ITEM: E2731L

FORTRAN language problem

Question:

I am running AIX V3.2.4 PMP.  We are using the FORTRAN compiler that
came on the PMP tape.  I am doing some work utilizing the block data
function.  The manual is unclear about how the block data function
works in certain cases and I would like someone to help me clear up
some questions.  


The questions I have are related to the use of Fortran "block
data" statements.  The Fortran language reference and users guide
from IBM are not specific on the following topics:

I have in the same directory many programs, subroutines and block
datas.

1.  For example, block data A has been written to initialize common
    block X in program AA.  Block data B has been written to initialize
    common block Y in program BB.  Since a block data is not a callable
    function, how does the linker know that block data B and common block Y
    belongs to program BB?  Is common block Y added to program AA?

2.  Block data A and block data B initialize the same common block,
    say X. However, the initialization done through block data A is for use
    by program AA, while the initialization done through block data B
    is for use by program BB.  How does the linker know which block data is
    used for each program?

3.  Is there a need to add the block data procedure in the same file
    as the main program, or is it possible to have them in two
    different Fortran (Unix) files and generate independent object files for
    use by the linker?

Response:

The answers to your questions are as follows:

1. The linker knows that block data A corresponds to named common
   block X and block data B corresponds to named common Y because
   of the use of named commons.  It matches up the name of the 
   common in the block data subprogram with the FORTRAN program 
   that references the named common.

2. This scenario is illegal.  The same named common block must not
   be referenced in two different block data subprograms.  If the
   same named common is referenced in two separate block data
   subprograms the first subprogram is used for data initialization.

3. It is possible to have the block data subprograms in different
   files and then link the appropriate block data subprogram file to
   the FORTRAN program that references the named common.

You also indicated that you had the block data subprograms in different
files and that you were compiling them to object modules and keeping
them in a library.  This will work fine unless you have two block data
subprograms that reference the same named common block (this will result
in the use of the first subprogram for the named common block as 
described above).  As a workaround, you could put the subprograms that 
reference the same named common in different libraries and then link to 
the appropriate library, OR you could use the FORTRAN INCLUDE statement 
in the program to include the .f file that contains the appropriate 
block data subprogram.  For example, you could use

  include 'blocka.f'

in program AA and

  include 'blockb.f'

in program BB where blocka.f contains block data subprogram A and 
blockb.f contains block data subprogram B.



Support Line: FORTRAN language problem ITEM: E2731L

Dated: October 1993 Category: N/A

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