS_$@ only apply to commands in current
kbuild makefile.
$(CFLAGS_$@) specifies per-file options for $(CC). The $@
part has a literal value which specifies the file that it is for.
# drivers/scsi/Makefile
CFLAGS_gdth.o = # -DDEBUG_GDTH=2 -D__SERIAL__ -D__COM2__ \
These three lines specify compilation flags for aha152x.o,
gdth.o, and seagate.o
$(AFLAGS_$@) is a similar feature for source files in assembly
# arch/arm/kernel/Makefile
AFLAGS_head-armv.o := -DTEXTADDR=$(TEXTADDR) -traditional
AFLAGS_head-armo.o := -DTEXTADDR=$(TEXTADDR) -traditional
--- 3.9 Dependency tracking
Kbuild tracks dependencies on the following:
1) All prerequisite files (both *.c and *.h)
2) CONFIG_ options used in all prerequisite files
3) Command-line used to compile target
Thus, if you change an option to $(CC) all affected files will
be re-compiled.
--- 3.10 Special Rules
Special rules are used when the kbuild infrastructure does
not provide the required support. A typical example is
header files generated during the build process.
Another example is the architecture specific Makefiles which
needs special rules to prepare boot images etc.
Special rules are written as normal Make rules.
Kbuild is not executing in the directory where the Makefile is
located, so all special rules shall provide a relative
path to prerequisite files and target files.
Two variables are used when defining special rules:
$(src) is a relative path which points to the directory
where the Makefile is located. Always use $(src) when
referring to files located in the src tree.
$(obj) is a relative path which points to the directory
where the target is saved. Always use $(obj) when
referring to generated files.
$(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/
$(CPP) -DCHIP=810 - < $< | ... $(src)/
This is a special rule, following the normal syntax
required by make.
The target file depends on two prerequisite files. References
to the target file are prefixed with $(obj), references
to prerequisites are referenced with $(src) (because they are not
generated files).
--- 3.11 $(CC) support functions
The kernel may be build with several different versions of
$(CC), each supporting a unique set of features and options.
kbuild provide basic support to check for valid options for $(CC).
$(CC) is useally the gcc compiler, but other alternatives are
as-option is used to check if $(CC) when used to compile
assembler (*.S) files supports the given option. An optional
second option may be specified if first option are not supported.
cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),)
In the above example cflags-y will be assinged the the option
-Wa$(comma)-isa=$(isa-y) if it is supported by $(CC).
The second argument is optional, and if supplied will be used
if first argument is not supported.
ld-option is used to check if $(CC) when used to link object files
supports the given option. An optional second option may be
specified if first option are not supported.
vsyscall-flags += $(call ld-option, -Wl$(comma)--hash-style=sysv)
In the above example vsyscall-flags will be assigned the option
-Wl$(comma)--hash-style=sysv if it is supported by $(CC).
The second argument is optional, and if supplied will be used
if first argument is not supported.
cc-option is used to check if $(CC) support a given option, and not
supported to use an optional second option.
cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586)
In the above example cflags-y will be assigned the option
-march=pentium-mmx if supported by $(CC), otherwise -march-i586.
The second argument to cc-option is optional, and if omitted
cflags-y will be assigned no value if first option is not supported.
cc-option-yn is used to check if gcc supports a given option
and return 'y' if supported, otherwise 'n'.
biarch := $(call cc-option-yn, -m32)
aflags-$(biarch) += -a32
cflags-$(biarch) += -m32
In the above example $(biarch) is set to y if $(CC) supports the -m32
option. When $(biarch) equals to y the expanded variables $(aflags-y)
and $(cflags-y) will be assigned the values -a32 and -m32.
gcc version >= 3.0 shifted type of options used to speify
alignment of functions, loops etc. $(cc-option-align) whrn used
as prefix to the align options will select the right prefix:
gcc < 3.00
cc-option-align = -malign
gcc >= 3.00
cc-option-align = -falign
CFLAGS += $(cc-option-align)-functions=4
In the above example the option -falign-functions=4 is used for
gcc >= 3.00. For gcc < 3.00 -malign-functions=4 is used.
cc-version return a numerical version of the $(CC) compiler version.
