Comparing HP C and HP Pascal

The longint type in HP Pascal is a 64-bit signed integer. A corresponding HP C type could be any structure or array of 2 words; however, HP C cannot directly operate on such an object.

By default, HP C enumerations are allocated 4 bytes of storage, while HP Pascal enumerations use the following scheme:

If the default enumeration specifier is modified with a char or short type specifier, 1 or 2 bytes of storage are allocated. See Table 3-1 for a description of the sized enumerated types.

This is important if the items are packed. For example, a 25-element enumeration in HP Pascal can use 1 byte and be on a byte boundary, so you must use the HP C type char or a sized enum declaration char enum.

HP C always indexes arrays from zero, while HP Pascal arrays can have lower bounds of any user-defined scalar value. This is only important when passing an array using an index to subscript the array. When passing the subscript between HP C and HP Pascal, you must adjust the subscript accordingly. HP C always passes a pointer to the first element of an array. To pass an array by value, enclose the array in a struct and pass the struct.

HP C char arrays are packed one character per byte, as are HP Pascal arrays (even if PACKED is not used). HP Pascal permits certain string operations with a packed array of char when the lower bound is one.

The HP Pascal type STRING [n] uses a string descriptor that consists of the following: a word containing the current length of the string, n bytes for the characters, and an extra byte allocated by the HP Pascal compiler. Thus, the HP Pascal type STRING[10] corresponds to the following HP C structure:

     typedef struct {
        int cur_len;                   /*  4 bytes  */
        char chars [10];               /* 10 bytes  */
        char extra_byte;               /*  1 byte   */
     } STRING_10;

which is initialized like this:

     STRING_10 this_string = {
        0,                              /* The current length */
        {0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, /* The 10 bytes       */
        0                               /* The null byte      */
     };

Both the C structure and the Pascal string are 4-byte aligned.

HP Pascal also has a generic string type in which the maximum length is unknown at compile time. Objects of this type have the same structure as in Note 5 above; the objects are only used as VAR formal parameters.

A variable of this type is a pointer to a character array if the string is null-terminated; HP Pascal will not handle the null byte in any special way. An HP C parameter of type "pointer to char" corresponds to an HP Pascal VAR parameter of type "packed array of char." However, the type definition of that VAR parameter must have the bounds specified.

The size is equal to the size of all members plus any padding needed for the alignment. (See Chapter 2 for details on alignment.) The alignment is that of the member with the strictest alignment requirement.

A union corresponds directly to an untagged HP Pascal variant record. For example, the HP C union:

     typedef union {
        int i;
        float r;
        unsigned char c;
     } UNIONTYPE;

corresponds to:

     TYPE
        UNIONTYPE = RECORD CASE INTEGER OF
           1 : (i : INTEGER);
           2 : (r : REAL);
           3 : (c : CHAR);
     END;

The tagged HP Pascal variant record:

     TYPE
        TAGGED_UNIONTYPE = RECORD CASE tag : INTEGER OF
           1 : (i : INTEGER);
           2 : (r : REAL);
     END;

corresponds to this HP C structure:

     typedef struct {
        int tag;
        union {
           int i;
           float r;
        };
     } TAGGED_UNIONTYPE;

HP Pascal subranges with a negative value as their lower bound have enough bits allocated to contain the upper bound, with an extra bit for the sign. Thus, the HP C structure:

     typedef struct {
        int b1 : 1;
        int b2 : 2;
        int b3 : 3;
        int b4 : 4;
        int b5 : 5;
        int b6 : 6;
        int b7 : 7;
     } BITS;

corresponds to the following untagged HP Pascal record:

     TYPE
        BITS = PACKED RECORD
           b1 : BOOLEAN;
           b2 : -2 ..  1;
           b3 : -4 ..  3;
           b4 : -8 ..  7;
           b5 : -16 .. 15;
           b6 : -32 .. 31;
           b7 : -64 .. 63;
        END;

Unsigned bit-fields map onto HP Pascal packed record fields whose types are the appropriate subranges. For example, the HP C structure:

     typedef struct {
        unsigned int b1 : 1;
        unsigned int b2 : 2;
        unsigned int b3 : 3;
        unsigned int b4 : 4;
        unsigned int b5 : 5;
        unsigned int b6 : 6;
        unsigned int b7 : 7;
     } BITS;

corresponds to this untagged HP Pascal record:

     TYPE
        BITS = PACKED RECORD
           b1 : 0 ..   1;
           b2 : 0 ..   3;
           b3 : 0 ..   7;
           b4 : 0 ..  15;
           b5 : 0 ..  31;
           b6 : 0 ..  63;
           b7 : 0 .. 127;
           END;

The type void, when applied to a function declaration, corresponds to an HP Pascal procedure.

HP Pascal allocates one byte for Boolean variables, and only accesses the rightmost bit to determine its value. HP Pascal uses a 1 to represent true and zero for false; HP C interprets any nonzero value as true and interprets zero as false.

HP Pascal sets are packed arrays of unsigned bits. For example, given the HP Pascal set:

     TYPE
         SET_10 = SET OF 0 .. 9;
     VAR s: SET_10;

the corresponding HP C struct would be:

     typedef struct {
          unsigned int b0 : 1;
          unsigned int b1 : 1;
          unsigned int b2 : 1;
          unsigned int b3 : 1;
          unsigned int b4 : 1;
          unsigned int b5 : 1;
          unsigned int b6 : 1;
          unsigned int b7 : 1;
          unsigned int b8 : 1;
          unsigned int b9 : 1;
     } SET_10;
 
     SET_10 s;

Also, the following operation in HP Pascal:

     s := s + [9];

has the following corresponding HP C code:

     s.b9 = 1;

HP C and HP Pascal file types and I/O operations do not correspond.

