COMMON block question

[Paul van Delst <pvandelst(at)ncep.noaa.gov>]

Hey there,

I am doing the following in an initialisation function:

  COMMON rtm_transmittance_data, predictor_index, $
                                 transmittance_coefficients

  result = read_transmittance_coefficients( 'transmittance_coefficients', $
                                            predictor_index,              $
                                            transmittance_coefficients    )
  IF ( result NE SUCCESS ) THEN $
    MESSAGE, 'Error reading transmittance coefficients.', $
             /NONAME, /NOPRINT

If I run the function I see the following:

IDL> print, initialize_rtm()
% Compiled module: COMPUTE_ABSORBER_SPACE.
% Compiled module: READ_TRANSMITTANCE_COEFFICIENTS.
% Compiled module: OPEN_COEFFICIENT_FILE.
% Compiled module: READ_SPECTRAL_COEFFICIENTS.
       1
IDL> common rtm_transmittance_data
IDL> help
% At  $MAIN$                 
PREDICTOR_INDEX (RTM_TRANSMITTANCE_DATA)
                LONG      = Array[6, 142, 3]
TRANSMITTANCE_COEFFICIENTS (RTM_TRANSMITTANCE_DATA)
                DOUBLE    = Array[6, 301, 142, 3]
Compiled Procedures:
    $MAIN$

Compiled Functions:
  COMPUTE_ABSORBER_SPACE  INITIALIZE_RTM          OPEN_COEFFICIENT_FILE
    READ_SPECTRAL_COEFFICIENTS      READ_TRANSMITTANCE_COEFFICIENTS


How does IDL go about creating COMMON blocks when the size/type of the common block
elements are not known until, in this case, they're read in. Understand I'm coming from
Fortran-90 background where I declare everything up front in its own module (sort of like
a f77/IDL common block but better). Does IDL simply create pointers to the data elements
up front and then fills in the blanks after the data has actually been read in/defined?
The documentation doesn't give much info. 

Thanks for any insights. All metphysical replies forwarded to DF. :o)

paulv

-- 
Paul van Delst                 A little learning is a dangerous thing;
CIMSS @ NOAA/NCEP              Drink deep, or taste not the Pierian spring;
Ph:    (301) 763-8000 x7274    There shallow draughts intoxicate the brain,
Fax:   (301) 763-8545          And drinking largely sobers us again.
Email: pvandelst@ncep.noaa.gov                    Alexander Pope.