The format is <major><minor> where both are two digits. So for example
gcc 3.41 would return 0341.
cc-version is useful when a specific $(CC) version is faulty in one
area, for example the -mregparm=3 were broken in some gcc version
even though the option was accepted by gcc.
cflags-y += $(shell \
if [ $(call cc-version) -ge 0300 ] ; then \
echo "-mregparm=3"; fi ;)
In the above example -mregparm=3 is only used for gcc version greater
than or equal to gcc 3.0.
cc-ifversion test the version of $(CC) and equals last argument if
version expression is true.
EXTRA_CFLAGS := $(call cc-ifversion, -lt, 0402, -O1)
In this example EXTRA_CFLAGS will be assigned the value -O1 if the
$(CC) version is less than 4.2.
cc-ifversion takes all the shell operators:
-eq, -ne, -lt, -le, -gt, and -ge
The third parameter may be a text as in this example, but it may also
be an expanded variable or a macro.
=== 4 Host Program support
Kbuild supports building executables on the host for use during the
compilation stage.
Two steps are required in order to use a host executable.
The first step is to tell kbuild that a host program exists. This is
done utilising the variable hostprogs-y.
The second step is to add an explicit dependency to the executable.
This can be done in two ways. Either add the dependency in a rule,
or utilise the variable $(always).
Both possibilities are described in the following.
--- 4.1 Simple Host Program
In some cases there is a need to compile and run a program on the
computer where the build is running.
The following line tells kbuild that the program bin2hex shall be
built on the build host.
hostprogs-y := bin2hex
Kbuild assumes in the above example that bin2hex is made from a single
c-source file named bin2hex.c located in the same directory as
the Makefile.
--- 4.2 Composite Host Programs
Host programs can be made up based on composite objects.
The syntax used to define composite objects for host programs is
similar to the syntax used for kernel objects.
$(<executeable>-objs) list all objects used to link the final
hostprogs-y := lxdialog
lxdialog-objs := checklist.o lxdialog.o
Objects with extension .o are compiled from the corresponding .c
files. In the above example checklist.c is compiled to checklist.o
and lxdialog.c is compiled to lxdialog.o.
Finally the two .o files are linked to the executable, lxdialog.
Note: The syntax <executable>-y is not permitted for host-programs.
--- 4.3 Defining shared libraries
Objects with extension .so are considered shared libraries, and
will be compiled as position independent objects.
Kbuild provides support for shared libraries, but the usage
shall be restricted.
In the following example the shared library is used
to link the executable conf.
hostprogs-y := conf
conf-objs := conf.o
libkconfig-objs := expr.o type.o
Shared libraries always require a corresponding -objs line, and
in the example above the shared library libkconfig is composed by
the two objects expr.o and type.o.
expr.o and type.o will be built as position independent code and
linked as a shared library C++ is not supported for
shared libraries.
--- 4.4 Using C++ for host programs
kbuild offers support for host programs written in C++. This was
introduced solely to support kconfig, and is not recommended
for general use.
hostprogs-y := qconf
qconf-cxxobjs := qconf.o
In the example above the executable is composed of the C++ file - identified by $(qconf-cxxobjs).
If qconf is composed by a mixture of .c and .cc files, then an
additional line can be used to identify this.
hostprogs-y := qconf
qconf-cxxobjs := qconf.o
qconf-objs := check.o
--- 4.5 Controlling compiler options for host programs
When compiling host programs, it is possible to set specific flags.
The programs will always be compiled utilising $(HOSTCC) passed
the options specified in $(HOSTCFLAGS).
To set flags that will take effect for all host programs created
in that Makefile use the variable HOST_EXTRACFLAGS.
