kbuild: doc: merge 'Special Rules' and 'Custom kbuild commands' sections

[mirror_ubuntu-hirsute-kernel.git] / Documentation / kbuild / makefiles.rst

======================
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 <deleted>
           --- 3.9 Dependency tracking
           --- 3.10 Custom Rules
           --- 3.11 $(CC) support functions
           --- 3.12 $(LD) support functions
           --- 3.13 Script Invocation

        === 4 Host Program support
           --- 4.1 Simple Host Program
           --- 4.2 Composite Host Programs
           --- 4.3 Using C++ for host programs
           --- 4.4 Controlling compiler options for host programs
           --- 4.5 When host programs are actually built

        === 5 Userspace Program support
           --- 5.1 Simple Userspace Program
           --- 5.2 Composite Userspace Programs
           --- 5.3 Controlling compiler options for userspace programs
           --- 5.4 When userspace programs are actually built

        === 6 Kbuild clean infrastructure

        === 7 Architecture Makefiles
           --- 7.1 Set variables to tweak the build to the architecture
           --- 7.2 Add prerequisites to archheaders
           --- 7.3 Add prerequisites to archprepare
           --- 7.4 List directories to visit when descending
           --- 7.5 Architecture-specific boot images
           --- 7.6 Building non-kbuild targets
           --- 7.7 Commands useful for building a boot image
           --- 7.8 <deleted>
           --- 7.9 Preprocessing linker scripts
           --- 7.10 Generic header files
           --- 7.11 Post-link pass

        === 8 Kbuild syntax for exported headers
                --- 8.1 no-export-headers
                --- 8.2 generic-y
                --- 8.3 generated-y
                --- 8.4 mandatory-y

        === 9 Kbuild Variables
        === 10 Makefile language
        === 11 Credits
        === 12 TODO

1 Overview
==========

The Makefiles have five parts::

        Makefile                    the top Makefile.
        .config                     the kernel configuration file.
        arch/$(SRCARCH)/Makefile    the arch Makefile.
        scripts/Makefile.*          common rules etc. for all kbuild Makefiles.
        kbuild Makefiles            exist in every subdirectory

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/$(SRCARCH)/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 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 introduces 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:

        Example::

                obj-y += foo.o

        This tells 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:

        Example::

                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 $(obj-y) lists.  These lists depend on the kernel
        configuration.

        Kbuild compiles all the $(obj-y) files.  It then calls
        "$(AR) rcSTP" to merge these files into one built-in.a file.
        This is a thin archive without a symbol table. It will be later
        linked into vmlinux by scripts/link-vmlinux.sh

        The order of files in $(obj-y) is significant.  Duplicates in
        the lists are allowed: the first instance will be linked into
        built-in.a 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 your disks are renumbered.

        Example::

                #drivers/isdn/i4l/Makefile
                # Makefile for the kernel ISDN subsystem and device drivers.
                # Each configuration option enables a list of files.
                obj-$(CONFIG_ISDN_I4L)         += isdn.o
                obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o

3.3 Loadable module goals - obj-m
---------------------------------

        $(obj-m) specifies 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).

        Example::

                #drivers/isdn/i4l/Makefile
                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; however,
        kbuild needs to know which object files you want to build your
        module from, so you have to tell it by setting a $(<module_name>-y)
        variable.

        Example::

                #drivers/isdn/i4l/Makefile
                obj-$(CONFIG_ISDN_I4L) += isdn.o
                isdn-y := 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 $(isdn-y) and then run
        "$(LD) -r" on the list of these files to generate isdn.o.

        Due to kbuild recognizing $(<module_name>-y) for composite objects,
        you can use the value of a `CONFIG_` symbol to optionally include an
        object file as part of a composite object.

        Example::

                #fs/ext2/Makefile
                obj-$(CONFIG_EXT2_FS) += ext2.o
                ext2-y := balloc.o dir.o file.o ialloc.o inode.o ioctl.o \
                          namei.o super.o symlink.o
                ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o xattr_user.o \
                                                xattr_trusted.o

        In this example, xattr.o, xattr_user.o and xattr_trusted.o are 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.a, 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.a 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 additionally listed in
        lib-y will not be included in the library, since they will
        be accessible anyway.
        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.a and a lib.a file.

