diff --git a/lib/fmt.c b/lib/fmt.c
index 6d8b085..8937dc3 100644
--- a/lib/fmt.c
+++ b/lib/fmt.c
@@ -1807,15 +1807,15 @@ static int print_bcs (hcl_fmtout_t* fmtout, const hcl_bch_t* ptr, hcl_oow_t len)
 		ucsptr = ucsbuf;
 		while (ucslen > 0)
 		{
-			hcl->io.outarg.ptr = ucsptr;
-			hcl->io.outarg.len = ucslen;
+			hcl->io.udo_arg.ptr = ucsptr;
+			hcl->io.udo_arg.len = ucslen;
 
-			if (hcl->io.printer(hcl, HCL_IO_WRITE, &hcl->io.outarg) <= -1) return -1;
-			if (hcl->io.outarg.xlen <= 0) return 0; /* end of stream. but not failure */
+			if (hcl->io.udo_wrtr(hcl, HCL_IO_WRITE, &hcl->io.udo_arg) <= -1) return -1;
+			if (hcl->io.udo_arg.xlen <= 0) return 0; /* end of stream. but not failure */
 
-			HCL_ASSERT (hcl, hcl->io.outarg.xlen <= len);
-			ucsptr += hcl->io.outarg.xlen;
-			ucslen -= hcl->io.outarg.xlen;
+			HCL_ASSERT (hcl, hcl->io.udo_arg.xlen <= len);
+			ucsptr += hcl->io.udo_arg.xlen;
+			ucslen -= hcl->io.udo_arg.xlen;
 		}
 
 		ptr += bcslen;
@@ -1827,15 +1827,15 @@ static int print_bcs (hcl_fmtout_t* fmtout, const hcl_bch_t* ptr, hcl_oow_t len)
 	optr = (hcl_bch_t*)ptr;
 	while (len > 0)
 	{
-		hcl->io.outarg.ptr = optr;
-		hcl->io.outarg.len = len;
+		hcl->io.udo_arg.ptr = optr;
+		hcl->io.udo_arg.len = len;
 
-		if (hcl->io.printer(hcl, HCL_IO_WRITE, &hcl->io.outarg) <= -1) return -1;
-		if (hcl->io.outarg.xlen <= 0) return 0; /* end of stream. but not failure */
+		if (hcl->io.udo_wrtr(hcl, HCL_IO_WRITE, &hcl->io.udo_arg) <= -1) return -1;
+		if (hcl->io.udo_arg.xlen <= 0) return 0; /* end of stream. but not failure */
 
-		HCL_ASSERT (hcl, hcl->io.outarg.xlen <= len);
-		optr += hcl->io.outarg.xlen;
-		len -= hcl->io.outarg.xlen;
+		HCL_ASSERT (hcl, hcl->io.udo_arg.xlen <= len);
+		optr += hcl->io.udo_arg.xlen;
+		len -= hcl->io.udo_arg.xlen;
 	}
 #endif
 #else
@@ -1844,15 +1844,15 @@ static int print_bcs (hcl_fmtout_t* fmtout, const hcl_bch_t* ptr, hcl_oow_t len)
 	optr = (hcl_bch_t*)ptr;
 	while (len > 0)
 	{
-		hcl->io.outarg.ptr = optr;
-		hcl->io.outarg.len = len;
+		hcl->io.udo_arg.ptr = optr;
+		hcl->io.udo_arg.len = len;
 
-		if (hcl->io.printer(hcl, HCL_IO_WRITE_BYTES, &hcl->io.outarg) <= -1) return -1;
-		if (hcl->io.outarg.xlen <= 0) return 0; /* end of stream. but not failure */
+		if (hcl->io.udo_wrtr(hcl, HCL_IO_WRITE_BYTES, &hcl->io.udo_arg) <= -1) return -1;
+		if (hcl->io.udo_arg.xlen <= 0) return 0; /* end of stream. but not failure */
 
-		HCL_ASSERT (hcl, hcl->io.outarg.xlen <= len);
-		optr += hcl->io.outarg.xlen;
-		len -= hcl->io.outarg.xlen;
+		HCL_ASSERT (hcl, hcl->io.udo_arg.xlen <= len);
+		optr += hcl->io.udo_arg.xlen;
+		len -= hcl->io.udo_arg.xlen;
 	}
 #endif
 
