Query systemd's user bus to check for a service's existence.

service-query.c

#include <stdio.h>
#include <stdlib.h>
#include <systemd/sd-bus.h>

static sd_bus_error error = SD_BUS_ERROR_NULL;
static sd_bus_message *response;
static sd_bus *bus;
static int result;

static const char *path;

const char* service_name = "org.freedesktop.systemd1",
		  * service_path = "/org/freedesktop/systemd1";

const char* interface = "org.freedesktop.systemd1.Manager",
		  * operation_id = "GetUnit";

const char* signature = "s",
		  * serviceId = "connector.service";

// Just because I prefer to look at cleaner API names.
#define RpcQueryMethod sd_bus_call_method
#define RpcBusConnect sd_bus_open_user
#define GetErrMessage strerror

int main(int argc, char *argv[]) {
	if (( result = -RpcBusConnect( &bus ))) {
		fprintf(stderr, "Failed to connect to system bus: %s\n", GetErrMessage(result));
	}
	else {
		if (( result = RpcQueryMethod(bus, service_name, service_path, interface, 
									  operation_id, &error, &response, signature, 
									  serviceId )))
		{
			printf( "Service Manager: %s\n", error.message);
		}
		else {
			/* Parse the response message */
			if (( result = sd_bus_message_read( response, "o", &path ))) {
				printf("Failed to parse response message: %s\n", strerror(-result));
			}
			else {
				puts("Got unit?");
			}
		}
	}
	
	sd_bus_error_free(&error);
	sd_bus_message_unref(response);
	
	sd_bus_unref(bus);
	return result? EXIT_SUCCESS: EXIT_FAILURE;
}