5 This document describes the Linux kernel Makefiles.
10 The Makefiles have five parts::
12 Makefile the top Makefile.
13 .config the kernel configuration file.
14 arch/$(SRCARCH)/Makefile the arch Makefile.
15 scripts/Makefile.* common rules etc. for all kbuild Makefiles.
16 kbuild Makefiles exist in every subdirectory
18 The top Makefile reads the .config file, which comes from the kernel
19 configuration process.
21 The top Makefile is responsible for building two major products: vmlinux
22 (the resident kernel image) and modules (any module files).
23 It builds these goals by recursively descending into the subdirectories of
24 the kernel source tree.
26 The list of subdirectories which are visited depends upon the kernel
27 configuration. The top Makefile textually includes an arch Makefile
28 with the name arch/$(SRCARCH)/Makefile. The arch Makefile supplies
29 architecture-specific information to the top Makefile.
31 Each subdirectory has a kbuild Makefile which carries out the commands
32 passed down from above. The kbuild Makefile uses information from the
33 .config file to construct various file lists used by kbuild to build
34 any built-in or modular targets.
36 scripts/Makefile.* contains all the definitions/rules etc. that
37 are used to build the kernel based on the kbuild makefiles.
42 People have four different relationships with the kernel Makefiles.
44 *Users* are people who build kernels. These people type commands such as
45 ``make menuconfig`` or ``make``. They usually do not read or edit
46 any kernel Makefiles (or any other source files).
48 *Normal developers* are people who work on features such as device
49 drivers, file systems, and network protocols. These people need to
50 maintain the kbuild Makefiles for the subsystem they are
51 working on. In order to do this effectively, they need some overall
52 knowledge about the kernel Makefiles, plus detailed knowledge about the
53 public interface for kbuild.
55 *Arch developers* are people who work on an entire architecture, such
56 as sparc or x86. Arch developers need to know about the arch Makefile
57 as well as kbuild Makefiles.
59 *Kbuild developers* are people who work on the kernel build system itself.
60 These people need to know about all aspects of the kernel Makefiles.
62 This document is aimed towards normal developers and arch developers.
68 Most Makefiles within the kernel are kbuild Makefiles that use the
69 kbuild infrastructure. This chapter introduces the syntax used in the
72 The preferred name for the kbuild files are ``Makefile`` but ``Kbuild`` can
73 be used and if both a ``Makefile`` and a ``Kbuild`` file exists, then the ``Kbuild``
76 Section `Goal definitions`_ is a quick intro; further chapters provide
77 more details, with real examples.
82 Goal definitions are the main part (heart) of the kbuild Makefile.
83 These lines define the files to be built, any special compilation
84 options, and any subdirectories to be entered recursively.
86 The most simple kbuild makefile contains one line:
92 This tells kbuild that there is one object in that directory, named
93 foo.o. foo.o will be built from foo.c or foo.S.
95 If foo.o shall be built as a module, the variable obj-m is used.
96 Therefore the following pattern is often used:
100 obj-$(CONFIG_FOO) += foo.o
102 $(CONFIG_FOO) evaluates to either y (for built-in) or m (for module).
103 If CONFIG_FOO is neither y nor m, then the file will not be compiled
106 Built-in object goals - obj-y
107 -----------------------------
109 The kbuild Makefile specifies object files for vmlinux
110 in the $(obj-y) lists. These lists depend on the kernel
113 Kbuild compiles all the $(obj-y) files. It then calls
114 ``$(AR) rcSTP`` to merge these files into one built-in.a file.
115 This is a thin archive without a symbol table. It will be later
116 linked into vmlinux by scripts/link-vmlinux.sh
118 The order of files in $(obj-y) is significant. Duplicates in
119 the lists are allowed: the first instance will be linked into
120 built-in.a and succeeding instances will be ignored.
122 Link order is significant, because certain functions
123 (module_init() / __initcall) will be called during boot in the
124 order they appear. So keep in mind that changing the link
125 order may e.g. change the order in which your SCSI
126 controllers are detected, and thus your disks are renumbered.
130 #drivers/isdn/i4l/Makefile
131 # Makefile for the kernel ISDN subsystem and device drivers.
132 # Each configuration option enables a list of files.
133 obj-$(CONFIG_ISDN_I4L) += isdn.o
134 obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
136 Loadable module goals - obj-m
137 -----------------------------
139 $(obj-m) specifies object files which are built as loadable
142 A module may be built from one source file or several source
143 files. In the case of one source file, the kbuild makefile
144 simply adds the file to $(obj-m).
148 #drivers/isdn/i4l/Makefile
149 obj-$(CONFIG_ISDN_PPP_BSDCOMP) += isdn_bsdcomp.o
151 Note: In this example $(CONFIG_ISDN_PPP_BSDCOMP) evaluates to "m"
153 If a kernel module is built from several source files, you specify
154 that you want to build a module in the same way as above; however,
155 kbuild needs to know which object files you want to build your
156 module from, so you have to tell it by setting a $(<module_name>-y)
161 #drivers/isdn/i4l/Makefile
162 obj-$(CONFIG_ISDN_I4L) += isdn.o
163 isdn-y := isdn_net_lib.o isdn_v110.o isdn_common.o
165 In this example, the module name will be isdn.o. Kbuild will
166 compile the objects listed in $(isdn-y) and then run
167 ``$(LD) -r`` on the list of these files to generate isdn.o.
169 Due to kbuild recognizing $(<module_name>-y) for composite objects,
170 you can use the value of a ``CONFIG_`` symbol to optionally include an
171 object file as part of a composite object.
176 obj-$(CONFIG_EXT2_FS) += ext2.o
177 ext2-y := balloc.o dir.o file.o ialloc.o inode.o ioctl.o \
178 namei.o super.o symlink.o
179 ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o xattr_user.o \
182 In this example, xattr.o, xattr_user.o and xattr_trusted.o are only
183 part of the composite object ext2.o if $(CONFIG_EXT2_FS_XATTR)
186 Note: Of course, when you are building objects into the kernel,
187 the syntax above will also work. So, if you have CONFIG_EXT2_FS=y,
188 kbuild will build an ext2.o file for you out of the individual
189 parts and then link this into built-in.a, as you would expect.