        Example::

                #arch/x86/lib/Makefile
                lib-y    := delay.o

        This will create a library lib.a based on delay.o. For kbuild to
        actually recognize that there is a lib.a being built, the directory
        shall be listed in libs-y.

        See also "7.4 List directories to visit when descending".

        Use 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
        them.

        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.

        Example::

                #fs/Makefile
                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 uses this information not only to decide that it needs to visit
        the directory, but also to decide whether or not to link objects from
        the directory into vmlinux.

        When Kbuild descends into the directory with 'y', all built-in objects
        from that directory are combined into the built-in.a, which will be
        eventually linked into vmlinux.

        When Kbuild descends into the directory with 'm', in contrast, nothing
        from that directory will be linked into vmlinux. If the Makefile in
        that directory specifies obj-y, those objects will be left orphan.
        It is very likely a bug of the Makefile or of dependencies in Kconfig.

        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
---------------------

    ccflags-y, asflags-y and ldflags-y
        These three flags apply only to the kbuild makefile in which they
        are assigned. They are used for all the normal cc, as and ld
        invocations happening during a recursive build.
        Note: Flags with the same behaviour were previously named:
        EXTRA_CFLAGS, EXTRA_AFLAGS and EXTRA_LDFLAGS.
        They are still supported but their usage is deprecated.

        ccflags-y specifies options for compiling with $(CC).

        Example::

                # drivers/acpi/acpica/Makefile
                ccflags-y                       := -Os -D_LINUX -DBUILDING_ACPICA
                ccflags-$(CONFIG_ACPI_DEBUG)    += -DACPI_DEBUG_OUTPUT

        This variable is necessary because the top Makefile owns the
        variable $(KBUILD_CFLAGS) and uses it for compilation flags for the
        entire tree.

        asflags-y specifies assembler options.

        Example::

                #arch/sparc/kernel/Makefile
                asflags-y := -ansi

        ldflags-y specifies options for linking with $(LD).

        Example::

                #arch/cris/boot/compressed/Makefile
                ldflags-y += -T $(srctree)/$(src)/decompress_$(arch-y).lds

    subdir-ccflags-y, subdir-asflags-y
        The two flags listed above are similar to ccflags-y and asflags-y.
        The difference is that the subdir- variants have effect for the kbuild
        file where they are present and all subdirectories.
        Options specified using subdir-* are added to the commandline before
        the options specified using the non-subdir variants.

        Example::

                subdir-ccflags-y := -Werror

    ccflags-remove-y, asflags-remove-y
        These flags are used to remove particular flags for the compiler,
        assembler invocations.

        Example::

                ccflags-remove-$(CONFIG_MCOUNT) += -pg

    CFLAGS_$@, AFLAGS_$@
        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.

        CFLAGS_$@ has the higher priority than ccflags-remove-y; CFLAGS_$@
        can re-add compiler flags that were removed by ccflags-remove-y.

        Example::

                # drivers/scsi/Makefile
                CFLAGS_aha152x.o =   -DAHA152X_STAT -DAUTOCONF
                CFLAGS_gdth.o    = # -DDEBUG_GDTH=2 -D__SERIAL__ -D__COM2__ \
                                     -DGDTH_STATISTICS

        These two lines specify compilation flags for aha152x.o and gdth.o.

        $(AFLAGS_$@) is a similar feature for source files in assembly
        languages.

        AFLAGS_$@ has the higher priority than asflags-remove-y; AFLAGS_$@
        can re-add assembler flags that were removed by asflags-remove-y.

        Example::

                # arch/arm/kernel/Makefile
                AFLAGS_head.o        := -DTEXT_OFFSET=$(TEXT_OFFSET)
                AFLAGS_crunch-bits.o := -Wa,-mcpu=ep9312
                AFLAGS_iwmmxt.o      := -Wa,-mcpu=iwmmxt


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 Custom Rules
------------------

        Custom 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 are the architecture-specific Makefiles which
        need custom rules to prepare boot images etc.

        Custom rules are written as normal Make rules.
        Kbuild is not executing in the directory where the Makefile is
        located, so all custom rules shall use a relative
        path to prerequisite files and target files.

        Two variables are used when defining custom rules:

        $(src)
            $(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)
            $(obj) is a relative path which points to the directory
            where the target is saved. Always use $(obj) when
            referring to generated files.

            Example::

                #drivers/scsi/Makefile
                $(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl
                        $(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl

            This is a custom 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).

