Using gdb with emacs

[Richard Stallman]

    What you, in effect, want is for GUD to support relative file names
    which are not relative to the default directory of the buffer from
    which you set the breakpoint.  This invalidates the assumptions on
    which Emacs primitives such as expand-file-name and file-relative-name
    base their default operation.

These primitives use the value of the variable default-directory,
which Lisp code can easily bind.  So these primitives don't limit
anything.  As long as there is a clear convention for what the
default directory should be, the code in gud.el should be able
to handle it.