xref: /linux-6.15/Documentation/kbuild/makefiles.rst

======================
Linux Kernel Makefiles
======================

This document describes the Linux kernel Makefiles.

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.

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 x86.  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.


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 `Goal definitions`_ is a quick intro; further chapters provide
more details, with real examples.

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.

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

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.

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 `List directories to visit when descending`_.

Use of lib-y is normally restricted to ``lib/`` and ``arch/*/lib``.

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.

Kbuild also supports dedicated syntax, subdir-y and subdir-m, for
descending into subdirectories. It is a good fit when you know they
do not contain kernel-space objects at all. A typical usage is to let
Kbuild descend into subdirectories to build tools.

Examples::

  # scripts/Makefile
  subdir-$(CONFIG_GCC_PLUGINS) += gcc-plugins
  subdir-$(CONFIG_MODVERSIONS) += genksyms
  subdir-$(CONFIG_SECURITY_SELINUX) += selinux

Unlike obj-y/m, subdir-y/m does not need the trailing slash since this
syntax is always used for directories.

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".

Non-builtin vmlinux targets - extra-y
-------------------------------------

extra-y specifies targets which are needed for building vmlinux,
but not combined into built-in.a.

Examples are:

1) vmlinux linker script

   The linker script for vmlinux is located at
   arch/$(SRCARCH)/kernel/vmlinux.lds

Example::

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

$(extra-y) should only contain targets needed for vmlinux.

Kbuild skips extra-y when vmlinux is apparently not a final goal.
(e.g. ``make modules``, or building external modules)

If you intend to build targets unconditionally, always-y (explained
in the next section) is the correct syntax to use.

Always built goals - always-y
-----------------------------

always-y specifies targets which are literally always built when
Kbuild visits the Makefile.

Example::

  # ./Kbuild
  offsets-file := include/generated/asm-offsets.h
  always-y += $(offsets-file)

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.

  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 $(src)/decompress_$(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

  This line specify compilation flags for aha152x.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

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.

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 the directory where the Makefile is located. Always use $(src) when
  referring to files located in the src tree.

$(obj)
  $(obj) is the directory where the target is saved. Always use $(obj) when
  referring to generated files. Use $(obj) for pattern rules that need to work
  for both generated files and real sources (VPATH will help to find the
  prerequisites not only in the object tree but also in the source tree).

  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).

$(srcroot)
  $(srcroot) refers to the root of the source you are building, which can be
  either the kernel source or the external modules source, depending on whether
  KBUILD_EXTMOD is set. This can be either a relative or an absolute path, but
  if KBUILD_ABS_SRCTREE=1 is set, it is always an absolute path.

$(srctree)
  $(srctree) refers to the root of the kernel source tree. When building the
  kernel, this is the same as $(srcroot).

$(objtree)
  $(objtree) refers to the root of the kernel object tree. It is ``.`` when
  building the kernel, but it is different when building external modules.

$(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 unset, 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=``.

Command change detection
------------------------

When the rule is evaluated, timestamps are compared between the target
and its prerequisite files. GNU Make updates the target when any of the
prerequisites is newer than that.

The target should be rebuilt also when the command line has changed
since the last invocation. This is not supported by Make itself, so
Kbuild achieves this by a kind of meta-programming.

if_changed is the macro used for this purpose, in the following form::

  quiet_cmd_<command> = ...
        cmd_<command> = ...

  <target>: <source(s)> FORCE
          $(call if_changed,<command>)

Any target that utilizes if_changed must be listed in $(targets),
otherwise the command line check will fail, and the target will
always be built.

If the target is already listed in the recognized syntax such as
obj-y/m, lib-y/m, extra-y/m, always-y/m, hostprogs, userprogs, Kbuild
automatically adds it to $(targets). Otherwise, the target must be
explicitly added to $(targets).

Assignments to $(targets) are without $(obj)/ prefix. if_changed may be
used in conjunction with custom rules as defined in `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, objcopy)

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.