        $(kecho)
            echoing information to user in a rule is often a good practice
            but when execution "make -s" one does not expect to see any output
            except for warnings/errors.
            To support this kbuild defines $(kecho) which will echo out the
            text following $(kecho) to stdout except if "make -s" is used.

        Example::

                # arch/arm/Makefile
                $(BOOT_TARGETS): vmlinux
                        $(Q)$(MAKE) $(build)=$(boot) MACHINE=$(MACHINE) $(boot)/$@
                        @$(kecho) '  Kernel: $(boot)/$@ is ready'

        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

        Example::

                # lib/Makefile
                quiet_cmd_crc32 = GEN     $@
                      cmd_crc32 = $< > $@

                $(obj)/crc32table.h: $(obj)/gen_crc32table
                        $(call cmd,crc32)

        When updating the $(obj)/crc32table.h target, the line:

                  GEN     lib/crc32table.h

        will be displayed with "make KBUILD_VERBOSE=0".

3.11 $(CC) support functions
----------------------------

        The kernel may be built with several different versions of
        $(CC), each supporting a unique set of features and options.
        kbuild provides basic support to check for valid options for $(CC).
        $(CC) is usually the gcc compiler, but other alternatives are
        available.

    as-option
        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 the first option is not supported.

        Example::

                #arch/sh/Makefile
                cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),)

        In the above example, cflags-y will be assigned 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.

    as-instr
        as-instr checks if the assembler reports a specific instruction
        and then outputs either option1 or option2
        C escapes are supported in the test instruction
        Note: as-instr-option uses KBUILD_AFLAGS for assembler options

    cc-option
        cc-option is used to check if $(CC) supports a given option, and if
        not supported to use an optional second option.

        Example::

                #arch/x86/Makefile
                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.
        Note: cc-option uses KBUILD_CFLAGS for $(CC) options

   cc-option-yn
        cc-option-yn is used to check if gcc supports a given option
        and return 'y' if supported, otherwise 'n'.

        Example::

                #arch/ppc/Makefile
                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 'y', the expanded variables $(aflags-y)
        and $(cflags-y) will be assigned the values -a32 and -m32,
        respectively.
        Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options

    cc-disable-warning
        cc-disable-warning checks if gcc supports a given warning and returns
        the commandline switch to disable it. This special function is needed,
        because gcc 4.4 and later accept any unknown -Wno-* option and only
        warn about it if there is another warning in the source file.

        Example::

                KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)

        In the above example, -Wno-unused-but-set-variable will be added to
        KBUILD_CFLAGS only if gcc really accepts it.

    cc-ifversion
        cc-ifversion tests the version of $(CC) and equals the fourth parameter
        if version expression is true, or the fifth (if given) if the version
        expression is false.

        Example::

                #fs/reiserfs/Makefile
                ccflags-y := $(call cc-ifversion, -lt, 0402, -O1)

        In this example, ccflags-y 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.

    cc-cross-prefix
        cc-cross-prefix is used to check if there exists a $(CC) in path with
        one of the listed prefixes. The first prefix where there exist a
        prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found
        then nothing is returned.
        Additional prefixes are separated by a single space in the
        call of cc-cross-prefix.
        This functionality is useful for architecture Makefiles that try
        to set CROSS_COMPILE to well-known values but may have several
        values to select between.
        It is recommended only to try to set CROSS_COMPILE if it is a cross
        build (host arch is different from target arch). And if CROSS_COMPILE
        is already set then leave it with the old value.

        Example::

                #arch/m68k/Makefile
                ifneq ($(SUBARCH),$(ARCH))
                        ifeq ($(CROSS_COMPILE),)
                               CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-)
                        endif
                endif

3.12 $(LD) support functions
----------------------------

    ld-option
        ld-option is used to check if $(LD) supports the supplied option.
        ld-option takes two options as arguments.
        The second argument is an optional option that can be used if the
        first option is not supported by $(LD).

        Example::

                #Makefile
                LDFLAGS_vmlinux += $(call ld-option, -X)

3.13 Script invocation
----------------------

        Make rules may invoke scripts to build the kernel. The rules shall
        always provide the appropriate interpreter to execute the script. They
        shall not rely on the execute bits being set, and shall not invoke the
        script directly. For the convenience of manual script invocation, such
        as invoking ./scripts/checkpatch.pl, it is recommended to set execute
        bits on the scripts nonetheless.

