makefiles.txt

讲述linux的初始化过程

Linux Kernel Makefiles
2000-September-14
Michael Elizabeth Chastain, <[e-mail]>



=== Table of Contents

This document describes the Linux kernel Makefiles.

  1  Overview
  2  Who does what
  3  Makefile language
  4  Variables passed down from the top
  5  The structure of an arch Makefile
     5.1  Architecture-specific variables
     5.2  Vmlinux build variables
     5.3  Post-vmlinux goals
     5.4  Mandatory arch-specific goals
  6  The structure of a subdirectory Makefile
     6.1  Comments
     6.2  Goal definitions
     6.3  Adapter section
     6.4  Rules.make section
     6.5  Special rules
  7  Rules.make variables
     7.1  Subdirectories
     7.2  Object file goals
     7.3  Library file goals
     7.4  Loadable module goals
     7.5  Multi-part modules
     7.6  Compilation flags
     7.7  Miscellaneous variables
  8  New-style variables
     8.1  New variables
     8.2  Converting to old-style
  9  Compatibility with Linux Kernel 2.2
 10  Credits


=== 1 Overview

The Makefiles have five parts:

    Makefile: the top Makefile.
    .config: the kernel configuration file.
    arch/*/Makefile: the arch Makefiles.
    Subdirectory Makefiles: there are about 300 of these.
    Rules.make: the common rules for all subdirectory Makefiles.

The top Makefile reads the .config file, which comes from the
kernel configuration process.

The top Makefile is responsible for building two major products: vmlinux
(the resident kernel image) and modules (any module files).  It builds
these goals by recursively descending into the subdirectories of the
kernel source tree.  The list of subdirectories which are visited depends
upon the kernel configuration.

The top Makefile textually includes an arch Makefile with the name
arch/$(ARCH)/Makefile.  The arch Makefile supplies architecture-specific
information to the top Makefile.

Each subdirectory has a Makefile which carries out the commands passed
down from above.  The subdirectory Makefile uses information from the
.config file to construct various file lists, and then it textually
includes the common rules in Rules.make.

Rules.make defines rules which are common to all the subdirectory
Makefiles.  It has a public interface in the form of certain variable
lists.  It then declares rules based on those lists.



=== 2 Who does what

People have four different relationships with the kernel Makefiles.

*Users* are people who build kernels.  These people type commands such as
"make menuconfig" or "make bzImage".  They usually do not read or edit
any kernel Makefiles (or any other source files).

*Normal developers* are people who work on features such as device
drivers, file systems, and network protocols.  These people need to
maintain the subdirectory Makefiles for the subsystem that they are
working on.  In order to do this effectively, they need some overall
knowledge about the kernel Makefiles, plus detailed knowledge about the
public interface for Rules.make.

*Arch developers* are people who work on an entire architecture, such
as sparc or ia64.  Arch developers need to know about the arch Makefiles
as well as subdirectory Makefiles.

*Kbuild developers* are people who work on the kernel build system itself.
These people need to know about all aspects of the kernel Makefiles.

This document is aimed towards normal developers and arch developers.



=== 3 Makefile language

The kernel Makefiles are designed to run with Gnu Make.  The Makefiles
use only the documented features of Gnu Make, but they do use many
Gnu extensions.

Gnu Make supports elementary list-processing functions.  The kernel
Makefiles use a novel style of list building and manipulation with few
"if" statements.

Gnu Make has two assignment operators, ":=" and "=".  ":=" performs
immediate evaluation of the right-hand side and stores an actual string
into the left-hand side.  "=" is like a formula definition; it stores the
right-hand side in an unevaluated form and then evaluates this form each
time the left-hand side is used.

There are some cases where "=" is appropriate.  Usually, though, ":="
is the right choice.

All of the examples in this document were drawn from actual kernel
sources.  The examples have been reformatted (white space changed, lines
split), but are otherwise exactly the same.



=== 4 Variables passed down from the top

The top Makefile exports the following variables:

    VERSION, PATCHLEVEL, SUBLEVEL, EXTRAVERSION

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

	$(VERSION), $(PATCHLEVEL), and $(SUBLEVEL) define the basic
	three-part version number, such as "2", "4", and "0".  These three
	values are always numeric.

	$(EXTRAVERSION) defines an even tinier sublevel for pre-patches
	or additional patches.	It is usually some non-numeric string
	such as "-pre4", and is often blank.

    KERNELRELEASE

	$(KERNELRELEASE) is a single string such as "2.4.0-pre4", suitable
	for constructing installation directory names or showing in
	version strings.  Some arch Makefiles use it for this purpose.

