output/null: Add a null module that discards all data.

This is useful for testing purposes.

Makefile.am

@@ -92,7 +92,8 @@ libsigrok_la_SOURCES += \
 	src/output/hex.c \
 	src/output/ols.c \
 	src/output/srzip.c \
-	src/output/vcd.c
+	src/output/vcd.c \
+	src/output/null.c
 
 # Transform modules
 libsigrok_la_SOURCES += \

src/output/null.c

@@ -0,0 +1,46 @@
+/*
+ * This file is part of the libsigrok project.
+ *
+ * Copyright (C) 2018 Uwe Hermann <uwe@hermann-uwe.de>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <config.h>
+#include <glib.h>
+#include <libsigrok/libsigrok.h>
+#include "libsigrok-internal.h"
+
+#define LOG_PREFIX "output/null"
+
+static int receive(const struct sr_output *o, const struct sr_datafeed_packet *packet,
+		GString **out)
+{
+	(void)o;
+	(void)packet;
+
+	*out = NULL;
+
+	return SR_OK;
+}
+
+SR_PRIV struct sr_output_module output_null = {
+	.id = "null",
+	.name = "Null output",
+	.desc = "Null output (discards all data)",
+	.exts = NULL,
+	.flags = 0,
+	.options = NULL,
+	.receive = receive,
+};

src/output/output.c

@@ -64,6 +64,7 @@ extern SR_PRIV struct sr_output_module output_csv;
 extern SR_PRIV struct sr_output_module output_analog;
 extern SR_PRIV struct sr_output_module output_srzip;
 extern SR_PRIV struct sr_output_module output_wav;
+extern SR_PRIV struct sr_output_module output_null;
 /* @endcond */
 
 static const struct sr_output_module *output_module_list[] = {
@@ -78,6 +79,7 @@ static const struct sr_output_module *output_module_list[] = {
 	&output_analog,
 	&output_srzip,
 	&output_wav,
+	&output_null,
 	NULL,
 };