$(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 $(CC) 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 $(CC) 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 $(CC) really accepts it.

gcc-min-version
  gcc-min-version tests if the value of $(CONFIG_GCC_VERSION) is greater than
  or equal to the provided value and evaluates to y if so.

  Example::

    cflags-$(call gcc-min-version, 70100) := -foo

  In this example, cflags-y will be assigned the value -foo if $(CC) is gcc and
  $(CONFIG_GCC_VERSION) is >= 7.1.

clang-min-version
  clang-min-version tests if the value of $(CONFIG_CLANG_VERSION) is greater
  than or equal to the provided value and evaluates to y if so.

  Example::

    cflags-$(call clang-min-version, 110000) := -foo

  In this example, cflags-y will be assigned the value -foo if $(CC) is clang
  and $(CONFIG_CLANG_VERSION) is >= 11.0.0.

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

$(RUSTC) support functions
--------------------------

rustc-min-version
  rustc-min-version tests if the value of $(CONFIG_RUSTC_VERSION) is greater
  than or equal to the provided value and evaluates to y if so.

  Example::

    rustflags-$(call rustc-min-version, 108500) := -Cfoo

  In this example, rustflags-y will be assigned the value -Cfoo if
  $(CONFIG_RUSTC_VERSION) is >= 1.85.0.

$(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)

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),
and $(PYTHON3) to refer to interpreters for the respective
scripts.

Example::

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

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.

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.

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.

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

Using Rust for host programs
----------------------------

Kbuild offers support for host programs written in Rust. However,
since a Rust toolchain is not mandatory for kernel compilation,
it may only be used in scenarios where Rust is required to be
available (e.g. when  ``CONFIG_RUST`` is enabled).

Example::

  hostprogs     := target
  target-rust   := y

Kbuild will compile ``target`` using ``target.rs`` as the crate root,
located in the same directory as the ``Makefile``. The crate may
consist of several source files (see ``samples/rust/hostprogs``).

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``.

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.

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``.

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.

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.

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

To specify libraries linked to a userspace program, you can use
``<executable>-userldlibs``. The ``userldlibs`` syntax specifies libraries
linked to all userspace programs created in the current Makefile.

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

From command line, :ref:`USERCFLAGS and USERLDFLAGS <userkbuildflags>` will also be used.

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.

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.

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.

Note 1: arch/$(SRCARCH)/Makefile cannot use ``subdir-``, because that file is
included in the top level makefile. Instead, arch/$(SRCARCH)/Kbuild can use
``subdir-``.

Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will
be visited during ``make clean``.

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 scripts/head-object-list.txt.

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

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 `Non-builtin vmlinux targets - extra-y`_.

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_RUSTFLAGS
  $(RUSTC) compiler flags

  Default value - see top level Makefile.

  Append or modify as required per architecture.

  Often, the KBUILD_RUSTFLAGS variable depends on the configuration.

  Note that target specification file generation (for ``--target``)
  is handled in ``scripts/generate_rust_target.rs``.

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_RUSTFLAGS_KERNEL
  $(RUSTC) options specific for built-in

  $(KBUILD_RUSTFLAGS_KERNEL) contains extra Rust compiler flags used to
  compile resident kernel code.

KBUILD_RUSTFLAGS_MODULE
  Options for $(RUSTC) when building modules

  $(KBUILD_RUSTFLAGS_MODULE) is used to add arch-specific options that
  are used for $(RUSTC).

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

  The objects listed in scripts/head-object-list.txt are exceptions;
  they are placed before the other objects.

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.

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.

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.

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.

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

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.

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

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

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

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.

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 `generic-y`_ for further info on syntax etc.

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.

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.

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.

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>

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

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.

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.

INSTALL_DTBS_PATH
  This variable specifies a prefix for relocations required by build
  roots. It defines a place for installing the device tree blobs. Like
  INSTALL_MOD_PATH, it isn't defined in the Makefile, but can be passed
  by the user if desired. Otherwise it defaults to the kernel install
  path.

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.

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>

TODO
====

- Generating offset header files.
- Add more variables to chapters 7 or 9?