I asked some time ago this question How can I use one function from main application and bootloader? (embedded) and started to implement proposed solution but ran into a few problems.
On my cortex M4 I, have 2 seperate applications - bootloader and user application. Now I had some (many) functions which were the same for both apps. So I compiled them only for bootloader, then created an array of function pointers at specified address, which is known for user application. So in application, I didn't compile the files with those functions again, but I use those pointers whenever needed.
This is example of code I tried to make common for both applications:
static uint8_t m_var_1;
// Sends events to the application.
static void send_event(fs_op_t const * const p_op, fs_ret_t result)
Yes you are right, the code will attempt to access the global variable at the same location as it is linked for loader. This is because linking involves replacing all occurrences of identifiers (including function names and variable names) by the addresses determined after compiling.
In your application, the variable, even if it does exist there too, is likely to be at a different address. The calling of the functions happens to work, because they are located in ROM and cannot be different for application and loader. Calling them via const pointers, which are also stored in ROM, bypasses the problem.
The solution is using a file system simulator, if you can find one for your hardware.
Otherwise you will hate having to do the following.
Part 1, setup:
Part 2, access:
Part 3, resuing values stored by other system part
(e.g. you want to leave some kind of message from loader, to be read my application)
To do all that, intense study of your linker features and syntax is needed.
So I recommend not to try, if you can get around it. Consider using an existing file system simulator; because that is basically what above means.