Access linker symbol from .C file

Altera_Forum · ‎06-09-2005

I'm trying to access a linker symbol from my C file. The idea is to find out where the end of my code and R/O memory is in flash so I can use the remainder for storage. I have the following in my .c file:

.C file:

extern int __ram_rodata_end;

void *p = &__ram_rodata_end;

The .map file shows the symbol __ram_rodata_end like this:

0x01014f20 PROVIDE (__ram_rodata_end, <code 336> (.))

But the linker gives me the error below. If I use a symbol that's in reach (e.g. __sbss_end) everything works and p points to the end of data. The format in the map file is exactly the same so I know that's not the problem.

Linker error:

../test.c:33: Unable to reach __ram_rodata_end (at 0x01014f20) from the global pointer (at 0x0101ed34) because the offset (-40468) is out of the allowed range, -32678 to 32767.

Here's the assembly code causing the problem:

145 0100 040080D0 addi r2,gp,%gprel(__ram_rodata_end)

146 0104 150680E0 stw r2,24(fp)

Is there an easy way of getting the value of a linker symbol into a C variable? I'm not above using inline assembly with a global or static variable to get the value.

Thanks,

Andrew

Altera_Forum · ‎06-10-2005

Ok, I'll answer my own question. To access a linker symbol that's out of reach use the following code:

extern int __ram_rodata_end[];

printf("%x\n", __ram_rodata_end);

That'll print out the same value you'll see in the .map file. The trick is to use an array declaration which forces GCC to use full-range addressing.

Thanks to David Brooks on comp.arch.embedded for the answer.

Andrew

Altera_Forum · ‎06-26-2005

The problem you had is related to the fact that you declared the extern variable as "int", that means the compiler will generate GP-relative addressing, that is not the right one in your case.

Take a look at the post "Disabling GP-addressing in generated source code" on this forum...

bye

PJ