4.7.3 Passing Parameters by Reference
If the parameter is passed by reference (e.g., Pascal
parameters), then the symbol descriptor is v if it is in the
argument list, or a if it in a register. Other than the fact
that these contain the address of the parameter rather than the
parameter itself, they are identical to p and R,
respectively. I believe a is an AIX invention; v is
supported by all stabs-using systems as far as I know.