    ARCH

	This variable defines the target architecture, such as "i386",
	"arm", or "sparc".   Many subdirectory Makefiles test $(ARCH)
	to determine which files to compile.

	By default, the top Makefile sets $(ARCH) to be the same as the
	host system system architecture.  For a cross build, a user may
	override the value of $(ARCH) on the command line:

	    make ARCH=m68k ...

    TOPDIR, HPATH
	
	$(TOPDIR) is the path to the top of the kernel source tree.
	Subdirectory Makefiles need this so that they can include
	$(TOPDIR)/Rules.make.

	$(HPATH) is equal to $(TOPDIR)/include.  A few arch Makefiles
	need to use this to do special things using include files.

    SUBDIRS

	$(SUBDIRS) is a list of directories which the top Makefile
	enters in order to build either vmlinux or modules.  The actual
	directories in $(SUBDIRS) depend on the kernel configuration.
	The top Makefile defines this variable, and the arch Makefile
	extends it.

    HEAD, CORE_FILES, NETWORKS, DRIVERS, LIBS
    LINKFLAGS

	$(HEAD), $(CORE_FILES), $(NETWORKS), $(DRIVERS), and $(LIBS)
	specify lists of object files and libraries to be linked into
	vmlinux.
	
	The files in $(HEAD) are linked first in vmlinux.

	$(LINKFLAGS) specifies the flags to build vmlinux.

	The top Makefile and the arch Makefile jointly define these
	variables.  The top Makefile defines $(CORE_FILES), $(NETWORKS),
	$(DRIVERS), and $(LIBS).  The arch Makefile defines $(HEAD)
	and $(LINKFLAGS), and extends $(CORE_FILES) and $(LIBS).

	Note: there are more names here than necessary.  $(NETWORKS),
	$(DRIVERS), and even $(LIBS) could be subsumed into $(CORE_FILES).

    CPP, CC, AS, LD, AR, NM, STRIP, OBJCOPY, OBJDUMP
    CPPFLAGS, CFLAGS, CFLAGS_KERNEL, MODFLAGS, AFLAGS, LDFLAGS
    PERL
    GENKSYMS

	These variables specify the commands and flags that Rules.make
	uses to build goal files from source files.

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

	$(MODFLAGS) contains extra C compiler flags used to compile code
	for loadable kernel modules.  In the future, this flag may be
	renamed to the more regular name $(CFLAGS_MODULE).

	$(AFLAGS) contains assembler flags.

	$(GENKSYMS) contains the command used to generate kernel symbol
	signatures when CONFIG_MODVERSIONS is enabled.	The genksyms
	command comes from the modutils package.

    CROSS_COMPILE

	This variable is a prefix path for other variables such as $(CC),
	$(AS), and $(LD).  The arch Makefiles sometimes use and set this
	variable explicitly.  Subdirectory Makefiles don't need to worry
	about it.

	The user may override $(CROSS_COMPILE) on the command line if
	desired.

    HOSTCC, HOSTCFLAGS

	These variables define the C compiler and C compiler flags to
	be used for compiling host side programs.  These are separate
	variables because the target architecture can be different from
	the host architecture.

	If your Makefile compiles and runs a program that is executed
	during the course of building the kernel, then it should use
	$(HOSTCC) and $(HOSTCFLAGS).

	For example, the subdirectory drivers/pci has a helper program
	named gen-devlist.c.  This program reads a list of PCI ID's and
	generates C code in the output files classlist.h and devlist.h.

	Suppose that a user has an i386 computer and wants to build a
	kernel for an ia64 machine.  Then the user would use an ia64
	cross-compiler for most of the compilation, but would use a
	native i386 host compiler to compile drivers/pci/gen-devlist.c.

	For another example, kbuild helper programs such as
	scripts/mkdep.c and scripts/lxdialog/*.c are compiled with
	$(HOSTCC) rather than $(CC).

    ROOT_DEV, SVGA_MODE, RAMDISK

	End users edit these variables to specify certain information
	about the configuration of their kernel.  These variables
	are ancient!  They are also specific to the i386 architecture.
	They really should be replaced with CONFIG_* options.

    MAKEBOOT

	This variable is defined and used only inside the main arch
	Makefiles.  The top Makefile should not export it.

    INSTALL_PATH

	This variable defines a place for the arch Makefiles to install
	the resident kernel image and System.map file.

    INSTALL_MOD_PATH, MODLIB

	$(INSTALL_MOD_PATH) specifies a prefix to $(MODLIB) for module
	installation.  This variable is not defined in the Makefile but
	may be passed in by the user if desired.

	$(MODLIB) specifies the directory for module installation.
	The top Makefile defines $(MODLIB) to
	$(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE).  The user may
	override this value on the command line if desired.

