Readonly variables are assigned the PSECT attribute NOWRT and are
  stored in the PSECT $READONLY$ which is a nonwritable data area.
  Other programs can access the PSECT directly, but none of the
  information can be overwritten.  You can use the readonly modifier
  with the storage-class keywords [extern], static, globaldef, and
  globaldef{"name"}.

  You can use both the readonly and noshare modifiers with the
  [extern] and the globaldef{"name"} specifiers.  If you use both
  modifiers with either the static or the globaldef specifiers, the
  compiler ignores noshare and accepts readonly.