191 Library file goals - lib-y
192 --------------------------
194 Objects listed with obj-* are used for modules, or
195 combined in a built-in.a for that specific directory.
196 There is also the possibility to list objects that will
197 be included in a library, lib.a.
198 All objects listed with lib-y are combined in a single
199 library for that directory.
200 Objects that are listed in obj-y and additionally listed in
201 lib-y will not be included in the library, since they will
202 be accessible anyway.
203 For consistency, objects listed in lib-m will be included in lib.a.
205 Note that the same kbuild makefile may list files to be built-in
206 and to be part of a library. Therefore the same directory
207 may contain both a built-in.a and a lib.a file.
211 #arch/x86/lib/Makefile
214 This will create a library lib.a based on delay.o. For kbuild to
215 actually recognize that there is a lib.a being built, the directory
216 shall be listed in libs-y.
218 See also `List directories to visit when descending`_.
220 Use of lib-y is normally restricted to ``lib/`` and ``arch/*/lib``.
222 Descending down in directories
223 ------------------------------
225 A Makefile is only responsible for building objects in its own
226 directory. Files in subdirectories should be taken care of by
227 Makefiles in these subdirs. The build system will automatically
228 invoke make recursively in subdirectories, provided you let it know of
231 To do so, obj-y and obj-m are used.
232 ext2 lives in a separate directory, and the Makefile present in fs/
233 tells kbuild to descend down using the following assignment.
238 obj-$(CONFIG_EXT2_FS) += ext2/
240 If CONFIG_EXT2_FS is set to either "y" (built-in) or "m" (modular)
241 the corresponding obj- variable will be set, and kbuild will descend
242 down in the ext2 directory.
244 Kbuild uses this information not only to decide that it needs to visit
245 the directory, but also to decide whether or not to link objects from
246 the directory into vmlinux.
248 When Kbuild descends into the directory with "y", all built-in objects
249 from that directory are combined into the built-in.a, which will be
250 eventually linked into vmlinux.
252 When Kbuild descends into the directory with "m", in contrast, nothing
253 from that directory will be linked into vmlinux. If the Makefile in
254 that directory specifies obj-y, those objects will be left orphan.
255 It is very likely a bug of the Makefile or of dependencies in Kconfig.
257 Kbuild also supports dedicated syntax, subdir-y and subdir-m, for
258 descending into subdirectories. It is a good fit when you know they
259 do not contain kernel-space objects at all. A typical usage is to let
260 Kbuild descend into subdirectories to build tools.
265 subdir-$(CONFIG_GCC_PLUGINS) += gcc-plugins
266 subdir-$(CONFIG_MODVERSIONS) += genksyms
267 subdir-$(CONFIG_SECURITY_SELINUX) += selinux
269 Unlike obj-y/m, subdir-y/m does not need the trailing slash since this
270 syntax is always used for directories.
272 It is good practice to use a ``CONFIG_`` variable when assigning directory
273 names. This allows kbuild to totally skip the directory if the
274 corresponding ``CONFIG_`` option is neither "y" nor "m".
276 Non-builtin vmlinux targets - extra-y
277 -------------------------------------
279 extra-y specifies targets which are needed for building vmlinux,
280 but not combined into built-in.a.
284 1) vmlinux linker script
286 The linker script for vmlinux is located at
287 arch/$(SRCARCH)/kernel/vmlinux.lds
291 # arch/x86/kernel/Makefile
292 extra-y += vmlinux.lds
294 $(extra-y) should only contain targets needed for vmlinux.
296 Kbuild skips extra-y when vmlinux is apparently not a final goal.
297 (e.g. ``make modules``, or building external modules)
299 If you intend to build targets unconditionally, always-y (explained
300 in the next section) is the correct syntax to use.
302 Always built goals - always-y
303 -----------------------------
305 always-y specifies targets which are literally always built when
306 Kbuild visits the Makefile.
311 offsets-file := include/generated/asm-offsets.h
312 always-y += $(offsets-file)
317 ccflags-y, asflags-y and ldflags-y
318 These three flags apply only to the kbuild makefile in which they
319 are assigned. They are used for all the normal cc, as and ld
320 invocations happening during a recursive build.
321 Note: Flags with the same behaviour were previously named:
322 EXTRA_CFLAGS, EXTRA_AFLAGS and EXTRA_LDFLAGS.
323 They are still supported but their usage is deprecated.
325 ccflags-y specifies options for compiling with $(CC).
329 # drivers/acpi/acpica/Makefile
330 ccflags-y := -Os -D_LINUX -DBUILDING_ACPICA
331 ccflags-$(CONFIG_ACPI_DEBUG) += -DACPI_DEBUG_OUTPUT
333 This variable is necessary because the top Makefile owns the
334 variable $(KBUILD_CFLAGS) and uses it for compilation flags for the
337 asflags-y specifies assembler options.
341 #arch/sparc/kernel/Makefile
344 ldflags-y specifies options for linking with $(LD).
348 #arch/cris/boot/compressed/Makefile
349 ldflags-y += -T $(srctree)/$(src)/decompress_$(arch-y).lds
351 subdir-ccflags-y, subdir-asflags-y
352 The two flags listed above are similar to ccflags-y and asflags-y.
353 The difference is that the subdir- variants have effect for the kbuild
354 file where they are present and all subdirectories.
355 Options specified using subdir-* are added to the commandline before
356 the options specified using the non-subdir variants.
360 subdir-ccflags-y := -Werror
362 ccflags-remove-y, asflags-remove-y
363 These flags are used to remove particular flags for the compiler,
364 assembler invocations.
368 ccflags-remove-$(CONFIG_MCOUNT) += -pg
371 CFLAGS_$@ and AFLAGS_$@ only apply to commands in current
374 $(CFLAGS_$@) specifies per-file options for $(CC). The $@
375 part has a literal value which specifies the file that it is for.
377 CFLAGS_$@ has the higher priority than ccflags-remove-y; CFLAGS_$@
378 can re-add compiler flags that were removed by ccflags-remove-y.
382 # drivers/scsi/Makefile
383 CFLAGS_aha152x.o = -DAHA152X_STAT -DAUTOCONF
385 This line specify compilation flags for aha152x.o.
387 $(AFLAGS_$@) is a similar feature for source files in assembly
390 AFLAGS_$@ has the higher priority than asflags-remove-y; AFLAGS_$@
391 can re-add assembler flags that were removed by asflags-remove-y.
