Index: uspace/lib/c/generic/async.c
===================================================================
--- uspace/lib/c/generic/async.c	(revision b946bf83ff435bbd24129b5a1e51abdaa1b57956)
+++ uspace/lib/c/generic/async.c	(revision f6bffeebe3a4e9f347a03931d2c463a8718cd991)
@@ -1572,12 +1572,14 @@
  * @param dst     Address of the beginning of the destination buffer.
  * @param size    Size of the destination buffer.
+ * @param flags   Flags to control the data transfer.
  *
  * @return Zero on success or a negative error code from errno.h.
  *
  */
-int async_data_read_start(int phoneid, void *dst, size_t size)
-{
-	return async_req_2_0(phoneid, IPC_M_DATA_READ, (sysarg_t) dst,
-	    (sysarg_t) size);
+int
+async_data_read_start_flexible(int phoneid, void *dst, size_t size, int flags)
+{
+	return async_req_3_0(phoneid, IPC_M_DATA_READ, (sysarg_t) dst,
+	    (sysarg_t) size, (sysarg_t) flags);
 }
 
@@ -1669,12 +1671,15 @@
  * @param src     Address of the beginning of the source buffer.
  * @param size    Size of the source buffer.
+ * @param flags   Flags to control the data transfer.
  *
  * @return Zero on success or a negative error code from errno.h.
  *
  */
-int async_data_write_start(int phoneid, const void *src, size_t size)
-{
-	return async_req_2_0(phoneid, IPC_M_DATA_WRITE, (sysarg_t) src,
-	    (sysarg_t) size);
+int
+async_data_write_start_flexible(int phoneid, const void *src, size_t size,
+    int flags)
+{
+	return async_req_3_0(phoneid, IPC_M_DATA_WRITE, (sysarg_t) src,
+	    (sysarg_t) size, (sysarg_t) flags);
 }
 
Index: uspace/lib/c/generic/vfs/vfs.c
===================================================================
--- uspace/lib/c/generic/vfs/vfs.c	(revision b946bf83ff435bbd24129b5a1e51abdaa1b57956)
+++ uspace/lib/c/generic/vfs/vfs.c	(revision f6bffeebe3a4e9f347a03931d2c463a8718cd991)
@@ -378,5 +378,6 @@
 	
 	req = async_send_1(vfs_phone, VFS_IN_READ, fildes, &answer);
-	rc = async_data_read_start(vfs_phone, (void *)buf, nbyte);
+	rc = async_data_read_start_flexible(vfs_phone, (void *) buf, nbyte,
+	    IPC_XF_RESTRICT);
 	if (rc != EOK) {
 		vfs_exchange_end(vfs_phone);
@@ -407,5 +408,6 @@
 	
 	req = async_send_1(vfs_phone, VFS_IN_WRITE, fildes, &answer);
-	rc = async_data_write_start(vfs_phone, (void *)buf, nbyte);
+	rc = async_data_write_start_flexible(vfs_phone, (void *) buf, nbyte,
+	    IPC_XF_RESTRICT);
 	if (rc != EOK) {
 		vfs_exchange_end(vfs_phone);
Index: uspace/lib/c/include/async.h
===================================================================
--- uspace/lib/c/include/async.h	(revision b946bf83ff435bbd24129b5a1e51abdaa1b57956)
+++ uspace/lib/c/include/async.h	(revision f6bffeebe3a4e9f347a03931d2c463a8718cd991)
@@ -340,5 +340,8 @@
 	    (arg4), (answer))
 
-extern int async_data_read_start(int, void *, size_t);
+#define async_data_read_start(p, buf, len) \
+	async_data_read_start_flexible((p), (buf), (len), IPC_XF_NONE)
+
+extern int async_data_read_start_flexible(int, void *, size_t, int);
 extern bool async_data_read_receive(ipc_callid_t *, size_t *);
 extern int async_data_read_finalize(ipc_callid_t, const void *, size_t);
@@ -379,5 +382,8 @@
 	    (arg4), (answer))
 
-extern int async_data_write_start(int, const void *, size_t);
+#define async_data_write_start(p, buf, len) \
+	async_data_write_start_flexible((p), (buf), (len), IPC_XF_NONE)
+
+extern int async_data_write_start_flexible(int, const void *, size_t, int);
 extern bool async_data_write_receive(ipc_callid_t *, size_t *);
 extern int async_data_write_finalize(ipc_callid_t, void *, size_t);