@@ -1869,15 +1869,15 @@ static int print_ucs (hcl_fmtout_t* fmtout, const hcl_uch_t* ptr, hcl_oow_t len)
 	optr = (hcl_uch_t*)ptr;
 	while (len > 0)
 	{
-		hcl->io.outarg.ptr = optr;
-		hcl->io.outarg.len = len;
+		hcl->io.udo_arg.ptr = optr;
+		hcl->io.udo_arg.len = len;
 
-		if (hcl->io.printer(hcl, HCL_IO_WRITE, &hcl->io.outarg) <= -1) return -1;
-		if (hcl->io.outarg.xlen <= 0) return 0; /* end of stream. but not failure */
+		if (hcl->io.udo_wrtr(hcl, HCL_IO_WRITE, &hcl->io.udo_arg) <= -1) return -1;
+		if (hcl->io.udo_arg.xlen <= 0) return 0; /* end of stream. but not failure */
 
-		HCL_ASSERT (hcl, hcl->io.outarg.xlen <= len);
-		optr += hcl->io.outarg.xlen;
-		len -= hcl->io.outarg.xlen;
+		HCL_ASSERT (hcl, hcl->io.udo_arg.xlen <= len);
+		optr += hcl->io.udo_arg.xlen;
+		len -= hcl->io.udo_arg.xlen;
 	}
 #else
 	hcl_oow_t bcslen, ucslen;
@@ -1892,15 +1892,15 @@ static int print_ucs (hcl_fmtout_t* fmtout, const hcl_uch_t* ptr, hcl_oow_t len)
 		bcsptr = bcsbuf;
 		while (bcslen > 0)
 		{
-			hcl->io.outarg.ptr = bcsptr;
-			hcl->io.outarg.len = bcslen;
+			hcl->io.udo_arg.ptr = bcsptr;
+			hcl->io.udo_arg.len = bcslen;
 
-			if (hcl->io.printer(hcl, HCL_IO_WRITE, &hcl->io.outarg) <= -1) return -1;
-			if (hcl->io.outarg.xlen <= 0) return 0; /* end of stream. but not failure */
+			if (hcl->io.udo_wrtr(hcl, HCL_IO_WRITE, &hcl->io.udo_arg) <= -1) return -1;
+			if (hcl->io.udo_arg.xlen <= 0) return 0; /* end of stream. but not failure */
 
-			HCL_ASSERT (hcl, hcl->io.outarg.xlen <= len);
-			bcsptr += hcl->io.outarg.xlen;
-			bcslen -= hcl->io.outarg.xlen;
+			HCL_ASSERT (hcl, hcl->io.udo_arg.xlen <= len);
+			bcsptr += hcl->io.udo_arg.xlen;
+			bcslen -= hcl->io.udo_arg.xlen;
 		}
 
 		ptr += ucslen;
diff --git a/lib/hcl-s.c b/lib/hcl-s.c
index b09bb07..cced7c9 100644
--- a/lib/hcl-s.c
+++ b/lib/hcl-s.c
@@ -572,7 +572,7 @@ static int print_handler (hcl_t* hcl, hcl_io_cmd_t cmd, void* arg)
 		case HCL_IO_WRITE:
 		{
 			worker_hcl_xtn_t* xtn = (worker_hcl_xtn_t*)hcl_getxtn(hcl);
-			hcl_io_outarg_t* outarg = (hcl_io_outarg_t*)arg;
+			hcl_io_udoarg_t* outarg = (hcl_io_udoarg_t*)arg;
 
 			if (hcl_server_proto_feed_reply(xtn->proto, outarg->ptr, outarg->len, 0) <= -1)
 			{
@@ -591,7 +591,7 @@ static int print_handler (hcl_t* hcl, hcl_io_cmd_t cmd, void* arg)
 		case HCL_IO_WRITE_BYTES:
 		{
 			worker_hcl_xtn_t* xtn = (worker_hcl_xtn_t*)hcl_getxtn(hcl);
-			hcl_io_outarg_t* outarg = (hcl_io_outarg_t*)arg;
+			hcl_io_udoarg_t* outarg = (hcl_io_udoarg_t*)arg;
 
 			if (hcl_server_proto_feed_reply_bytes(xtn->proto, outarg->ptr, outarg->len, 0) <= -1)
 			{
diff --git a/lib/hcl.h b/lib/hcl.h
index 6e95f3b..9bcc427 100644
--- a/lib/hcl.h
+++ b/lib/hcl.h
@@ -1268,8 +1268,8 @@ struct hcl_io_sciarg_t
 	/*-----------------------------------------------------------------*/
 };
 
