Using modules in Fortran: Undefined reference to `parse_' - c

Question

I am trying to use the parse routine described here to parse a string in Fortran90. When following the link in the document one can download a .zip with two f90-files, which I have done. I have then compiled them gfortran -c precmod.f90 && gfortran -c stringmod.f90. I have also added use strings to my program.
Despite of this, I receive the following error when compiling (gfortran stringmod.o precmod.o calcs.o server.o):
calcs.o: In function `calculate_':
calcs.f90:(.text+0x174): undefined reference to `parse_'
collect2: error: ld returned exit status 1
calcs.f90 is shown below and server.o is a server written in C that should be invoked by calcs.
program name
use strings
use iso_c_binding, only: C_CHAR, C_NULL_CHAR, C_INT
implicit none
! type declaration statements
character(255) query
integer calc, ans, portnum, calculate
interface
subroutine server(portnum) bind(C, name="server")
use iso_c_binding, only: c_int
integer(kind=c_int), value :: portnum
end subroutine server
end interface
! executable statements
print *, "Please provide me with a port number. Plz. <3"
read "(1i9)", portnum
call server(portnum)
end program name
function calculate(query)
implicit none
character(255) query, op
integer length, i, calculate
integer, dimension (:,:), allocatable :: inputarray
call parse(query, ' ', inputarray, length)
do i=1,size(inputarray)
print *, inputarray(i, 1)
end do
calculate = 5
end function calculate
I have tried to add public to the top of stringmod.f90.

Answer 1

When we have something like
program
end program
function func()
end function
then the function func is an external function, even when this is given in the same source code file as the main program. This external function knows nothing about the main program, and in turn the main program knows little about the external function.
Part of this lack of knowledge is that, in the example of the question, the fact that the subroutine parse has an explicit interface (through the module) in the main program is irrelevant to calculate.
That is, the function calculate has its own scope and has no host. To gain access to the module procedure parse it can itself use the module strings:
function calculate(query)
use strings, only : parse
implicit none
end function
There is a hint to this lack of being aware that parse is a module procedure in strings. The name decoration in parse_ (that single trailing underscore) is a common way to mangle an external procedure. Module procedures (without bind(c)) generally have much more elaborate symbol names.
Finally, I'll repeat something from the comments. Earlier, I said that the main program knows little about the external function. In the main program we have the declaration
integer ... calculate
which says that the external function calculate (which has an implicit interface) has a return type of integer. The function, in this case, could instead be made an internal function
program
use strings, only : parse
contains
integer function calculate
end function
end program
and not only does the function calculate have an explicit interface (removing also the need for that return declaration in the main program) but it also has access to parse through host association.