From c2cf7c80183478d574f9d1be4a04c7fbf3943502 Mon Sep 17 00:00:00 2001
From: Ian Abbott <abbotti@mev.co.uk>
Date: Tue, 8 Nov 2016 14:15:35 +0000
Subject: [PATCH] demo/cmd: check subdevice exists and supports 'read' commands

Check the subdevice actually exists and that it claims to support 'read'
commands (SDF_CMD_READ subdevice flag set), bailing out early if not.
---
 demo/cmd.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/demo/cmd.c b/demo/cmd.c
index f15cfe7..d024f6a 100644
--- a/demo/cmd.c
+++ b/demo/cmd.c
@@ -96,6 +96,20 @@ int main(int argc, char *argv[])
 		fprintf(stderr, "defaulted to subdevice %d\n", options.subdevice);
 	}
 
+	/* Check subdevice exists. */
+	ret = comedi_get_n_subdevices(dev);
+	if(ret <= options.subdevice){
+		fprintf(stderr, "subdevice %d does not exist\n", options.subdevice);
+		exit(1);
+	}
+
+	/* Check subdevice supports 'read' commands. */
+	ret = comedi_get_subdevice_flags(dev, options.subdevice);
+	if(ret < 0 || !(ret & SDF_CMD_READ)) {
+		fprintf(stderr, "subdevice %d does not support 'read' commands\n", options.subdevice);
+		exit(1);
+	}
+
 	// Print numbers for clipped inputs
 	comedi_set_global_oor_behavior(COMEDI_OOR_NUMBER);