-typedef struct hcl_io_inarg_t hcl_io_inarg_t;
-struct hcl_io_inarg_t
+typedef struct hcl_io_udiarg_t hcl_io_udiarg_t;
+struct hcl_io_udiarg_t
 {
 	/**
 	 * [OUT] I/O handle set by a handler.
@@ -1290,8 +1290,8 @@ struct hcl_io_inarg_t
 	hcl_oow_t xlen;
 };
 
-typedef struct hcl_io_outarg_t hcl_io_outarg_t;
-struct hcl_io_outarg_t
+typedef struct hcl_io_udoarg_t hcl_io_udoarg_t;
+struct hcl_io_udoarg_t
 {
 	/**
 	 * [OUT] I/O handle set by a handler.
@@ -1328,7 +1328,7 @@ struct hcl_io_outarg_t
 typedef int (*hcl_io_impl_t) (
 	hcl_t*        hcl,
 	hcl_io_cmd_t  cmd,
-	void*         arg /* one of hcl_io_sciarg_t*, hcl_io_inarg_t*, hcl_io_outarg_t* */
+	void*         arg /* one of hcl_io_sciarg_t*, hcl_io_udiarg_t*, hcl_io_udoarg_t* */
 );
 
 /* =========================================================================
@@ -1706,7 +1706,7 @@ struct hcl_t
 		hcl_dbgi_t* dbgi;
 	} code;
 
-	/* == PRINTER == */
+	/* == PRINTER to udo stream == */
 	struct
 	{
 		struct
@@ -1717,7 +1717,7 @@ struct hcl_t
 		} s;
 		hcl_oop_t e; /* top entry being printed */
 	} p;
-	/* == PRINTER == */
+	/* == PRINTER to udo stream == */
 
 	struct
 	{
@@ -1750,12 +1750,12 @@ struct hcl_t
 	struct
 	{
 		/* input handler */
-		hcl_io_impl_t scanner;
-		hcl_io_inarg_t inarg;
+		hcl_io_impl_t udi_rdr;
+		hcl_io_udiarg_t udi_arg;
 
 		/* output handler */
-		hcl_io_impl_t printer;
-		hcl_io_outarg_t outarg;
+		hcl_io_impl_t udo_wrtr;
+		hcl_io_udoarg_t udo_arg;
 	} io;
 
 #if defined(HCL_INCLUDE_COMPILER)
@@ -2243,8 +2243,8 @@ HCL_EXPORT hcl_ooch_t* hcl_readbasesrraw (
 HCL_EXPORT int hcl_attachio (
 	hcl_t*          hcl,
 	hcl_io_impl_t   sci_rdr, /* source code input handler */
-	hcl_io_impl_t   scanner, /* user data input handler */
-	hcl_io_impl_t   printer /* user data output handler */
+	hcl_io_impl_t   udi_rdr, /* user data input handler */
+	hcl_io_impl_t   udo_wrtr /* user data output handler */
 );
 
 HCL_EXPORT int hcl_attachiostdwithbcstr (
diff --git a/lib/print.c b/lib/print.c
index 18a90ae..aafe666 100644
--- a/lib/print.c
+++ b/lib/print.c
@@ -778,7 +778,7 @@ int hcl_outfmtobj (hcl_t* hcl, hcl_bitmask_t mask, hcl_oop_t obj, hcl_outbfmt_t
 
 int hcl_print (hcl_t* hcl, hcl_oop_t obj)
 {
-	HCL_ASSERT (hcl, hcl->io.printer != HCL_NULL);
+	HCL_ASSERT (hcl, hcl->io.udo_wrtr != HCL_NULL);
 	/*return hcl_outfmtobj(hcl, HCL_LOG_APP | HCL_LOG_FATAL, obj);*/
 	return hcl_prbfmt(hcl, "%O", obj);
 }
diff --git a/lib/read.c b/lib/read.c
index 2afed97..96a8c86 100644
--- a/lib/read.c
+++ b/lib/read.c
@@ -2669,7 +2669,7 @@ default callback for on_eof?
 
