GNU LD: How to override a symbol value (an address) defined by the linker script specified using -T

Question

My usecase is as follows:

• I am using a typical SDK that comes with Makefile based projects
• I belive the linker is patched gcc. gcc --version gives me

Answer 1

While waiting for someone to respond, I did resolve the issue. There are few issues with the problem here and I thought of explaining my findings for someone who might do the same mistake.

First of all Any options to be passed to the linker must be specified with -Xlinker or with -Wl. Hence both 2 and 3 won't work in the above case. The corrected 2 and 3 would be as follows:

1. Is correct already

2. gcc $(OBJS) -l$(Lib1) -l$(Lib2) -nostdlib -lgcc -L$(library_path) -g -msmall-mode -mconst-switch-tables -mas-mode -mno-initc -Wl,--start-group,--end-group,-T,$(PATH_TO_Linker.ld),--gc-sections -Xlinker --defsym=SYMBOL_RAM_START=$(VALUE_TO_OVERRIDE) -o$(OUTPUT).elf

3. gcc $(OBJS) -l$(Lib1) -l$(Lib2) -Xlinker --defsym=SYMBOL_RAM_START=$(VALUE_TO_OVERRIDE) -nostdlib -lgcc -L$(library_path) -g -msmall-mode -mconst-switch-tables -mas-mode -mno-initc -Wl,--start-group,--end-group,-T,$(PATH_TO_Linker.ld),--gc-sections -o$(OUTPUT).elf

Now for the case of options 1 & 2 above, --defsym comes after linker script and SYMBOL_RAM_START was already defined by the linker script. It does override it. But the overriden value will not be used, because the sections have already been defined as the linker script has already been used.

For the case of option 3 above, the SYMBOL_RAM_START was defined before the linker script was read by the linker. Hence when linker script is parsed, the value specified in the script overrides it.

Solution:

In order for this to work, the linker script will need to conditionally initialize the symbol SYMBOL_RAM_START, something like below:

SYMBOL_RAM_START = DEFINED( SYMBOL_RAM_START ) ? SYMBOL_RAM_START : DEFAULT_VALUE ;

Given the above in the linker script, when the SYMBOL_RAM_START was defined before the linker script is included (as shows in option 3 above) it did work. But in the end I had to patch the linker script.

This solution doesn't really override the symbol, but provides a way in which a symbol can be defined so that it can be overridden.