Linux Kernel Architecture

Mauerer runapp02.tex V2 - 09/04/2008 6:09pm Page 1158

Appendix B: Working with the Source Code

CFLAGS_MODULE = $(MODFLAGS)

AFLAGS_MODULE = $(MODFLAGS)

LDFLAGS_MODULE = -r

CFLAGS_KERNEL =

AFLAGS_KERNEL =

...

TheCROSS_COMPILEprefix that precedes a definition is normally left blank. It must be assigned an appro-

priate value (ia64-linux-, for example) if the kernel is to be compiled for a different architecture.^4 As a

result, two different toolsets are used for the host and the target.

All other Makefiles mayneveruse the names of the tools directly, but must always employ the variables

defined here.

The main Makefile declares theARCHvariable to indicate the architecture for which the kernel is compiled.

It contains a value that is automatically detected and that is compatible with the directory names inarch/.

For example,ARCHis set toi386for IA-32 because the architecture-specific files reside inarch/i386/.

If the kernel is to be cross-compiled,ARCHmust be modified accordingly. For example, the following

calls are required to configure and compile the kernel for ARM systems (assuming that the appropriate

toolchain is available):

make ARCH=arm menuconfig

make ARCH=arm CROSS_COMPILE=arm-linux-

In addition to these definitions, the Makefile includes the statements needed to descend recursively into

the individual subdirectories and to compile the files they contain with the help of the local Makefile.

This appendix doesn’t go into the implementation details of this mechanism because it involves a large

number ofmakesubtleties.

Driver and SubsystemMakefiles

The Makefiles in the driver and subsystem directories are used to compile the correct files — in accor-

dance with the configuration in.config— and to direct the compilation flow to the required subdirec-

tories. The Kbuild framework makes the creation of such Makefiles relatively easy. Only the following

line is needed to generate an object file for permanent compilation into the kernel (regardless of the

configuration):

obj-y = file.o

By reference to the filename, Kbuild automatically detects that the source file isfile.cand invokes the

C compiler with the appropriate options to generate the binary object file if it is not already present or if

the source file has been modified after generation of an old version of the object file. The generated file is

also automatically included when the kernel is linked by the linker.

This approach can also be adopted if there are several object files. The specified files must then be sepa-

rated by blanks.

(^4) This can be set explicitly in the Makefile, specified by means of a shell variable in the environment, or passed as a parameter for
make.