gcc: Linking error in mixed language programming using C and Fortran

Based on this SO post I want to create software with gcc which uses mixed language source code, e.g. C and Fortran. While the linking of the generated object files works fine, the linker gives me the following error if I use a static or shared library instead of the object file:

/usr/bin/ld: /tmp/ccdqJ6hY.o: in function `MAIN__':hello.f90:(.text+0x76): undefined reference to `fun'collect2: error: ld returned 1 exit status

This is my C library source code file fun.c:

#include <stdio.h>void fun(void) {  printf("Hello from C function.\n");}

This is my Fortran source code hello.f90:

program hello  print *, "Hello from Fortran program."  call fun()end

This is my build script:

#! /bin/sh#gcc -c fun.c                     # Create object file of C libraryar r libfun.a fun.o              # Create static C library gcc -shared -o libfun.so fun.o   # Create shared/dynamic linked C library#gfortran -fno-underscoring -o hello1 fun.o hello.f90                      # This works.#gfortran -fno-underscoring -o hello2 -L. -Wl,-rpath=`pwd` -lfun hello.f90 # This fails.#gfortran -fno-underscoring -o hello3 -L. libfun.a hello.f90               # This fails.

The symbol name fun is fine in the libraries as indicated by nm:

$ nm libfun.afun.o:0000000000000000 T fun

I'm using gcc version gcc (Debian 10.2.1-6) 10.2.1 20210110 on Debian GNU/Linux 11 (bullseye).

I suppose that something with my gcc configuration might be broken, but I have no glue, what.Could you give me any advice, please?