Fortran Program units and file layout External procedures
Example
An external procedure is one which is defined outside another program unit, or by a means other than Fortran.
The function contained in a file like
integer function f()
implicit none
end function f
is an external function.
For external procedures, their existence may be declared by using an interface block (to given an explicit interface)
program prog
implicit none
interface
integer function f()
end interface
end program prog
or by a declaration statement to give an implicit interface
program prog
implicit none
integer, external :: f
end program prog
or even
program prog
implicit none
integer f
external f
end program prog
The external attribute is not necessary:
program prog
implicit none
integer i
integer f
i = f() ! f is now an external function
end program prog