        Kbuild provides variables $(CONFIG_SHELL), $(AWK), $(PERL),
        $(PYTHON) and $(PYTHON3) to refer to interpreters for the respective
        scripts.

        Example::

                #Makefile
                cmd_depmod = $(CONFIG_SHELL) $(srctree)/scripts/depmod.sh $(DEPMOD) \
                             $(KERNELRELEASE)

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".

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-y".
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.

        Example::

                hostprogs := 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.
        $(<executable>-objs) lists all objects used to link the final
        executable.

        Example::

                #scripts/lxdialog/Makefile
                hostprogs     := 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 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.

        Example::

                #scripts/kconfig/Makefile
                hostprogs     := qconf
                qconf-cxxobjs := qconf.o

        In the example above the executable is composed of the C++ file
        qconf.cc - identified by $(qconf-cxxobjs).

        If qconf is composed of a mixture of .c and .cc files, then an
        additional line can be used to identify this.

        Example::

                #scripts/kconfig/Makefile
                hostprogs     := qconf
                qconf-cxxobjs := qconf.o
                qconf-objs    := check.o

4.4 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 $(KBUILD_HOSTCFLAGS).
        To set flags that will take effect for all host programs created
        in that Makefile, use the variable HOST_EXTRACFLAGS.

        Example::

                #scripts/lxdialog/Makefile
                HOST_EXTRACFLAGS += -I/usr/include/ncurses

        To set specific flags for a single file the following construction
        is used:

        Example::

                #arch/ppc64/boot/Makefile
                HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE)

        It is also possible to specify additional options to the linker.

        Example::

                #scripts/kconfig/Makefile
                HOSTLDLIBS_qconf := -L$(QTDIR)/lib

        When linking qconf, it will be passed the extra option
        "-L$(QTDIR)/lib".

4.5 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 custom rule.

        Example::

                #drivers/pci/Makefile
                hostprogs := 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 custom rules must be prefixed with $(obj).

        (2) Use always-y

        When there is no suitable custom rule, and the host program
        shall be built when a makefile is entered, the always-y
        variable shall be used.

        Example::

                #scripts/lxdialog/Makefile
                hostprogs     := lxdialog
                always-y      := $(hostprogs)

        Kbuild provides the following shorthand for this:

                hostprogs-always-y := lxdialog

        This will tell kbuild to build lxdialog even if not referenced in
        any rule.

5 Userspace Program support
===========================

Just like host programs, Kbuild also supports building userspace executables
for the target architecture (i.e. the same architecture as you are building
the kernel for).

The syntax is quite similar. The difference is to use "userprogs" instead of
"hostprogs".

5.1 Simple Userspace Program
----------------------------

        The following line tells kbuild that the program bpf-direct shall be
        built for the target architecture.

        Example::

                userprogs := bpf-direct

        Kbuild assumes in the above example that bpf-direct is made from a
        single C source file named bpf-direct.c located in the same directory
        as the Makefile.

5.2 Composite Userspace Programs
--------------------------------

        Userspace programs can be made up based on composite objects.
        The syntax used to define composite objects for userspace programs is
        similar to the syntax used for kernel objects.
        $(<executable>-objs) lists all objects used to link the final
        executable.

        Example::

                #samples/seccomp/Makefile
                userprogs      := bpf-fancy
                bpf-fancy-objs := bpf-fancy.o bpf-helper.o

        Objects with extension .o are compiled from the corresponding .c
        files. In the above example, bpf-fancy.c is compiled to bpf-fancy.o
        and bpf-helper.c is compiled to bpf-helper.o.

        Finally, the two .o files are linked to the executable, bpf-fancy.
        Note: The syntax <executable>-y is not permitted for userspace programs.

5.3 Controlling compiler options for userspace programs
-------------------------------------------------------

        When compiling userspace programs, it is possible to set specific flags.
        The programs will always be compiled utilising $(CC) passed
        the options specified in $(KBUILD_USERCFLAGS).
        To set flags that will take effect for all userspace programs created
        in that Makefile, use the variable userccflags.

        Example::

                # samples/seccomp/Makefile
                userccflags += -I usr/include

        To set specific flags for a single file the following construction
        is used:

        Example::

                bpf-helper-userccflags += -I user/include

        It is also possible to specify additional options to the linker.

