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using namespace std;
using namespace __gnu_cxx;
  
template<>
struct __gnu_cxx::hash<const off64_t> {
  size_t operator() (const off64_t o) const {
    return(o&0xFF);
  };
};

hash_map<const off64_t, char, hash<const off64_t>, eq_offset> block_map;

with this error:

server.cpp:58: error: specialization of 'template<class _Key> struct __gnu_cxx::hash' in different namespace

which is the __gnu_cxx::hash template specialization.