Building Linux modules.

greplinux · 09-29-2010, 08:05 AM

I am facing a problem while building a kernel module.

file1.c:

Quote:

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

int __init sam_init(void)
{
printk(KERN_INFO "Hello, world - this is the kernel speaking\n");
return 0;
}

module_init(sam_init);

file2.c

Quote:

#include <linux/kernel.h>
#include <linux/module.h>
#include "sam.h"
void __exit sam_exit(void)
{
printk(KERN_INFO "Short is the life of a kernel module\n");
}

module_exit(sam_exit);

Makefile

Quote:

obj-m += file1file2.o
file1file2-objs := file1.o file2.o

all:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

clean:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

The header file 'sam.h' is an empty file.

If I compile the above every thing goes fine. In case, I add a global variable in the header file like 'int global', I am getting the following error

Quote:

/home/greplinux/file2.o

.bss+0x0): multiple definition of `global'
/home/greplinux/file1.o

.bss+0x0): first defined here
make[2]: *** [/home/greplinux/file1file2.o] Error 1

Can you guys please explain the logic behind this error. I know, I am doing some thing wrong but not sure about it.

Thanks,

theNbomr · 09-29-2010, 10:39 AM

Header files are treated just like inline code in each source file. Including the same header in each source module will result in a variable named 'global' in each object module. This causes the linker to gag, as you have shown.
It is better to either not define variables in header files, or use conditionals in the header to prevent your circumstance. The compiler should create an instance of the variable in exactly one source module compilation. In all others, it should see merely a declaration.

--- rod.

Tinkster · 09-29-2010, 11:50 AM

Moved: This thread is more suitable in <PROGRAMMING> and has been moved accordingly to help your thread/question get the exposure it deserves.

jf.argentino · 09-29-2010, 03:45 PM

To add a precision:
Declare the global variable you want to share in the header with the keyword "extern" (or in any peace of code that need to access it, since you can declare the same symbol many times). Then _define_ it in only one code file (one symbol can be defined only once).
As it goes for function, you're declaring them (just their signatures) before using it: tell the compiler that you're using a symbol, whatever it contents. Then you're defining the function, or you're using the function definition provided by a library, to tell the linker what the function does.

2 remarks:

Shared datas generally need to be protected against concurrent accesses.

Adding the "volatile" keyword to the variable declaration is a good idea, thus avoiding the compiler to optimize access to it, since optimization could break your algorithm (your assumption of "when" your application can access to the variable).