    CONFIG_SHELL

	This variable is private between Makefile and Rules.make.
	Arch makefiles and subdirectory Makefiles should never use this.

    MODVERFILE

	An internal variable.  This doesn't need to be exported, as it
	is never used outside of the top Makefile.

    MAKE, MAKEFILES

	Some variables internal to Gnu Make.

	$(MAKEFILES) in particular is used to force the arch Makefiles
	and subdirectory Makefiles to read $(TOPDIR)/.config without
	including it explicitly.  (This was an implementation hack and
	could be fixed).



=== 5 The structure of an arch Makefile



--- 5.1 Architecture-specific variables

The top Makefile includes one arch Makefile file, arch/$(ARCH)/Makefile.
This section describes the functions of the arch Makefile.

An arch Makefile extends some of the top Makefile's variables with
architecture-specific values.

    SUBDIRS

	The top Makefile defines $(SUBDIRS).  The arch Makefile extends
	$(SUBDIRS) with a list of architecture-specific directories.

	Example:

		# arch/alpha/Makefile

		SUBDIRS := $(SUBDIRS) arch/alpha/kernel arch/alpha/mm \
		           arch/alpha/lib arch/alpha/math-emu

	This list may depend on the configuration:

		# arch/arm/Makefile

		ifeq ($(CONFIG_ARCH_ACORN),y)
		SUBDIRS         += drivers/acorn
		...
		endif

    CPP, CC, AS, LD, AR, NM, STRIP, OBJCOPY, OBJDUMP
    CPPFLAGS, CFLAGS, CFLAGS_KERNEL, MODFLAGS, AFLAGS, LDFLAGS

	The top Makefile defines these variables, and the arch Makefile
	extends them.

	Many arch Makefiles dynamically run the target C compiler to
	probe what options it supports:

		# arch/i386/Makefile

		# prevent gcc from keeping the stack 16 byte aligned
		CFLAGS += $(shell if $(CC) -mpreferred-stack-boundary=2 \
			  -S -o /dev/null -xc /dev/null >/dev/null 2>&1; \
			  then echo "-mpreferred-stack-boundary=2"; fi)

	And, of course, $(CFLAGS) can depend on the configuration:

		# arch/i386/Makefile

		ifdef CONFIG_M386
		CFLAGS += -march=i386
		endif

		ifdef CONFIG_M486
		CFLAGS += -march=i486
		endif

		ifdef CONFIG_M586
		CFLAGS += -march=i586
		endif

	Some arch Makefiles redefine the compilation commands in order
	to add architecture-specific flags:

		# arch/s390/Makefile

		LD=$(CROSS_COMPILE)ld -m elf_s390
		OBJCOPY=$(CROSS_COMPILE)objcopy -O binary -R .note -R .comment -S



--- 5.2 Vmlinux build variables

An arch Makefile co-operates with the top Makefile to define variables
which specify how to build the vmlinux file.  Note that there is no
corresponding arch-specific section for modules; the module-building
machinery is all architecture-independent.

    HEAD, CORE_FILES, LIBS
    LINKFLAGS

	The top Makefile defines the architecture-independent core of
	thse variables, and the arch Makefile extends them.  Note that the
	arch Makefile defines (not just extends) $(HEAD) and $(LINKFLAGS).

	Example:

		# arch/m68k/Makefile

		ifndef CONFIG_SUN3
		LINKFLAGS = -T $(TOPDIR)/arch/m68k/vmlinux.lds
		else
		LINKFLAGS = -T $(TOPDIR)/arch/m68k/vmlinux-sun3.lds -N
		endif

		...

		ifndef CONFIG_SUN3
		HEAD := arch/m68k/kernel/head.o
		else
		HEAD := arch/m68k/kernel/sun3-head.o
		endif

		SUBDIRS += arch/m68k/kernel arch/m68k/mm arch/m68k/lib
		CORE_FILES := arch/m68k/kernel/kernel.o arch/m68k/mm/mm.o $(CORE_FILES)
		LIBS += arch/m68k/lib/lib.a



--- 5.3 Post-vmlinux goals

An arch Makefile specifies goals that take the vmlinux file, compress
it, wrap it in bootstrapping code, and copy the resulting files somewhere.
This includes various kinds of installation commands.

These post-vmlinux goals are not standardized across different
architectures.  Here is a list of these goals and the architectures
that support each of them (as of kernel version 2.4.0-test6-pre5):

    balo		mips
    bootimage		alpha
    bootpfile		alpha, ia64
    bzImage		i386, m68k
    bzdisk		i386
    bzlilo		i386