All HP C parameters are passed by value except arrays and functions, which are always passed as pointers. Reference parameters to HP Pascal can be implemented in two ways: first, by passing the address of an object by applying the address operator & to the variable; second, by declaring a variable to be a pointer to such a type, assigning an address to the pointer variable, and passing the pointer.

If an HP Pascal procedure or function has a parameter that is an array by value, it can be called from HP C by passing a struct that contains an array of the corresponding type.

Be careful when passing strings to HP Pascal. If the routine expects a packed array of char, be sure to pass a char array. If the routine expects a user-defined string, pass the structure declared in Note 5 above.

The examples below are HP Pascal and HP C source files that show the parameter passing rules. The HP Pascal file contains 2 subroutines, pass_char_arrays() and pass_a_string(). The HP C file contains the main line routine that calls these two subroutines and displays the results. The HP C program is annotated with the expected results.

The following is the HP Pascal procedure called from HP C:

$subprogram$
program p;
const len = 10;
type
  pac_10  = packed array [1..10] of char;
  string_10 = string [len];
 
  function pass_char_arrays (a: pac_10;
    var b: pac_10;
        c: string_10;
           var d: string_10) : integer;
       var
    i : integer;
    ret_val : integer;
       begin
    ret_val := 0;
           for i := 1 to len - 1 do
           begin
        if ( a[i] <> "a" ) then
     ret_val := 1;
               a[i]    := "z";
               if ( b[i] <> "b" ) then
                   ret_val := 256;
               b[i]    := "y";
       end;
 
       for i := 1 to strlen (c) do
       begin
           if ( c[i] <> "c" ) then
        ret_val := 65536;
           c[i] := "x";
       end;
 
       for i := 1 to strlen (d) do
       begin
           if ( d[i] <> "d" ) then
               ret_val := maxint;
            d[i] := "w";
        end;
 pass_char_arrays := ret_val;
    end;
 
function pass_a_string (var a: string) : integer;
  var
      i       : integer;
      ret_val : integer;
  begin
       ret_val := 0;
       for i := 1 to strlen (a) do
       begin
    if (a[i] <> "x" ) then
        ret_val := maxint;
           a[i] := "q";
        end;
 pass_a_string := ret_val;
    end;
 
begin
end.

The following HP C main program calls the HP Pascal procedure:

#include <stdio.h>
#include <string.h>
static struct string_10 {
   int cur_len;
   char chars[10];
};
/* a Pascal routine */
extern int pass_char_arrays (/* pac10,
                                var pac10,
                                string_10,
                                var string[10] */);
main(void)
{
   static struct string_10 a, b, c, d;
   int ret_val;
   strcpy (a.chars, "aaaaaaaaa");
   strcpy (b.chars, "bbbbbbbbb");
   strcpy (c.chars, "ccccccccc");
   c.cur_len = strlen (c.chars);
   strcpy (d.chars, "ddddddddd");
   d.cur_len = 5;
   ret_val = pass_char_arrays (a.chars, b.chars, &c, &d);
 
   printf ("a: %s\n", a.chars);           /* prints aaaaaaaaa */
   printf ("b: %s\n", b.chars);           /* prints yyyyyyyyy */
   printf ("c: %s\n", c.chars); /* value parm prints xxxxxxxxx */
   printf ("d: %s\n", d.chars);           /* prints wwwwwdddd */
   printf ("return mask: %d\n", ret_val); /* print 0 */
 
   ret_val = pass_a_string (&c);
   printf ("c: %s\n", c.chars);           /* prints qqqqqqqqq */
   printf ("return mask: %d\n", ret_val); /* print 0 */
   return 0;
}

The program produces the following output:

a: aaaaaaaaa
b: yyyyyyyyy
c: xxxxxxxxx
d: wwwwwdddd
return mask: 0
c: qqqqqqqqq
return mask: 0

The routine pass_a_string() expects a generic string (described in Note 6 above), so you must pass an extra argument. The extra argument consists of a value parameter containing the maximum length of the char array.

HP Pascal routines do not maintain a null byte at the end of HP C strings. HP Pascal determines the current length of the string by maintaining the length in a 4-byte integer preceding the character data. When an HP Pascal procedure or function (that takes as a parameter a string by reference) is called, the following code is necessary if the Pascal routine modifies the string:

    pass_a_string (a, temp);  /* From note 2 above */
    a.chars[a.cur_len] = "\0";

In non-ANSI mode, HP C promotes most float (32-bit) arguments to double (64-bit). Therefore, all arithmetic using objects defined as float is actually using double code. Float code is only used when the float objects are stored.

In ANSI mode where function prototypes have been declared with a float parameter, no automatic promotion is performed. If the prototype is within the current scope, floats will not be automatically promoted.

To call an HP Pascal routine that expects an argument of type REAL (32-bits), you may either declare a function prototype in ANSI mode, use the +r command line option in non-ANSI mode to always pass floats as floats, or declare the actual parameter as a struct with a float as its only field, such as:

     typedef struct {float f;} PASCAL_REAL_ARG;

HP Pascal global data can usually only be accessed by HP C if the data is declared at the outermost level. HP Pascal stores the names of the objects in lowercase letters.

For example, the HP Pascal global:

     PROGRAM example;
 
     VAR
        PASCAL_GLOBAL: INTEGER;
 
     BEGIN END.

is accessed by HP C with this declaration:

     extern int pascal_global;