395 # arch/arm/kernel/Makefile
396 AFLAGS_head.o := -DTEXT_OFFSET=$(TEXT_OFFSET)
397 AFLAGS_crunch-bits.o := -Wa,-mcpu=ep9312
398 AFLAGS_iwmmxt.o := -Wa,-mcpu=iwmmxt
403 Kbuild tracks dependencies on the following:
405 1) All prerequisite files (both ``*.c`` and ``*.h``)
406 2) ``CONFIG_`` options used in all prerequisite files
407 3) Command-line used to compile target
409 Thus, if you change an option to $(CC) all affected files will
415 Custom rules are used when the kbuild infrastructure does
416 not provide the required support. A typical example is
417 header files generated during the build process.
418 Another example are the architecture-specific Makefiles which
419 need custom rules to prepare boot images etc.
421 Custom rules are written as normal Make rules.
422 Kbuild is not executing in the directory where the Makefile is
423 located, so all custom rules shall use a relative
424 path to prerequisite files and target files.
426 Two variables are used when defining custom rules:
429 $(src) is a relative path which points to the directory
430 where the Makefile is located. Always use $(src) when
431 referring to files located in the src tree.
434 $(obj) is a relative path which points to the directory
435 where the target is saved. Always use $(obj) when
436 referring to generated files.
440 #drivers/scsi/Makefile
441 $(obj)/53c8xx_d.h: $(src)/53c7,8xx.scr $(src)/script_asm.pl
442 $(CPP) -DCHIP=810 - < $< | ... $(src)/script_asm.pl
444 This is a custom rule, following the normal syntax
447 The target file depends on two prerequisite files. References
448 to the target file are prefixed with $(obj), references
449 to prerequisites are referenced with $(src) (because they are not
453 echoing information to user in a rule is often a good practice
454 but when execution ``make -s`` one does not expect to see any output
455 except for warnings/errors.
456 To support this kbuild defines $(kecho) which will echo out the
457 text following $(kecho) to stdout except if ``make -s`` is used.
462 $(BOOT_TARGETS): vmlinux
463 $(Q)$(MAKE) $(build)=$(boot) MACHINE=$(MACHINE) $(boot)/$@
464 @$(kecho) ' Kernel: $(boot)/$@ is ready'
466 When kbuild is executing with KBUILD_VERBOSE unset, then only a shorthand
467 of a command is normally displayed.
468 To enable this behaviour for custom commands kbuild requires
469 two variables to be set::
471 quiet_cmd_<command> - what shall be echoed
472 cmd_<command> - the command to execute
477 quiet_cmd_crc32 = GEN $@
480 $(obj)/crc32table.h: $(obj)/gen_crc32table
483 When updating the $(obj)/crc32table.h target, the line::
487 will be displayed with ``make KBUILD_VERBOSE=``.
489 Command change detection
490 ------------------------
492 When the rule is evaluated, timestamps are compared between the target
493 and its prerequisite files. GNU Make updates the target when any of the
494 prerequisites is newer than that.
496 The target should be rebuilt also when the command line has changed
497 since the last invocation. This is not supported by Make itself, so
498 Kbuild achieves this by a kind of meta-programming.
500 if_changed is the macro used for this purpose, in the following form::
502 quiet_cmd_<command> = ...
505 <target>: <source(s)> FORCE
506 $(call if_changed,<command>)
508 Any target that utilizes if_changed must be listed in $(targets),
509 otherwise the command line check will fail, and the target will
512 If the target is already listed in the recognized syntax such as
513 obj-y/m, lib-y/m, extra-y/m, always-y/m, hostprogs, userprogs, Kbuild
514 automatically adds it to $(targets). Otherwise, the target must be
515 explicitly added to $(targets).
517 Assignments to $(targets) are without $(obj)/ prefix. if_changed may be
518 used in conjunction with custom rules as defined in `Custom Rules`_.
520 Note: It is a typical mistake to forget the FORCE prerequisite.
521 Another common pitfall is that whitespace is sometimes significant; for
522 instance, the below will fail (note the extra space after the comma)::
524 target: source(s) FORCE
526 **WRONG!** $(call if_changed, objcopy)
529 if_changed should not be used more than once per target.
530 It stores the executed command in a corresponding .cmd
531 file and multiple calls would result in overwrites and
532 unwanted results when the target is up to date and only the
533 tests on changed commands trigger execution of commands.
535 $(CC) support functions
536 -----------------------
538 The kernel may be built with several different versions of
539 $(CC), each supporting a unique set of features and options.
540 kbuild provides basic support to check for valid options for $(CC).
541 $(CC) is usually the gcc compiler, but other alternatives are
545 as-option is used to check if $(CC) -- when used to compile
546 assembler (``*.S``) files -- supports the given option. An optional
547 second option may be specified if the first option is not supported.
552 cflags-y += $(call as-option,-Wa$(comma)-isa=$(isa-y),)
554 In the above example, cflags-y will be assigned the option
555 -Wa$(comma)-isa=$(isa-y) if it is supported by $(CC).
556 The second argument is optional, and if supplied will be used
557 if first argument is not supported.
560 as-instr checks if the assembler reports a specific instruction
561 and then outputs either option1 or option2
562 C escapes are supported in the test instruction
563 Note: as-instr-option uses KBUILD_AFLAGS for assembler options
566 cc-option is used to check if $(CC) supports a given option, and if
567 not supported to use an optional second option.
572 cflags-y += $(call cc-option,-march=pentium-mmx,-march=i586)
574 In the above example, cflags-y will be assigned the option
575 -march=pentium-mmx if supported by $(CC), otherwise -march=i586.
576 The second argument to cc-option is optional, and if omitted,
577 cflags-y will be assigned no value if first option is not supported.
578 Note: cc-option uses KBUILD_CFLAGS for $(CC) options
581 cc-option-yn is used to check if gcc supports a given option
582 and return "y" if supported, otherwise "n".
