diff --git a/include/iscsi-private.h b/include/iscsi-private.h
index dfea31c..dccd8c3 100644
--- a/include/iscsi-private.h
+++ b/include/iscsi-private.h
@@ -288,10 +288,10 @@ void iscsi_pdu_set_datasn(struct iscsi_pdu *pdu, uint32_t datasn);
 void iscsi_pdu_set_bufferoffset(struct iscsi_pdu *pdu, uint32_t bufferoffset);
 void iscsi_cancel_pdus(struct iscsi_context *iscsi);
 int iscsi_pdu_add_data(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
-		       unsigned char *dptr, int dsize);
+		       const unsigned char *dptr, int dsize);
 int iscsi_queue_pdu(struct iscsi_context *iscsi, struct iscsi_pdu *pdu);
 int iscsi_add_data(struct iscsi_context *iscsi, struct iscsi_data *data,
-		   unsigned char *dptr, int dsize, int pdualignment);
+		   const unsigned char *dptr, int dsize, int pdualignment);
 
 struct scsi_task;
 void iscsi_pdu_set_cdb(struct iscsi_pdu *pdu, struct scsi_task *task);
diff --git a/lib/pdu.c b/lib/pdu.c
index f9bdbab..8c8fc99 100644
--- a/lib/pdu.c
+++ b/lib/pdu.c
@@ -169,7 +169,7 @@ iscsi_tcp_free_pdu(struct iscsi_context *iscsi, struct iscsi_pdu *pdu)
 
 int
 iscsi_add_data(struct iscsi_context *iscsi, struct iscsi_data *data,
-	       unsigned char *dptr, int dsize, int pdualignment)
+	       const unsigned char *dptr, int dsize, int pdualignment)
 {
 	size_t len, aligned;
 
@@ -216,7 +216,7 @@ iscsi_add_data(struct iscsi_context *iscsi, struct iscsi_data *data,
 
 int
 iscsi_pdu_add_data(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
-		   unsigned char *dptr, int dsize)
+		   const unsigned char *dptr, int dsize)
 {
 	if (pdu == NULL) {
 		iscsi_set_error(iscsi, "trying to add data to NULL pdu");