 /* ------------------------------------------------------------------------ */
 
-/* TODO: rename compiler to something else that can include reader, printer, and compiler
+/* TODO: rename compiler to something else that can include reader, udo_wrtr, and compiler
  * move compiler intialization/finalization here to more common place */
 
 static void gc_compiler_cb (hcl_t* hcl)
@@ -2786,13 +2786,13 @@ static int init_compiler (hcl_t* hcl)
 	return 0;
 }
 
-int hcl_attachio (hcl_t* hcl, hcl_io_impl_t sci_rdr, hcl_io_impl_t scanner, hcl_io_impl_t printer)
+int hcl_attachio (hcl_t* hcl, hcl_io_impl_t sci_rdr, hcl_io_impl_t udi_rdr, hcl_io_impl_t udo_wrtr)
 {
 	int n;
 	int inited_compiler = 0;
 	hcl_io_sciarg_t new_sciarg;
-	hcl_io_inarg_t new_inarg;
-	hcl_io_outarg_t new_outarg;
+	hcl_io_udiarg_t new_udiarg;
+	hcl_io_udoarg_t new_udoarg;
 
 	if (!hcl->c)
 	{
@@ -2813,10 +2813,10 @@ int hcl_attachio (hcl_t* hcl, hcl_io_impl_t sci_rdr, hcl_io_impl_t scanner, hcl_
 		if (n <= -1) goto oops;
 	}
 
-	if (scanner)
+	if (udi_rdr)
 	{
-		HCL_MEMSET (&new_inarg, 0, HCL_SIZEOF(new_inarg));
-		n = scanner(hcl, HCL_IO_OPEN, &new_inarg);
+		HCL_MEMSET (&new_udiarg, 0, HCL_SIZEOF(new_udiarg));
+		n = udi_rdr(hcl, HCL_IO_OPEN, &new_udiarg);
 		if (n <= -1)
 		{
 			sci_rdr (hcl, HCL_IO_CLOSE, &new_sciarg);
@@ -2824,14 +2824,14 @@ int hcl_attachio (hcl_t* hcl, hcl_io_impl_t sci_rdr, hcl_io_impl_t scanner, hcl_
 		}
 	}
 
-	if (printer)
+	if (udo_wrtr)
 	{
 		/* open the new output stream */
-		HCL_MEMSET (&new_outarg, 0, HCL_SIZEOF(new_outarg));
-		n = printer(hcl, HCL_IO_OPEN, &new_outarg);
+		HCL_MEMSET (&new_udoarg, 0, HCL_SIZEOF(new_udoarg));
+		n = udo_wrtr(hcl, HCL_IO_OPEN, &new_udoarg);
 		if (n <= -1)
 		{
-			if (scanner) scanner (hcl, HCL_IO_CLOSE, &new_inarg);
+			if (udi_rdr) udi_rdr (hcl, HCL_IO_CLOSE, &new_udiarg);
 			if (sci_rdr) sci_rdr (hcl, HCL_IO_CLOSE, &new_sciarg);
 			goto oops;
 		}
@@ -2848,26 +2848,26 @@ int hcl_attachio (hcl_t* hcl, hcl_io_impl_t sci_rdr, hcl_io_impl_t scanner, hcl_
 		hcl->c->sci_arg = new_sciarg;
 	}
 
-	if (scanner)
+	if (udi_rdr)
 	{
-		if (hcl->io.scanner)
+		if (hcl->io.udi_rdr)
 		{
 			/* close the old input stream */
-			hcl->io.scanner (hcl, HCL_IO_CLOSE, &hcl->io.inarg);
+			hcl->io.udi_rdr (hcl, HCL_IO_CLOSE, &hcl->io.udi_arg);
 		}
-		hcl->io.scanner = scanner;
-		hcl->io.inarg = new_inarg;
+		hcl->io.udi_rdr = udi_rdr;
+		hcl->io.udi_arg = new_udiarg;
 	}
 
