5 This document describes the Linux kernel Makefiles.
11 === 3 The kbuild files
12 --- 3.1 Goal definitions
13 --- 3.2 Built-in object goals - obj-y
14 --- 3.3 Loadable module goals - obj-m
16 --- 3.5 Library file goals - lib-y
17 --- 3.6 Descending down in directories
18 --- 3.7 Non-builtin vmlinux targets - extra-y
19 --- 3.8 Always built goals - always-y
20 --- 3.9 Compilation flags
21 --- 3.10 Dependency tracking
23 --- 3.12 Command change detection
24 --- 3.13 $(CC) support functions
25 --- 3.14 $(LD) support functions
26 --- 3.15 Script Invocation
28 === 4 Host Program support
29 --- 4.1 Simple Host Program
30 --- 4.2 Composite Host Programs
31 --- 4.3 Using C++ for host programs
32 --- 4.4 Controlling compiler options for host programs
33 --- 4.5 When host programs are actually built
35 === 5 Userspace Program support
36 --- 5.1 Simple Userspace Program
37 --- 5.2 Composite Userspace Programs
38 --- 5.3 Controlling compiler options for userspace programs
39 --- 5.4 When userspace programs are actually built
41 === 6 Kbuild clean infrastructure
43 === 7 Architecture Makefiles
44 --- 7.1 Set variables to tweak the build to the architecture
45 --- 7.2 Add prerequisites to archheaders
46 --- 7.3 Add prerequisites to archprepare
47 --- 7.4 List directories to visit when descending
48 --- 7.5 Architecture-specific boot images
49 --- 7.6 Building non-kbuild targets
50 --- 7.7 Commands useful for building a boot image
52 --- 7.9 Preprocessing linker scripts
53 --- 7.10 Generic header files
54 --- 7.11 Post-link pass
56 === 8 Kbuild syntax for exported headers
57 --- 8.1 no-export-headers
62 === 9 Kbuild Variables
63 === 10 Makefile language
70 The Makefiles have five parts::
72 Makefile the top Makefile.
73 .config the kernel configuration file.
74 arch/$(SRCARCH)/Makefile the arch Makefile.
75 scripts/Makefile.* common rules etc. for all kbuild Makefiles.
76 kbuild Makefiles exist in every subdirectory
78 The top Makefile reads the .config file, which comes from the kernel
79 configuration process.
81 The top Makefile is responsible for building two major products: vmlinux
82 (the resident kernel image) and modules (any module files).
83 It builds these goals by recursively descending into the subdirectories of
84 the kernel source tree.
85 The list of subdirectories which are visited depends upon the kernel
86 configuration. The top Makefile textually includes an arch Makefile
87 with the name arch/$(SRCARCH)/Makefile. The arch Makefile supplies
88 architecture-specific information to the top Makefile.
90 Each subdirectory has a kbuild Makefile which carries out the commands
91 passed down from above. The kbuild Makefile uses information from the
92 .config file to construct various file lists used by kbuild to build
93 any built-in or modular targets.
95 scripts/Makefile.* contains all the definitions/rules etc. that
96 are used to build the kernel based on the kbuild makefiles.
102 People have four different relationships with the kernel Makefiles.
104 *Users* are people who build kernels. These people type commands such as
105 "make menuconfig" or "make". They usually do not read or edit
106 any kernel Makefiles (or any other source files).
108 *Normal developers* are people who work on features such as device
109 drivers, file systems, and network protocols. These people need to
110 maintain the kbuild Makefiles for the subsystem they are
111 working on. In order to do this effectively, they need some overall
112 knowledge about the kernel Makefiles, plus detailed knowledge about the
113 public interface for kbuild.
115 *Arch developers* are people who work on an entire architecture, such
116 as sparc or ia64. Arch developers need to know about the arch Makefile
117 as well as kbuild Makefiles.
119 *Kbuild developers* are people who work on the kernel build system itself.
120 These people need to know about all aspects of the kernel Makefiles.
122 This document is aimed towards normal developers and arch developers.
128 Most Makefiles within the kernel are kbuild Makefiles that use the
129 kbuild infrastructure. This chapter introduces the syntax used in the
131 The preferred name for the kbuild files are 'Makefile' but 'Kbuild' can
132 be used and if both a 'Makefile' and a 'Kbuild' file exists, then the 'Kbuild'
135 Section 3.1 "Goal definitions" is a quick intro; further chapters provide
136 more details, with real examples.
141 Goal definitions are the main part (heart) of the kbuild Makefile.
142 These lines define the files to be built, any special compilation
143 options, and any subdirectories to be entered recursively.
145 The most simple kbuild makefile contains one line:
151 This tells kbuild that there is one object in that directory, named
152 foo.o. foo.o will be built from foo.c or foo.S.
154 If foo.o shall be built as a module, the variable obj-m is used.
155 Therefore the following pattern is often used:
159 obj-$(CONFIG_FOO) += foo.o
161 $(CONFIG_FOO) evaluates to either y (for built-in) or m (for module).
162 If CONFIG_FOO is neither y nor m, then the file will not be compiled
165 3.2 Built-in object goals - obj-y
166 ---------------------------------
168 The kbuild Makefile specifies object files for vmlinux
169 in the $(obj-y) lists. These lists depend on the kernel
172 Kbuild compiles all the $(obj-y) files. It then calls
173 "$(AR) rcSTP" to merge these files into one built-in.a file.
174 This is a thin archive without a symbol table. It will be later
175 linked into vmlinux by scripts/link-vmlinux.sh
177 The order of files in $(obj-y) is significant. Duplicates in
178 the lists are allowed: the first instance will be linked into
179 built-in.a and succeeding instances will be ignored.
181 Link order is significant, because certain functions
182 (module_init() / __initcall) will be called during boot in the
183 order they appear. So keep in mind that changing the link
184 order may e.g. change the order in which your SCSI
185 controllers are detected, and thus your disks are renumbered.
189 #drivers/isdn/i4l/Makefile
190 # Makefile for the kernel ISDN subsystem and device drivers.
191 # Each configuration option enables a list of files.
