Horrible indistinguishable Fortran strings

strings.f90

program strings

    implicit none

    character(len=20) :: a, b

    print *, "strings: ", a, b
    if (a == b) then
        print *, "strings are equal"
    endif

    print *, "len(): ", len(a), len(b)
    if (len(a) == len(b)) then
        print *, "strings are equal length"
    endif

    print *, "len_trim(): ", len_trim(a), len_trim(b)
    if (len_trim(a) == len_trim(b)) then
        print *, "strings are equal len_trim", len_trim(a), len_trim(b)
    endif

    a = "foo"
    b = "foo   "

    print *, "strings: ", a, b
    if (a == b) then
        print *, "strings are equal"
    endif

    print *, "len(): ", len(a), len(b)
    if (len(a) == len(b)) then
        print *, "strings are equal length"
    endif

    print *, "len_trim(): ", len_trim(a), len_trim(b)
    if (len_trim(a) == len_trim(b)) then
        print *, "strings are equal len_trim", len_trim(a), len_trim(b)
    endif

end program

Output is:

 strings are equal
 strings are equal length          20          20
 strings are equal len_trim           3           3

$ gfortran string.f90 
$ ./a.out 
 strings: jSD¬�Е3ÿ�À@Ж3
 len():           20          20
 strings are equal length
 len_trim():           20          20
 strings are equal len_trim          20          20
 strings: foo                 foo                 
 strings are equal
 len():           20          20
 strings are equal length
 len_trim():            3           3
 strings are equal len_trim           3           3
$ gfortran -O2 string.f90 
$ ./a.out 
 strings: Å÷ª�ÿÿÿÿ ZIª�ØôG«�P¨Ö
 len():           20          20
 strings are equal length
 len_trim():           20          20
 strings are equal len_trim          20          20
 strings: foo                 foo                 
 strings are equal
 len():           20          20
 strings are equal length
 len_trim():            3           3
 strings are equal len_trim           3           3