        Example::

                # net/bpfilter/Makefile
                bpfilter_umh-userldflags += -static

        When linking bpfilter_umh, it will be passed the extra option -static.

5.4 When userspace programs are actually built
----------------------------------------------

        Kbuild builds userspace programs only when told to do so.
        There are two ways to do this.

        (1) Add it as the prerequisite of another file

        Example::

                #net/bpfilter/Makefile
                userprogs := bpfilter_umh
                $(obj)/bpfilter_umh_blob.o: $(obj)/bpfilter_umh

        $(obj)/bpfilter_umh is built before $(obj)/bpfilter_umh_blob.o

        (2) Use always-y

        Example::

                userprogs := binderfs_example
                always-y := $(userprogs)

        Kbuild provides the following shorthand for this:

                userprogs-always-y := binderfs_example

        This will tell Kbuild to build binderfs_example when it visits this
        Makefile.

6 Kbuild clean infrastructure
=============================

"make clean" deletes most generated files in the obj tree where the kernel
is compiled. This includes generated files such as host programs.
Kbuild knows targets listed in $(hostprogs), $(always-y), $(always-m),
$(always-), $(extra-y), $(extra-) 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
source tree when "make clean" is executed.

Additional files or directories can be specified in kbuild makefiles by use of
$(clean-files).

        Example::

                #lib/Makefile
                clean-files := crc32table.h

When executing "make clean", the file "crc32table.h" will be deleted.
Kbuild will assume files to be in the same relative directory as the
Makefile, except if prefixed with $(objtree).

To exclude certain files or directories from make clean, use the
$(no-clean-files) variable.

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.

        Example::

                #arch/x86/boot/Makefile
                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 build the
final bootimage there is an optional target named archclean:

        Example::

                #arch/x86/Makefile
                archclean:
                        $(Q)$(MAKE) $(clean)=arch/x86/boot

When "make clean" is executed, make will descend down in arch/x86/boot,
and clean as usual. The Makefile located in arch/x86/boot/ may use
the subdir- trick to descend further down.

Note 1: arch/$(SRCARCH)/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".

7 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
arch/$(SRCARCH)/Makefile contains what is required to set up kbuild
for said architecture.
To do so, arch/$(SRCARCH)/Makefile sets up a number of variables and defines
a few targets.

When kbuild executes, the following steps are followed (roughly):

1) Configuration of the kernel => produce .config
2) Store kernel version in include/linux/version.h
3) Updating all other prerequisites to the target prepare:
   - Additional prerequisites are specified in arch/$(SRCARCH)/Makefile
4) Recursively descend down in all directories listed in
   init-* core* drivers-* net-* libs-* and build all targets.
   - The values of the above variables are expanded in arch/$(SRCARCH)/Makefile.
5) All object files are then linked and the resulting file vmlinux is
   located at the root of the obj tree.
   The very first objects linked are listed in head-y, assigned by
   arch/$(SRCARCH)/Makefile.
6) 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


7.1 Set variables to tweak the build to the architecture
--------------------------------------------------------

    KBUILD_LDFLAGS
        Generic $(LD) options

        Flags used for all invocations of the linker.
        Often specifying the emulation is sufficient.

        Example::

                #arch/s390/Makefile
                KBUILD_LDFLAGS         := -m elf_s390

        Note: ldflags-y can be used to further customise
        the flags used. See section 3.7.

    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 image.
        LDFLAGS_vmlinux uses the LDFLAGS_$@ support.

        Example::

                #arch/x86/Makefile
                LDFLAGS_vmlinux := -e stext

    OBJCOPYFLAGS
        objcopy flags

        When $(call if_changed,objcopy) is used to translate a .o file,
        the flags specified in OBJCOPYFLAGS will be used.
        $(call if_changed,objcopy) is often used to generate raw binaries on
        vmlinux.

        Example::

                #arch/s390/Makefile
                OBJCOPYFLAGS := -O binary

                #arch/s390/boot/Makefile
                $(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.

    KBUILD_AFLAGS
        Assembler flags

        Default value - see top level Makefile
        Append or modify as required per architecture.

        Example::

                #arch/sparc64/Makefile
                KBUILD_AFLAGS += -m64 -mcpu=ultrasparc

    KBUILD_CFLAGS
        $(CC) compiler flags

        Default value - see top level Makefile
        Append or modify as required per architecture.

