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
15 --- 3.4 Objects which export symbols
16 --- 3.5 Library file goals - lib-y
17 --- 3.6 Descending down in directories
18 --- 3.7 Compilation flags
19 --- 3.8 Command line dependency
20 --- 3.9 Dependency tracking
21 --- 3.10 Special Rules
22 --- 3.11 $(CC) support functions
23 --- 3.12 $(LD) support functions
25 === 4 Host Program support
26 --- 4.1 Simple Host Program
27 --- 4.2 Composite Host Programs
28 --- 4.3 Using C++ for host programs
29 --- 4.4 Controlling compiler options for host programs
30 --- 4.5 When host programs are actually built
32 === 5 Userspace Program support
33 --- 5.1 Simple Userspace Program
34 --- 5.2 Composite Userspace Programs
35 --- 5.3 Controlling compiler options for userspace programs
36 --- 5.4 When userspace programs are actually built
38 === 6 Kbuild clean infrastructure
40 === 7 Architecture Makefiles
41 --- 7.1 Set variables to tweak the build to the architecture
42 --- 7.2 Add prerequisites to archheaders:
43 --- 7.3 Add prerequisites to archprepare:
44 --- 7.4 List directories to visit when descending
45 --- 7.5 Architecture-specific boot images
46 --- 7.6 Building non-kbuild targets
47 --- 7.7 Commands useful for building a boot image
48 --- 7.8 Custom kbuild commands
49 --- 7.9 Preprocessing linker scripts
50 --- 7.10 Generic header files
51 --- 7.11 Post-link pass
53 === 8 Kbuild syntax for exported headers
54 --- 8.1 no-export-headers
59 === 9 Kbuild Variables
60 === 10 Makefile language
67 The Makefiles have five parts::
69 Makefile the top Makefile.
70 .config the kernel configuration file.
71 arch/$(ARCH)/Makefile the arch Makefile.
72 scripts/Makefile.* common rules etc. for all kbuild Makefiles.
73 kbuild Makefiles there are about 500 of these.
75 The top Makefile reads the .config file, which comes from the kernel
76 configuration process.
78 The top Makefile is responsible for building two major products: vmlinux
79 (the resident kernel image) and modules (any module files).
80 It builds these goals by recursively descending into the subdirectories of
81 the kernel source tree.
82 The list of subdirectories which are visited depends upon the kernel
83 configuration. The top Makefile textually includes an arch Makefile
84 with the name arch/$(ARCH)/Makefile. The arch Makefile supplies
85 architecture-specific information to the top Makefile.
87 Each subdirectory has a kbuild Makefile which carries out the commands
88 passed down from above. The kbuild Makefile uses information from the
89 .config file to construct various file lists used by kbuild to build
90 any built-in or modular targets.
92 scripts/Makefile.* contains all the definitions/rules etc. that
93 are used to build the kernel based on the kbuild makefiles.
99 People have four different relationships with the kernel Makefiles.
101 *Users* are people who build kernels. These people type commands such as
102 "make menuconfig" or "make". They usually do not read or edit
103 any kernel Makefiles (or any other source files).
105 *Normal developers* are people who work on features such as device
106 drivers, file systems, and network protocols. These people need to
107 maintain the kbuild Makefiles for the subsystem they are
108 working on. In order to do this effectively, they need some overall
109 knowledge about the kernel Makefiles, plus detailed knowledge about the
110 public interface for kbuild.
112 *Arch developers* are people who work on an entire architecture, such
113 as sparc or ia64. Arch developers need to know about the arch Makefile
114 as well as kbuild Makefiles.
116 *Kbuild developers* are people who work on the kernel build system itself.
117 These people need to know about all aspects of the kernel Makefiles.
119 This document is aimed towards normal developers and arch developers.
125 Most Makefiles within the kernel are kbuild Makefiles that use the
126 kbuild infrastructure. This chapter introduces the syntax used in the
128 The preferred name for the kbuild files are 'Makefile' but 'Kbuild' can
129 be used and if both a 'Makefile' and a 'Kbuild' file exists, then the 'Kbuild'
132 Section 3.1 "Goal definitions" is a quick intro, further chapters provide
133 more details, with real examples.
138 Goal definitions are the main part (heart) of the kbuild Makefile.
139 These lines define the files to be built, any special compilation
140 options, and any subdirectories to be entered recursively.
142 The most simple kbuild makefile contains one line:
148 This tells kbuild that there is one object in that directory, named
149 foo.o. foo.o will be built from foo.c or foo.S.
151 If foo.o shall be built as a module, the variable obj-m is used.
152 Therefore the following pattern is often used:
156 obj-$(CONFIG_FOO) += foo.o
158 $(CONFIG_FOO) evaluates to either y (for built-in) or m (for module).
159 If CONFIG_FOO is neither y nor m, then the file will not be compiled
162 3.2 Built-in object goals - obj-y
163 ---------------------------------
165 The kbuild Makefile specifies object files for vmlinux
166 in the $(obj-y) lists. These lists depend on the kernel
169 Kbuild compiles all the $(obj-y) files. It then calls
170 "$(AR) rcSTP" to merge these files into one built-in.a file.
171 This is a thin archive without a symbol table. It will be later
172 linked into vmlinux by scripts/link-vmlinux.sh
174 The order of files in $(obj-y) is significant. Duplicates in
175 the lists are allowed: the first instance will be linked into
176 built-in.a and succeeding instances will be ignored.
178 Link order is significant, because certain functions
179 (module_init() / __initcall) will be called during boot in the
180 order they appear. So keep in mind that changing the link
181 order may e.g. change the order in which your SCSI
182 controllers are detected, and thus your disks are renumbered.
186 #drivers/isdn/i4l/Makefile
187 # Makefile for the kernel ISDN subsystem and device drivers.
188 # Each configuration option enables a list of files.