587 biarch := $(call cc-option-yn, -m32)
588 aflags-$(biarch) += -a32
589 cflags-$(biarch) += -m32
591 In the above example, $(biarch) is set to y if $(CC) supports the -m32
592 option. When $(biarch) equals "y", the expanded variables $(aflags-y)
593 and $(cflags-y) will be assigned the values -a32 and -m32,
596 Note: cc-option-yn uses KBUILD_CFLAGS for $(CC) options
599 cc-disable-warning checks if gcc supports a given warning and returns
600 the commandline switch to disable it. This special function is needed,
601 because gcc 4.4 and later accept any unknown -Wno-* option and only
602 warn about it if there is another warning in the source file.
606 KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
608 In the above example, -Wno-unused-but-set-variable will be added to
609 KBUILD_CFLAGS only if gcc really accepts it.
612 gcc-min-version tests if the value of $(CONFIG_GCC_VERSION) is greater than
613 or equal to the provided value and evaluates to y if so.
617 cflags-$(call gcc-min-version, 70100) := -foo
619 In this example, cflags-y will be assigned the value -foo if $(CC) is gcc and
620 $(CONFIG_GCC_VERSION) is >= 7.1.
623 clang-min-version tests if the value of $(CONFIG_CLANG_VERSION) is greater
624 than or equal to the provided value and evaluates to y if so.
628 cflags-$(call clang-min-version, 110000) := -foo
630 In this example, cflags-y will be assigned the value -foo if $(CC) is clang
631 and $(CONFIG_CLANG_VERSION) is >= 11.0.0.
634 cc-cross-prefix is used to check if there exists a $(CC) in path with
635 one of the listed prefixes. The first prefix where there exist a
636 prefix$(CC) in the PATH is returned - and if no prefix$(CC) is found
637 then nothing is returned.
639 Additional prefixes are separated by a single space in the
640 call of cc-cross-prefix.
642 This functionality is useful for architecture Makefiles that try
643 to set CROSS_COMPILE to well-known values but may have several
644 values to select between.
646 It is recommended only to try to set CROSS_COMPILE if it is a cross
647 build (host arch is different from target arch). And if CROSS_COMPILE
648 is already set then leave it with the old value.
653 ifneq ($(SUBARCH),$(ARCH))
654 ifeq ($(CROSS_COMPILE),)
655 CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu-)
659 $(LD) support functions
660 -----------------------
663 ld-option is used to check if $(LD) supports the supplied option.
664 ld-option takes two options as arguments.
666 The second argument is an optional option that can be used if the
667 first option is not supported by $(LD).
672 LDFLAGS_vmlinux += $(call ld-option, -X)
677 Make rules may invoke scripts to build the kernel. The rules shall
678 always provide the appropriate interpreter to execute the script. They
679 shall not rely on the execute bits being set, and shall not invoke the
680 script directly. For the convenience of manual script invocation, such
681 as invoking ./scripts/checkpatch.pl, it is recommended to set execute
682 bits on the scripts nonetheless.
684 Kbuild provides variables $(CONFIG_SHELL), $(AWK), $(PERL),
685 and $(PYTHON3) to refer to interpreters for the respective
691 cmd_depmod = $(CONFIG_SHELL) $(srctree)/scripts/depmod.sh $(DEPMOD) \
697 Kbuild supports building executables on the host for use during the
700 Two steps are required in order to use a host executable.
702 The first step is to tell kbuild that a host program exists. This is
703 done utilising the variable ``hostprogs``.
705 The second step is to add an explicit dependency to the executable.
706 This can be done in two ways. Either add the dependency in a rule,
707 or utilise the variable ``always-y``.
708 Both possibilities are described in the following.
713 In some cases there is a need to compile and run a program on the
714 computer where the build is running.
716 The following line tells kbuild that the program bin2hex shall be
717 built on the build host.
723 Kbuild assumes in the above example that bin2hex is made from a single
724 c-source file named bin2hex.c located in the same directory as
727 Composite Host Programs
728 -----------------------
730 Host programs can be made up based on composite objects.
731 The syntax used to define composite objects for host programs is
732 similar to the syntax used for kernel objects.
733 $(<executable>-objs) lists all objects used to link the final
738 #scripts/lxdialog/Makefile
739 hostprogs := lxdialog
740 lxdialog-objs := checklist.o lxdialog.o
742 Objects with extension .o are compiled from the corresponding .c
743 files. In the above example, checklist.c is compiled to checklist.o
744 and lxdialog.c is compiled to lxdialog.o.
746 Finally, the two .o files are linked to the executable, lxdialog.
747 Note: The syntax <executable>-y is not permitted for host-programs.
749 Using C++ for host programs
750 ---------------------------
752 kbuild offers support for host programs written in C++. This was
753 introduced solely to support kconfig, and is not recommended
758 #scripts/kconfig/Makefile
760 qconf-cxxobjs := qconf.o
762 In the example above the executable is composed of the C++ file
763 qconf.cc - identified by $(qconf-cxxobjs).
765 If qconf is composed of a mixture of .c and .cc files, then an
766 additional line can be used to identify this.
770 #scripts/kconfig/Makefile
772 qconf-cxxobjs := qconf.o
773 qconf-objs := check.o
775 Using Rust for host programs
776 ----------------------------
778 Kbuild offers support for host programs written in Rust. However,
779 since a Rust toolchain is not mandatory for kernel compilation,
780 it may only be used in scenarios where Rust is required to be
781 available (e.g. when ``CONFIG_RUST`` is enabled).
788 Kbuild will compile ``target`` using ``target.rs`` as the crate root,
789 located in the same directory as the ``Makefile``. The crate may
790 consist of several source files (see ``samples/rust/hostprogs``).
792 Controlling compiler options for host programs
793 ----------------------------------------------
795 When compiling host programs, it is possible to set specific flags.
796 The programs will always be compiled utilising $(HOSTCC) passed
797 the options specified in $(KBUILD_HOSTCFLAGS).
799 To set flags that will take effect for all host programs created
800 in that Makefile, use the variable HOST_EXTRACFLAGS.
804 #scripts/lxdialog/Makefile
805 HOST_EXTRACFLAGS += -I/usr/include/ncurses
807 To set specific flags for a single file the following construction
812 #arch/ppc64/boot/Makefile
813 HOSTCFLAGS_piggyback.o := -DKERNELBASE=$(KERNELBASE)
815 It is also possible to specify additional options to the linker.
819 #scripts/kconfig/Makefile
820 HOSTLDLIBS_qconf := -L$(QTDIR)/lib
822 When linking qconf, it will be passed the extra option
825 When host programs are actually built
826 -------------------------------------
828 Kbuild will only build host-programs when they are referenced
831 This is possible in two ways:
833 (1) List the prerequisite explicitly in a custom rule.