        Often, the KBUILD_CFLAGS variable depends on the configuration.

        Example::

                #arch/x86/boot/compressed/Makefile
                cflags-$(CONFIG_X86_32) := -march=i386
                cflags-$(CONFIG_X86_64) := -mcmodel=small
                KBUILD_CFLAGS += $(cflags-y)

        Many arch Makefiles dynamically run the target C compiler to
        probe supported options::

                #arch/x86/Makefile

                ...
                cflags-$(CONFIG_MPENTIUMII)     += $(call cc-option,\
                                                -march=pentium2,-march=i686)
                ...
                # Disable unit-at-a-time mode ...
                KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time)
                ...


        The first example utilises the trick that a config option expands
        to 'y' when selected.

    KBUILD_AFLAGS_KERNEL
        Assembler options specific for built-in

        $(KBUILD_AFLAGS_KERNEL) contains extra C compiler flags used to compile
        resident kernel code.

    KBUILD_AFLAGS_MODULE
        Assembler options specific for modules

        $(KBUILD_AFLAGS_MODULE) is used to add arch-specific options that
        are used for assembler.

        From commandline AFLAGS_MODULE shall be used (see kbuild.rst).

    KBUILD_CFLAGS_KERNEL
        $(CC) options specific for built-in

        $(KBUILD_CFLAGS_KERNEL) contains extra C compiler flags used to compile
        resident kernel code.

    KBUILD_CFLAGS_MODULE
        Options for $(CC) when building modules

        $(KBUILD_CFLAGS_MODULE) is used to add arch-specific options that
        are used for $(CC).
        From commandline CFLAGS_MODULE shall be used (see kbuild.rst).

    KBUILD_LDFLAGS_MODULE
        Options for $(LD) when linking modules

        $(KBUILD_LDFLAGS_MODULE) is used to add arch-specific options
        used when linking modules. This is often a linker script.

        From commandline LDFLAGS_MODULE shall be used (see kbuild.rst).

    KBUILD_LDS

        The linker script with full path. Assigned by the top-level Makefile.

    KBUILD_LDS_MODULE

        The module linker script with full path. Assigned by the top-level
        Makefile and additionally by the arch Makefile.

    KBUILD_VMLINUX_OBJS

        All object files for vmlinux. They are linked to vmlinux in the same
        order as listed in KBUILD_VMLINUX_OBJS.

    KBUILD_VMLINUX_LIBS

        All .a "lib" files for vmlinux. KBUILD_VMLINUX_OBJS and
        KBUILD_VMLINUX_LIBS together specify all the object files used to
        link vmlinux.

7.2 Add prerequisites to archheaders
------------------------------------

        The archheaders: rule is used to generate header files that
        may be installed into user space by "make header_install".

        It is run before "make archprepare" when run on the
        architecture itself.


7.3 Add prerequisites to archprepare
------------------------------------

        The archprepare: rule is used to list prerequisites that need to be
        built before starting to descend down in the subdirectories.
        This is usually used for header files containing assembler constants.

        Example::

                #arch/arm/Makefile
                archprepare: maketools

        In this example, the file target maketools will be processed
        before descending down in the subdirectories.
        See also chapter XXX-TODO that describes how kbuild supports
        generating offset header files.


7.4 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, core-y, libs-y, drivers-y
            $(head-y) lists objects to be linked first in vmlinux.

            $(libs-y) lists directories where a lib.a archive can be located.

            The rest list directories where a built-in.a object file can be
            located.

            Then the rest follows in this order:

                $(core-y), $(libs-y), $(drivers-y)

            The top level Makefile defines values for all generic directories,
            and arch/$(SRCARCH)/Makefile only adds architecture-specific
            directories.

            Example::

                # arch/sparc/Makefile
                core-y                 += arch/sparc/

                libs-y                 += arch/sparc/prom/
                libs-y                 += arch/sparc/lib/

                drivers-$(CONFIG_PM) += arch/sparc/power/
                drivers-$(CONFIG_OPROFILE)      += arch/sparc/oprofile/

7.5 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/$(SRCARCH)/.

        Kbuild does not provide any smart way to support building a
        target specified in boot/. Therefore arch/$(SRCARCH)/Makefile shall
        call make manually to build a target in boot/.

