kernel module spanning multiple files

[amit mehta]

Whereas for a simple single file hello world kernel module, i see that
the symbols in the object (hello_world.ko) for the init and exit routines.
<snip>

[root at localhost #] nm hello_world.ko
0000000000000000 r ____versions
000000000000000c r __mod_license17
0000000000000020 r __mod_srcversion30
0000000000000060 r __mod_vermagic5
0000000000000000 r __mod_version18
0000000000000043 r __module_depends
0000000000000000 D __this_module
0000000000000000 T cleanup_module
0000000000000000 t hello_exit  < --- --------
0000000000000000 t hello_init   < -- ---------
0000000000000000 T init_module
                 U printk
<snip>

Regards,
amit

On Wed, Jun 15, 2011 at 3:29 PM, amit mehta <gmate.amit at gmail.com> wrote:
> Hi,
>
> After looking at some Makefiles under Linux Driver sources
> which uses multiple files to create the target, i tried to write
> a simple hello world kernel module spanning multiple files for learning purpose.
> But I'm observing a strange behavior. After inserting the module,
> I don't see the prints getting logged into the syslog(/var/log/message)
> or in the dmesg. Maybe its just a C programming mistake, or is due to
> an incorrect Makefile but as of now I'm clueless.
>
> Here are the code snippet from the four files(hello_multi.c, helper.c,
> myhdr.h, Makefile)
>
> [root at localhost #] cat hello_multi.c
> #include <linux/kernel.h>
> #include <linux/module.h>
> #include <linux/init.h>
> #include "myhdr.h"
> static int __init hello_init(void)
> {
>        printk(KERN_WARNING "Hello world\n");
>        foo(); //defined in anther file
>        return 0;
> }
>
> static void __exit hello_exit(void)
> {
>        printk(KERN_WARNING "goodbye world\n");
> }
>
> module_init(hello_init);
> module_exit(hello_exit);
>
> MODULE_LICENSE("GPL v2");
>
> [root at localhost #] cat helper.c
> #include <linux/kernel.h>
> #include "myhdr.h"
> void foo(void)
> {
>        printk(KERN_WARNING "inside foo\n");
> }
>
> [root at localhost #] cat myhdr.h
> #ifndef __MY_HDR_H
> #define __MY_HDR_H
> void foo(void);
> #endif
>
> [root at localhost #] cat Makefile
> obj-m += hello_multi.o
> hello_multi-objs := helper.o
> KERNELDIR=/lib/modules/$(shell uname -r)/build
> PWD=$(shell pwd)
>
> modules:
>        make -C $(KERNELDIR) M=$(PWD) modules
>
> modules_install:
>        make -C $(KERNELDIR) M=$(PWD) modules_install
>
> clean:
>        rm -rf *.o *.ko *~ core .depend .tmp_versions .*.cmd *.mod.c
> .PHONY: modules modules_install clean
>
> Invoking 'make modules' completes with success and i am able to
> insert this module.
>
> [root at localhost #]  make modules
> make -C /lib/modules/2.6.31-23-generic/build
> M=/home/superman/programming/ldd/hello_multi modules
> make[1]: Entering directory `/usr/src/linux-headers-2.6.31-23-generic'
>  CC [M]  /home/superman/programming/ldd/hello_multi/helper.o
>  LD [M]  /home/superman/programming/ldd/hello_multi/hello_multi.o
>  Building modules, stage 2.
>  MODPOST 1 modules
>  CC      /home/superman/programming/ldd/hello_multi/hello_multi.mod.o
>  LD [M]  /home/superman/programming/ldd/hello_multi/hello_multi.ko
> make[1]: Leaving directory `/usr/src/linux-headers-2.6.31-23-generic'
> [root at localhost #] dmesg -c
> [root at localhost #] insmod hello_multi.ko
> [root at localhost #] dmesg|tail    <-- -- No messages here
>
> [root at localhost #] cat /proc/modules |grep -i hello
> hello_multi 727 0 - Live 0xffffffffa020b000 (PN
>
> [root at localhost #] nm hello_multi.ko
> 0000000000000000 r ____versions
> 0000000000000000 r __mod_srcversion26
> 0000000000000040 r __mod_vermagic5
> 0000000000000023 r __module_depends
> 0000000000000000 D __this_module
> 0000000000000000 T foo
>                 U printk
>
> Regards,
> amit
>