837 #drivers/pci/Makefile
838 hostprogs := gen-devlist
839 $(obj)/devlist.h: $(src)/pci.ids $(obj)/gen-devlist
840 ( cd $(obj); ./gen-devlist ) < $<
842 The target $(obj)/devlist.h will not be built before
843 $(obj)/gen-devlist is updated. Note that references to
844 the host programs in custom rules must be prefixed with $(obj).
848 When there is no suitable custom rule, and the host program
849 shall be built when a makefile is entered, the always-y
850 variable shall be used.
854 #scripts/lxdialog/Makefile
855 hostprogs := lxdialog
856 always-y := $(hostprogs)
858 Kbuild provides the following shorthand for this::
860 hostprogs-always-y := lxdialog
862 This will tell kbuild to build lxdialog even if not referenced in
865 Userspace Program support
866 =========================
868 Just like host programs, Kbuild also supports building userspace executables
869 for the target architecture (i.e. the same architecture as you are building
872 The syntax is quite similar. The difference is to use ``userprogs`` instead of
875 Simple Userspace Program
876 ------------------------
878 The following line tells kbuild that the program bpf-direct shall be
879 built for the target architecture.
883 userprogs := bpf-direct
885 Kbuild assumes in the above example that bpf-direct is made from a
886 single C source file named bpf-direct.c located in the same directory
889 Composite Userspace Programs
890 ----------------------------
892 Userspace programs can be made up based on composite objects.
893 The syntax used to define composite objects for userspace programs is
894 similar to the syntax used for kernel objects.
895 $(<executable>-objs) lists all objects used to link the final
900 #samples/seccomp/Makefile
901 userprogs := bpf-fancy
902 bpf-fancy-objs := bpf-fancy.o bpf-helper.o
904 Objects with extension .o are compiled from the corresponding .c
905 files. In the above example, bpf-fancy.c is compiled to bpf-fancy.o
906 and bpf-helper.c is compiled to bpf-helper.o.
908 Finally, the two .o files are linked to the executable, bpf-fancy.
909 Note: The syntax <executable>-y is not permitted for userspace programs.
911 Controlling compiler options for userspace programs
912 ---------------------------------------------------
914 When compiling userspace programs, it is possible to set specific flags.
915 The programs will always be compiled utilising $(CC) passed
916 the options specified in $(KBUILD_USERCFLAGS).
918 To set flags that will take effect for all userspace programs created
919 in that Makefile, use the variable userccflags.
923 # samples/seccomp/Makefile
924 userccflags += -I usr/include
926 To set specific flags for a single file the following construction
931 bpf-helper-userccflags += -I user/include
933 It is also possible to specify additional options to the linker.
937 # net/bpfilter/Makefile
938 bpfilter_umh-userldflags += -static
940 To specify libraries linked to a userspace program, you can use
941 ``<executable>-userldlibs``. The ``userldlibs`` syntax specifies libraries
942 linked to all userspace programs created in the current Makefile.
944 When linking bpfilter_umh, it will be passed the extra option -static.
946 From command line, :ref:`USERCFLAGS and USERLDFLAGS <userkbuildflags>` will also be used.
948 When userspace programs are actually built
949 ------------------------------------------
951 Kbuild builds userspace programs only when told to do so.
952 There are two ways to do this.
954 (1) Add it as the prerequisite of another file
958 #net/bpfilter/Makefile
959 userprogs := bpfilter_umh
960 $(obj)/bpfilter_umh_blob.o: $(obj)/bpfilter_umh
962 $(obj)/bpfilter_umh is built before $(obj)/bpfilter_umh_blob.o
968 userprogs := binderfs_example
969 always-y := $(userprogs)
971 Kbuild provides the following shorthand for this::
973 userprogs-always-y := binderfs_example
975 This will tell Kbuild to build binderfs_example when it visits this
978 Kbuild clean infrastructure
979 ===========================
981 ``make clean`` deletes most generated files in the obj tree where the kernel
982 is compiled. This includes generated files such as host programs.
983 Kbuild knows targets listed in $(hostprogs), $(always-y), $(always-m),
984 $(always-), $(extra-y), $(extra-) and $(targets). They are all deleted
985 during ``make clean``. Files matching the patterns ``*.[oas]``, ``*.ko``, plus
986 some additional files generated by kbuild are deleted all over the kernel
987 source tree when ``make clean`` is executed.
989 Additional files or directories can be specified in kbuild makefiles by use of
995 clean-files := crc32table.h
997 When executing ``make clean``, the file ``crc32table.h`` will be deleted.
998 Kbuild will assume files to be in the same relative directory as the
1001 To exclude certain files or directories from make clean, use the
1002 $(no-clean-files) variable.
1004 Usually kbuild descends down in subdirectories due to ``obj-* := dir/``,
1005 but in the architecture makefiles where the kbuild infrastructure
1006 is not sufficient this sometimes needs to be explicit.
1010 #arch/x86/boot/Makefile
1011 subdir- := compressed
1013 The above assignment instructs kbuild to descend down in the
1014 directory compressed/ when ``make clean`` is executed.
1016 Note 1: arch/$(SRCARCH)/Makefile cannot use ``subdir-``, because that file is
1017 included in the top level makefile. Instead, arch/$(SRCARCH)/Kbuild can use
1020 Note 2: All directories listed in core-y, libs-y, drivers-y and net-y will
1021 be visited during ``make clean``.
1023 Architecture Makefiles
1024 ======================
1026 The top level Makefile sets up the environment and does the preparation,
1027 before starting to descend down in the individual directories.
1029 The top level makefile contains the generic part, whereas
1030 arch/$(SRCARCH)/Makefile contains what is required to set up kbuild
1031 for said architecture.
