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I have a program that calls a subroutine which then calls a function. I am somewhat confused by Fortran's requirements for function type declaration. I have declared the type in the function (i.e. real function foo(...)), and the program works whether or not I declare the function in the subroutine declaration section.

My specific question is, will not declaring the function in the subroutine potentially lead to unexpected behavior in future? I have also seen the interface block and am wondering if this is necessary as well.

More generally, I am also interested in what Fortran is doing "behind the scenes" and why it would be more or less important to declare the function or use an interface block.

EDIT: Some sample code:

program foo
  real :: a,b,c

  call bar(a,b,c)
end program foo

subroutine bar(a,b,c)
  real :: a,b,c

  c = baz(a,b)
end subroutine bar

real function baz(a,b)
  real :: a,b

  baz = a*b
end function baz
astay13
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1 Answers1

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The best approach is to declare the function in the function, then to place the function in a module. Then "use" the function from any main program or procedure (subroutine or function) that calls that function. That way the calling program or procedure will be aware of the interface to the function and will generate the correct code. In Fortran terminology, the interface is explicit. If the function is called from a procedure in the same module, you don't have to "use" it because procedures in a module are aware of each other. See Computing the cross product of two vectors in Fortran 90 for an example. Typically there is no need to use an interface unless you are calling a procedure to which you lack the source code, or which is in another language, e.g., C accessed via the ISO C Binding.

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M. S. B.
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  • I currently have the functions and subroutines defined in the same file as the main program, but have not defined a module. Do I technically need a module for the program to "see" the subroutines or functions? If so, why does the program even work? Thanks for your help! – astay13 Jul 11 '11 at 22:05
  • Yes, you should, and "for historical reasons", respectively. have a module at the start of the file which contains the subroutine & function, and use modulename in the main program for the call to the subroutine. – Jonathan Dursi Jul 11 '11 at 22:50
  • @astay, your program may be working by implicit typing, whereby "baz" starting with "b" is is assumed to be a real. This is an risky and obsolete approach. Check and see what happens if you change the name to ibaz, in which case the caller may implicitly type it as an integer function, disagreeing with the actual. – M. S. B. Jul 11 '11 at 23:26
  • @M.S.B., you are right, the program does not work anymore with the name change. I think I will just put everything in a module. Thanks for all your help! – astay13 Jul 12 '11 at 13:25