192 obj-$(CONFIG_ISDN_I4L) += isdn.o
193 obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
195 3.3 Loadable module goals - obj-m
196 ---------------------------------
198 $(obj-m) specifies object files which are built as loadable
201 A module may be built from one source file or several source
202 files. In the case of one source file, the kbuild makefile
203 simply adds the file to $(obj-m).
207 #drivers/isdn/i4l/Makefile
208 obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
210 Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm'
212 If a kernel module is built from several source files, you specify
213 that you want to build a module in the same way as above; however,
214 kbuild needs to know which object files you want to build your
215 module from, so you have to tell it by setting a $(<module_name>-y)
220 #drivers/isdn/i4l/Makefile
221 obj-$(CONFIG_ISDN_I4L) += isdn.o
222 isdn-y := isdn_net_lib.o isdn_v110.o isdn_common.o
224 In this example, the module name will be isdn.o. Kbuild will
225 compile the objects listed in $(isdn-y) and then run
226 "$(LD) -r" on the list of these files to generate isdn.o.
228 Due to kbuild recognizing $(<module_name>-y) for composite objects,
229 you can use the value of a `CONFIG_` symbol to optionally include an
230 object file as part of a composite object.
235 obj-$(CONFIG_EXT2_FS) += ext2.o
236 ext2-y := balloc.o dir.o file.o ialloc.o inode.o ioctl.o \
237 namei.o super.o symlink.o
238 ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o xattr_user.o \
241 In this example, xattr.o, xattr_user.o and xattr_trusted.o are only
242 part of the composite object ext2.o if $(CONFIG_EXT2_FS_XATTR)
245 Note: Of course, when you are building objects into the kernel,
246 the syntax above will also work. So, if you have CONFIG_EXT2_FS=y,
247 kbuild will build an ext2.o file for you out of the individual
248 parts and then link this into built-in.a, as you would expect.
250 3.5 Library file goals - lib-y
251 ------------------------------
253 Objects listed with obj-* are used for modules, or
254 combined in a built-in.a for that specific directory.
255 There is also the possibility to list objects that will
256 be included in a library, lib.a.
257 All objects listed with lib-y are combined in a single
258 library for that directory.
259 Objects that are listed in obj-y and additionally listed in
260 lib-y will not be included in the library, since they will
261 be accessible anyway.
262 For consistency, objects listed in lib-m will be included in lib.a.
264 Note that the same kbuild makefile may list files to be built-in
265 and to be part of a library. Therefore the same directory
266 may contain both a built-in.a and a lib.a file.
270 #arch/x86/lib/Makefile
273 This will create a library lib.a based on delay.o. For kbuild to
274 actually recognize that there is a lib.a being built, the directory
275 shall be listed in libs-y.
277 See also "7.4 List directories to visit when descending".
279 Use of lib-y is normally restricted to `lib/` and `arch/*/lib`.
281 3.6 Descending down in directories
282 ----------------------------------
284 A Makefile is only responsible for building objects in its own
285 directory. Files in subdirectories should be taken care of by
286 Makefiles in these subdirs. The build system will automatically
287 invoke make recursively in subdirectories, provided you let it know of
290 To do so, obj-y and obj-m are used.
291 ext2 lives in a separate directory, and the Makefile present in fs/
292 tells kbuild to descend down using the following assignment.
297 obj-$(CONFIG_EXT2_FS) += ext2/
299 If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular)
300 the corresponding obj- variable will be set, and kbuild will descend
301 down in the ext2 directory.
303 Kbuild uses this information not only to decide that it needs to visit
304 the directory, but also to decide whether or not to link objects from
305 the directory into vmlinux.
307 When Kbuild descends into the directory with 'y', all built-in objects
308 from that directory are combined into the built-in.a, which will be
309 eventually linked into vmlinux.
311 When Kbuild descends into the directory with 'm', in contrast, nothing
312 from that directory will be linked into vmlinux. If the Makefile in
313 that directory specifies obj-y, those objects will be left orphan.
314 It is very likely a bug of the Makefile or of dependencies in Kconfig.
316 Kbuild also supports dedicated syntax, subdir-y and subdir-m, for
317 descending into subdirectories. It is a good fit when you know they
318 do not contain kernel-space objects at all. A typical usage is to let
319 Kbuild descend into subdirectories to build tools.
324 subdir-$(CONFIG_GCC_PLUGINS) += gcc-plugins
325 subdir-$(CONFIG_MODVERSIONS) += genksyms
326 subdir-$(CONFIG_SECURITY_SELINUX) += selinux
328 Unlike obj-y/m, subdir-y/m does not need the trailing slash since this
329 syntax is always used for directories.
331 It is good practice to use a `CONFIG_` variable when assigning directory
332 names. This allows kbuild to totally skip the directory if the
333 corresponding `CONFIG_` option is neither 'y' nor 'm'.
335 3.7 Non-builtin vmlinux targets - extra-y
336 -----------------------------------------
338 extra-y specifies targets which are needed for building vmlinux,
339 but not combined into built-in.a.
345 Some objects must be placed at the head of vmlinux. They are
346 directly linked to vmlinux without going through built-in.a
347 A typical use-case is an object that contains the entry point.
349 arch/$(SRCARCH)/Makefile should specify such objects as head-y.
352 Given that we can control the section order in the linker script,
353 why do we need head-y?
355 2) vmlinux linker script
357 The linker script for vmlinux is located at
358 arch/$(SRCARCH)/kernel/vmlinux.lds
362 # arch/x86/kernel/Makefile
363 extra-y := head_$(BITS).o
364 extra-y += head$(BITS).o
366 extra-y += platform-quirks.o
367 extra-y += vmlinux.lds
369 $(extra-y) should only contain targets needed for vmlinux.
371 Kbuild skips extra-y when vmlinux is apparently not a final goal.
372 (e.g. 'make modules', or building external modules)
374 If you intend to build targets unconditionally, always-y (explained
375 in the next section) is the correct syntax to use.
377 3.8 Always built goals - always-y
378 ---------------------------------
380 always-y specifies targets which are literally always built when
381 Kbuild visits the Makefile.
385 offsets-file := include/generated/asm-offsets.h
386 always-y += $(offsets-file)
388 3.9 Compilation flags
389 ---------------------
391 ccflags-y, asflags-y and ldflags-y
392 These three flags apply only to the kbuild makefile in which they
393 are assigned. They are used for all the normal cc, as and ld
394 invocations happening during a recursive build.
