gfortran question about optional subroutine arguments

Tobias Burnus burnus@net-b.de
Thu Mar 14 14:41:00 GMT 2013


Mark Hounschell wrote:
> When I use -std=f95 this fortran77 code will not compile.

Well, then the code is not a valid Fortran 77 code* but uses some vendor 
extension; using vendor extensions is very common with old so-called 
"Fortran 77" code. (* valid = valid according to the Fortran 77 
standard, cf. http://gcc.gnu.org/wiki/GFortranStandards ) Using -std=gnu 
and -std=legacy, gfortran enables support for some more or less common 
vendor extensions.

What I meant before is that using -std=gnu (the default) doesn't disable 
the support for features which are in -std=f2003. Similarly for 
-std=legacy, which should also add only new features without disabling 
others.

> Using the default "gnu" mostly just lots of warnings show up. Is 
> "if(present(arg2)) arg2 = 0" available when using gnu?

Yes, it is definitely available with both -std=gnu and -std=legacy. 
However, it requires you declare the dummy argument as OPTIONAL.

> Maybe my warnings can be suppressed with other switches.

Well, using -std=gnu or -std=legacy to silence those warnings is fine; 
as written, they essentially just allow more features, which should 
usually not break things.

>> Hmm, I have not seen the ",)" syntax before. I assume that that is some
>> vendor extension, where the compiler does something - or a typo and it
>> should be "arg1)" without ",".
>>
>
> No, the "," is how this old compiler knows I'm not passing that arg.
> The same could be done as
>              call (arg1,,arg3)
>
> Indicating arg2 was not passed. Also, the very last arg can always be 
> left out without using the ",".

Seems as if this is a vendor extension, which only few compilers 
support. I just tried the compilers of PGI, PathScale, NAG, SUN, Cray, 
and Intel - and only Intel's ifort seems to support this extension.

And, doing some digging, I also found out that GCC's g77 supports this 
feature: 
http://gcc.gnu.org/onlinedocs/gcc-3.4.6/g77/Ugly-Null-Arguments.html 
(g77 is the predecessor of gfortran. gfortran was written from scratch 
to support Fortran 90+. It has most but not not all legacy features of 
g77. [On the other hand, it has some vendor extensions which g77 does 
not have.])

Thus, one option would be to use GCC 3.4.6's g77. However, GCC 3.x and 
g77 are no longer supported and g77 also only supports very few Fortran 
90 features. Thus, I think it would be best to change the code to be 
fully Fortran conform.

Actually, I wonder how ifort and g77 pass those 'null arguments'; or 
rather, how to use then in the called procedure. Something PRESENT() 
does not seem to exist. g77's documentation does not tell this (but one 
can look at the source code) and Intel's documentation does not mention 
this feature.

Regarding gfortran: In principle, one could add support for null 
arguments; however, given that they seem to be only in few old codes, I 
don't think that this has a high priority.


Tobias



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