HOST_EXTRACFLAGS += -I/usr/include/ncurses
To set specific flags for a single file the following construction
is used:
It is also possible to specify additional options to the linker.
When linking qconf it will be passed the extra option "-L$(QTDIR)/lib".
--- 4.6 When host programs are actually built
Kbuild will only build host-programs when they are referenced
as a prerequisite.
This is possible in two ways:
(1) List the prerequisite explicitly in a special rule.
hostprogs-y := gen-devlist
$(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist
( cd $(obj); ./gen-devlist ) < $<
The target $(obj)/devlist.h will not be built before
$(obj)/gen-devlist is updated. Note that references to
the host programs in special rules must be prefixed with $(obj).
(2) Use $(always)
When there is no suitable special rule, and the host program
shall be built when a makefile is entered, the $(always)
variable shall be used.
hostprogs-y := lxdialog
always := $(hostprogs-y)
This will tell kbuild to build lxdialog even if not referenced in
any rule.
--- 4.7 Using hostprogs-$(CONFIG_FOO)
A typcal pattern in a Kbuild file lok like this:
hostprogs-$(CONFIG_KALLSYMS) += kallsyms
Kbuild knows about both 'y' for built-in and 'm' for module.
So if a config symbol evaluate to 'm', kbuild will still build
the binary. In other words Kbuild handle hostprogs-m exactly
like hostprogs-y. But only hostprogs-y is recommend used
when no CONFIG symbol are involved.
=== 5 Kbuild clean infrastructure
"make clean" deletes most generated files in the src tree where the kernel
is compiled. This includes generated files such as host programs.
Kbuild knows targets listed in $(hostprogs-y), $(hostprogs-m), $(always),
$(extra-y) and $(targets). They are all deleted during "make clean".
Files matching the patterns "*.[oas]", "*.ko", plus some additional files
generated by kbuild are deleted all over the kernel src tree when
"make clean" is executed.
Additional files can be specified in kbuild makefiles by use of $(clean-files).
clean-files := devlist.h classlist.h
When executing "make clean", the two files "devlist.h classlist.h" will
be deleted. Kbuild will assume files to be in same relative directory as the
Makefile except if an absolute path is specified (path starting with '/').
To delete a directory hirachy use:
clean-dirs := $(objtree)/debian/
This will delete the directory debian, including all subdirectories.
Kbuild will assume the directories to be in the same relative path as the
Makefile if no absolute path is specified (path does not start with '/').
Usually kbuild descends down in subdirectories due to "obj-* := dir/",
but in the architecture makefiles where the kbuild infrastructure
is not sufficient this sometimes needs to be explicit.
subdir- := compressed/
The above assignment instructs kbuild to descend down in the
directory compressed/ when "make clean" is executed.
To support the clean infrastructure in the Makefiles that builds the
final bootimage there is an optional target named archclean:
$(Q)$(MAKE) $(clean)=arch/i386/boot
When "make clean" is executed, make will descend down in arch/i386/boot,
and clean as usual. The Makefile located in arch/i386/boot/ may use
the subdir- trick to descend further down.
Note 1: arch/$(ARCH)/Makefile cannot use "subdir-", because that file is
included in the top level makefile, and the kbuild infrastructure
is not operational at that point.
Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will
be visited during "make clean".
=== 6 Architecture Makefiles
The top level Makefile sets up the environment and does the preparation,
before starting to descend down in the individual directories.
The top level makefile contains the generic part, whereas the
arch/$(ARCH)/Makefile contains what is required to set-up kbuild
to the said architecture.
To do so arch/$(ARCH)/Makefile sets a number of variables, and defines
a few targets.
When kbuild executes the following steps are followed (roughly):
1) Configuration of the kernel => produced .config
2) Store kernel version in include/linux/version.h
3) Symlink include/asm to include/asm-$(ARCH)
4) Updating all other prerequisites to the target prepare:
- Additional prerequisites are specified in arch/$(ARCH)/Makefile
5) Recursively descend down in all directories listed in
init-* core* drivers-* net-* libs-* and build all targets.