395 Note: Flags with the same behaviour were previously named:
396 EXTRA_CFLAGS, EXTRA_AFLAGS and EXTRA_LDFLAGS.
397 They are still supported but their usage is deprecated.
399 ccflags-y specifies options for compiling with $(CC).
403 # drivers/acpi/acpica/Makefile
404 ccflags-y := -Os -D_LINUX -DBUILDING_ACPICA
405 ccflags-$(CONFIG_ACPI_DEBUG) += -DACPI_DEBUG_OUTPUT
407 This variable is necessary because the top Makefile owns the
408 variable $(KBUILD_CFLAGS) and uses it for compilation flags for the
411 asflags-y specifies assembler options.
415 #arch/sparc/kernel/Makefile
418 ldflags-y specifies options for linking with $(LD).
422 #arch/cris/boot/compressed/Makefile
423 ldflags-y += -T $(srctree)/$(src)/decompress_$(arch-y).lds
425 subdir-ccflags-y, subdir-asflags-y
426 The two flags listed above are similar to ccflags-y and asflags-y.
427 The difference is that the subdir- variants have effect for the kbuild
428 file where they are present and all subdirectories.
429 Options specified using subdir-* are added to the commandline before
430 the options specified using the non-subdir variants.
434 subdir-ccflags-y := -Werror
436 ccflags-remove-y, asflags-remove-y
437 These flags are used to remove particular flags for the compiler,
438 assembler invocations.
442 ccflags-remove-$(CONFIG_MCOUNT) += -pg
445 CFLAGS_$@ and AFLAGS_$@ only apply to commands in current
448 $(CFLAGS_$@) specifies per-file options for $(CC). The $@
449 part has a literal value which specifies the file that it is for.
451 CFLAGS_$@ has the higher priority than ccflags-remove-y; CFLAGS_$@
452 can re-add compiler flags that were removed by ccflags-remove-y.
456 # drivers/scsi/Makefile
457 CFLAGS_aha152x.o = -DAHA152X_STAT -DAUTOCONF
459 This line specify compilation flags for aha152x.o.
461 $(AFLAGS_$@) is a similar feature for source files in assembly
464 AFLAGS_$@ has the higher priority than asflags-remove-y; AFLAGS_$@
465 can re-add assembler flags that were removed by asflags-remove-y.
469 # arch/arm/kernel/Makefile
470 AFLAGS_head.o := -DTEXT_OFFSET=$(TEXT_OFFSET)
471 AFLAGS_crunch-bits.o := -Wa,-mcpu=ep9312
472 AFLAGS_iwmmxt.o := -Wa,-mcpu=iwmmxt
475 3.10 Dependency tracking
476 ------------------------
478 Kbuild tracks dependencies on the following:
480 1) All prerequisite files (both `*.c` and `*.h`)
481 2) `CONFIG_` options used in all prerequisite files
482 3) Command-line used to compile target
484 Thus, if you change an option to $(CC) all affected files will
490 Custom rules are used when the kbuild infrastructure does
491 not provide the required support. A typical example is
492 header files generated during the build process.
493 Another example are the architecture-specific Makefiles which
494 need custom rules to prepare boot images etc.
496 Custom rules are written as normal Make rules.
497 Kbuild is not executing in the directory where the Makefile is
498 located, so all custom rules shall use a relative
499 path to prerequisite files and target files.
501 Two variables are used when defining custom rules:
504 $(src) is a relative path which points to the directory
505 where the Makefile is located. Always use $(src) when
506 referring to files located in the src tree.
509 $(obj) is a relative path which points to the directory
510 where the target is saved. Always use $(obj) when
511 referring to generated files.
515 #drivers/scsi/Makefile
516 $(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl
517 $(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl
519 This is a custom rule, following the normal syntax
522 The target file depends on two prerequisite files. References
523 to the target file are prefixed with $(obj), references
524 to prerequisites are referenced with $(src) (because they are not
528 echoing information to user in a rule is often a good practice
529 but when execution "make -s" one does not expect to see any output
530 except for warnings/errors.
531 To support this kbuild defines $(kecho) which will echo out the
532 text following $(kecho) to stdout except if "make -s" is used.
537 $(BOOT_TARGETS): vmlinux
538 $(Q)$(MAKE) $(build)=$(boot) MACHINE=$(MACHINE) $(boot)/$@
539 @$(kecho) ' Kernel: $(boot)/$@ is ready'
541 When kbuild is executing with KBUILD_VERBOSE=0, then only a shorthand
542 of a command is normally displayed.
543 To enable this behaviour for custom commands kbuild requires
544 two variables to be set::
546 quiet_cmd_<command> - what shall be echoed
547 cmd_<command> - the command to execute
552 quiet_cmd_crc32 = GEN $@
555 $(obj)/crc32table.h: $(obj)/gen_crc32table
558 When updating the $(obj)/crc32table.h target, the line:
562 will be displayed with "make KBUILD_VERBOSE=0".
564 3.12 Command change detection
565 -----------------------------
567 When the rule is evaluated, timestamps are compared between the target
568 and its prerequisite files. GNU Make updates the target when any of the
569 prerequisites is newer than that.
571 The target should be rebuilt also when the command line has changed
572 since the last invocation. This is not supported by Make itself, so
573 Kbuild achieves this by a kind of meta-programming.
575 if_changed is the macro used for this purpose, in the following form::
577 quiet_cmd_<command> = ...
580 <target>: <source(s)> FORCE
581 $(call if_changed,<command>)
583 Any target that utilizes if_changed must be listed in $(targets),
584 otherwise the command line check will fail, and the target will
587 If the target is already listed in the recognized syntax such as
588 obj-y/m, lib-y/m, extra-y/m, always-y/m, hostprogs, userprogs, Kbuild
589 automatically adds it to $(targets). Otherwise, the target must be
590 explicitly added to $(targets).
592 Assignments to $(targets) are without $(obj)/ prefix. if_changed may be
593 used in conjunction with custom rules as defined in "3.11 Custom Rules".
595 Note: It is a typical mistake to forget the FORCE prerequisite.
596 Another common pitfall is that whitespace is sometimes significant; for
597 instance, the below will fail (note the extra space after the comma)::
599 target: source(s) FORCE
601 **WRONG!** $(call if_changed, objcopy)
604 if_changed should not be used more than once per target.
