linux driver compile error, cannot use $(shell pwd)/xxx as include path

When I compile my linux drive, I find a problem:

I have some ways to define the include path

A relative path by $(shell pwd), but it doesn't work:

MOD_INC=$(shell pwd)/include
EXTRA_CFLAGS+=-I$(MOD_INC)

I found: $(shell pwd) == /home/myname/prj_svn

B abosulte path, it works:

MOD_INC=/home/myname/prj_svn/include
EXTRA_CFLAGS+=-I$(MOD_INC)

C define a path, it works:

PRJ_PATH=/home/myname/prj_svn
MOD_INC=$(PRJ_PATH)/include
EXTRA_CFLAGS+=-I$(MOD_INC)

I don't know why?

I try them in 开发者_如何转开发an application project, all of them can work.

thanks for your help!

Make does quite a lot of directory changing as it builds; i wouldn't want to rely on the value of pwd at any particular point in time.

How about defining an environment variable called something like MODULE_HOME, set to /home/myname/prj_svn, and writing all your paths relative to that?