1033 To do so, arch/$(SRCARCH)/Makefile sets up a number of variables and defines
1036 When kbuild executes, the following steps are followed (roughly):
1038 1) Configuration of the kernel => produce .config
1040 2) Store kernel version in include/linux/version.h
1042 3) Updating all other prerequisites to the target prepare:
1044 - Additional prerequisites are specified in arch/$(SRCARCH)/Makefile
1046 4) Recursively descend down in all directories listed in
1047 init-* core* drivers-* net-* libs-* and build all targets.
1049 - The values of the above variables are expanded in arch/$(SRCARCH)/Makefile.
1051 5) All object files are then linked and the resulting file vmlinux is
1052 located at the root of the obj tree.
1053 The very first objects linked are listed in scripts/head-object-list.txt.
1055 6) Finally, the architecture-specific part does any required post processing
1056 and builds the final bootimage.
1058 - This includes building boot records
1059 - Preparing initrd images and the like
1061 Set variables to tweak the build to the architecture
1062 ----------------------------------------------------
1065 Generic $(LD) options
1067 Flags used for all invocations of the linker.
1068 Often specifying the emulation is sufficient.
1073 KBUILD_LDFLAGS := -m elf_s390
1075 Note: ldflags-y can be used to further customise
1076 the flags used. See `Non-builtin vmlinux targets - extra-y`_.
1079 Options for $(LD) when linking vmlinux
1081 LDFLAGS_vmlinux is used to specify additional flags to pass to
1082 the linker when linking the final vmlinux image.
1084 LDFLAGS_vmlinux uses the LDFLAGS_$@ support.
1089 LDFLAGS_vmlinux := -e stext
1094 When $(call if_changed,objcopy) is used to translate a .o file,
1095 the flags specified in OBJCOPYFLAGS will be used.
1097 $(call if_changed,objcopy) is often used to generate raw binaries on
1103 OBJCOPYFLAGS := -O binary
1105 #arch/s390/boot/Makefile
1106 $(obj)/image: vmlinux FORCE
1107 $(call if_changed,objcopy)
1109 In this example, the binary $(obj)/image is a binary version of
1110 vmlinux. The usage of $(call if_changed,xxx) will be described later.
1115 Default value - see top level Makefile.
1117 Append or modify as required per architecture.
1121 #arch/sparc64/Makefile
1122 KBUILD_AFLAGS += -m64 -mcpu=ultrasparc
1125 $(CC) compiler flags
1127 Default value - see top level Makefile.
1129 Append or modify as required per architecture.
1131 Often, the KBUILD_CFLAGS variable depends on the configuration.
1135 #arch/x86/boot/compressed/Makefile
1136 cflags-$(CONFIG_X86_32) := -march=i386
1137 cflags-$(CONFIG_X86_64) := -mcmodel=small
1138 KBUILD_CFLAGS += $(cflags-y)
1140 Many arch Makefiles dynamically run the target C compiler to
1141 probe supported options::
1146 cflags-$(CONFIG_MPENTIUMII) += $(call cc-option,\
1147 -march=pentium2,-march=i686)
1149 # Disable unit-at-a-time mode ...
1150 KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time)
1154 The first example utilises the trick that a config option expands
1155 to "y" when selected.
1158 $(RUSTC) compiler flags
1160 Default value - see top level Makefile.
1162 Append or modify as required per architecture.
1164 Often, the KBUILD_RUSTFLAGS variable depends on the configuration.
1166 Note that target specification file generation (for ``--target``)
1167 is handled in ``scripts/generate_rust_target.rs``.
1169 KBUILD_AFLAGS_KERNEL
1170 Assembler options specific for built-in
1172 $(KBUILD_AFLAGS_KERNEL) contains extra C compiler flags used to compile
1173 resident kernel code.
1175 KBUILD_AFLAGS_MODULE
1176 Assembler options specific for modules
1178 $(KBUILD_AFLAGS_MODULE) is used to add arch-specific options that
1179 are used for assembler.
1181 From commandline AFLAGS_MODULE shall be used (see kbuild.rst).
1183 KBUILD_CFLAGS_KERNEL
1184 $(CC) options specific for built-in
1186 $(KBUILD_CFLAGS_KERNEL) contains extra C compiler flags used to compile
1187 resident kernel code.
1189 KBUILD_CFLAGS_MODULE
1190 Options for $(CC) when building modules
1192 $(KBUILD_CFLAGS_MODULE) is used to add arch-specific options that
1195 From commandline CFLAGS_MODULE shall be used (see kbuild.rst).
1197 KBUILD_RUSTFLAGS_KERNEL
1198 $(RUSTC) options specific for built-in
1200 $(KBUILD_RUSTFLAGS_KERNEL) contains extra Rust compiler flags used to
1201 compile resident kernel code.
1203 KBUILD_RUSTFLAGS_MODULE
1204 Options for $(RUSTC) when building modules
1206 $(KBUILD_RUSTFLAGS_MODULE) is used to add arch-specific options that
1207 are used for $(RUSTC).
1209 From commandline RUSTFLAGS_MODULE shall be used (see kbuild.rst).
1211 KBUILD_LDFLAGS_MODULE
1212 Options for $(LD) when linking modules
1214 $(KBUILD_LDFLAGS_MODULE) is used to add arch-specific options
1215 used when linking modules. This is often a linker script.
1217 From commandline LDFLAGS_MODULE shall be used (see kbuild.rst).
1220 The linker script with full path. Assigned by the top-level Makefile.
1223 All object files for vmlinux. They are linked to vmlinux in the same
1224 order as listed in KBUILD_VMLINUX_OBJS.
1226 The objects listed in scripts/head-object-list.txt are exceptions;
1227 they are placed before the other objects.
1230 All .a ``lib`` files for vmlinux. KBUILD_VMLINUX_OBJS and
1231 KBUILD_VMLINUX_LIBS together specify all the object files used to
1234 Add prerequisites to archheaders
1235 --------------------------------
1237 The archheaders: rule is used to generate header files that
1238 may be installed into user space by ``make header_install``.
1240 It is run before ``make archprepare`` when run on the
1241 architecture itself.
1243 Add prerequisites to archprepare
1244 --------------------------------
1246 The archprepare: rule is used to list prerequisites that need to be
1247 built before starting to descend down in the subdirectories.
1249 This is usually used for header files containing assembler constants.
1254 archprepare: maketools
1256 In this example, the file target maketools will be processed
1257 before descending down in the subdirectories.
