Unknown symbol when compiling kernel module from multiple files - makefile

Question

I am writing a kernel module that is comprised of several source files,
One of these source files has a function that needs to be used by the other objects in the same module.
It is defined in my file named ModemAPI.c
static void LogMessage ( char *format, ...)
This c file should be (together with other files) compiled into one kernel module, its makefile looks like this:
obj-m += ModemAPI.o
ModemAPI-objs := ../Common/StateMachine.o ../Common/ElementsPool.o
When I compile this kernel module, I get a warning during linking that the above function "LogMessage" is undefined and when I try to load the module I get an error saying it has an unknown symbol in it (of course LogMessage).
EDIT: Just to make clear, the function "LogMessage" is declared and implemented in the file ModemAPI.c, moreover it is exported via EXPORT_SYMBOL
EXPORT_SYMBOL(LogMessage);
In the files that use the function (such as StateMachine.c), it is declared via extern
extern void LogMessage ( char *format, ...);
The module compiles, the problem is in the linking stage.
Does anyone have any idea what could be the problem with this?
Thanks,
Roy.

Answer 1

In the Makefile try
obj-m += Module.o
Module-objs := ../Common/StateMachine.o ../Common/ElementsPool.o ../Common/ModemAPI.o
some times the order of the .o files are important

Answer 2

Answer for : moreover it is exported via EXPORT_SYMBOL
EXPORT_SYMBOL() makes LogMessage() is accessible to loadable kernel modules.
For eample, vmalloc() is exported to use in kernel modules http://lxr.free-electrons.com/source/mm/vmalloc.c#L1708
But still you have to include the vmalloc.h in loadable kernel module source.so do not confuse with extern and EXPORT_SYMBOL.
Solution
In your Makefile, modify as below
obj-m += ModemAPI.c StateMachine.c
i.e ModemAPI.c which has LogMessage() should be compiled first.
Change proposed to Makefile
obj-m += Mymodule.o
Mymodule-objs := ../Common/ModemAPI.o ../Common/StateMachine.o ../Common/ElementsPool.o
EDIT : 2
static in static void LogMessage ( char *format, ...) restricts the use of LogMessage() in other files. This concept is called static functions.

Answer 3

I believe the issue is that ModemAPI.c is not getting compiled.
obj-m += ModemAPI.o
ModemAPI-objs := ../Common/StateMachine.o ../Common/ElementsPool.o
Normally obj-m += ModemAPI.o tells make to use ModemAPI.c (I think that is the default for the linux make system), but adding the ModemAPI-objs tells make that ModemAPI.o is built using the objects ../Common/StateMachine.o ../Common/ElementsPool.o instead.
Try renaming the object or your ModemAPI.c file:
obj-m += modem.o #something not named ModemAPI.o
ModemAPI-objs := ModemAPI.o ../Common/StateMachine.o ../Common/ElementsPool.o
or
obj-m += ModemAPI.o
ModemAPI-objs := main.o ../Common/StateMachine.o ../Common/ElementsPool.o
where ModemAPI.c has been renamed to main.c

Answer 4

Make sure, that your kernel is compiled with support for module loading (e.g., if /proc/modules exists, you can be sure, it is)
Make sure, you are compiling against the same kernel, where you are trying to load the module
define obj-m += ... and [my-module-ko]-objs := [all xx.o object list]