- The value of the above variables are extended in arch/$(ARCH)/Makefile.
6) All object files are then linked and the resulting file vmlinux is
located at the root of the src tree.
The very first objects linked are listed in head-y, assigned by
7) Finally the architecture specific part does any required post processing
and builds the final bootimage.
- This includes building boot records
- Preparing initrd images and the like
--- 6.1 Set variables to tweak the build to the architecture
LDFLAGS Generic $(LD) options
Flags used for all invocations of the linker.
Often specifying the emulation is sufficient.
LDFLAGS := -m elf_s390
Note: EXTRA_LDFLAGS and LDFLAGS_$@ can be used to further customise
the flags used. See chapter 7.
LDFLAGS_MODULE Options for $(LD) when linking modules
LDFLAGS_MODULE is used to set specific flags for $(LD) when
linking the .ko files used for modules.
Default is "-r", for relocatable output.
LDFLAGS_vmlinux Options for $(LD) when linking vmlinux
LDFLAGS_vmlinux is used to specify additional flags to pass to
the linker when linking the final vmlinux.
LDFLAGS_vmlinux uses the LDFLAGS_$@ support.
LDFLAGS_vmlinux := -e stext
OBJCOPYFLAGS objcopy flags
When $(call if_changed,objcopy) is used to translate a .o file,
then the flags specified in OBJCOPYFLAGS will be used.
$(call if_changed,objcopy) is often used to generate raw binaries on
$(obj)/image: vmlinux FORCE
$(call if_changed,objcopy)
In this example the binary $(obj)/image is a binary version of
vmlinux. The usage of $(call if_changed,xxx) will be described later.
AFLAGS $(AS) assembler flags
Default value - see top level Makefile
Append or modify as required per architecture.
AFLAGS += -m64 -mcpu=ultrasparc
CFLAGS $(CC) compiler flags
Default value - see top level Makefile
Append or modify as required per architecture.
Often the CFLAGS variable depends on the configuration.
cflags-$(CONFIG_M386) += -march=i386
CFLAGS += $(cflags-y)
Many arch Makefiles dynamically run the target C compiler to
probe supported options:
cflags-$(CONFIG_MPENTIUMII) += $(call cc-option,\
# Disable unit-at-a-time mode ...
CFLAGS += $(call cc-option,-fno-unit-at-a-time)
The first examples utilises the trick that a config option expands
to 'y' when selected.
CFLAGS_KERNEL $(CC) options specific for built-in
$(CFLAGS_KERNEL) contains extra C compiler flags used to compile
resident kernel code.
CFLAGS_MODULE $(CC) options specific for modules
$(CFLAGS_MODULE) contains extra C compiler flags used to compile code
for loadable kernel modules.
--- 6.2 Add prerequisites to archprepare:
The archprepare: rule is used to list prerequisites that needs to be
built before starting to descend down in the subdirectories.
This is usual header files containing assembler constants.
archprepare: maketools
In this example the file target maketools will be processed
before descending down in the subdirectories.
See also chapter XXX-TODO that describe how kbuild supports
generating offset header files.
--- 6.3 List directories to visit when descending
An arch Makefile cooperates with the top Makefile to define variables
which specify how to build the vmlinux file. Note that there is no
corresponding arch-specific section for modules; the module-building
machinery is all architecture-independent.
head-y, init-y, core-y, libs-y, drivers-y, net-y
$(head-y) list objects to be linked first in vmlinux.
$(libs-y) list directories where a lib.a archive can be located.
The rest list directories where a built-in.o object file can be located.
$(init-y) objects will be located after $(head-y).
Then the rest follows in this order:
$(core-y), $(libs-y), $(drivers-y) and $(net-y).