189 obj-$(CONFIG_ISDN_I4L) += isdn.o
190 obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
192 3.3 Loadable module goals - obj-m
193 ---------------------------------
195 $(obj-m) specifies object files which are built as loadable
198 A module may be built from one source file or several source
199 files. In the case of one source file, the kbuild makefile
200 simply adds the file to $(obj-m).
204 #drivers/isdn/i4l/Makefile
205 obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
207 Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to 'm'
209 If a kernel module is built from several source files, you specify
210 that you want to build a module in the same way as above; however,
211 kbuild needs to know which object files you want to build your
212 module from, so you have to tell it by setting a $(<module_name>-y)
217 #drivers/isdn/i4l/Makefile
218 obj-$(CONFIG_ISDN_I4L) += isdn.o
219 isdn-y := isdn_net_lib.o isdn_v110.o isdn_common.o
221 In this example, the module name will be isdn.o. Kbuild will
222 compile the objects listed in $(isdn-y) and then run
223 "$(LD) -r" on the list of these files to generate isdn.o.
225 Due to kbuild recognizing $(<module_name>-y) for composite objects,
226 you can use the value of a `CONFIG_` symbol to optionally include an
227 object file as part of a composite object.
232 obj-$(CONFIG_EXT2_FS) += ext2.o
233 ext2-y := balloc.o dir.o file.o ialloc.o inode.o ioctl.o \
234 namei.o super.o symlink.o
235 ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o xattr_user.o \
238 In this example, xattr.o, xattr_user.o and xattr_trusted.o are only
239 part of the composite object ext2.o if $(CONFIG_EXT2_FS_XATTR)
242 Note: Of course, when you are building objects into the kernel,
243 the syntax above will also work. So, if you have CONFIG_EXT2_FS=y,
244 kbuild will build an ext2.o file for you out of the individual
245 parts and then link this into built-in.a, as you would expect.
247 3.4 Objects which export symbols
248 --------------------------------
250 No special notation is required in the makefiles for
251 modules exporting symbols.
253 3.5 Library file goals - lib-y
254 ------------------------------
256 Objects listed with obj-* are used for modules, or
257 combined in a built-in.a for that specific directory.
258 There is also the possibility to list objects that will
259 be included in a library, lib.a.
260 All objects listed with lib-y are combined in a single
261 library for that directory.
262 Objects that are listed in obj-y and additionally listed in
263 lib-y will not be included in the library, since they will
264 be accessible anyway.
265 For consistency, objects listed in lib-m will be included in lib.a.
267 Note that the same kbuild makefile may list files to be built-in
268 and to be part of a library. Therefore the same directory
269 may contain both a built-in.a and a lib.a file.
273 #arch/x86/lib/Makefile
276 This will create a library lib.a based on delay.o. For kbuild to
277 actually recognize that there is a lib.a being built, the directory
278 shall be listed in libs-y.
280 See also "6.4 List directories to visit when descending".
282 Use of lib-y is normally restricted to `lib/` and `arch/*/lib`.
284 3.6 Descending down in directories
285 ----------------------------------
287 A Makefile is only responsible for building objects in its own
288 directory. Files in subdirectories should be taken care of by
289 Makefiles in these subdirs. The build system will automatically
290 invoke make recursively in subdirectories, provided you let it know of
293 To do so, obj-y and obj-m are used.
294 ext2 lives in a separate directory, and the Makefile present in fs/
295 tells kbuild to descend down using the following assignment.
300 obj-$(CONFIG_EXT2_FS) += ext2/
302 If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular)
303 the corresponding obj- variable will be set, and kbuild will descend
304 down in the ext2 directory.
306 Kbuild uses this information not only to decide that it needs to visit
307 the directory, but also to decide whether or not to link objects from
308 the directory into vmlinux.
310 When Kbuild descends into the directory with 'y', all built-in objects
311 from that directory are combined into the built-in.a, which will be
312 eventually linked into vmlinux.
314 When Kbuild descends into the directory with 'm', in contrast, nothing
315 from that directory will be linked into vmlinux. If the Makefile in
316 that directory specifies obj-y, those objects will be left orphan.
317 It is very likely a bug of the Makefile or of dependencies in Kconfig.
319 It is good practice to use a `CONFIG_` variable when assigning directory
320 names. This allows kbuild to totally skip the directory if the
321 corresponding `CONFIG_` option is neither 'y' nor 'm'.
323 3.7 Compilation flags
324 ---------------------
326 ccflags-y, asflags-y and ldflags-y
327 These three flags apply only to the kbuild makefile in which they
328 are assigned. They are used for all the normal cc, as and ld
329 invocations happening during a recursive build.
330 Note: Flags with the same behaviour were previously named:
331 EXTRA_CFLAGS, EXTRA_AFLAGS and EXTRA_LDFLAGS.
332 They are still supported but their usage is deprecated.
334 ccflags-y specifies options for compiling with $(CC).
338 # drivers/acpi/acpica/Makefile
339 ccflags-y := -Os -D_LINUX -DBUILDING_ACPICA
340 ccflags-$(CONFIG_ACPI_DEBUG) += -DACPI_DEBUG_OUTPUT
342 This variable is necessary because the top Makefile owns the
343 variable $(KBUILD_CFLAGS) and uses it for compilation flags for the
346 asflags-y specifies assembler options.
350 #arch/sparc/kernel/Makefile
353 ldflags-y specifies options for linking with $(LD).
357 #arch/cris/boot/compressed/Makefile
358 ldflags-y += -T $(srctree)/$(src)/decompress_$(arch-y).lds
360 subdir-ccflags-y, subdir-asflags-y
361 The two flags listed above are similar to ccflags-y and asflags-y.
362 The difference is that the subdir- variants have effect for the kbuild
363 file where they are present and all subdirectories.
364 Options specified using subdir-* are added to the commandline before
365 the options specified using the non-subdir variants.
369 subdir-ccflags-y := -Werror
371 ccflags-remove-y, asflags-remove-y
372 These flags are used to remove particular flags for the compiler,
373 assembler invocations.
