I have the following program:
program hello
implicit none
integer :: a
a = 5
call print_hello() ! Prints F
call print_hello(a) ! Prints T
! call print_hello(something_recognized_as_not_present) ! Prints F
contains
subroutine print_hello(some_variable)
implicit none
integer, optional :: some_variable
print *, "Hello. some_variable is present? ", present(some_variable)
end subroutine print_hello
end program hello
Is there any way to create something along the lines of something_recognized_as_not_present? My reason for wanting this is that I’m trying to expose a Fortran subroutine that has optional arguments to C, and I’d like to set it up such that when C passes nullptr for an argument, Fortran interprets that as not present. If it was only 1 optional argument I could do an if statement along with C_ASSOCIATED, but there are over 10 so when you consider all their possible combinations thats >1024 if statements 
I managed to figure something out, the following code does what I want as far as the example goes, it’s yet to be determined if this will work for my application but it looks promising:
program hello
implicit none
integer :: a
integer, pointer :: b
print *, "b is associated? ", associated(b) ! Prints T, for some reason
b => null()
print *, "b is associated? ", associated(b) ! Prints F
a = 5
call print_hello() ! Prints F
call print_hello(a) ! Prints T
call print_hello(b) ! Prints F
contains
subroutine print_hello(some_variable)
implicit none
integer, optional :: some_variable
print *, "Hello. some_variable is present? ", present(some_variable)
end subroutine print_hello
end program hello
edit: Compiler Explorer
Since An unallocated variable passed as an argument is not PRESENT, your something_recognized_as_not_present could be an allocatable scalar that has not been allocated, as in the following code
program hello
implicit none
integer :: a
integer, allocatable :: b
a = 5
call print_hello() ! prints "variable not present"
call print_hello(a) ! prints "variable is 5"
call print_hello(b) ! prints "variable not present"
allocate (b, source=3)
call print_hello(b) ! prints "variable is 3"
contains
subroutine print_hello(some_variable)
implicit none
integer, optional :: some_variable
if (present(some_variable)) then
print*,"variable is", some_variable
else
print*,"variable not present"
end if
end subroutine print_hello
end program hello
Quoting fortran: Detecting null-pointer passed as dummy argument - Stack Overflow, the reason your code in the 2nd post works is that
A nonpointer, nonallocatable, optional dummy argument will be treated as not present if associated with a disassociated pointer actual argument.
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