The top level Makefile define values for all generic directories,
and arch/$(ARCH)/Makefile only adds architecture specific directories.
core-y += arch/sparc64/kernel/
libs-y += arch/sparc64/prom/ arch/sparc64/lib/
drivers-$(CONFIG_OPROFILE) += arch/sparc64/oprofile/
--- 6.4 Architecture specific boot images
An arch Makefile specifies goals that take the vmlinux file, compress
it, wrap it in bootstrapping code, and copy the resulting files
somewhere. This includes various kinds of installation commands.
The actual goals are not standardized across architectures.
It is common to locate any additional processing in a boot/
directory below arch/$(ARCH)/.
Kbuild does not provide any smart way to support building a
target specified in boot/. Therefore arch/$(ARCH)/Makefile shall
call make manually to build a target in boot/.
The recommended approach is to include shortcuts in
arch/$(ARCH)/Makefile, and use the full path when calling down
into the arch/$(ARCH)/boot/Makefile.
boot := arch/i386/boot
bzImage: vmlinux
$(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
"$(Q)$(MAKE) $(build)=<dir>" is the recommended way to invoke
make in a subdirectory.
There are no rules for naming of the architecture specific targets,
but executing "make help" will list all relevant targets.
To support this $(archhelp) must be defined.
define archhelp
echo '* bzImage - Image (arch/$(ARCH)/boot/bzImage)'
When make is executed without arguments, the first goal encountered
will be built. In the top level Makefile the first goal present
is all:.
An architecture shall always per default build a bootable image.
In "make help" the default goal is highlighted with a '*'.
Add a new prerequisite to all: to select a default goal different
from vmlinux.
all: bzImage
When "make" is executed without arguments, bzImage will be built.
--- 6.5 Building non-kbuild targets
extra-y specify additional targets created in the current
directory, in addition to any targets specified by obj-*.
Listing all targets in extra-y is required for two purposes:
1) Enable kbuild to check changes in command lines
- When $(call if_changed,xxx) is used
2) kbuild knows what files to delete during "make clean"
extra-y := head.o init_task.o
In this example extra-y is used to list object files that
shall be built, but shall not be linked as part of built-in.o.
--- 6.6 Commands useful for building a boot image
Kbuild provides a few macros that are useful when building a
boot image.
if_changed is the infrastructure used for the following commands.
target: source(s) FORCE
$(call if_changed,ld/objcopy/gzip)
When the rule is evaluated it is checked to see if any files
needs an update, or the commandline has changed since last
invocation. The latter will force a rebuild if any options
to the executable have changed.
Any target that utilises if_changed must be listed in $(targets),
otherwise the command line check will fail, and the target will
always be built.
Assignments to $(targets) are without $(obj)/ prefix.
if_changed may be used in conjunction with custom commands as
defined in 6.7 "Custom kbuild commands".
Note: It is a typical mistake to forget the FORCE prerequisite.
Another common pitfall is that whitespace is sometimes
significant; for instance, the below will fail (note the extra space
after the comma):
target: source(s) FORCE
#WRONG!# $(call if_changed, ld/objcopy/gzip)
Link target. Often LDFLAGS_$@ is used to set specific options to ld.
Copy binary. Uses OBJCOPYFLAGS usually specified in
OBJCOPYFLAGS_$@ may be used to set additional options.
Compress target. Use maximum compression to compress target.
LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary
LDFLAGS_setup := -Ttext 0x0 -s --oformat binary -e begtext
targets += setup setup.o bootsect bootsect.o
$(obj)/setup $(obj)/bootsect: %: %.o FORCE
$(call if_changed,ld)
In this example there are two possible targets, requiring different
options to the linker. the linker options are specified using the
LDFLAGS_$@ syntax - one for each potential target.
$(targets) are assinged all potential targets, herby kbuild knows
the targets and will:
1) check for commandline changes
2) delete target during make clean
The ": %: %.o" part of the prerequisite is a shorthand that
free us from listing the setup.o and bootsect.o files.
Note: It is a common mistake to forget the "target :=" assignment,
resulting in the target file being recompiled for no
obvious reason.
