Linker script: insert absolute address of the function to the generated code

Question

I have a question related to gcc's linker.

I'm working with embedded stuff (PIC32), but PIC32's compiler and linker are based on gcc, so, main things should be common for "regular" gcc linker and PIC32 linker.

In order to save flash space (which is often insufficient on microcontrollers) I need to put several large functions in the bootloader, and application should call these functions just by pointers.

So, I need to create vector table in the generated code.

I'm trying to get absolute address of the function and write it to the generated code, but still getting errors.

In the example below I'm trying to get address of the function _formatted_write.

Code:

.user_addr_table _USER_ADDR_TABLE_ADDR :
{
   KEEP(*(.user_addr_table))
   LONG((ABSOLUTE(_formatted_write))); /* _formatted_write is a name of my function */
} > user_addr_table

Linker returns error: "unresolvable symbol '_formatted_write' referenced in expression".

I have noticed that if I write some garbage instead of _formatted_write, then it returns error "undefined symbol .....", so, _formatted_write is known by linker.

It makes me think I should perform some additional steps to make it "resolvable". But still no idea how to do that.

Answer 1

For the case that someone has the same problem (like me some hours ago), there is a still better solution:

Let "bootloader.out" be the bootloader-binary. Then we can generate with

nm -g bootloader.out | grep '^[0-9,a-e]\{8\} T' | awk '{printf "PROVIDE(%s = 0x%s);\n",$3,$1}' > bootloader.inc

a file that we can include in the linker script of the application with

INCLUDE bootloader.inc

The linker knows now the addresses of all bootloader-functions and we can link against it without to have the actual function code in the application. All what we need there is a declaration for every bootloader-function which we want to execute.