Linux Kernel Architecture

Mauerer runapp02.tex V2 - 09/04/2008 6:09pm Page 1157

Appendix B: Working with the Source Code

The Main Makefile

The main Makefile is key to kernel compilation. It defines the call paths for the C compiler, linker, and so

on. The following distinction must be made between two toolchain alternatives:

❑ A toolchain to generatelocalprograms that execute on the host that compiles the kernel.

Examples of such programs are themenuconfigbinaries or tools for analyzing module symbols.

❑ A toolchain to generate the kernel itself.

The toolchains are usually identical. Differences arise only when a kernel is cross-compiled; in other

words, when a specific architecture is used to compile a kernel for a different architecture. This method

is applied if the target computer is either an embedded system with few resources (e.g., an ARM or MIPS

handheld device) or a very old and slow computer (aclassic Sparc or m68 Mac). In this case, a cross-

compiler (and appropriate cross-binutils) must be available for the toolchain responsible for creating the

kernel so that the desired code can be generated.

The local tools are defined as follows:

wolfgang@meitner>cat Makefile

...

HOSTCC = gcc

HOSTCXX = g++

HOSTCFLAGS = -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer

HOSTCXXFLAGS = -O2

...

The kernel tools are defined as follows:

wolfgang@meitner>cat Makefile

...

CROSS_COMPILE=

AS = $(CROSS_COMPILE)as

LD = $(CROSS_COMPILE)ld

CC = $(CROSS_COMPILE)gcc

CPP = $(CC) -E

AR = $(CROSS_COMPILE)ar

NM = $(CROSS_COMPILE)nm

STRIP = $(CROSS_COMPILE)strip

OBJCOPY = $(CROSS_COMPILE)objcopy

OBJDUMP = $(CROSS_COMPILE)objdump

AWK = awk

GENKSYMS = scripts/genksyms/genksyms

DEPMOD = /sbin/depmod

KALLSYMS = scripts/kallsyms

PERL = perl

CHECK = sparse

CHECKFLAGS := -D__linux__ -Dlinux -D__STDC__ -Dunix -D__unix__ -Wbitwise $(CF)

MODFLAGS = -DMODULE