status.i4.v = dio_device_expression_c(device_indices.i4a.r,
					      errors.i2a.r, num_devices.i4.v)

	This routine returns the expression (if any) which is used to
	calculate a device reading.

	device_indices	device index or array of device indices
	expressions	returned device expression or array of
			device expressions (allow DIO_DEVICE_EXPRESSION_LEN
			characters per device)
			(A value of NULL may be passed if no expressions
			 are needed.)
	errors		returned ACNET status value or array of status values
	num_devices	number of devices

	This function returns status values as follows:

	OK			success
	DIO_BADARG		invalid number of devices passed
	DIO_MEMFAIL		memory allocation failure
	negative value		ACNET format error indicating overall failure
	positive value		number of devices in error indicating partial

	This function requires the following include files:

	diolib_h, acnet_errors_h

	Related functions:

	dio_device_has_expression_c, error_in_list(_c)

	C/C++ usage:

	short	errors[NUM_DEVICES];
	int	status;
	static const int	device_indices[NUM_DEVICES] = {14430, 14431};
	int	num_devices = NUM_DEVICES;

	status = dio_device_expression_c(device_indices,expressions,