377 ccflags-remove-$(CONFIG_MCOUNT) += -pg
380 CFLAGS_$@ and AFLAGS_$@ only apply to commands in current
383 $(CFLAGS_$@) specifies per-file options for $(CC). The $@
384 part has a literal value which specifies the file that it is for.
386 CFLAGS_$@ has the higher priority than ccflags-remove-y; CFLAGS_$@
387 can re-add compiler flags that were removed by ccflags-remove-y.
391 # drivers/scsi/Makefile
392 CFLAGS_aha152x.o = -DAHA152X_STAT -DAUTOCONF
393 CFLAGS_gdth.o = # -DDEBUG_GDTH=2 -D__SERIAL__ -D__COM2__ \
396 These two lines specify compilation flags for aha152x.o and gdth.o.
398 $(AFLAGS_$@) is a similar feature for source files in assembly
401 AFLAGS_$@ has the higher priority than asflags-remove-y; AFLAGS_$@
402 can re-add assembler flags that were removed by asflags-remove-y.
406 # arch/arm/kernel/Makefile
407 AFLAGS_head.o := -DTEXT_OFFSET=$(TEXT_OFFSET)
408 AFLAGS_crunch-bits.o := -Wa,-mcpu=ep9312
409 AFLAGS_iwmmxt.o := -Wa,-mcpu=iwmmxt
412 3.9 Dependency tracking
413 -----------------------
415 Kbuild tracks dependencies on the following:
417 1) All prerequisite files (both `*.c` and `*.h`)
418 2) `CONFIG_` options used in all prerequisite files
419 3) Command-line used to compile target
421 Thus, if you change an option to $(CC) all affected files will
427 Special rules are used when the kbuild infrastructure does
428 not provide the required support. A typical example is
429 header files generated during the build process.
430 Another example are the architecture-specific Makefiles which
431 need special rules to prepare boot images etc.
433 Special rules are written as normal Make rules.
434 Kbuild is not executing in the directory where the Makefile is
435 located, so all special rules shall provide a relative
436 path to prerequisite files and target files.
438 Two variables are used when defining special rules:
441 $(src) is a relative path which points to the directory
442 where the Makefile is located. Always use $(src) when
443 referring to files located in the src tree.
446 $(obj) is a relative path which points to the directory
447 where the target is saved. Always use $(obj) when
448 referring to generated files.
452 #drivers/scsi/Makefile
453 $(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl
454 $(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl
456 This is a special rule, following the normal syntax
459 The target file depends on two prerequisite files. References
460 to the target file are prefixed with $(obj), references
461 to prerequisites are referenced with $(src) (because they are not
465 echoing information to user in a rule is often a good practice
466 but when execution "make -s" one does not expect to see any output
467 except for warnings/errors.
468 To support this kbuild defines $(kecho) which will echo out the
469 text following $(kecho) to stdout except if "make -s" is used.
473 #arch/blackfin/boot/Makefile
474 $(obj)/vmImage: $(obj)/vmlinux.gz
475 $(call if_changed,uimage)
476 @$(kecho) 'Kernel: $@ is ready'
479 3.11 $(CC) support functions
480 ----------------------------
482 The kernel may be built with several different versions of
483 $(CC), each supporting a unique set of features and options.
484 kbuild provides basic support to check for valid options for $(CC).
485 $(CC) is usually the gcc compiler, but other alternatives are
489 as-option is used to check if $(CC) -- when used to compile
490 assembler (`*.S`) files -- supports the given option. An optional
491 second option may be specified if the first option is not supported.
496 cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),)
498 In the above example, cflags-y will be assigned the option
499 -Wa$(comma)-isa=$(isa-y) if it is supported by $(CC).
500 The second argument is optional, and if supplied will be used
501 if first argument is not supported.
504 as-instr checks if the assembler reports a specific instruction
505 and then outputs either option1 or option2
506 C escapes are supported in the test instruction
507 Note: as-instr-option uses KBUILD_AFLAGS for assembler options
510 cc-option is used to check if $(CC) supports a given option, and if
511 not supported to use an optional second option.
516 cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586)
518 In the above example, cflags-y will be assigned the option
519 -march=pentium-mmx if supported by $(CC), otherwise -march=i586.
520 The second argument to cc-option is optional, and if omitted,
521 cflags-y will be assigned no value if first option is not supported.
522 Note: cc-option uses KBUILD_CFLAGS for $(CC) options
525 cc-option-yn is used to check if gcc supports a given option
526 and return 'y' if supported, otherwise 'n'.
531 biarch := $(call cc-option-yn, -m32)
532 aflags-$(biarch) += -a32
533 cflags-$(biarch) += -m32
535 In the above example, $(biarch) is set to y if $(CC) supports the -m32
536 option. When $(biarch) equals 'y', the expanded variables $(aflags-y)
537 and $(cflags-y) will be assigned the values -a32 and -m32,
539 Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options
542 cc-disable-warning checks if gcc supports a given warning and returns
543 the commandline switch to disable it. This special function is needed,
544 because gcc 4.4 and later accept any unknown -Wno-* option and only
545 warn about it if there is another warning in the source file.
549 KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
551 In the above example, -Wno-unused-but-set-variable will be added to
552 KBUILD_CFLAGS only if gcc really accepts it.
555 cc-ifversion tests the version of $(CC) and equals the fourth parameter
556 if version expression is true, or the fifth (if given) if the version
561 #fs/reiserfs/Makefile
562 ccflags-y := $(call cc-ifversion, -lt, 0402, -O1)
564 In this example, ccflags-y will be assigned the value -O1 if the
565 $(CC) version is less than 4.2.
566 cc-ifversion takes all the shell operators:
567 -eq, -ne, -lt, -le, -gt, and -ge
568 The third parameter may be a text as in this example, but it may also
569 be an expanded variable or a macro.
