Undefined symbol when using insmod

Question

I write a linux-kernel module, but when I run this command insmod ./...ko , the kernel posts an error: Undefined symbol ...

After that, I searched for a long time, all the solutions are to use EXPORT_SYMBOL() , so I write it in kernel codes and rebuild the kernel. Interesting things happen, the kernel says exports duplicate symbol ... (owned by kernel) . It seems that the symbol is exported more than once, but if so, why can I not use it before?

This is related code in my module:

extern struct task_struct *find_task_by_vpid(pid_t nr);
target_tsk = pid == -1 ? current : find_task_by_vpid(pid);

and I changed the kernel file linux/sched.h to this format:

extern struct task_struct *find_task_by_vpid(pid_t nr);
EXPORT_SYMBOL_GPL(find_task_by_vpid);

Answer 1

Symbol exporting ( EXPORT_SYMBOL ) should be performed in a source file ( .c ), not in a header file ( .h ).

This is because exporting symbol is a definition of (some other) symbol, and header files a not suitable for such things.