how to define linux kernel variable accessed by several source file?

Question

I have searched the answer to my question for hours, to no avail.

My question: I want to define a variable that can be accessed(w/r)(shared) by the two files in linux kernel: source/arch/x86/kvm/x86.c and source/kernel/sched/core.c .

My failed attempt: I tried to use export_symbol to define a global var in x86.c . But the compile error message says:

the var is undefined reference

Is there any other solution? I am new to linux kernel programming. Thanks in advance.

Answer 1

When you use want to use a global variable in kernel modules, you should use the EXPORT_SYMBOL() or EXPORT_SYMBOL_GPL() or EXPORT_SYMBOL_GPL_FUTURE(): Eg:

 int myvar;
 EXPORT_SYMBOL(myvar);

you should then use

extern int myvar

in the other file where you want to use it, before you use it.

Answer 2

You can think of kernel symbols (either functions or data objects, variables) and their visibility at three different levels in the kernel source code:

"static", - visible only within their own source file
"external" - visible to any other code built into the kernel itself
"exported" - visible and available to any loadable module. 

Also you may want to consider how other module should use your exported symbols, typically done with one of:

EXPORT_SYMBOL() - exports to any loadable module, or
EXPORT_SYMBOL_GPL() - exports only to GPL-licensed modules.