How to place a variable at a given absolute address in memory (with GCC)

c variables gcc arm keil
Bas van Dijk picture Bas van Dijk · Nov 1, 2010 · Viewed 60.6k times · Source

The RealView ARM C Compiler supports placing a variable at a given memory address using the variable attribute at(address):

int var __attribute__((at(0x40001000)));
var = 4;   // changes the memory located at 0x40001000

Does GCC have a similar variable attribute?

Answer

Prof. Falken picture Prof. Falken · Nov 1, 2010

I don't know, but you can easily create a workaround like this:

int *var = (int*)0x40001000;
*var = 4;

It's not exactly the same thing, but in most situations a perfect substitute. It will work with any compiler, not just GCC.

If you use GCC, I assume you also use GNU ld (although it is not a certainty, of course) and ld has support for placing variables wherever you want them.

I imagine letting the linker do that job is pretty common.

Inspired by answer by @rib, I'll add that if the absolute address is for some control register, I'd add volatile to the pointer definition. If it is just RAM, it doesn't matter.

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