605 It stores the executed command in a corresponding .cmd
606 file and multiple calls would result in overwrites and
607 unwanted results when the target is up to date and only the
608 tests on changed commands trigger execution of commands.
610 3.13 $(CC) support functions
611 ----------------------------
613 The kernel may be built with several different versions of
614 $(CC), each supporting a unique set of features and options.
615 kbuild provides basic support to check for valid options for $(CC).
616 $(CC) is usually the gcc compiler, but other alternatives are
620 as-option is used to check if $(CC) -- when used to compile
621 assembler (`*.S`) files -- supports the given option. An optional
622 second option may be specified if the first option is not supported.
627 cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),)
629 In the above example, cflags-y will be assigned the option
630 -Wa$(comma)-isa=$(isa-y) if it is supported by $(CC).
631 The second argument is optional, and if supplied will be used
632 if first argument is not supported.
635 as-instr checks if the assembler reports a specific instruction
636 and then outputs either option1 or option2
637 C escapes are supported in the test instruction
638 Note: as-instr-option uses KBUILD_AFLAGS for assembler options
641 cc-option is used to check if $(CC) supports a given option, and if
642 not supported to use an optional second option.
647 cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586)
649 In the above example, cflags-y will be assigned the option
650 -march=pentium-mmx if supported by $(CC), otherwise -march=i586.
651 The second argument to cc-option is optional, and if omitted,
652 cflags-y will be assigned no value if first option is not supported.
653 Note: cc-option uses KBUILD_CFLAGS for $(CC) options
656 cc-option-yn is used to check if gcc supports a given option
657 and return 'y' if supported, otherwise 'n'.
662 biarch := $(call cc-option-yn, -m32)
663 aflags-$(biarch) += -a32
664 cflags-$(biarch) += -m32
666 In the above example, $(biarch) is set to y if $(CC) supports the -m32
667 option. When $(biarch) equals 'y', the expanded variables $(aflags-y)
668 and $(cflags-y) will be assigned the values -a32 and -m32,
670 Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options
673 cc-disable-warning checks if gcc supports a given warning and returns
674 the commandline switch to disable it. This special function is needed,
675 because gcc 4.4 and later accept any unknown -Wno-* option and only
676 warn about it if there is another warning in the source file.
680 KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
682 In the above example, -Wno-unused-but-set-variable will be added to
683 KBUILD_CFLAGS only if gcc really accepts it.
686 gcc-min-version tests if the value of $(CONFIG_GCC_VERSION) is greater than
687 or equal to the provided value and evaluates to y if so.
691 cflags-$(call gcc-min-version, 70100) := -foo
693 In this example, cflags-y will be assigned the value -foo if $(CC) is gcc and
694 $(CONFIG_GCC_VERSION) is >= 7.1.
697 clang-min-version tests if the value of $(CONFIG_CLANG_VERSION) is greater
698 than or equal to the provided value and evaluates to y if so.
702 cflags-$(call clang-min-version, 110000) := -foo
704 In this example, cflags-y will be assigned the value -foo if $(CC) is clang
705 and $(CONFIG_CLANG_VERSION) is >= 11.0.0.
708 cc-cross-prefix is used to check if there exists a $(CC) in path with
709 one of the listed prefixes. The first prefix where there exist a
710 prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found
711 then nothing is returned.
712 Additional prefixes are separated by a single space in the
713 call of cc-cross-prefix.
714 This functionality is useful for architecture Makefiles that try
715 to set CROSS_COMPILE to well-known values but may have several
716 values to select between.
717 It is recommended only to try to set CROSS_COMPILE if it is a cross
718 build (host arch is different from target arch). And if CROSS_COMPILE
719 is already set then leave it with the old value.
724 ifneq ($(SUBARCH),$(ARCH))
725 ifeq ($(CROSS_COMPILE),)
726 CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-)
730 3.14 $(LD) support functions
731 ----------------------------
734 ld-option is used to check if $(LD) supports the supplied option.
735 ld-option takes two options as arguments.
736 The second argument is an optional option that can be used if the
737 first option is not supported by $(LD).
742 LDFLAGS_vmlinux += $(call ld-option, -X)
744 3.15 Script invocation
745 ----------------------
747 Make rules may invoke scripts to build the kernel. The rules shall
748 always provide the appropriate interpreter to execute the script. They
749 shall not rely on the execute bits being set, and shall not invoke the
750 script directly. For the convenience of manual script invocation, such
751 as invoking ./scripts/checkpatch.pl, it is recommended to set execute
752 bits on the scripts nonetheless.
754 Kbuild provides variables $(CONFIG_SHELL), $(AWK), $(PERL),
755 and $(PYTHON3) to refer to interpreters for the respective
761 cmd_depmod = $(CONFIG_SHELL) $(srctree)/scripts/depmod.sh $(DEPMOD) \
764 4 Host Program support
765 ======================
767 Kbuild supports building executables on the host for use during the
769 Two steps are required in order to use a host executable.
771 The first step is to tell kbuild that a host program exists. This is
772 done utilising the variable "hostprogs".
774 The second step is to add an explicit dependency to the executable.
775 This can be done in two ways. Either add the dependency in a rule,
776 or utilise the variable "always-y".
777 Both possibilities are described in the following.
779 4.1 Simple Host Program
780 -----------------------
782 In some cases there is a need to compile and run a program on the
783 computer where the build is running.
784 The following line tells kbuild that the program bin2hex shall be
785 built on the build host.
791 Kbuild assumes in the above example that bin2hex is made from a single
792 c-source file named bin2hex.c located in the same directory as
795 4.2 Composite Host Programs
796 ---------------------------
798 Host programs can be made up based on composite objects.
799 The syntax used to define composite objects for host programs is
800 similar to the syntax used for kernel objects.
801 $(<executable>-objs) lists all objects used to link the final
806 #scripts/lxdialog/Makefile
807 hostprogs := lxdialog
808 lxdialog-objs := checklist.o lxdialog.o
810 Objects with extension .o are compiled from the corresponding .c
811 files. In the above example, checklist.c is compiled to checklist.o
812 and lxdialog.c is compiled to lxdialog.o.