572 cc-cross-prefix is used to check if there exists a $(CC) in path with
573 one of the listed prefixes. The first prefix where there exist a
574 prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found
575 then nothing is returned.
576 Additional prefixes are separated by a single space in the
577 call of cc-cross-prefix.
578 This functionality is useful for architecture Makefiles that try
579 to set CROSS_COMPILE to well-known values but may have several
580 values to select between.
581 It is recommended only to try to set CROSS_COMPILE if it is a cross
582 build (host arch is different from target arch). And if CROSS_COMPILE
583 is already set then leave it with the old value.
588 ifneq ($(SUBARCH),$(ARCH))
589 ifeq ($(CROSS_COMPILE),)
590 CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-)
594 3.12 $(LD) support functions
595 ----------------------------
598 ld-option is used to check if $(LD) supports the supplied option.
599 ld-option takes two options as arguments.
600 The second argument is an optional option that can be used if the
601 first option is not supported by $(LD).
606 LDFLAGS_vmlinux += $(call ld-option, -X)
609 4 Host Program support
610 ======================
612 Kbuild supports building executables on the host for use during the
614 Two steps are required in order to use a host executable.
616 The first step is to tell kbuild that a host program exists. This is
617 done utilising the variable "hostprogs".
619 The second step is to add an explicit dependency to the executable.
620 This can be done in two ways. Either add the dependency in a rule,
621 or utilise the variable "always-y".
622 Both possibilities are described in the following.
624 4.1 Simple Host Program
625 -----------------------
627 In some cases there is a need to compile and run a program on the
628 computer where the build is running.
629 The following line tells kbuild that the program bin2hex shall be
630 built on the build host.
636 Kbuild assumes in the above example that bin2hex is made from a single
637 c-source file named bin2hex.c located in the same directory as
640 4.2 Composite Host Programs
641 ---------------------------
643 Host programs can be made up based on composite objects.
644 The syntax used to define composite objects for host programs is
645 similar to the syntax used for kernel objects.
646 $(<executable>-objs) lists all objects used to link the final
651 #scripts/lxdialog/Makefile
652 hostprogs := lxdialog
653 lxdialog-objs := checklist.o lxdialog.o
655 Objects with extension .o are compiled from the corresponding .c
656 files. In the above example, checklist.c is compiled to checklist.o
657 and lxdialog.c is compiled to lxdialog.o.
659 Finally, the two .o files are linked to the executable, lxdialog.
660 Note: The syntax <executable>-y is not permitted for host-programs.
662 4.3 Using C++ for host programs
663 -------------------------------
665 kbuild offers support for host programs written in C++. This was
666 introduced solely to support kconfig, and is not recommended
671 #scripts/kconfig/Makefile
673 qconf-cxxobjs := qconf.o
675 In the example above the executable is composed of the C++ file
676 qconf.cc - identified by $(qconf-cxxobjs).
678 If qconf is composed of a mixture of .c and .cc files, then an
679 additional line can be used to identify this.
683 #scripts/kconfig/Makefile
685 qconf-cxxobjs := qconf.o
686 qconf-objs := check.o
688 4.4 Controlling compiler options for host programs
689 --------------------------------------------------
691 When compiling host programs, it is possible to set specific flags.
692 The programs will always be compiled utilising $(HOSTCC) passed
693 the options specified in $(KBUILD_HOSTCFLAGS).
694 To set flags that will take effect for all host programs created
695 in that Makefile, use the variable HOST_EXTRACFLAGS.
699 #scripts/lxdialog/Makefile
700 HOST_EXTRACFLAGS += -I/usr/include/ncurses
702 To set specific flags for a single file the following construction
707 #arch/ppc64/boot/Makefile
708 HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE)
710 It is also possible to specify additional options to the linker.
714 #scripts/kconfig/Makefile
715 HOSTLDLIBS_qconf := -L$(QTDIR)/lib
717 When linking qconf, it will be passed the extra option
720 4.5 When host programs are actually built
721 -----------------------------------------
723 Kbuild will only build host-programs when they are referenced
725 This is possible in two ways:
727 (1) List the prerequisite explicitly in a special rule.
731 #drivers/pci/Makefile
732 hostprogs := gen-devlist
733 $(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist
734 ( cd $(obj); ./gen-devlist ) < $<
736 The target $(obj)/devlist.h will not be built before
737 $(obj)/gen-devlist is updated. Note that references to
738 the host programs in special rules must be prefixed with $(obj).
742 When there is no suitable special rule, and the host program
743 shall be built when a makefile is entered, the always-y
744 variable shall be used.
748 #scripts/lxdialog/Makefile
749 hostprogs := lxdialog
750 always-y := $(hostprogs)
752 Kbuild provides the following shorthand for this:
754 hostprogs-always-y := lxdialog
756 This will tell kbuild to build lxdialog even if not referenced in
759 5 Userspace Program support
760 ===========================
762 Just like host programs, Kbuild also supports building userspace executables
763 for the target architecture (i.e. the same architecture as you are building
766 The syntax is quite similar. The difference is to use "userprogs" instead of
769 5.1 Simple Userspace Program
770 ----------------------------
772 The following line tells kbuild that the program bpf-direct shall be
773 built for the target architecture.
777 userprogs := bpf-direct
779 Kbuild assumes in the above example that bpf-direct is made from a
780 single C source file named bpf-direct.c located in the same directory
783 5.2 Composite Userspace Programs
784 --------------------------------
786 Userspace programs can be made up based on composite objects.
787 The syntax used to define composite objects for userspace programs is
788 similar to the syntax used for kernel objects.
789 $(<executable>-objs) lists all objects used to link the final
794 #samples/seccomp/Makefile
795 userprogs := bpf-fancy
796 bpf-fancy-objs := bpf-fancy.o bpf-helper.o
798 Objects with extension .o are compiled from the corresponding .c
799 files. In the above example, bpf-fancy.c is compiled to bpf-fancy.o
800 and bpf-helper.c is compiled to bpf-helper.o.
