#define SUM_EXTRA_CNT ((MD4_SUM_LENGTH + EXTRA_LEN - 1) / EXTRA_LEN)
#define REQ_EXTRA(f,ndx) ((union file_extras*)(f) - (ndx))
#define SUM_EXTRA_CNT ((MD4_SUM_LENGTH + EXTRA_LEN - 1) / EXTRA_LEN)
#define REQ_EXTRA(f,ndx) ((union file_extras*)(f) - (ndx))