--- 6.7 Custom kbuild commands
When kbuild is executing with KBUILD_VERBOSE=0 then only a shorthand
of a command is normally displayed.
To enable this behaviour for custom commands kbuild requires
two variables to be set:
quiet_cmd_<command> - what shall be echoed
cmd_<command> - the command to execute
quiet_cmd_image = BUILD $@
cmd_image = $(obj)/tools/build $(BUILDFLAGS) \
$(obj)/vmlinux.bin > $@
targets += bzImage
$(obj)/bzImage: $(obj)/vmlinux.bin $(obj)/tools/build FORCE
$(call if_changed,image)
@echo 'Kernel: $@ is ready'
When updating the $(obj)/bzImage target the line:
BUILD arch/i386/boot/bzImage
will be displayed with "make KBUILD_VERBOSE=0".
--- 6.8 Preprocessing linker scripts
When the vmlinux image is build the linker script:
arch/$(ARCH)/kernel/ is used.
The script is a preprocessed variant of the file
located in the same directory.
kbuild knows .lds file and includes a rule *lds.S -> *lds.
always :=
export += -P -C -U$(ARCH)
The assigment to $(always) is used to tell kbuild to build the
The assignment to $( tell kbuild to use the
specified options when building the target
When building the *.lds target kbuild used the variakles:
CPPFLAGS : Set in top-level Makefile
EXTRA_CPPFLAGS : May be set in the kbuild makefile
CPPFLAGS_$(@F) : Target specific flags.
Note that the full filename is used in this
The kbuild infrastructure for *lds file are used in several
architecture specific files.
=== 7 Kbuild Variables
The top Makefile exports the following variables:
These variables define the current kernel version. A few arch
Makefiles actually use these values directly; they should use
$(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic
three-part version number, such as "2", "4", and "0". These three
values are always numeric.
$(EXTRAVERSION) defines an even tinier sublevel for pre-patches
or additional patches. It is usually some non-numeric string
such as "-pre4", and is often blank.
$(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable
for constructing installation directory names or showing in
version strings. Some arch Makefiles use it for this purpose.
This variable defines the target architecture, such as "i386",
"arm", or "sparc". Some kbuild Makefiles test $(ARCH) to
determine which files to compile.
By default, the top Makefile sets $(ARCH) to be the same as the
host system architecture. For a cross build, a user may
override the value of $(ARCH) on the command line:
make ARCH=m68k ...
This variable defines a place for the arch Makefiles to install
the resident kernel image and file.
Use this for architecture specific install targets.
$(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module
installation. This variable is not defined in the Makefile but
may be passed in by the user if desired.
$(MODLIB) specifies the directory for module installation.
The top Makefile defines $(MODLIB) to
$(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE). The user may
override this value on the command line if desired.
If this variable is specified, will cause modules to be stripped
after they are installed. If INSTALL_MOD_STRIP is '1', then the
default option --strip-debug will be used. Otherwise,
INSTALL_MOD_STRIP will used as the option(s) to the strip command.
=== 8 Makefile language
The kernel Makefiles are designed to run with GNU Make. The Makefiles
use only the documented features of GNU Make, but they do use many
GNU extensions.
GNU Make supports elementary list-processing functions. The kernel
Makefiles use a novel style of list building and manipulation with few
"if" statements.
GNU Make has two assignment operators, ":=" and "=". ":=" performs
immediate evaluation of the right-hand side and stores an actual string
into the left-hand side. "=" is like a formula definition; it stores the
right-hand side in an unevaluated form and then evaluates this form each
time the left-hand side is used.
There are some cases where "=" is appropriate. Usually, though, ":="
is the right choice.
=== 9 Credits
Original version made by Michael Elizabeth Chastain, <>
Updates by Kai Germaschewski <>
Updates by Sam Ravnborg <>
=== 10 TODO
- Describe how kbuild support shipped files with _shipped.
- Generating offset header files.
- Add more variables to section 7?