814 Finally, the two .o files are linked to the executable, lxdialog.
815 Note: The syntax <executable>-y is not permitted for host-programs.
817 4.3 Using C++ for host programs
818 -------------------------------
820 kbuild offers support for host programs written in C++. This was
821 introduced solely to support kconfig, and is not recommended
826 #scripts/kconfig/Makefile
828 qconf-cxxobjs := qconf.o
830 In the example above the executable is composed of the C++ file
831 qconf.cc - identified by $(qconf-cxxobjs).
833 If qconf is composed of a mixture of .c and .cc files, then an
834 additional line can be used to identify this.
838 #scripts/kconfig/Makefile
840 qconf-cxxobjs := qconf.o
841 qconf-objs := check.o
843 4.4 Controlling compiler options for host programs
844 --------------------------------------------------
846 When compiling host programs, it is possible to set specific flags.
847 The programs will always be compiled utilising $(HOSTCC) passed
848 the options specified in $(KBUILD_HOSTCFLAGS).
849 To set flags that will take effect for all host programs created
850 in that Makefile, use the variable HOST_EXTRACFLAGS.
854 #scripts/lxdialog/Makefile
855 HOST_EXTRACFLAGS += -I/usr/include/ncurses
857 To set specific flags for a single file the following construction
862 #arch/ppc64/boot/Makefile
863 HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE)
865 It is also possible to specify additional options to the linker.
869 #scripts/kconfig/Makefile
870 HOSTLDLIBS_qconf := -L$(QTDIR)/lib
872 When linking qconf, it will be passed the extra option
875 4.5 When host programs are actually built
876 -----------------------------------------
878 Kbuild will only build host-programs when they are referenced
880 This is possible in two ways:
882 (1) List the prerequisite explicitly in a custom rule.
886 #drivers/pci/Makefile
887 hostprogs := gen-devlist
888 $(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist
889 ( cd $(obj); ./gen-devlist ) < $<
891 The target $(obj)/devlist.h will not be built before
892 $(obj)/gen-devlist is updated. Note that references to
893 the host programs in custom rules must be prefixed with $(obj).
897 When there is no suitable custom rule, and the host program
898 shall be built when a makefile is entered, the always-y
899 variable shall be used.
903 #scripts/lxdialog/Makefile
904 hostprogs := lxdialog
905 always-y := $(hostprogs)
907 Kbuild provides the following shorthand for this:
909 hostprogs-always-y := lxdialog
911 This will tell kbuild to build lxdialog even if not referenced in
914 5 Userspace Program support
915 ===========================
917 Just like host programs, Kbuild also supports building userspace executables
918 for the target architecture (i.e. the same architecture as you are building
921 The syntax is quite similar. The difference is to use "userprogs" instead of
924 5.1 Simple Userspace Program
925 ----------------------------
927 The following line tells kbuild that the program bpf-direct shall be
928 built for the target architecture.
932 userprogs := bpf-direct
934 Kbuild assumes in the above example that bpf-direct is made from a
935 single C source file named bpf-direct.c located in the same directory
938 5.2 Composite Userspace Programs
939 --------------------------------
941 Userspace programs can be made up based on composite objects.
942 The syntax used to define composite objects for userspace programs is
943 similar to the syntax used for kernel objects.
944 $(<executable>-objs) lists all objects used to link the final
949 #samples/seccomp/Makefile
950 userprogs := bpf-fancy
951 bpf-fancy-objs := bpf-fancy.o bpf-helper.o
953 Objects with extension .o are compiled from the corresponding .c
954 files. In the above example, bpf-fancy.c is compiled to bpf-fancy.o
955 and bpf-helper.c is compiled to bpf-helper.o.
957 Finally, the two .o files are linked to the executable, bpf-fancy.
958 Note: The syntax <executable>-y is not permitted for userspace programs.
960 5.3 Controlling compiler options for userspace programs
961 -------------------------------------------------------
963 When compiling userspace programs, it is possible to set specific flags.
964 The programs will always be compiled utilising $(CC) passed
965 the options specified in $(KBUILD_USERCFLAGS).
966 To set flags that will take effect for all userspace programs created
967 in that Makefile, use the variable userccflags.
971 # samples/seccomp/Makefile
972 userccflags += -I usr/include
974 To set specific flags for a single file the following construction
979 bpf-helper-userccflags += -I user/include
981 It is also possible to specify additional options to the linker.
985 # net/bpfilter/Makefile
986 bpfilter_umh-userldflags += -static
988 When linking bpfilter_umh, it will be passed the extra option -static.
990 From command line, :ref:`USERCFLAGS and USERLDFLAGS <userkbuildflags>` will also be used.
992 5.4 When userspace programs are actually built
993 ----------------------------------------------
995 Kbuild builds userspace programs only when told to do so.
996 There are two ways to do this.
998 (1) Add it as the prerequisite of another file
1002 #net/bpfilter/Makefile
1003 userprogs := bpfilter_umh
1004 $(obj)/bpfilter_umh_blob.o: $(obj)/bpfilter_umh
1006 $(obj)/bpfilter_umh is built before $(obj)/bpfilter_umh_blob.o
1012 userprogs := binderfs_example
1013 always-y := $(userprogs)
1015 Kbuild provides the following shorthand for this:
1017 userprogs-always-y := binderfs_example
1019 This will tell Kbuild to build binderfs_example when it visits this
1022 6 Kbuild clean infrastructure
1023 =============================
1025 "make clean" deletes most generated files in the obj tree where the kernel
1026 is compiled. This includes generated files such as host programs.
1027 Kbuild knows targets listed in $(hostprogs), $(always-y), $(always-m),
1028 $(always-), $(extra-y), $(extra-) and $(targets). They are all deleted
1029 during "make clean". Files matching the patterns "*.[oas]", "*.ko", plus
1030 some additional files generated by kbuild are deleted all over the kernel
1031 source tree when "make clean" is executed.
1033 Additional files or directories can be specified in kbuild makefiles by use of
1039 clean-files := crc32table.h
1041 When executing "make clean", the file "crc32table.h" will be deleted.
1042 Kbuild will assume files to be in the same relative directory as the
1043 Makefile, except if prefixed with $(objtree).
1045 To exclude certain files or directories from make clean, use the
1046 $(no-clean-files) variable.