-	if (printer)
+	if (udo_wrtr)
 	{
-		if (hcl->io.printer)
+		if (hcl->io.udo_wrtr)
 		{
 			/* close the old output stream */
-			hcl->io.printer (hcl, HCL_IO_CLOSE, &hcl->io.outarg);
+			hcl->io.udo_wrtr (hcl, HCL_IO_CLOSE, &hcl->io.udo_arg);
 		}
-		hcl->io.printer = printer;
-		hcl->io.outarg = new_outarg;
+		hcl->io.udo_wrtr = udo_wrtr;
+		hcl->io.udo_arg = new_udoarg;
 	}
 
 	if (sci_rdr)
@@ -2890,7 +2890,7 @@ oops:
 
 void hcl_flushio (hcl_t* hcl)
 {
-	if (hcl->io.printer) hcl->io.printer (hcl, HCL_IO_FLUSH, &hcl->io.outarg);
+	if (hcl->io.udo_wrtr) hcl->io.udo_wrtr (hcl, HCL_IO_FLUSH, &hcl->io.udo_arg);
 }
 
 void hcl_detachio (hcl_t* hcl)
@@ -2922,16 +2922,16 @@ void hcl_detachio (hcl_t* hcl)
 
 	}
 
-	if (hcl->io.scanner)
+	if (hcl->io.udi_rdr)
 	{
-		hcl->io.scanner (hcl, HCL_IO_CLOSE, &hcl->io.inarg);
-		hcl->io.scanner = HCL_NULL; /* ready for another attachment */
+		hcl->io.udi_rdr (hcl, HCL_IO_CLOSE, &hcl->io.udi_arg);
+		hcl->io.udi_rdr = HCL_NULL; /* ready for another attachment */
 	}
 
-	if (hcl->io.printer)
+	if (hcl->io.udo_wrtr)
 	{
-		hcl->io.printer (hcl, HCL_IO_CLOSE, &hcl->io.outarg);
-		hcl->io.printer = HCL_NULL; /* ready for another attachment */
+		hcl->io.udo_wrtr (hcl, HCL_IO_CLOSE, &hcl->io.udo_arg);
+		hcl->io.udo_wrtr = HCL_NULL; /* ready for another attachment */
 	}
 }
 
diff --git a/lib/std.c b/lib/std.c
index 1de8e52..87813a4 100644
--- a/lib/std.c
+++ b/lib/std.c
@@ -3396,7 +3396,7 @@ static int sci_handler (hcl_t* hcl, hcl_io_cmd_t cmd, void* arg)
 }
 
 /* --------------------------------------------------------------------- */
-static HCL_INLINE int open_in_stream (hcl_t* hcl, hcl_io_inarg_t* arg)
+static HCL_INLINE int open_in_stream (hcl_t* hcl, hcl_io_udiarg_t* arg)
 {
 	xtn_t* xtn = GET_XTN(hcl);
 	bb_t* bb = HCL_NULL;
@@ -3438,7 +3438,7 @@ oops:
 	return -1;
 }
 
-static HCL_INLINE int close_in_stream (hcl_t* hcl, hcl_io_inarg_t* arg)
+static HCL_INLINE int close_in_stream (hcl_t* hcl, hcl_io_udiarg_t* arg)
 {
 	/*xtn_t* xtn = GET_XTN(hcl);*/
 	bb_t* bb;
@@ -3453,7 +3453,7 @@ static HCL_INLINE int close_in_stream (hcl_t* hcl, hcl_io_inarg_t* arg)
 	return 0;
 }
 
-static HCL_INLINE int read_in_stream (hcl_t* hcl, hcl_io_inarg_t* arg)
+static HCL_INLINE int read_in_stream (hcl_t* hcl, hcl_io_udiarg_t* arg)
 {
 	/*xtn_t* xtn = GET_XTN(hcl);*/
 	bb_t* bb;
@@ -3508,13 +3508,13 @@ static int scan_handler (hcl_t* hcl, hcl_io_cmd_t cmd, void* arg)
 	switch (cmd)
 	{
 		case HCL_IO_OPEN:
-			return open_in_stream(hcl, (hcl_io_inarg_t*)arg);
+			return open_in_stream(hcl, (hcl_io_udiarg_t*)arg);
 
 		case HCL_IO_CLOSE:
-			return close_in_stream(hcl, (hcl_io_inarg_t*)arg);
+			return close_in_stream(hcl, (hcl_io_udiarg_t*)arg);
 
 		case HCL_IO_READ:
-			return read_in_stream(hcl, (hcl_io_inarg_t*)arg);
+			return read_in_stream(hcl, (hcl_io_udiarg_t*)arg);
 
 		case HCL_IO_FLUSH:
 			/* no effect on an input stream */
@@ -3528,7 +3528,7 @@ static int scan_handler (hcl_t* hcl, hcl_io_cmd_t cmd, void* arg)
 
