Kernel module with multiple files - unknown symbol

Question

Hello stackoverflowers :)

For the last several hours I've been trying to compile+load a multiple file module. The compilation emits a strange warning and the module fails to load. Here are the module, Makefile, compilation output and dmesg.

header:

// header.h

#ifndef _HEADER_H
#define _HEADER_H
void do_module_func(void);
void do_other_func(void);
#endif

'main' module file:

//mymodule.c

#include <linux/module.h>
#include <linux/kernel.h>
#include "header.h"

void do_module_func(void)
{
    printk(KERN_INFO "module_func\n");
}

static int mymodule_init(void)
{
    printk(KERN_INFO "Hello world\n");
    do_other_func();
    return 0;
}
module_init(mymodule_init);


static void mymodule_exit(void)
{
    printk(KERN_INFO "Goodbye, cruel world\n");
}
module_exit(mymodule_exit);

MODULE_LICENSE("GPL")

other c file, which calls do_module_func() that sits in the 'main' module

//other_file.c

#include "header.h"
#include <linux/kernel.h>

void do_other_func(void)
{
    printk(KERN_INFO "other_func\n");
    do_module_func();
}

Makefile

//Makefile

obj-m := mymodule.o
mymodule-objs := other_file.o

CROSS:=arm-unknown-linux-gnueabi-
KERNEL:= ~/work/linux-davinci-2.6.38/
ARCH:=arm

PWD:=$(shell pwd)

all:
    $(MAKE) CROSS_COMPILE=$(CROSS) ARCH=$(ARCH) -C $(KERNEL) M=$(PWD) modules

clean:
    $(MAKE) CROSS_COMPILE=$(CROSS) ARCH=$(ARCH) -C $(KERNEL) M=$(PWD) clean

I'm cross compiling but I believe this shouldn't be a problem. make output:

make CROSS_COMPILE....
make[1]: Entering directory .../linux-davinci-2.6.38
  CC [M] .../other_file.o
  LD [M] .../mymodule.o
  Building modules, stage 2.
  MODPOST 1 modules
WARNING: "do_module_func" [.../mymodule.o] undefined! <--- warning here
  CC .../mymodule.mod.o
  LD [M] .../mymodule.ko
make[1]: Leaving directory .../linux-davinci-2.6.38

insmod output:

can't insert 'mymodule.ko': unknown symbol in module, or unknown parameter

dmesg:

mymodule: Unknown symbol do_mdule_func (err 0)

Thus the module compiles with a (linkage?) warning and the module doesn't load.

Now, I see that in the make output there appears to be a linkage attempt after compiling other_file.c, but shouldn't there be a compilation of also mymodule.c before the linkage?

Thanks! :)

Answer 1

Turns out the problem was in the Makefile. The 'trick' is that you define in obj-m the module that will be compiled (into a .ko) and in the -objs you write all the source files.
Thus the definitions in this Makefile turn to:

obj-m := moduleko.o
moduleko-objs := other_file.o mymodule.o

and this is compiled into moduleko.ko.