        The recommended approach is to include shortcuts in
        arch/$(SRCARCH)/Makefile, and use the full path when calling down
        into the arch/$(SRCARCH)/boot/Makefile.

        Example::

                #arch/x86/Makefile
                boot := arch/x86/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 architecture-specific targets,
        but executing "make help" will list all relevant targets.
        To support this, $(archhelp) must be defined.

        Example::

                #arch/x86/Makefile
                define archhelp
                  echo  '* bzImage      - Compressed kernel image (arch/x86/boot/bzImage)'
                endif

        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.

        Example::

                #arch/x86/Makefile
                all: bzImage

        When "make" is executed without arguments, bzImage will be built.

7.6 Building non-kbuild targets
-------------------------------

    extra-y
        extra-y specifies 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"

        Example::

                #arch/x86/kernel/Makefile
                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.a.

7.7 Commands useful for building a boot image
---------------------------------------------

    Kbuild provides a few macros that are useful when building a
    boot image.

    if_changed
        if_changed is the infrastructure used for the following commands.

        Usage::

                target: source(s) FORCE
                        $(call if_changed,ld/objcopy/gzip/...)

        When the rule is evaluated, it is checked to see if any files
        need an update, or the command line has changed since the 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 rules as
        defined in "3.10 Custom Rules".

        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/...)

        Note:
              if_changed should not be used more than once per target.
              It stores the executed command in a corresponding .cmd

        file and multiple calls would result in overwrites and
        unwanted results when the target is up to date and only the
        tests on changed commands trigger execution of commands.

    ld
        Link target. Often, LDFLAGS_$@ is used to set specific options to ld.

        Example::

                #arch/x86/boot/Makefile
                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 assigned all potential targets, by which 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
        frees us from listing the setup.o and bootsect.o files.

        Note:
              It is a common mistake to forget the "targets :=" assignment,
              resulting in the target file being recompiled for no
              obvious reason.

    objcopy
        Copy binary. Uses OBJCOPYFLAGS usually specified in
        arch/$(SRCARCH)/Makefile.
        OBJCOPYFLAGS_$@ may be used to set additional options.

    gzip
        Compress target. Use maximum compression to compress target.

        Example::

                #arch/x86/boot/compressed/Makefile
                $(obj)/vmlinux.bin.gz: $(vmlinux.bin.all-y) FORCE
                        $(call if_changed,gzip)

    dtc
        Create flattened device tree blob object suitable for linking
        into vmlinux. Device tree blobs linked into vmlinux are placed
        in an init section in the image. Platform code *must* copy the
        blob to non-init memory prior to calling unflatten_device_tree().

        To use this command, simply add `*.dtb` into obj-y or targets, or make
        some other target depend on `%.dtb`

        A central rule exists to create `$(obj)/%.dtb` from `$(src)/%.dts`;
        architecture Makefiles do no need to explicitly write out that rule.

        Example::

                targets += $(dtb-y)
                DTC_FLAGS ?= -p 1024

7.9 Preprocessing linker scripts
--------------------------------

        When the vmlinux image is built, the linker script
        arch/$(SRCARCH)/kernel/vmlinux.lds is used.
        The script is a preprocessed variant of the file vmlinux.lds.S
        located in the same directory.
        kbuild knows .lds files and includes a rule `*lds.S` -> `*lds`.

        Example::

                #arch/x86/kernel/Makefile
                extra-y := vmlinux.lds

        The assignment to extra-y is used to tell kbuild to build the
        target vmlinux.lds.
        The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the
        specified options when building the target vmlinux.lds.

        When building the `*.lds` target, kbuild uses the variables::

                KBUILD_CPPFLAGS : Set in top-level Makefile
                cppflags-y      : May be set in the kbuild makefile
                CPPFLAGS_$(@F)  : Target-specific flags.
                                Note that the full filename is used in this
                                assignment.

        The kbuild infrastructure for `*lds` files is used in several
        architecture-specific files.

7.10 Generic header files
-------------------------

        The directory include/asm-generic contains the header files
        that may be shared between individual architectures.
        The recommended approach how to use a generic header file is
        to list the file in the Kbuild file.
        See "8.2 generic-y" for further info on syntax etc.