 /* --------------------------------------------------------------------- */
 
-static HCL_INLINE int open_out_stream (hcl_t* hcl, hcl_io_outarg_t* arg)
+static HCL_INLINE int open_out_stream (hcl_t* hcl, hcl_io_udoarg_t* arg)
 {
 	xtn_t* xtn = GET_XTN(hcl);
 	FILE* fp;
@@ -3552,7 +3552,7 @@ static HCL_INLINE int open_out_stream (hcl_t* hcl, hcl_io_outarg_t* arg)
 	return 0;
 }
 
-static HCL_INLINE int close_out_stream (hcl_t* hcl, hcl_io_outarg_t* arg)
+static HCL_INLINE int close_out_stream (hcl_t* hcl, hcl_io_udoarg_t* arg)
 {
 	/*xtn_t* xtn = GET_XTN(hcl);*/
 	FILE* fp;
@@ -3564,7 +3564,7 @@ static HCL_INLINE int close_out_stream (hcl_t* hcl, hcl_io_outarg_t* arg)
 	return 0;
 }
 
-static HCL_INLINE int write_out_stream (hcl_t* hcl, hcl_io_outarg_t* arg)
+static HCL_INLINE int write_out_stream (hcl_t* hcl, hcl_io_udoarg_t* arg)
 {
 	/*xtn_t* xtn = GET_XTN(hcl);*/
 	const hcl_ooch_t* ptr;
@@ -3604,7 +3604,7 @@ static HCL_INLINE int write_out_stream (hcl_t* hcl, hcl_io_outarg_t* arg)
 	return 0;
 }
 
-static HCL_INLINE int write_bytes_out_stream (hcl_t* hcl, hcl_io_outarg_t* arg)
+static HCL_INLINE int write_bytes_out_stream (hcl_t* hcl, hcl_io_udoarg_t* arg)
 {
 	/*xtn_t* xtn = GET_XTN(hcl);*/
 	const hcl_uint8_t* ptr;
@@ -3621,7 +3621,7 @@ static HCL_INLINE int write_bytes_out_stream (hcl_t* hcl, hcl_io_outarg_t* arg)
 	return 0;
 }
 
-static HCL_INLINE int flush_out_stream (hcl_t* hcl, hcl_io_outarg_t* arg)
+static HCL_INLINE int flush_out_stream (hcl_t* hcl, hcl_io_udoarg_t* arg)
 {
 	FILE* fp;
 
@@ -3637,19 +3637,19 @@ static int print_handler (hcl_t* hcl, hcl_io_cmd_t cmd, void* arg)
 	switch (cmd)
 	{
 		case HCL_IO_OPEN:
-			return open_out_stream(hcl, (hcl_io_outarg_t*)arg);
+			return open_out_stream(hcl, (hcl_io_udoarg_t*)arg);
 
 		case HCL_IO_CLOSE:
-			return close_out_stream(hcl, (hcl_io_outarg_t*)arg);
+			return close_out_stream(hcl, (hcl_io_udoarg_t*)arg);
 
 		case HCL_IO_WRITE:
-			return write_out_stream(hcl, (hcl_io_outarg_t*)arg);
+			return write_out_stream(hcl, (hcl_io_udoarg_t*)arg);
 
 		case HCL_IO_WRITE_BYTES:
-			return write_bytes_out_stream(hcl, (hcl_io_outarg_t*)arg);
+			return write_bytes_out_stream(hcl, (hcl_io_udoarg_t*)arg);
 
 		case HCL_IO_FLUSH:
-			return flush_out_stream(hcl, (hcl_io_outarg_t*)arg);
+			return flush_out_stream(hcl, (hcl_io_udoarg_t*)arg);
 
 		default:
 			hcl_seterrnum (hcl, HCL_EINTERN);