Problems Interfacing Modula-2 with GNU Readline library

[Alice Osako]

    I am working on a
      medium-sized project in Modula-2 (specifically, an implementation
      of the MAL interpreter - see https://github.com/kanaka/mal for
      details), and one of the soft requirements/recommended
      implementation details is editable input and input history. For
      this purpose, I intend to use the existing Readline library, and
      to this end I created a simple definition module as a binding. 
      
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      DEFINITION MODULE FOR "C" readline;
      
      FROM SYSTEM IMPORT ADDRESS;
      
      EXPORT UNQUALIFIED readline, add_history;
      
      PROCEDURE readline (prompt: ARRAY OF CHAR) : ADDRESS;
      
      PROCEDURE add_history (line: ARRAY OF CHAR);
      
      END readline.
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    
      However, I am running into a problem when testing these bindings.
      When I run the following test program:
      
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      MODULE rl_test;
      
      FROM SYSTEM IMPORT ADDRESS;
      
      FROM DynamicStrings IMPORT String, Fin, Length,
                                                            
      InitStringCharStar,
                                                            CopyOut,
      Equal;
      
      FROM StrIO IMPORT WriteString, WriteLn;
      
      FROM readline IMPORT readline, add_history;
      
      VAR
         input, exitCondition: String;
         temp: ARRAY [0..1024] OF CHAR;
      
      BEGIN
         exitCondition := InitString("exit");
         WHILE NOT Equal(input, exitCondition) DO
            input := InitStringCharStar(readline("Write something: "));
      
            IF Length(input) > 0 THEN
               WriteString("You Wrote: ");
               CopyOut(temp, input);
      
               WriteString(temp);
               WriteLn;
               add_history(temp);
            ELSE
               WriteString("No input");
               WriteLn;
            END;
            Fin(input);
         END;
      
      END rl_test.
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    
      I expected that this would result in something like the following:
      
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Write something: foo
    You Wrote: foo
    Write something: bar
    You Wrote: bar
    Write something:
      No input
    Write something: bar
    You Wrote: bar
    Write something: quux
    You Wrote: quux
    Write something: exit
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    
      However, the output I actually got was:
      
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Write something: foo
      Write something: bar
      Write something: 
      Write something: bar
      Write something: quux
      Write something: exit
      You Wrote: foo
      You Wrote: bar
      No input
      You Wrote: bar
      You Wrote: quux
      You Wrote: exit
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      
      So my issue is, why is the response not getting flushed until
      after the input is completed?
      
      Oddly enough, when I add 
      
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      FROM InOut IMPORT WriteCard;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    
      and insert this following the input but before the IF statement:
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
         
      WriteCard(Length(input), 3);
            WriteLn;
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      
      The result is different:
      
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Write something: foo
        3Write something: bar
      
      You Wrote: foo
        3Write something: 
      
      You Wrote: bar
        0Write something: bar
      
      No input
        3Write something: quux
      
      You Wrote: bar
        4Write something: exit
      
      You Wrote: quux
        4
      You Wrote: exit
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      
      Why this particular delayed interleaving is happening, and why the
      WriteLn is getting ignored,  is puzzling to me.
      
      Is there some known issue with interfacing the C libraries, or the
      Readline library specifically, that I need to be aware of?