The attributes flag_values, flag_masks and flag_meanings are intended to make variables that contain flag values self describing. Status codes and Boolean (binary) condition flags may be expressed with different combinations of flag_values and flag_masks attribute definitions.

The flag_values and flag_meanings attributes describe a status flag consisting of mutually exclusive coded values. The flag_values attribute is the same type as the variable to which it is attached, and contains a list of the possible flag values. The flag_meanings attribute is a string whose value is a blank separated list of descriptive words or phrases, one for each flag value. If multi-word phrases are used to describe the flag values, then the words within a phrase should be connected with underscores. The following example illustrates the use of flag values to express a speed quality with an enumerated status code.

  byte current_speed_qc(time, depth, lat, lon) ;
    current_speed_qc:long_name = "Current Speed Quality" ;
    current_speed_qc:standard_name = "sea_water_speed status_flag" ;
    current_speed_qc:_FillValue = -128b ;
    current_speed_qc:valid_range = 0b, 2b-127b, 127b ;
    current_speed_qc:flag_values = 0b, 1b, 2b ;
    current_speed_qc:flag_meanings = "quality_good sensor_nonfunctional 
                                      outside_valid_range" ;
      

The flag_masks and flag_meanings attributes describe a number of independent Boolean conditions using bit field notation by setting unique bits in each flag_masks value. The flag_masks attribute is the same type as the variable to which it is attached, and contains a list of values matching unique bit fields. The flag_meanings attribute is defined as above, one for each flag_masks value. A flagged condition is identified by performing a bitwise AND of the variable value and each flag_masks value; a non-zero result indicates a true condition. Thus, any or all of the flagged conditions may be true, depending on the variable bit settings. The following example illustrates the use of flag_masks to express six sensor status conditions.

  byte sensor_status_qc(time, depth, lat, lon) ;
    sensor_status_qc:long_name = "Sensor Status" ;
    sensor_status_qc:_FillValue = 0b ;
    sensor_status_qc:valid_range = 1b, 63b ;
    sensor_status_qc:flag_masks = 1b, 2b, 4b, 8b, 16b, 32b ;
    sensor_status_qc:flag_meanings = "low_battery processor_fault
                                      memory_fault disk_fault
                                      software_fault
                                      maintenance_required" ;
      

The flag_masks, flag_values and flag_meanings attributes, used together, describe a blend of independent Boolean conditions and enumerated status codes. The flag_masks and flag_values attributes are both the same type as the variable to which they are attached. A flagged condition is identified by a bitwise AND of the variable value and each flag_masks value; a result that matches the flag_values value indicates a true condition. Repeated flag_masks define a bit field mask that identifies a number of status conditions with different flag_values. The flag_meanings attribute is defined as above, one for each flag_masks bit field and flag_values definition. Each flag_values and flag_masks value must coincide with a flag_meanings value. The following example illustrates the use of flag_masks and flag_values to express two sensor status conditions and one enumerated status code.

  byte sensor_status_qc(time, depth, lat, lon) ;
    sensor_status_qc:long_name = "Sensor Status" ;
    sensor_status_qc:_FillValue = 0b ;
    sensor_status_qc:valid_range = 1b, 15b ;
    sensor_status_qc:flag_masks = 1b, 2b, 12b, 12b, 12b ;
    sensor_status_qc:flag_values = 1b, 2b, 4b, 8b, 12b ;
    sensor_status_qc:flag_meanings =
         "low_battery
          hardware_fault
          offline_mode calibration_mode maintenance_mode" ;
      

In this case, mutually exclusive values are blended with Boolean values to maximize use of the available bits in a flag value. The table below represents the four binary digits (bits) expressed by the sensor_status_qc variable in the previous example.

Bit 0 and Bit 1 are Boolean values indicating a low battery condition and a hardware fault, respectively. The next two bits (Bit 2 and Bit 3) express an enumeration indicating abnormal sensor operating modes. Thus, if Bit 0 is set, the battery is low and if Bit 1 is set, there is a hardware fault - independent of the current sensor operating mode.

The remaining bits (Bit 2 and Bit 3) are decoded as follows:

The "12b" flag mask is repeated in the sensor_status_qc flag_masks definition to explicitly declare the recommended bit field masks to repeatedly AND with the variable value while searching for matching enumerated values. An application determines if any of the conditions declared in the flag_meanings list are true by simply iterating through each of the flag_masks and AND'ing them with the variable. When a result is equal to the corresponding flag_values element, that condition is true. The repeated flag_masks enable a simple mechanism for clients to detect all possible conditions.