如何理解 Linux 内核模块的“构建单独文件”？

Question

Excerpted from the Linux kernel documentation :

2.4 Building Separate Files

It is possible to build single files that are part of a module.
This works equally well for the kernel, a module, and even for
external modules.

Example (The module foo.ko, consist of bar.o and baz.o):
    make -C $KDIR M=$PWD bar.lst
    make -C $KDIR M=$PWD baz.o
    make -C $KDIR M=$PWD foo.ko
    make -C $KDIR M=$PWD /

I can't understand what these commands mean.

make -C $KDIR M=$PWD bar.lst

Does it mean: look for bar.c and compile it into bar.1st?

make -C $KDIR M=$PWD foo.ko

Does it mean: look for all *.o and link them into foo.ko? Does it also look for *.1st?

make -C $KDIR M=$PWD /

What does it mean?

Answer 1

First let's understand

make -C $KDIR M=$PWD [target]

here target can be anything defined as target (ie bar.o : )

mostly we use dependency in make file.

in given example three files

foo.c bar.c baz.c

foo.c has module_init ie entry point

the statement states that we can build all files at once. or single files individually and link them together finally.

in a nutshell you can try compiling just bar.c without compiling other two files. producing only bar.o

once all .o files are generated you can create.ko file.

refer linux/fs/ufs/Makefile

in you can compile once ufs.ko or balloc.o cylinder.o dir.o file.o ialloc.o inode.o etc. and compile ufs to create ufs.ko

Answer 2

Those commands all mean "do whatever the makefile says is necessary to build the named target". What the set of actions involved in doing that is I can't say and will depend entirely on what the specific target is.

If you want to know, specifically, what make will do for a given command give it the -n argument.