gcc - Linking binary against functions/data in specific location in memory

Question

I'm currently in the process of writing an intermediate-memory bootloader for an ATMega.

I'd like to place a section of commonly used functions and data in a specific location in memory, such that:

• limited size of the bootloader section is not overcome
• library functions, drivers, etc, are not reproduced by the application section and thus wasting space

For illustrative purposes, a map of the desired memory layout is below:

Following some help in this thread on avrfreaks, I'm to the point where I've been able to move all code (in my bootloader + library development environment - applications will be developed in separate projects) not tagged with __attribute__ ((section(".boot"))) to the shared library section successfully via means of a custom linker script.

It was suggested in the avrfreaks thread that I can link my applications by using avr-objcopy --strip-all --keep-symbol=fred --keep-symbol=greg ... boot.elf dummy.elf to create a symbol reference of what I have in my shared library, and then linking my applications against this memory layout with avr-gcc -o app.elf -Wl,--just-symbols=dummy.elf app1.o app2.o ....

The problem I face here is that I need to specify each symbol I want to keep in my dumy.elf. I can use the keep-symbols=<file> directive to specify a text file list of symbols to keep, but I still must generate this list.

I've noticed that there is a bunch of symbols that I don't want to include (stuff like C environment set-up code that is common in name, but different in functionality, for both the bootloader and application) that seems to start with the prefix '_' (but of course, there are some useful and large library functions with the same prefix, e.g. *printf and math routines). Perhaps there won't be conflicts if I link my application against the existing runtime code in the application/bootloader?

How can I generate a list of symbols for my library section that contains the code that I've written (maybe some sed magic and scanning header files)/excludes any symbols that may conflict in linking the application?

The project can be viewed in its current state at this github repository.

Edit: I want to make clear that I could tag everything I want to be in the shared library section with __attribute__ ((section(".library"))), but as I also want to share some rather large libc stuff (vsprintf, etc) between the bootloader and application, this becomes cumbersome very quickly. As such, I've elected to put everything not tagged as boot in the library memory region via a linker script.

Perhaps I just need some advice on my linker script, as I'm not super sure what I'm doing there.

Answer 1

Consider using -R <file> as linker option (gcc -Wl,-R -Wl,<file>).

This will generate references to (global) symbols in <file> just as if they were linked normally, but not include the referenced code.