[systemd-devel] sd_bus_message_read() failing with -ENXIO and -EINVAL

[Erik Falor]

I wish to use sd-bus to query upowerd for my battery's remaining percentage. I
have begun by adapting the example program from Lennart's blog post introducing
the sd-bus API:

	http://0pointer.net/blog/the-new-sd-bus-api-of-systemd.html

However, I cannot succeed in adapting it to my needs. I can read the value of
the Percentage property with

	$ busctl introspect org.freedesktop.UPower /org/freedesktop/UPower/devices/battery_BAT0

This shows me that the Percentage is a double value, and that the Get() method
of the org.freedesktop.DBus.Properties interface returns a variant value.

I can also retrieve the desired datum with busctl:

	$ busctl call org.freedesktop.UPower               \
		/org/freedesktop/UPower/devices/battery_BAT0   \
		org.freedesktop.DBus.Properties Get            \
		ss org.freedesktop.UPower.Device Percentage

I can make the equivalent method call using the sd-bus API from the C program,
but the program errors out at the call to sd_bus_message_read().

When I tell sd_bus_message_read() that my message is of variant type "v", it
fails with -EINVAL. When I instead give it "d", it fails with -ENXIO.

What does sd_bus_message_read() expect in the 2nd argument? Or am I doing
something else wrong?



/*
 gcc bus-client.c -o bus-client -lsystemd
 */
#include <stdio.h>
#include <stdlib.h>
#include <systemd/sd-bus.h>

int main() {
	sd_bus_error error = SD_BUS_ERROR_NULL;
	sd_bus_message *m = NULL;
	sd_bus *bus = NULL;
	int r;
	double p;

	/* Connect to the system bus */
	r = sd_bus_open_system(&bus);
	if (r < 0) {
		fprintf(stderr, "Failed to connect to system bus: %s\n", strerror(-r));
		goto finish;
	}

	/* The following call to sd_bus_call_method() should be equivalent to this cmd:
		busctl call org.freedesktop.UPower             \
		/org/freedesktop/UPower/devices/battery_BAT0   \
		org.freedesktop.DBus.Properties Get            \
		ss org.freedesktop.UPower.Device Percentage
	*/
	r = sd_bus_call_method(bus,
			"org.freedesktop.UPower",                         /* service to contact */
			"/org/freedesktop/UPower/devices/battery_BAT0",   /* object path */
			"org.freedesktop.DBus.Properties",                /* interface name */
			"Get",                                            /* method name */
			&error,                                           /* object to return error in */
			&m,                                               /* return message on success */
			"ss"                                              /* input signature */
			,"org.freedesktop.UPower.Device"                  /* first argument */
			,"Percentage"                                     /* second argument */
			);

	if (r < 0) {
		fprintf(stderr, "Failed to issue method call: %s\n", error.message);
		goto finish;
	}

	/*  XXX This is where the failure is occurring XXX  */
	r = sd_bus_message_read(m, "v", &p);
	if (r < 0) {
		fprintf(stderr, "Failed to parse response message: (%d) %s\n", r, strerror(-r));
		goto finish;
	}

	printf("Battery %f%%", p);

finish:
	sd_bus_error_free(&error);
	sd_bus_message_unref(m);
	sd_bus_unref(bus);

	return r < 0 ? EXIT_FAILURE : EXIT_SUCCESS;
}

/*
 * Running this program results in the following output:
 *
 * % ./bus-client
 * Failed to parse response message: (-22) Invalid argument
 *
 */

-- 
Erik Falor
Registered Linux User #445632                    http://unnovative.net
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