1259 See also chapter XXX-TODO that describes how kbuild supports
1260 generating offset header files.
1262 List directories to visit when descending
1263 -----------------------------------------
1265 An arch Makefile cooperates with the top Makefile to define variables
1266 which specify how to build the vmlinux file. Note that there is no
1267 corresponding arch-specific section for modules; the module-building
1268 machinery is all architecture-independent.
1270 core-y, libs-y, drivers-y
1271 $(libs-y) lists directories where a lib.a archive can be located.
1273 The rest list directories where a built-in.a object file can be
1276 Then the rest follows in this order:
1278 $(core-y), $(libs-y), $(drivers-y)
1280 The top level Makefile defines values for all generic directories,
1281 and arch/$(SRCARCH)/Makefile only adds architecture-specific
1286 # arch/sparc/Makefile
1287 core-y += arch/sparc/
1289 libs-y += arch/sparc/prom/
1290 libs-y += arch/sparc/lib/
1292 drivers-$(CONFIG_PM) += arch/sparc/power/
1294 Architecture-specific boot images
1295 ---------------------------------
1297 An arch Makefile specifies goals that take the vmlinux file, compress
1298 it, wrap it in bootstrapping code, and copy the resulting files
1299 somewhere. This includes various kinds of installation commands.
1300 The actual goals are not standardized across architectures.
1302 It is common to locate any additional processing in a boot/
1303 directory below arch/$(SRCARCH)/.
1305 Kbuild does not provide any smart way to support building a
1306 target specified in boot/. Therefore arch/$(SRCARCH)/Makefile shall
1307 call make manually to build a target in boot/.
1309 The recommended approach is to include shortcuts in
1310 arch/$(SRCARCH)/Makefile, and use the full path when calling down
1311 into the arch/$(SRCARCH)/boot/Makefile.
1316 boot := arch/x86/boot
1318 $(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
1320 ``$(Q)$(MAKE) $(build)=<dir>`` is the recommended way to invoke
1321 make in a subdirectory.
1323 There are no rules for naming architecture-specific targets,
1324 but executing ``make help`` will list all relevant targets.
1325 To support this, $(archhelp) must be defined.
1331 echo '* bzImage - Compressed kernel image (arch/x86/boot/bzImage)'
1334 When make is executed without arguments, the first goal encountered
1335 will be built. In the top level Makefile the first goal present
1338 An architecture shall always, per default, build a bootable image.
1339 In ``make help``, the default goal is highlighted with a ``*``.
1341 Add a new prerequisite to all: to select a default goal different
1349 When ``make`` is executed without arguments, bzImage will be built.
1351 Commands useful for building a boot image
1352 -----------------------------------------
1354 Kbuild provides a few macros that are useful when building a
1358 Link target. Often, LDFLAGS_$@ is used to set specific options to ld.
1362 #arch/x86/boot/Makefile
1363 LDFLAGS_bootsect := -Ttext 0x0 -s --oformat binary
1364 LDFLAGS_setup := -Ttext 0x0 -s --oformat binary -e begtext
1366 targets += setup setup.o bootsect bootsect.o
1367 $(obj)/setup $(obj)/bootsect: %: %.o FORCE
1368 $(call if_changed,ld)
1370 In this example, there are two possible targets, requiring different
1371 options to the linker. The linker options are specified using the
1372 LDFLAGS_$@ syntax - one for each potential target.
1374 $(targets) are assigned all potential targets, by which kbuild knows
1375 the targets and will:
1377 1) check for commandline changes
1378 2) delete target during make clean
1380 The ``: %: %.o`` part of the prerequisite is a shorthand that
1381 frees us from listing the setup.o and bootsect.o files.
1384 It is a common mistake to forget the ``targets :=`` assignment,
1385 resulting in the target file being recompiled for no
1389 Copy binary. Uses OBJCOPYFLAGS usually specified in
1390 arch/$(SRCARCH)/Makefile.
1392 OBJCOPYFLAGS_$@ may be used to set additional options.
1395 Compress target. Use maximum compression to compress target.
1399 #arch/x86/boot/compressed/Makefile
1400 $(obj)/vmlinux.bin.gz: $(vmlinux.bin.all-y) FORCE
1401 $(call if_changed,gzip)
1404 Create flattened device tree blob object suitable for linking
1405 into vmlinux. Device tree blobs linked into vmlinux are placed
1406 in an init section in the image. Platform code *must* copy the
1407 blob to non-init memory prior to calling unflatten_device_tree().
1409 To use this command, simply add ``*.dtb`` into obj-y or targets, or make
1410 some other target depend on ``%.dtb``
1412 A central rule exists to create ``$(obj)/%.dtb`` from ``$(src)/%.dts``;
1413 architecture Makefiles do no need to explicitly write out that rule.
1418 DTC_FLAGS ?= -p 1024
1420 Preprocessing linker scripts
1421 ----------------------------
1423 When the vmlinux image is built, the linker script
1424 arch/$(SRCARCH)/kernel/vmlinux.lds is used.
1426 The script is a preprocessed variant of the file vmlinux.lds.S
1427 located in the same directory.
1429 kbuild knows .lds files and includes a rule ``*lds.S`` -> ``*lds``.
1433 #arch/x86/kernel/Makefile
1434 extra-y := vmlinux.lds
1436 The assignment to extra-y is used to tell kbuild to build the
1439 The assignment to $(CPPFLAGS_vmlinux.lds) tells kbuild to use the
1440 specified options when building the target vmlinux.lds.
1442 When building the ``*.lds`` target, kbuild uses the variables::
1444 KBUILD_CPPFLAGS : Set in top-level Makefile
1445 cppflags-y : May be set in the kbuild makefile
1446 CPPFLAGS_$(@F) : Target-specific flags.
1447 Note that the full filename is used in this
1450 The kbuild infrastructure for ``*lds`` files is used in several
1451 architecture-specific files.
1453 Generic header files
1454 --------------------
1456 The directory include/asm-generic contains the header files
1457 that may be shared between individual architectures.
1459 The recommended approach how to use a generic header file is
1460 to list the file in the Kbuild file.
