Putting Code of Files into Special Section with the GNU Linker

The GNU Linker (ld) is very, very powerful. This time I wanted to put all my Processor Expert generated code into its own dedicated section. This is useful for example to have a bootloader or a library inside a special area in FLASH. It was not obvious to me how to do this with the linker, with some search on the internet and some trial and errors, I finally managed that. And as always with exploring things, I have learnt something :-). So here is how I’m able to put the code of arbitrary files into its own dedicated section.

Code Section for Generated Code

For this, I need to change the linker file. As in my setup Processor Expert is generating it, I need to disable it so it does not change it anymore. This is a setting in the CPU component, under Build Options:

Disabled Generation of Linker File in Processor Expert

Next, I’m modifying the linker file, and creating a new section (m_text2) where I want to put my dedicated code:

Modified Linker File with new Section

Next, I’m extending the linker file with my placement. For example to place the Cpu.o into my dedicated section, I use a new placement .generatedCode just before the .text placement:

  .generatedCode : 
  { 
    . = ALIGN(4);
    *Cpu.o (.text .text*) 
    . = ALIGN(4); 
  }  > m_text2 /* new section for my special code files */ 

  /* The program code and other data goes into INTERNAL_FLASH */
  .text :
  {
   ...

Note that I’m using ‘*Cpu.o’, that means that any object file matching this will end up in this section. So if I want to have all the object files from the Processor Expert Generated_Code folder, I simply can use this:

  .generatedCode : 
  { 
    . = ALIGN(4);
    *Generated_Code/*.o (.text .text*) 
    . = ALIGN(4); 
  }  > m_text2 /* new section for my special code files */

So this might be already all what you need.

However, if you get linker errors about duplicated symbols, then it means that the linker is pulling in things twice. To avoid that objects get linked twice, I use the EXCLUDE_FILE directive, with a modified original .text and .text* placement:

  .generatedCode : 
  { 
    . = ALIGN(4);
    *Generated_Code/*.o (.text .text*) 
    . = ALIGN(4); 
  }  > m_text2 /* new section for my special code files */ 
  
  
  /* The program code and other data goes into INTERNAL_FLASH */
  .text :
  {
    . = ALIGN(4);

    /* original placement: */
    /* *(.text)  */         /* .text sections (code) */
    /* *(.text*) */          /* .text* sections (code) */

    /* modified placement: need to exclude the ones I have in .generatedCode above */
    *(EXCLUDE_FILE(*Generated_Code/*.o).text .text*)  /* .text and .text* sections (code) */

    *(.rodata)         /* .rodata sections (constants, strings, etc.) */
    *(.rodata*)        /* .rodata* sections (constants, strings, etc.) */
    *(.glue_7)         /* glue arm to thumb code */
    *(.glue_7t)        /* glue thumb to arm code */
    *(.eh_frame)

    KEEP (*(.init))
    KEEP (*(.fini))

    . = ALIGN(4);

    _etext = .;        /* define a global symbols at end of code */
  } > m_text

💡 It seems that EXCLUDE_FILE can only be used once in the linker file. All objects must be inside the (…) following the EXCLUDE_FILE directive.

Below another example (contributed by TsiChung, thank you!) which shows how to exclude certain object files not be linked: with adding .constdata to the list prevents things linked to RAM area.

.text : ALIGN(4)
{
   *(EXCLUDE_FILE(*my_wifi/*.o *lwip/*.o).text* .rodata .rodata* .constdata .constdata*)
   /* other content here */
} > FLASH

.data : ALIGN(4)
{
  /* other content here */
  *my_wifi/*.o(.text .text* .rodata .rodata*)
  /* other content here */
} > SRAM AT>FLASH

And in the linker .map file I can verify that things worked: