If you do not have a kernel tree handy, or have not yet configured and built that kernel, now is the time to go do it. You cannot build loadable modules for a 2.6 kernel without this tree on your filesystem. It is also helpful (though not required) to be actually running the kernel that you are building for. Once you have everything set up, creating a makefile for your module is straightforward. In fact, for the "hello world" example shown earlier in this chapter, a single line will suffice:
obj-m := hello.o
The assignment above states that there is one module to be built from the object file hello.o. The resulting module is named hello.kO after being built from the object file. If, instead, you have a module called module.ko that is generated from two source files (called, say, file1.c and file2.c), the correct incantation would be:
obj-m := module.o
module-objs := file1.o file2.o
Makefile:
# If KERNELRELEASE is defined, we've been invoked from the
# kernel build system and can use its language.
obj-m := hello.o
ifneq ($(KERNELRELEASE),)
obj-m := hello.o
# Otherwise we were called directly from the command
# line; invoke the kernel build system.
else
KERNELDIR ?= /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)
default:
$(MAKE) -C $(KERNELDIR) M=$(PWD) modules
endif
