verify_blk
ssize_t verify_blk(void *dst, size_t len, size_t expected);
ssize_t verify_blk(void *dst, size_t len, size_t expected);
nread2 = output->verify_blk(buf2, maxblk, (size_t)nread1);
virtual ssize_t verify_blk(void *dst, size_t len, size_t expected);