802 Finally, the two .o files are linked to the executable, bpf-fancy.
803 Note: The syntax <executable>-y is not permitted for userspace programs.
805 5.3 Controlling compiler options for userspace programs
806 -------------------------------------------------------
808 When compiling userspace programs, it is possible to set specific flags.
809 The programs will always be compiled utilising $(CC) passed
810 the options specified in $(KBUILD_USERCFLAGS).
811 To set flags that will take effect for all userspace programs created
812 in that Makefile, use the variable userccflags.
816 # samples/seccomp/Makefile
817 userccflags += -I usr/include
819 To set specific flags for a single file the following construction
824 bpf-helper-userccflags += -I user/include
826 It is also possible to specify additional options to the linker.
830 # net/bpfilter/Makefile
831 bpfilter_umh-userldflags += -static
833 When linking bpfilter_umh, it will be passed the extra option -static.
835 5.4 When userspace programs are actually built
836 ----------------------------------------------
838 Kbuild builds userspace programs only when told to do so.
839 There are two ways to do this.
841 (1) Add it as the prerequisite of another file
845 #net/bpfilter/Makefile
846 userprogs := bpfilter_umh
847 $(obj)/bpfilter_umh_blob.o: $(obj)/bpfilter_umh
849 $(obj)/bpfilter_umh is built before $(obj)/bpfilter_umh_blob.o
855 userprogs := binderfs_example
856 always-y := $(userprogs)
858 Kbuild provides the following shorthand for this:
860 userprogs-always-y := binderfs_example
862 This will tell Kbuild to build binderfs_example when it visits this
865 6 Kbuild clean infrastructure
866 =============================
868 "make clean" deletes most generated files in the obj tree where the kernel
869 is compiled. This includes generated files such as host programs.
870 Kbuild knows targets listed in $(hostprogs), $(always-y), $(always-m),
871 $(always-), $(extra-y), $(extra-) and $(targets). They are all deleted
872 during "make clean". Files matching the patterns "*.[oas]", "*.ko", plus
873 some additional files generated by kbuild are deleted all over the kernel
874 source tree when "make clean" is executed.
876 Additional files or directories can be specified in kbuild makefiles by use of
882 clean-files := crc32table.h
884 When executing "make clean", the file "crc32table.h" will be deleted.
885 Kbuild will assume files to be in the same relative directory as the
886 Makefile, except if prefixed with $(objtree).
888 To exclude certain files or directories from make clean, use the
889 $(no-clean-files) variable.
891 Usually kbuild descends down in subdirectories due to "obj-* := dir/",
892 but in the architecture makefiles where the kbuild infrastructure
893 is not sufficient this sometimes needs to be explicit.
897 #arch/x86/boot/Makefile
898 subdir- := compressed
900 The above assignment instructs kbuild to descend down in the
901 directory compressed/ when "make clean" is executed.
903 To support the clean infrastructure in the Makefiles that build the
904 final bootimage there is an optional target named archclean:
910 $(Q)$(MAKE) $(clean)=arch/x86/boot
912 When "make clean" is executed, make will descend down in arch/x86/boot,
913 and clean as usual. The Makefile located in arch/x86/boot/ may use
914 the subdir- trick to descend further down.
916 Note 1: arch/$(ARCH)/Makefile cannot use "subdir-", because that file is
917 included in the top level makefile, and the kbuild infrastructure
918 is not operational at that point.
920 Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will
921 be visited during "make clean".
923 7 Architecture Makefiles
924 ========================
926 The top level Makefile sets up the environment and does the preparation,
927 before starting to descend down in the individual directories.
928 The top level makefile contains the generic part, whereas
929 arch/$(ARCH)/Makefile contains what is required to set up kbuild
930 for said architecture.
931 To do so, arch/$(ARCH)/Makefile sets up a number of variables and defines
934 When kbuild executes, the following steps are followed (roughly):
936 1) Configuration of the kernel => produce .config
937 2) Store kernel version in include/linux/version.h
938 3) Updating all other prerequisites to the target prepare:
939 - Additional prerequisites are specified in arch/$(ARCH)/Makefile
940 4) Recursively descend down in all directories listed in
941 init-* core* drivers-* net-* libs-* and build all targets.
942 - The values of the above variables are expanded in arch/$(ARCH)/Makefile.
943 5) All object files are then linked and the resulting file vmlinux is
944 located at the root of the obj tree.
945 The very first objects linked are listed in head-y, assigned by
946 arch/$(ARCH)/Makefile.
947 6) Finally, the architecture-specific part does any required post processing
948 and builds the final bootimage.
949 - This includes building boot records
950 - Preparing initrd images and the like
953 7.1 Set variables to tweak the build to the architecture
954 --------------------------------------------------------
957 Generic $(LD) options
959 Flags used for all invocations of the linker.
960 Often specifying the emulation is sufficient.
965 KBUILD_LDFLAGS := -m elf_s390
967 Note: ldflags-y can be used to further customise
968 the flags used. See chapter 3.7.
971 Options for $(LD) when linking vmlinux
973 LDFLAGS_vmlinux is used to specify additional flags to pass to
974 the linker when linking the final vmlinux image.
975 LDFLAGS_vmlinux uses the LDFLAGS_$@ support.
980 LDFLAGS_vmlinux := -e stext
985 When $(call if_changed,objcopy) is used to translate a .o file,
986 the flags specified in OBJCOPYFLAGS will be used.
987 $(call if_changed,objcopy) is often used to generate raw binaries on
993 OBJCOPYFLAGS := -O binary
995 #arch/s390/boot/Makefile
996 $(obj)/image: vmlinux FORCE
997 $(call if_changed,objcopy)
999 In this example, the binary $(obj)/image is a binary version of
1000 vmlinux. The usage of $(call if_changed,xxx) will be described later.
