[arm-gnu] how to locate a const struct in memory (ld question)

["Andrew Kohlsmith (mailing lists account)" <aklists@xxxxxxxxxx>]

Good afternoon, everyone,

I'm working on an ARM7 (LPC2103) design and have a generic linker question 
regarding locating a data structure in ROM.

Essentially I have a bootloader in flash sector 0 which will never be 
overwritten. The bootloader is much smaller than the 4kB sector size, so I 
have padded it out with some low-level functions which are unlikely to ever 
need to be updated. (If the end up needing to be updated, the application can 
just use its own version instead of calling the bootloader's version.

I've built up a structure of function pointers which the bootloader has in its 
memory space:

bootloader_funcs.h:
typedef struct {
	void (* const cli)(void);
	void (* const sti)(void);
	void (* const doze)(void)
	void (* const oled_init)(void);
	int (* const timer_get_ms)(void);

	/* etc */
} bl_func_struct;

extern const bl_func_struct bl_funcs;

And then the bootloader has a .c file which actually allocates and initializes 
this structure:

bootloader_funcs.c:
const bl_func_struct bl_funcs = {
	.cli = cli,
	.sti = sti,
	.doze = doze,

	/* etc. */
};

Now this compiles and the code looks about right for what I'd expect. The main 
(upgradeable) application accesses this structure like this:

#include <bootloader_funcs.h>
#define cli bl_funcs.cli()
#define doze bl_funcs.doze()

etc.  I've experimented with using inline void cli(void) { bl.funcs.cli(); } 
and other means, and if anyone has any suggestions on how to better write this 
I'm open to suggestions. :-) I don't like the #defines, personally.

Now, much to my chagrin, I find that when I use -ffunction-sections and -fdata-
sections so that I can place the bl_funcs struct at a specific location that it 
does not work. I don't get a specific section for bl_funcs, although I do get 
specific sections for the various functions (cli, sti, etc.) and also for their 
variables.  Objdump -t shows me that bl_funcs has no section name at all:

0000e20 g     O        00000068 bl_funcs

'g' says it's global and O says it's an object, which I suppose is right.

Is there a way to specify a specific location for a specific object, or perhaps 
should I be declaring the structure differently so that I can get a section out 
of it for ld to put at a specific location?

Finally, right now the bootloader and the application are built at the same 
time, so the linker is resolving bl_funcs for the application's extern 
declaration. Do I use the linker PROVIDE() function to do this when the 
bootloader is removed from the image?

Thanks for your assistance,

-A.