[SPDK] Export file as a block device using SPDK

How can we export a file as a block device using SPDK? Actually, currently I can now export a spdk block device(linux aio or malloc) using nvmf_tgt or iscsi_tgt and can access it on initiator. But can anyone help me export a file as a block device to the initiator using nvmf_tgt or iscsi_tgt.