1005 Default value - see top level Makefile
1006 Append or modify as required per architecture.
1010 #arch/sparc64/Makefile
1011 KBUILD_AFLAGS += -m64 -mcpu=ultrasparc
1014 $(CC) compiler flags
1016 Default value - see top level Makefile
1017 Append or modify as required per architecture.
1019 Often, the KBUILD_CFLAGS variable depends on the configuration.
1023 #arch/x86/boot/compressed/Makefile
1024 cflags-$(CONFIG_X86_32) := -march=i386
1025 cflags-$(CONFIG_X86_64) := -mcmodel=small
1026 KBUILD_CFLAGS += $(cflags-y)
1028 Many arch Makefiles dynamically run the target C compiler to
1029 probe supported options::
1034 cflags-$(CONFIG_MPENTIUMII) += $(call cc-option,\
1035 -march=pentium2,-march=i686)
1037 # Disable unit-at-a-time mode ...
1038 KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time)
1042 The first example utilises the trick that a config option expands
1043 to 'y' when selected.
1045 KBUILD_AFLAGS_KERNEL
1046 Assembler options specific for built-in
1048 $(KBUILD_AFLAGS_KERNEL) contains extra C compiler flags used to compile
1049 resident kernel code.
1051 KBUILD_AFLAGS_MODULE
1052 Assembler options specific for modules
1054 $(KBUILD_AFLAGS_MODULE) is used to add arch-specific options that
1055 are used for assembler.
1057 From commandline AFLAGS_MODULE shall be used (see kbuild.rst).
1059 KBUILD_CFLAGS_KERNEL
1060 $(CC) options specific for built-in
1062 $(KBUILD_CFLAGS_KERNEL) contains extra C compiler flags used to compile
1063 resident kernel code.
1065 KBUILD_CFLAGS_MODULE
1066 Options for $(CC) when building modules
1068 $(KBUILD_CFLAGS_MODULE) is used to add arch-specific options that
1070 From commandline CFLAGS_MODULE shall be used (see kbuild.rst).
1072 KBUILD_LDFLAGS_MODULE
1073 Options for $(LD) when linking modules
1075 $(KBUILD_LDFLAGS_MODULE) is used to add arch-specific options
1076 used when linking modules. This is often a linker script.
1078 From commandline LDFLAGS_MODULE shall be used (see kbuild.rst).
1082 The linker script with full path. Assigned by the top-level Makefile.
1086 The module linker script with full path. Assigned by the top-level
1087 Makefile and additionally by the arch Makefile.
1091 All object files for vmlinux. They are linked to vmlinux in the same
1092 order as listed in KBUILD_VMLINUX_OBJS.
1096 All .a "lib" files for vmlinux. KBUILD_VMLINUX_OBJS and
1097 KBUILD_VMLINUX_LIBS together specify all the object files used to
1100 7.2 Add prerequisites to archheaders
1101 ------------------------------------
1103 The archheaders: rule is used to generate header files that
1104 may be installed into user space by "make header_install".
1106 It is run before "make archprepare" when run on the
1107 architecture itself.
1110 7.3 Add prerequisites to archprepare
1111 ------------------------------------
1113 The archprepare: rule is used to list prerequisites that need to be
1114 built before starting to descend down in the subdirectories.
1115 This is usually used for header files containing assembler constants.
1120 archprepare: maketools
1122 In this example, the file target maketools will be processed
1123 before descending down in the subdirectories.
1124 See also chapter XXX-TODO that describe how kbuild supports
1125 generating offset header files.
1128 7.4 List directories to visit when descending
1129 ---------------------------------------------
1131 An arch Makefile cooperates with the top Makefile to define variables
1132 which specify how to build the vmlinux file. Note that there is no
1133 corresponding arch-specific section for modules; the module-building
1134 machinery is all architecture-independent.
1137 head-y, init-y, core-y, libs-y, drivers-y, net-y
1138 $(head-y) lists objects to be linked first in vmlinux.
1140 $(libs-y) lists directories where a lib.a archive can be located.
1142 The rest list directories where a built-in.a object file can be
1145 $(init-y) objects will be located after $(head-y).
1147 Then the rest follows in this order:
1149 $(core-y), $(libs-y), $(drivers-y) and $(net-y).
1151 The top level Makefile defines values for all generic directories,
1152 and arch/$(ARCH)/Makefile only adds architecture-specific
1157 #arch/sparc64/Makefile
1158 core-y += arch/sparc64/kernel/
1159 libs-y += arch/sparc64/prom/ arch/sparc64/lib/
1160 drivers-$(CONFIG_OPROFILE) += arch/sparc64/oprofile/
1163 7.5 Architecture-specific boot images
1164 -------------------------------------
1166 An arch Makefile specifies goals that take the vmlinux file, compress
1167 it, wrap it in bootstrapping code, and copy the resulting files
1168 somewhere. This includes various kinds of installation commands.
1169 The actual goals are not standardized across architectures.
1171 It is common to locate any additional processing in a boot/
1172 directory below arch/$(ARCH)/.
1174 Kbuild does not provide any smart way to support building a
1175 target specified in boot/. Therefore arch/$(ARCH)/Makefile shall
1176 call make manually to build a target in boot/.
1178 The recommended approach is to include shortcuts in
1179 arch/$(ARCH)/Makefile, and use the full path when calling down
1180 into the arch/$(ARCH)/boot/Makefile.
1185 boot := arch/x86/boot
1187 $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
1189 "$(Q)$(MAKE) $(build)=<dir>" is the recommended way to invoke
1190 make in a subdirectory.
1192 There are no rules for naming architecture-specific targets,
1193 but executing "make help" will list all relevant targets.
1194 To support this, $(archhelp) must be defined.
1200 echo '* bzImage - Image (arch/$(ARCH)/boot/bzImage)'
1203 When make is executed without arguments, the first goal encountered
1204 will be built. In the top level Makefile the first goal present
1206 An architecture shall always, per default, build a bootable image.
1207 In "make help", the default goal is highlighted with a '*'.
1208 Add a new prerequisite to all: to select a default goal different
1216 When "make" is executed without arguments, bzImage will be built.
1218 7.6 Building non-kbuild targets
1219 -------------------------------
1222 extra-y specifies additional targets created in the current
1223 directory, in addition to any targets specified by `obj-*`.
