HP 3000 Manuals

HP Pascal/iX Programmer's Guide

READONLY Parameters 

A READONLY parameter is similar to a value parameter in that the routine
cannot directly modify its actual parameter, which can be a constant, an
expression, or a variable.  READONLY differs from a value parameter in
that the routine cannot modify the formal parameter:  you cannot assign a
value to the formal READONLY parameter, pass it to a VAR or ANYVAR
parameter, or pass it to either of the predefined functions addr,
baddress, or waddress.

A READONLY parameter is similar to a VAR or ANYVAR parameter in that its
actual parameter is passed by reference.  If the actual parameter is an
expression or constant, a copy of its value is passed by reference.

Example 

     PROGRAM prog;
     $STANDARD_LEVEL 'HP_MODCAL'$

     TYPE
        arraytype = ARRAY [1..10] OF integer;

     CONST
        arrayconst = arraytype [10 OF 0];

     VAR
        arrayvar : arraytype;

     FUNCTION arrayfunc : arraytype; EXTERNAL;

     PROCEDURE p (            valuep : arraytype;
                            VAR varp : arraytype;
                  READONLY readonlyp : arraytype); EXTERNAL;

     BEGIN
        p(arrayconst,   {value is passed}
          arrayconst,   {illegal -- must be a variable}
          arrayconst);  {address of copy of value is passed}

        p(arrayvar,     {value is passed}
          arrayvar,     {address is passed}
          arrayvar);    {address is passed}

        p(arrayfunc,    {value is passed}
          arrayfunc,    {illegal -- must be a variable}
          arrayfunc);   {address of copy of value is passed}
     END.

The comments in the preceding program explain the differences in passing
a constant (arrayconst), a variable (arrayvar), and an expression (a call
to the function arrayfunc) to a value parameter (valuep), a VAR parameter
(varp), and a READONLY parameter (readonlyp).

MPE/iX 5.0 Documentation