1462 See `generic-y`_ for further info on syntax etc.
1467 If the file arch/xxx/Makefile.postlink exists, this makefile
1468 will be invoked for post-link objects (vmlinux and modules.ko)
1469 for architectures to run post-link passes on. Must also handle
1472 This pass runs after kallsyms generation. If the architecture
1473 needs to modify symbol locations, rather than manipulate the
1474 kallsyms, it may be easier to add another postlink target for
1475 .tmp_vmlinux? targets to be called from link-vmlinux.sh.
1477 For example, powerpc uses this to check relocation sanity of
1478 the linked vmlinux file.
1480 Kbuild syntax for exported headers
1481 ==================================
1483 The kernel includes a set of headers that is exported to userspace.
1484 Many headers can be exported as-is but other headers require a
1485 minimal pre-processing before they are ready for user-space.
1487 The pre-processing does:
1489 - drop kernel-specific annotations
1490 - drop include of compiler.h
1491 - drop all sections that are kernel internal (guarded by ``ifdef __KERNEL__``)
1493 All headers under include/uapi/, include/generated/uapi/,
1494 arch/<arch>/include/uapi/ and arch/<arch>/include/generated/uapi/
1497 A Kbuild file may be defined under arch/<arch>/include/uapi/asm/ and
1498 arch/<arch>/include/asm/ to list asm files coming from asm-generic.
1500 See subsequent chapter for the syntax of the Kbuild file.
1505 no-export-headers is essentially used by include/uapi/linux/Kbuild to
1506 avoid exporting specific headers (e.g. kvm.h) on architectures that do
1507 not support it. It should be avoided as much as possible.
1512 If an architecture uses a verbatim copy of a header from
1513 include/asm-generic then this is listed in the file
1514 arch/$(SRCARCH)/include/asm/Kbuild like this:
1518 #arch/x86/include/asm/Kbuild
1519 generic-y += termios.h
1522 During the prepare phase of the build a wrapper include
1523 file is generated in the directory::
1525 arch/$(SRCARCH)/include/generated/asm
1527 When a header is exported where the architecture uses
1528 the generic header a similar wrapper is generated as part
1529 of the set of exported headers in the directory::
1533 The generated wrapper will in both cases look like the following:
1535 Example: termios.h::
1537 #include <asm-generic/termios.h>
1542 If an architecture generates other header files alongside generic-y
1543 wrappers, generated-y specifies them.
1545 This prevents them being treated as stale asm-generic wrappers and
1550 #arch/x86/include/asm/Kbuild
1551 generated-y += syscalls_32.h
1556 mandatory-y is essentially used by include/(uapi/)asm-generic/Kbuild
1557 to define the minimum set of ASM headers that all architectures must have.
1559 This works like optional generic-y. If a mandatory header is missing
1560 in arch/$(SRCARCH)/include/(uapi/)/asm, Kbuild will automatically
1561 generate a wrapper of the asm-generic one.
1566 The top Makefile exports the following variables:
1568 VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION
1569 These variables define the current kernel version. A few arch
1570 Makefiles actually use these values directly; they should use
1571 $(KERNELRELEASE) instead.
1573 $(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic
1574 three-part version number, such as "2", "4", and "0". These three
1575 values are always numeric.
1577 $(EXTRAVERSION) defines an even tinier sublevel for pre-patches
1578 or additional patches. It is usually some non-numeric string
1579 such as "-pre4", and is often blank.
1582 $(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable
1583 for constructing installation directory names or showing in
1584 version strings. Some arch Makefiles use it for this purpose.
1587 This variable defines the target architecture, such as "i386",
1588 "arm", or "sparc". Some kbuild Makefiles test $(ARCH) to
1589 determine which files to compile.
1591 By default, the top Makefile sets $(ARCH) to be the same as the
1592 host system architecture. For a cross build, a user may
1593 override the value of $(ARCH) on the command line::
1598 This variable specifies the directory in arch/ to build.
1600 ARCH and SRCARCH may not necessarily match. A couple of arch
1601 directories are biarch, that is, a single ``arch/*/`` directory supports
1602 both 32-bit and 64-bit.
1604 For example, you can pass in ARCH=i386, ARCH=x86_64, or ARCH=x86.
1605 For all of them, SRCARCH=x86 because arch/x86/ supports both i386 and
1609 This variable defines a place for the arch Makefiles to install
1610 the resident kernel image and System.map file.
1611 Use this for architecture-specific install targets.
1613 INSTALL_MOD_PATH, MODLIB
1614 $(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module
1615 installation. This variable is not defined in the Makefile but
1616 may be passed in by the user if desired.
1618 $(MODLIB) specifies the directory for module installation.
1619 The top Makefile defines $(MODLIB) to
1620 $(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE). The user may
1621 override this value on the command line if desired.
1624 If this variable is specified, it will cause modules to be stripped
1625 after they are installed. If INSTALL_MOD_STRIP is "1", then the
1626 default option --strip-debug will be used. Otherwise, the
1627 INSTALL_MOD_STRIP value will be used as the option(s) to the strip
1631 This variable specifies a prefix for relocations required by build
1632 roots. It defines a place for installing the device tree blobs. Like
1633 INSTALL_MOD_PATH, it isn't defined in the Makefile, but can be passed
1634 by the user if desired. Otherwise it defaults to the kernel install
1640 The kernel Makefiles are designed to be run with GNU Make. The Makefiles
1641 use only the documented features of GNU Make, but they do use many
1644 GNU Make supports elementary list-processing functions. The kernel
1645 Makefiles use a novel style of list building and manipulation with few
1648 GNU Make has two assignment operators, ``:=`` and ``=``. ``:=`` performs
1649 immediate evaluation of the right-hand side and stores an actual string
1650 into the left-hand side. ``=`` is like a formula definition; it stores the
1651 right-hand side in an unevaluated form and then evaluates this form each
1652 time the left-hand side is used.
1654 There are some cases where ``=`` is appropriate. Usually, though, ``:=``
1655 is the right choice.
1660 - Original version made by Michael Elizabeth Chastain, <mailto:mec@shout.net>
1661 - Updates by Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de>
1662 - Updates by Sam Ravnborg <sam@ravnborg.org>
1663 - Language QA by Jan Engelhardt <jengelh@gmx.de>
1668 - Describe how kbuild supports shipped files with _shipped.
1669 - Generating offset header files.
1670 - Add more variables to chapters 7 or 9?