fortran90 - How could a simple optional argument lead to data corruption?

Question

I have a FORTRAN code with a routine:

SUBROUTINE READ_NC_VALS(NCID, RECID, VARNAME, VARDATA)
integer ncid, recid
character*(*) varname
real*8 vardata
dimension vardata(15,45,75)

etc.

I want to add some flexibility to this code, and I thought I would do it by first adding an optional flag argument:

SUBROUTINE READ_NC_VALS(NCID, RECID, VARNAME, VARDATA, how_to_calculate)

! everything the same and then ...
logical, optional :: how_to_calculate

Now, at this point, I am not even using "how_to_calculate". I am just putting it into the code for testing. So I compile the code without a hitch. Then I run it, and I get an error in the subroutine. Specifically, some of the values in the code later on are "magically" altered from what they were without that optional argument. The new values don't make sense to the logic of the code, so it exits politely with an error message. Let me stress again that at this point, I am not even using this optional argument. So then, on a lark, I go back to all the places in the source that call this routine and, even though my new argument in optional, I put in values for it in all the calls. When I do that, the code runs fine. So, what's up? How can the mere presence of an unused optional argument in a subroutine result in other data being corrupted? And how can adding input parameters for this optional argument fix things again? This is being compiled with PGI, by the way.

Any ideas? Thanks.

BTW, sorry for not providing more code. My boss might not be too happy with me if I did that. I don't make the rules; I just work here.

Answer 1

Optional arguments in Fortran are implemented by passing 0 (a null pointer) for each optional argument that has no value provided by the calling subroutine. Because of this subroutines that take optional arguments have to:

• either have an explicit INTERFACE definition inside the calling subroutine
• or be a module-level subroutine (for them interfaces are generated automatically)

If you add an optional argument to a subroutine but it neither has an interface in the caller or is not a module-level subroutine, then the compiler will not generate the right calling sequence - it will pass less arguments than expected. This could pose a problem on Unix systems, where PGI passes the length of all CHARACTER*(*) arguments at the end of the argument list (on Windows it passes the length as the next argument after the address of the string). One missing argument would shift the length arguments placement in the stack (or put them in the wrong registers on x64) leading to the incorrect length of the VARNAME string being received by READ_NC_VALS. This could lead to all sorts of ill behaviour, including overwritting memory and "magically" changing values that should not change according to the program logic.