"But it seems it is not possible to use the at-Keyword to locate a struct in the codespace?!"

Wrong.

The manual clearly states that variables defined with _at_ cannot be initialised.

So you'll either have to do as Erik & Peter have suggested, or just make your 'C' a standard initialised definition (without _at_), and use the Linker to place the structure.

After all, it is the Linker's job to assign addresses; not the compiler's!