1225 Listing all targets in extra-y is required for two purposes:
1227 1) Enable kbuild to check changes in command lines
1229 - When $(call if_changed,xxx) is used
1231 2) kbuild knows what files to delete during "make clean"
1235 #arch/x86/kernel/Makefile
1236 extra-y := head.o init_task.o
1238 In this example, extra-y is used to list object files that
1239 shall be built, but shall not be linked as part of built-in.a.
1241 7.7 Commands useful for building a boot image
1242 ---------------------------------------------
1244 Kbuild provides a few macros that are useful when building a
1248 if_changed is the infrastructure used for the following commands.
1252 target: source(s) FORCE
1253 $(call if_changed,ld/objcopy/gzip/...)
1255 When the rule is evaluated, it is checked to see if any files
1256 need an update, or the command line has changed since the last
1257 invocation. The latter will force a rebuild if any options
1258 to the executable have changed.
1259 Any target that utilises if_changed must be listed in $(targets),
1260 otherwise the command line check will fail, and the target will
1262 Assignments to $(targets) are without $(obj)/ prefix.
1263 if_changed may be used in conjunction with custom commands as
1264 defined in 6.8 "Custom kbuild commands".
1266 Note: It is a typical mistake to forget the FORCE prerequisite.
1267 Another common pitfall is that whitespace is sometimes
1268 significant; for instance, the below will fail (note the extra space
1271 target: source(s) FORCE
1273 **WRONG!** $(call if_changed, ld/objcopy/gzip/...)
1276 if_changed should not be used more than once per target.
1277 It stores the executed command in a corresponding .cmd
1279 file and multiple calls would result in overwrites and
1280 unwanted results when the target is up to date and only the
1281 tests on changed commands trigger execution of commands.
1284 Link target. Often, LDFLAGS_$@ is used to set specific options to ld.
1288 #arch/x86/boot/Makefile
1289 LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary
1290 LDFLAGS_setup := -Ttext 0x0 -s --oformat binary -e begtext
1292 targets += setup setup.o bootsect bootsect.o
1293 $(obj)/setup $(obj)/bootsect: %: %.o FORCE
1294 $(call if_changed,ld)
1296 In this example, there are two possible targets, requiring different
1297 options to the linker. The linker options are specified using the
1298 LDFLAGS_$@ syntax - one for each potential target.
1299 $(targets) are assigned all potential targets, by which kbuild knows
1300 the targets and will:
1302 1) check for commandline changes
1303 2) delete target during make clean
1305 The ": %: %.o" part of the prerequisite is a shorthand that
1306 frees us from listing the setup.o and bootsect.o files.
1309 It is a common mistake to forget the "targets :=" assignment,
1310 resulting in the target file being recompiled for no
1314 Copy binary. Uses OBJCOPYFLAGS usually specified in
1315 arch/$(ARCH)/Makefile.
1316 OBJCOPYFLAGS_$@ may be used to set additional options.
1319 Compress target. Use maximum compression to compress target.
1323 #arch/x86/boot/compressed/Makefile
1324 $(obj)/vmlinux.bin.gz: $(vmlinux.bin.all-y) FORCE
1325 $(call if_changed,gzip)
1328 Create flattened device tree blob object suitable for linking
1329 into vmlinux. Device tree blobs linked into vmlinux are placed
1330 in an init section in the image. Platform code *must* copy the
1331 blob to non-init memory prior to calling unflatten_device_tree().
1333 To use this command, simply add `*.dtb` into obj-y or targets, or make
1334 some other target depend on `%.dtb`
1336 A central rule exists to create `$(obj)/%.dtb` from `$(src)/%.dts`;
1337 architecture Makefiles do no need to explicitly write out that rule.
1342 DTC_FLAGS ?= -p 1024
1344 7.8 Custom kbuild commands
1345 --------------------------
1347 When kbuild is executing with KBUILD_VERBOSE=0, then only a shorthand
1348 of a command is normally displayed.
1349 To enable this behaviour for custom commands kbuild requires
1350 two variables to be set::
1352 quiet_cmd_<command> - what shall be echoed
1353 cmd_<command> - the command to execute
1358 quiet_cmd_image = BUILD $@
1359 cmd_image = $(obj)/tools/build $(BUILDFLAGS) \
1360 $(obj)/vmlinux.bin > $@
1363 $(obj)/bzImage: $(obj)/vmlinux.bin $(obj)/tools/build FORCE
1364 $(call if_changed,image)
1365 @echo 'Kernel: $@ is ready'
1367 When updating the $(obj)/bzImage target, the line:
1369 BUILD arch/x86/boot/bzImage
1371 will be displayed with "make KBUILD_VERBOSE=0".
1374 7.9 Preprocessing linker scripts
1375 --------------------------------
1377 When the vmlinux image is built, the linker script
1378 arch/$(ARCH)/kernel/vmlinux.lds is used.
1379 The script is a preprocessed variant of the file vmlinux.lds.S
1380 located in the same directory.
1381 kbuild knows .lds files and includes a rule `*lds.S` -> `*lds`.
1385 #arch/x86/kernel/Makefile
1386 extra-y := vmlinux.lds
1389 export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH)
1391 The assignment to extra-y is used to tell kbuild to build the
1393 The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the
1394 specified options when building the target vmlinux.lds.
1396 When building the `*.lds` target, kbuild uses the variables::
1398 KBUILD_CPPFLAGS : Set in top-level Makefile
1399 cppflags-y : May be set in the kbuild makefile
1400 CPPFLAGS_$(@F) : Target-specific flags.
1401 Note that the full filename is used in this
1404 The kbuild infrastructure for `*lds` files is used in several
1405 architecture-specific files.
