Why would common block variables not preserve their values?

Question

First, I know that using common blocks is a bad idea in fortran (and programming in general). However, I'm updating someone else's code and I don't want to mess up things that are known to work.
Second, I know I should post something more specific then this. If I knew how to reduce this into something small, I would. However, since I know, and I don't think you'll appreciate 2500 lines of code, I can't post a specific example.
With that in mind, I can't describe my problem.

I'm updating someone else's fortran code. The guy used several (4) common blocks to set up global variables. For some reason when I call a function that use such a block, all it's value are 0. Has anyone encountered that before? Does anyone know why this might happen? How to reproduce this? Any starting point to check this would be helpful.

For what it worth, the said common block is declared as

common /set/ block,x,y,z,llx,lly,llz,ilx,ily,ilz,third,third2

block is a 4D array. x, y, and z are 1D array. llx,lly, and llz, are double precision types. The rest are integer types.

The common block(s) is (are) declared and initialized at the main program before any function is called.

They are zero even if you set them otherwise previously? Are the declared dimensions equal in all instances of the common block? — Vladimir F Героям слава, Feb 12 '13 at 12:44
Can you not upgrade the common blocks to modules? The only difference would be to `use` the module at places where the common blocks are currently employed. The major advantage is that you only need to declare the variables in one place and the order does not matter. — sigma, Feb 12 '13 at 14:43
@sigma I can, but this require caution. I prefer to avoid this if I can. — Yotam, Feb 12 '13 at 15:33
@VladimirF Yes and yes. The variable I test things with is llx — Yotam, Feb 12 '13 at 15:36
If you've got COMMON blocks do you also have any EQUIVALENCEing going on ? — High Performance Mark, Feb 12 '13 at 16:24
Equivalencing as in ``llx=0``? I do, but right before I call the function the value is "correct" and at the first line of the function it is 0 — Yotam, Feb 12 '13 at 16:36
Well, I meant as in `equivalence(llx,another_name)` but it seems you've got that covered. — High Performance Mark, Feb 12 '13 at 16:39

score 2 · Answer 1 · answered Feb 12 '13 at 14:04

2

Some compilers do initialize common variables to zero, so if you first invoke the function with the common block, you might find zeros everywhere (although you should not rely on that). But once you set some values for the common block variables in the program, those values should appear whenever you use the common block.

As of the variables in the common block: They can be of arbitrary type, as long as they are consistently defined at all places, where the common block is used.

answered Feb 12 '13 at 14:04

Bálint Aradi

3,754
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The OP claims the problem is in retaining the values, not in initialization. Otherwise the BSS segment is zeroed automatically in ELF executables regardless of the compiler. – Vladimir F Героям слава Feb 12 '13 at 15:47

score 1 · Answer 2 · answered Jun 24 '14 at 21:32

Can you compare your code with this tiny example? I think you might be missing something, like the "common" declaration inside the subroutine.

Note that you don't need to use the same name for the variable inside the subroutine (AA) as you have for main (GB). Just the common block name (myarray) has to be the same. However, nothing bad will happen if you replace AA by GB, and the final result would be a little bit cleaner to read.

program main
    real             GB(4)
    common /myarray/ GB

    integer  i
    real     B(4)

    GB=0
    write(*,*) 'GB',GB

    do i=1,4
        call AddSubR()
        write(*,*) 'GB',GB
    enddo

end program main

subroutine AddSubR()
    real             AA(4)
    common /myarray/ AA

    integer i

    do i=1,4
        AA(i) = AA(i)+1
    enddo

end subroutine AddSubR

Why would common block variables not preserve their values?

2 Answers2