I am writing a simple device driver in Linux. In the makefile we normally write first line as
obj-m += hello.o
where hello is the module written. I have seen in other makefiles the symbol += being replaced by := and some others. What is significant difference between the two and also what are the possible options?
Actually, Kbuild resets obj-m along with some other variables (see scripts/Makefile.build) before including user Makefile, so there is no difference between += and := assignments (assuming that there is the only hello.o line).
Personally I would prefer appending assignment (+=). This prevents some stupid errors like the following:
obj-m := hello.o
obj-m := world.o
If the last line is a copy-paste of the first one and you forget to change the assignment type to +=, then you'll get only world.o compiled (without hello.o).
I would also suggest you to read Kbuild documentation about how to write module makefiles.