1407 7.10 Generic header files
1408 -------------------------
1410 The directory include/asm-generic contains the header files
1411 that may be shared between individual architectures.
1412 The recommended approach how to use a generic header file is
1413 to list the file in the Kbuild file.
1414 See "7.2 generic-y" for further info on syntax etc.
1419 If the file arch/xxx/Makefile.postlink exists, this makefile
1420 will be invoked for post-link objects (vmlinux and modules.ko)
1421 for architectures to run post-link passes on. Must also handle
1424 This pass runs after kallsyms generation. If the architecture
1425 needs to modify symbol locations, rather than manipulate the
1426 kallsyms, it may be easier to add another postlink target for
1427 .tmp_vmlinux? targets to be called from link-vmlinux.sh.
1429 For example, powerpc uses this to check relocation sanity of
1430 the linked vmlinux file.
1432 8 Kbuild syntax for exported headers
1433 ------------------------------------
1435 The kernel includes a set of headers that is exported to userspace.
1436 Many headers can be exported as-is but other headers require a
1437 minimal pre-processing before they are ready for user-space.
1438 The pre-processing does:
1440 - drop kernel-specific annotations
1441 - drop include of compiler.h
1442 - drop all sections that are kernel internal (guarded by `ifdef __KERNEL__`)
1444 All headers under include/uapi/, include/generated/uapi/,
1445 arch/<arch>/include/uapi/ and arch/<arch>/include/generated/uapi/
1448 A Kbuild file may be defined under arch/<arch>/include/uapi/asm/ and
1449 arch/<arch>/include/asm/ to list asm files coming from asm-generic.
1450 See subsequent chapter for the syntax of the Kbuild file.
1452 8.1 no-export-headers
1453 ---------------------
1455 no-export-headers is essentially used by include/uapi/linux/Kbuild to
1456 avoid exporting specific headers (e.g. kvm.h) on architectures that do
1457 not support it. It should be avoided as much as possible.
1462 If an architecture uses a verbatim copy of a header from
1463 include/asm-generic then this is listed in the file
1464 arch/$(ARCH)/include/asm/Kbuild like this:
1468 #arch/x86/include/asm/Kbuild
1469 generic-y += termios.h
1472 During the prepare phase of the build a wrapper include
1473 file is generated in the directory::
1475 arch/$(ARCH)/include/generated/asm
1477 When a header is exported where the architecture uses
1478 the generic header a similar wrapper is generated as part
1479 of the set of exported headers in the directory::
1483 The generated wrapper will in both cases look like the following:
1485 Example: termios.h::
1487 #include <asm-generic/termios.h>
1492 If an architecture generates other header files alongside generic-y
1493 wrappers, generated-y specifies them.
1495 This prevents them being treated as stale asm-generic wrappers and
1500 #arch/x86/include/asm/Kbuild
1501 generated-y += syscalls_32.h
1506 mandatory-y is essentially used by include/(uapi/)asm-generic/Kbuild
1507 to define the minimum set of ASM headers that all architectures must have.
1509 This works like optional generic-y. If a mandatory header is missing
1510 in arch/$(ARCH)/include/(uapi/)/asm, Kbuild will automatically generate
1511 a wrapper of the asm-generic one.
1516 The top Makefile exports the following variables:
1518 VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION
1519 These variables define the current kernel version. A few arch
1520 Makefiles actually use these values directly; they should use
1521 $(KERNELRELEASE) instead.
1523 $(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic
1524 three-part version number, such as "2", "4", and "0". These three
1525 values are always numeric.
1527 $(EXTRAVERSION) defines an even tinier sublevel for pre-patches
1528 or additional patches. It is usually some non-numeric string
1529 such as "-pre4", and is often blank.
1532 $(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable
1533 for constructing installation directory names or showing in
1534 version strings. Some arch Makefiles use it for this purpose.
1537 This variable defines the target architecture, such as "i386",
1538 "arm", or "sparc". Some kbuild Makefiles test $(ARCH) to
1539 determine which files to compile.
1541 By default, the top Makefile sets $(ARCH) to be the same as the
1542 host system architecture. For a cross build, a user may
1543 override the value of $(ARCH) on the command line::
1549 This variable defines a place for the arch Makefiles to install
1550 the resident kernel image and System.map file.
1551 Use this for architecture-specific install targets.
1553 INSTALL_MOD_PATH, MODLIB
1554 $(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module
1555 installation. This variable is not defined in the Makefile but
1556 may be passed in by the user if desired.
1558 $(MODLIB) specifies the directory for module installation.
1559 The top Makefile defines $(MODLIB) to
1560 $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE). The user may
1561 override this value on the command line if desired.
1564 If this variable is specified, it will cause modules to be stripped
1565 after they are installed. If INSTALL_MOD_STRIP is '1', then the
1566 default option --strip-debug will be used. Otherwise, the
1567 INSTALL_MOD_STRIP value will be used as the option(s) to the strip
1571 10 Makefile language
1572 ====================
1574 The kernel Makefiles are designed to be run with GNU Make. The Makefiles
1575 use only the documented features of GNU Make, but they do use many
1578 GNU Make supports elementary list-processing functions. The kernel
1579 Makefiles use a novel style of list building and manipulation with few
1582 GNU Make has two assignment operators, ":=" and "=". ":=" performs
1583 immediate evaluation of the right-hand side and stores an actual string
1584 into the left-hand side. "=" is like a formula definition; it stores the
1585 right-hand side in an unevaluated form and then evaluates this form each
1586 time the left-hand side is used.
1588 There are some cases where "=" is appropriate. Usually, though, ":="
1589 is the right choice.
1594 - Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net>
1595 - Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de>
1596 - Updates by Sam Ravnborg <sam@ravnborg.org>
1597 - Language QA by Jan Engelhardt <jengelh@gmx.de>
1602 - Describe how kbuild supports shipped files with _shipped.
1603 - Generating offset header files.
1604 - Add more variables to section 7?