7.11 Post-link pass
-------------------

        If the file arch/xxx/Makefile.postlink exists, this makefile
        will be invoked for post-link objects (vmlinux and modules.ko)
        for architectures to run post-link passes on. Must also handle
        the clean target.

        This pass runs after kallsyms generation. If the architecture
        needs to modify symbol locations, rather than manipulate the
        kallsyms, it may be easier to add another postlink target for
        .tmp_vmlinux? targets to be called from link-vmlinux.sh.

        For example, powerpc uses this to check relocation sanity of
        the linked vmlinux file.

8 Kbuild syntax for exported headers
------------------------------------

The kernel includes a set of headers that is exported to userspace.
Many headers can be exported as-is but other headers require a
minimal pre-processing before they are ready for user-space.
The pre-processing does:

- drop kernel-specific annotations
- drop include of compiler.h
- drop all sections that are kernel internal (guarded by `ifdef __KERNEL__`)

All headers under include/uapi/, include/generated/uapi/,
arch/<arch>/include/uapi/ and arch/<arch>/include/generated/uapi/
are exported.

A Kbuild file may be defined under arch/<arch>/include/uapi/asm/ and
arch/<arch>/include/asm/ to list asm files coming from asm-generic.
See subsequent chapter for the syntax of the Kbuild file.

8.1 no-export-headers
---------------------

        no-export-headers is essentially used by include/uapi/linux/Kbuild to
        avoid exporting specific headers (e.g. kvm.h) on architectures that do
        not support it. It should be avoided as much as possible.

8.2 generic-y
-------------

        If an architecture uses a verbatim copy of a header from
        include/asm-generic then this is listed in the file
        arch/$(SRCARCH)/include/asm/Kbuild like this:

                Example::

                        #arch/x86/include/asm/Kbuild
                        generic-y += termios.h
                        generic-y += rtc.h

        During the prepare phase of the build a wrapper include
        file is generated in the directory::

                arch/$(SRCARCH)/include/generated/asm

        When a header is exported where the architecture uses
        the generic header a similar wrapper is generated as part
        of the set of exported headers in the directory::

                usr/include/asm

        The generated wrapper will in both cases look like the following:

                Example: termios.h::

                        #include <asm-generic/termios.h>

8.3 generated-y
---------------

        If an architecture generates other header files alongside generic-y
        wrappers, generated-y specifies them.

        This prevents them being treated as stale asm-generic wrappers and
        removed.

                Example::

                        #arch/x86/include/asm/Kbuild
                        generated-y += syscalls_32.h

8.4 mandatory-y
---------------

        mandatory-y is essentially used by include/(uapi/)asm-generic/Kbuild
        to define the minimum set of ASM headers that all architectures must have.

        This works like optional generic-y. If a mandatory header is missing
        in arch/$(SRCARCH)/include/(uapi/)/asm, Kbuild will automatically
        generate a wrapper of the asm-generic one.

9 Kbuild Variables
==================

The top Makefile exports the following variables:

    VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION
        These variables define the current kernel version.  A few arch
        Makefiles actually use these values directly; they should use
        $(KERNELRELEASE) instead.

        $(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
        $(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.

    ARCH
        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 ...

    SRCARCH
        This variable specifies the directory in arch/ to build.

        ARCH and SRCARCH may not necessarily match. A couple of arch
        directories are biarch, that is, a single `arch/*/` directory supports
        both 32-bit and 64-bit.

        For example, you can pass in ARCH=i386, ARCH=x86_64, or ARCH=x86.
        For all of them, SRCARCH=x86 because arch/x86/ supports both i386 and
        x86_64.

    INSTALL_PATH
        This variable defines a place for the arch Makefiles to install
        the resident kernel image and System.map file.
        Use this for architecture-specific install targets.

    INSTALL_MOD_PATH, MODLIB
        $(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.

    INSTALL_MOD_STRIP
        If this variable is specified, it 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, the
        INSTALL_MOD_STRIP value will be used as the option(s) to the strip
        command.


10 Makefile language
====================

The kernel Makefiles are designed to be 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.

11 Credits
==========

- Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net>
- Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de>
- Updates by Sam Ravnborg <sam@ravnborg.org>
- Language QA by Jan Engelhardt <jengelh@gmx.de>

12 TODO
=======

- Describe how kbuild supports shipped files with _shipped.
- Generating offset header files.
- Add more variables to chapters 7 or 9?