+ favail = bdirect_read_prepare(f, &fptr);
+ if (!favail)
+ die("bbcopy: source exhausted");
+ tavail = bdirect_write_prepare(t, &tptr);
+ n = MIN(l, favail);
+ n = MIN(n, tavail);
+ memcpy(tptr, fptr, n);
+ bdirect_read_commit(f, fptr + n);
+ bdirect_write_commit(t, tptr + n);
+ l -= n;
+ }