Writing nul character to file

What simulator are you using? writing NUL to a text file works just fine for me

use std.textio.all;
entity test_chars is   
end entity test_chars;
architecture test of test_chars is
begin
 
    process
        file     f : text;
        variable l : line;
    begin
        FILE_OPEN(f, "test.txt", write_mode);
        
        write(l, NUL & "This string is started and ended with a NUL" & NUL);
        
        -- confirm all chars
        for i in l'range loop
            write(OUTPUT, l.all(i) & " (" & integer'image(character'pos(l.all(i))) & ")" & LF);
        end loop;
        
        writeline(f, l);
        
        FILE_CLOSE(F);
        
        FILE_OPEN(f, "test.txt");
        
        readline(f,l);
        
        --confirm all chars again
        for i in l'range loop
            write(OUTPUT, l.all(i) & " (" & integer'image(character'pos(l.all(i))) & ")" & LF);
        end loop;
        
        FILE_CLOSE(f);
        
        wait;
    end process;
    
end architecture test;