I start liking the GNU linker (ld) more and more. Yes, that linker command file syntax needs some time to learn, but it is very powerful. I stumbled over an interesting way how to define linker symbols:

/* Linker file for GNU C Compiler */

/* Entry Point */
ENTRY(Reset_Handler)

HEAP_SIZE  = DEFINED(__heap_size__)  ? __heap_size__  : 0x00000400;
STACK_SIZE = DEFINED(__stack_size__) ? __stack_size__ : 0x00000400;

The interesting part is how the HEAP_SIZE and STACK_SIZE symbols are defined.

It checks if e.g. __heap_size__ is DEFINED, and if so, it uses that symbol, otherwise it is using 0x400. Very similar to the C/C++ ‘?’ operator. So I can overwrite the default of 0x400 with my value or symbol. The questions is: from where does the symbol come from?

I checked http://www.math.utah.edu/docs/info/ld_3.html which explains DEFINED as

DEFINED(symbol)

Return 1 if symbol is in the linker global symbol table and is defined, otherwise return 0. You can use this function to provide default values for symbols. For example, the following command-file fragment shows how to set a global symbol begin to the first location in the .text section–but if a symbol called begin already existed, its value is preserved:

SECTIONS{ ...
  .text : {
    begin = DEFINED(begin) ? begin : . ;
    ...
  }
... }

So this is really a cool way to overwrite a symbol in the linker file, without touching the linker file itself. After some digging, I have found a way how this can be done: with options in passed to the linker: It requires the -Xlinker option and the –defsym option:

-Xlinker --defsym=

So to set the __heap_size__ to 0x600, I use

-Xlinker --defsym=__heap_size__=0x600

If using the GNU ARM Eclipse plugins (e.g. Kinetis Design Studio or my DIY Eclipse IDE), the options is added to the ‘other linker flags’ of the Cross ARM C++ Linker settings:

An easy way to verify if things are working as intended is to check the linker map file, and search for that symbol. For my case above I get:

LOAD c:/freescale/kds_1.1.0/toolchain/bin/../lib/gcc/arm-none-eabi/4.8.0/m4/fp/v4-sp-d16/crti.o
LOAD c:/freescale/kds_1.1.0/toolchain/bin/../lib/gcc/arm-none-eabi/4.8.0/m4/fp/v4-sp-d16/crtbegin.o
LOAD c:/freescale/kds_1.1.0/toolchain/bin/../lib/gcc/arm-none-eabi/4.8.0/../../../../arm-none-eabi/lib/m4/fp/v4-sp-d16/crt0.o
                0x00000600                __heap_size__ = 0x600

So that worked as expected 🙂

Summary

If symbols are defined in the GNU linker file with

newSymbol  = DEFINED(symbol)  ? symbol  : default ;

Then I can overwrite it using a linker command:

-Xlinker --defsym=symbol=value

Happy Defining 🙂

Useful link:http://stackoverflow.com/questions/10032598/gnu-ld-how-to-override-a-symbol-value-an-address-defined-by-the-linker-script