$ more m.f
       print*,'a= ',a()
       print*,'b= ',b()
       end

$ more a.f
      function a()

      print*,'hello from a'
      a=5.0
      return
      end 

$ more b.f
      function b()

      print*,'hello from b'
      b=15.0
      return
      end 

$ flang -o m m.f a.f b.f
$ ./m
 a= hello from a    5.000000    
 b= hello from b    15.00000    

$ gfortran5 -o m m.f a.f b.f
$ ./m
/lib/libgcc_s.so.1: version GCC_4.6.0 required by /usr/local/lib/gcc5/libgfortran.so.3 not found

# compile with flang only a.f

$ flang -c a.f -o a.o
clang-3.9: ^[[0;1;35mwarning: ^[[0m-lflang: 'linker' input unused^[[0m
clang-3.9: ^[[0;1;35mwarning: ^[[0m-lflangmain: 'linker' input unused^[[0m
clang-3.9: ^[[0;1;35mwarning: ^[[0m-lflangrti: 'linker' input unused^[[0m
clang-3.9: ^[[0;1;35mwarning: ^[[0m-lompstub: 'linker' input unused^[[0m
clang-3.9: ^[[0;1;35mwarning: ^[[0m-lexecinfo: 'linker' input unused^[[0m
clang-3.9: ^[[0;1;35mwarning: ^[[0margument unused during compilation: '-L/usr/l
ocal/flang/lib'

# compile with gfortran only b.f
$ gfortran5 -c b.f -o b.o

$ flang -o m m.f a.o b.o
/usr/bin/ld: error: undefined symbol: _gfortran_st_write
>>> referenced by b.f
>>>               b.o:(b_)

/usr/bin/ld: error: undefined symbol: _gfortran_transfer_character_write
>>> referenced by b.f
>>>               b.o:(b_)

/usr/bin/ld: error: undefined symbol: _gfortran_st_write_done
>>> referenced by b.f
>>>               b.o:(b_)
clang-3.9: error: linker command failed with exit code 1 (use -v to see invocation)
$ flang -o m m.f a.o b.o /usr/local/lib/gcc5/libgfortran.so
$ ./m
/lib/libgcc_s.so.1: version GCC_4.6.0 required by /usr/local/lib/gcc5/libgfortran.so.3 not found

$ gfortran -o m m.f a.o b.o /usr/local/lib/gcc5/libgfortran.sogfortran[1@5 
a.o: In function `a_':
/home/db/a.f:3: undefined reference to `f90io_src_info03'
/home/db/a.f:3: undefined reference to `f90io_print_init'
/home/db/a.f:3: undefined reference to `f90io_sc_ch_ldw'
/home/db/a.f:3: undefined reference to `f90io_ldw_end'
collect2: error: ld returned 1 exit status

$ gfortran5 -o m m.f a.o b.o /usr/local/lib/gcc5/libgfortran.so /usr/local/flang /lib/libflang.so
//usr/local/flang/lib/libflangrti.so: undefined reference to `backtrace_symbols'
//usr/local/flang/lib/libflangrti.so: undefined reference to `backtrace'
collect2: error: ld returned 1 exit status


# Removing I/O from a.f and b.f

$ cat a.b
cat: a.b: No such file or directory
$ cat a.f
      function a()

C      print*,'hello from a'
      a=5.0
      return
      end 
$ cat b.f
      function b()

C     print*,'hello from b'
      b=15.0
      return
      end 

$ flang -c -o a.o a.
clang-3.9: warning: -lflang: 'linker' input unused
clang-3.9: warning: -lflangmain: 'linker' input unused
clang-3.9: warning: -lflangrti: 'linker' input unused
clang-3.9: warning: -lompstub: 'linker' input unused
clang-3.9: warning: -lexecinfo: 'linker' input unused
clang-3.9: warning: argument unused during compilation: '-L/usr/local/flang/lib'
$ gfortran5 -c -o b.o b.f
$ flang -o m m.f a.o b.o
$ ./m
 a=     5.000000    
 b=     15.00000    
$ gfortran5 -o m m.f a.o b.o
$ ./m
/lib/libgcc_s.so.1: version GCC_4.6.0 required by /usr/local/lib/gcc5/libgfortran.so.3 not found