1048 Usually kbuild descends down in subdirectories due to "obj-* := dir/",
1049 but in the architecture makefiles where the kbuild infrastructure
1050 is not sufficient this sometimes needs to be explicit.
1054 #arch/x86/boot/Makefile
1055 subdir- := compressed
1057 The above assignment instructs kbuild to descend down in the
1058 directory compressed/ when "make clean" is executed.
1060 Note 1: arch/$(SRCARCH)/Makefile cannot use "subdir-", because that file is
1061 included in the top level makefile. Instead, arch/$(SRCARCH)/Kbuild can use
1064 Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will
1065 be visited during "make clean".
1067 7 Architecture Makefiles
1068 ========================
1070 The top level Makefile sets up the environment and does the preparation,
1071 before starting to descend down in the individual directories.
1072 The top level makefile contains the generic part, whereas
1073 arch/$(SRCARCH)/Makefile contains what is required to set up kbuild
1074 for said architecture.
1075 To do so, arch/$(SRCARCH)/Makefile sets up a number of variables and defines
1078 When kbuild executes, the following steps are followed (roughly):
1080 1) Configuration of the kernel => produce .config
1081 2) Store kernel version in include/linux/version.h
1082 3) Updating all other prerequisites to the target prepare:
1083 - Additional prerequisites are specified in arch/$(SRCARCH)/Makefile
1084 4) Recursively descend down in all directories listed in
1085 init-* core* drivers-* net-* libs-* and build all targets.
1086 - The values of the above variables are expanded in arch/$(SRCARCH)/Makefile.
1087 5) All object files are then linked and the resulting file vmlinux is
1088 located at the root of the obj tree.
1089 The very first objects linked are listed in head-y, assigned by
1090 arch/$(SRCARCH)/Makefile.
1091 6) Finally, the architecture-specific part does any required post processing
1092 and builds the final bootimage.
1093 - This includes building boot records
1094 - Preparing initrd images and the like
1097 7.1 Set variables to tweak the build to the architecture
1098 --------------------------------------------------------
1101 Generic $(LD) options
1103 Flags used for all invocations of the linker.
1104 Often specifying the emulation is sufficient.
1109 KBUILD_LDFLAGS := -m elf_s390
1111 Note: ldflags-y can be used to further customise
1112 the flags used. See section 3.7.
1115 Options for $(LD) when linking vmlinux
1117 LDFLAGS_vmlinux is used to specify additional flags to pass to
1118 the linker when linking the final vmlinux image.
1119 LDFLAGS_vmlinux uses the LDFLAGS_$@ support.
1124 LDFLAGS_vmlinux := -e stext
1129 When $(call if_changed,objcopy) is used to translate a .o file,
1130 the flags specified in OBJCOPYFLAGS will be used.
1131 $(call if_changed,objcopy) is often used to generate raw binaries on
1137 OBJCOPYFLAGS := -O binary
1139 #arch/s390/boot/Makefile
1140 $(obj)/image: vmlinux FORCE
1141 $(call if_changed,objcopy)
1143 In this example, the binary $(obj)/image is a binary version of
1144 vmlinux. The usage of $(call if_changed,xxx) will be described later.
1149 Default value - see top level Makefile
1150 Append or modify as required per architecture.
1154 #arch/sparc64/Makefile
1155 KBUILD_AFLAGS += -m64 -mcpu=ultrasparc
1158 $(CC) compiler flags
1160 Default value - see top level Makefile
1161 Append or modify as required per architecture.
1163 Often, the KBUILD_CFLAGS variable depends on the configuration.
1167 #arch/x86/boot/compressed/Makefile
1168 cflags-$(CONFIG_X86_32) := -march=i386
1169 cflags-$(CONFIG_X86_64) := -mcmodel=small
1170 KBUILD_CFLAGS += $(cflags-y)
1172 Many arch Makefiles dynamically run the target C compiler to
1173 probe supported options::
1178 cflags-$(CONFIG_MPENTIUMII) += $(call cc-option,\
1179 -march=pentium2,-march=i686)
1181 # Disable unit-at-a-time mode ...
1182 KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time)
1186 The first example utilises the trick that a config option expands
1187 to 'y' when selected.
1189 KBUILD_AFLAGS_KERNEL
1190 Assembler options specific for built-in
1192 $(KBUILD_AFLAGS_KERNEL) contains extra C compiler flags used to compile
1193 resident kernel code.
1195 KBUILD_AFLAGS_MODULE
1196 Assembler options specific for modules
1198 $(KBUILD_AFLAGS_MODULE) is used to add arch-specific options that
1199 are used for assembler.
1201 From commandline AFLAGS_MODULE shall be used (see kbuild.rst).
1203 KBUILD_CFLAGS_KERNEL
1204 $(CC) options specific for built-in
1206 $(KBUILD_CFLAGS_KERNEL) contains extra C compiler flags used to compile
1207 resident kernel code.
1209 KBUILD_CFLAGS_MODULE
1210 Options for $(CC) when building modules
1212 $(KBUILD_CFLAGS_MODULE) is used to add arch-specific options that
1214 From commandline CFLAGS_MODULE shall be used (see kbuild.rst).
1216 KBUILD_LDFLAGS_MODULE
1217 Options for $(LD) when linking modules
1219 $(KBUILD_LDFLAGS_MODULE) is used to add arch-specific options
1220 used when linking modules. This is often a linker script.
1222 From commandline LDFLAGS_MODULE shall be used (see kbuild.rst).
1226 The linker script with full path. Assigned by the top-level Makefile.
1230 The module linker script with full path. Assigned by the top-level
1231 Makefile and additionally by the arch Makefile.
1235 All object files for vmlinux. They are linked to vmlinux in the same
1236 order as listed in KBUILD_VMLINUX_OBJS.
1240 All .a "lib" files for vmlinux. KBUILD_VMLINUX_OBJS and
1241 KBUILD_VMLINUX_LIBS together specify all the object files used to
1244 7.2 Add prerequisites to archheaders
1245 ------------------------------------
1247 The archheaders: rule is used to generate header files that
1248 may be installed into user space by "make header_install".
1250 It is run before "make archprepare" when run on the
1251 architecture itself.
1254 7.3 Add prerequisites to archprepare
1255 ------------------------------------
1257 The archprepare: rule is used to list prerequisites that need to be
1258 built before starting to descend down in the subdirectories.
1259 This is usually used for header files containing assembler constants.
1264 archprepare: maketools
1266 In this example, the file target maketools will be processed
1267 before descending down in the subdirectories.
1268 See also chapter XXX-TODO that describes how kbuild supports
1269 generating offset header files.
1272 7.4 List directories to visit when descending
1273 ---------------------------------------------
1275 An arch Makefile cooperates with the top Makefile to define variables
1276 which specify how to build the vmlinux file. Note that there is no
1277 corresponding arch-specific section for modules; the module-building
1278 machinery is all architecture-independent.
1281 head-y, core-y, libs-y, drivers-y
1282 $(head-y) lists objects to be linked first in vmlinux.
1284 $(libs-y) lists directories where a lib.a archive can be located.
1286 The rest list directories where a built-in.a object file can be
1289 Then the rest follows in this order:
1291 $(core-y), $(libs-y), $(drivers-y)
1293 The top level Makefile defines values for all generic directories,
1294 and arch/$(SRCARCH)/Makefile only adds architecture-specific
1299 # arch/sparc/Makefile
1300 core-y += arch/sparc/
1302 libs-y += arch/sparc/prom/
1303 libs-y += arch/sparc/lib/
1305 drivers-$(CONFIG_PM) += arch/sparc/power/
1307 7.5 Architecture-specific boot images
1308 -------------------------------------
1310 An arch Makefile specifies goals that take the vmlinux file, compress
1311 it, wrap it in bootstrapping code, and copy the resulting files
1312 somewhere. This includes various kinds of installation commands.
1313 The actual goals are not standardized across architectures.
1315 It is common to locate any additional processing in a boot/
1316 directory below arch/$(SRCARCH)/.
1318 Kbuild does not provide any smart way to support building a
1319 target specified in boot/. Therefore arch/$(SRCARCH)/Makefile shall
1320 call make manually to build a target in boot/.
1322 The recommended approach is to include shortcuts in
1323 arch/$(SRCARCH)/Makefile, and use the full path when calling down
1324 into the arch/$(SRCARCH)/boot/Makefile.
1329 boot := arch/x86/boot
1331 $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
1333 "$(Q)$(MAKE) $(build)=<dir>" is the recommended way to invoke
1334 make in a subdirectory.
1336 There are no rules for naming architecture-specific targets,
1337 but executing "make help" will list all relevant targets.
1338 To support this, $(archhelp) must be defined.
1344 echo '* bzImage - Compressed kernel image (arch/x86/boot/bzImage)'
1347 When make is executed without arguments, the first goal encountered
1348 will be built. In the top level Makefile the first goal present
1350 An architecture shall always, per default, build a bootable image.
1351 In "make help", the default goal is highlighted with a '*'.
1352 Add a new prerequisite to all: to select a default goal different
1360 When "make" is executed without arguments, bzImage will be built.
1362 7.7 Commands useful for building a boot image
1363 ---------------------------------------------
1365 Kbuild provides a few macros that are useful when building a
1369 Link target. Often, LDFLAGS_$@ is used to set specific options to ld.
1373 #arch/x86/boot/Makefile
1374 LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary
1375 LDFLAGS_setup := -Ttext 0x0 -s --oformat binary -e begtext
1377 targets += setup setup.o bootsect bootsect.o
1378 $(obj)/setup $(obj)/bootsect: %: %.o FORCE
1379 $(call if_changed,ld)
1381 In this example, there are two possible targets, requiring different
1382 options to the linker. The linker options are specified using the
1383 LDFLAGS_$@ syntax - one for each potential target.
1384 $(targets) are assigned all potential targets, by which kbuild knows
1385 the targets and will:
1387 1) check for commandline changes
1388 2) delete target during make clean
1390 The ": %: %.o" part of the prerequisite is a shorthand that
1391 frees us from listing the setup.o and bootsect.o files.
1394 It is a common mistake to forget the "targets :=" assignment,
1395 resulting in the target file being recompiled for no
1399 Copy binary. Uses OBJCOPYFLAGS usually specified in
1400 arch/$(SRCARCH)/Makefile.
1401 OBJCOPYFLAGS_$@ may be used to set additional options.
1404 Compress target. Use maximum compression to compress target.
1408 #arch/x86/boot/compressed/Makefile
1409 $(obj)/vmlinux.bin.gz: $(vmlinux.bin.all-y) FORCE
1410 $(call if_changed,gzip)
1413 Create flattened device tree blob object suitable for linking
1414 into vmlinux. Device tree blobs linked into vmlinux are placed
1415 in an init section in the image. Platform code *must* copy the
1416 blob to non-init memory prior to calling unflatten_device_tree().
1418 To use this command, simply add `*.dtb` into obj-y or targets, or make
1419 some other target depend on `%.dtb`
1421 A central rule exists to create `$(obj)/%.dtb` from `$(src)/%.dts`;
1422 architecture Makefiles do no need to explicitly write out that rule.
1427 DTC_FLAGS ?= -p 1024
1429 7.9 Preprocessing linker scripts
1430 --------------------------------
1432 When the vmlinux image is built, the linker script
1433 arch/$(SRCARCH)/kernel/vmlinux.lds is used.
1434 The script is a preprocessed variant of the file vmlinux.lds.S
1435 located in the same directory.
1436 kbuild knows .lds files and includes a rule `*lds.S` -> `*lds`.
1440 #arch/x86/kernel/Makefile
1441 extra-y := vmlinux.lds
1443 The assignment to extra-y is used to tell kbuild to build the
1445 The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the
1446 specified options when building the target vmlinux.lds.
1448 When building the `*.lds` target, kbuild uses the variables::
1450 KBUILD_CPPFLAGS : Set in top-level Makefile
1451 cppflags-y : May be set in the kbuild makefile
1452 CPPFLAGS_$(@F) : Target-specific flags.
1453 Note that the full filename is used in this
1456 The kbuild infrastructure for `*lds` files is used in several
1457 architecture-specific files.
1459 7.10 Generic header files
1460 -------------------------
1462 The directory include/asm-generic contains the header files
1463 that may be shared between individual architectures.
1464 The recommended approach how to use a generic header file is
1465 to list the file in the Kbuild file.
1466 See "8.2 generic-y" for further info on syntax etc.
1471 If the file arch/xxx/Makefile.postlink exists, this makefile
1472 will be invoked for post-link objects (vmlinux and modules.ko)
1473 for architectures to run post-link passes on. Must also handle
1476 This pass runs after kallsyms generation. If the architecture
1477 needs to modify symbol locations, rather than manipulate the
1478 kallsyms, it may be easier to add another postlink target for
1479 .tmp_vmlinux? targets to be called from link-vmlinux.sh.
1481 For example, powerpc uses this to check relocation sanity of
1482 the linked vmlinux file.
1484 8 Kbuild syntax for exported headers
1485 ------------------------------------
1487 The kernel includes a set of headers that is exported to userspace.
1488 Many headers can be exported as-is but other headers require a
1489 minimal pre-processing before they are ready for user-space.
1490 The pre-processing does:
1492 - drop kernel-specific annotations
1493 - drop include of compiler.h
1494 - drop all sections that are kernel internal (guarded by `ifdef __KERNEL__`)
1496 All headers under include/uapi/, include/generated/uapi/,
1497 arch/<arch>/include/uapi/ and arch/<arch>/include/generated/uapi/
1500 A Kbuild file may be defined under arch/<arch>/include/uapi/asm/ and
1501 arch/<arch>/include/asm/ to list asm files coming from asm-generic.
1502 See subsequent chapter for the syntax of the Kbuild file.
1504 8.1 no-export-headers
1505 ---------------------
1507 no-export-headers is essentially used by include/uapi/linux/Kbuild to
1508 avoid exporting specific headers (e.g. kvm.h) on architectures that do
1509 not support it. It should be avoided as much as possible.
1514 If an architecture uses a verbatim copy of a header from
1515 include/asm-generic then this is listed in the file
1516 arch/$(SRCARCH)/include/asm/Kbuild like this:
1520 #arch/x86/include/asm/Kbuild
1521 generic-y += termios.h
1524 During the prepare phase of the build a wrapper include
1525 file is generated in the directory::
1527 arch/$(SRCARCH)/include/generated/asm
1529 When a header is exported where the architecture uses
1530 the generic header a similar wrapper is generated as part
1531 of the set of exported headers in the directory::
1535 The generated wrapper will in both cases look like the following:
1537 Example: termios.h::
1539 #include <asm-generic/termios.h>
1544 If an architecture generates other header files alongside generic-y
1545 wrappers, generated-y specifies them.
1547 This prevents them being treated as stale asm-generic wrappers and
1552 #arch/x86/include/asm/Kbuild
1553 generated-y += syscalls_32.h
1558 mandatory-y is essentially used by include/(uapi/)asm-generic/Kbuild
1559 to define the minimum set of ASM headers that all architectures must have.
1561 This works like optional generic-y. If a mandatory header is missing
1562 in arch/$(SRCARCH)/include/(uapi/)/asm, Kbuild will automatically
1563 generate a wrapper of the asm-generic one.
1568 The top Makefile exports the following variables:
1570 VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION
1571 These variables define the current kernel version. A few arch
1572 Makefiles actually use these values directly; they should use
1573 $(KERNELRELEASE) instead.
1575 $(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic
1576 three-part version number, such as "2", "4", and "0". These three
1577 values are always numeric.
1579 $(EXTRAVERSION) defines an even tinier sublevel for pre-patches
1580 or additional patches. It is usually some non-numeric string
1581 such as "-pre4", and is often blank.
1584 $(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable
1585 for constructing installation directory names or showing in
1586 version strings. Some arch Makefiles use it for this purpose.
1589 This variable defines the target architecture, such as "i386",
1590 "arm", or "sparc". Some kbuild Makefiles test $(ARCH) to
1591 determine which files to compile.
1593 By default, the top Makefile sets $(ARCH) to be the same as the
1594 host system architecture. For a cross build, a user may
1595 override the value of $(ARCH) on the command line::
1600 This variable specifies the directory in arch/ to build.
1602 ARCH and SRCARCH may not necessarily match. A couple of arch
1603 directories are biarch, that is, a single `arch/*/` directory supports
1604 both 32-bit and 64-bit.
1606 For example, you can pass in ARCH=i386, ARCH=x86_64, or ARCH=x86.
1607 For all of them, SRCARCH=x86 because arch/x86/ supports both i386 and
1611 This variable defines a place for the arch Makefiles to install
1612 the resident kernel image and System.map file.
1613 Use this for architecture-specific install targets.
1615 INSTALL_MOD_PATH, MODLIB
1616 $(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module
1617 installation. This variable is not defined in the Makefile but
1618 may be passed in by the user if desired.
1620 $(MODLIB) specifies the directory for module installation.
1621 The top Makefile defines $(MODLIB) to
1622 $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE). The user may
1623 override this value on the command line if desired.
1626 If this variable is specified, it will cause modules to be stripped
1627 after they are installed. If INSTALL_MOD_STRIP is '1', then the
1628 default option --strip-debug will be used. Otherwise, the
1629 INSTALL_MOD_STRIP value will be used as the option(s) to the strip
1633 10 Makefile language
1634 ====================
1636 The kernel Makefiles are designed to be run with GNU Make. The Makefiles
1637 use only the documented features of GNU Make, but they do use many
1640 GNU Make supports elementary list-processing functions. The kernel
1641 Makefiles use a novel style of list building and manipulation with few
1644 GNU Make has two assignment operators, ":=" and "=". ":=" performs
1645 immediate evaluation of the right-hand side and stores an actual string
1646 into the left-hand side. "=" is like a formula definition; it stores the
1647 right-hand side in an unevaluated form and then evaluates this form each
1648 time the left-hand side is used.
1650 There are some cases where "=" is appropriate. Usually, though, ":="
1651 is the right choice.
1656 - Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net>
1657 - Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de>
1658 - Updates by Sam Ravnborg <sam@ravnborg.org>
1659 - Language QA by Jan Engelhardt <jengelh@gmx.de>
1664 - Describe how kbuild supports shipped files with _shipped.
1665 - Generating offset header files.
1666 - Add more variables to chapters 7 or 9?