[YLUG] Help with c++ pointers - Please

[Dominic Hibbs]

POINTER HELP REQUESTED

I am an assembly language programmer - I also program in many other
languages BUT, as many of you youngsters may not yet have learned, Knowledge
has a short shelf life and my C/C++ has been on the shelf too long.
In this - "long int" means 64 bit.

In assembly language there is no typing at all.  A 32 bit quantity may be
1. an integer,
2. a sixteen bit number followed by a byte with no defined value and a
    character,
3. half of a double,
4. four bytes of program.
5. a pointer to something,
6. or a pointer to a pointer
and it is up to the programmer how something is interpreted and what
alignment is used.

I am converting a program, written in assembler to C/C++ and I am stuck over
pointers. This data structure is a linked list of variables.

A block of memory consists of records made up of

Byte No. Content
(0..7)     a 32 bit pointer to
                a long int (8 Byte)
                or a double
        or (32 bit pointer and two 16 bit integers)
(8..15)     followed by one byte made up of
        2 bits of flags, 4 bits of a number and two more bit flag
        followed by an ASCIZ string of 4 n + 3 bytes long (n = 0..30)
(16..23) followed by a long int or Double
        or (32 bit pointer and two 16 bit integers)

Bits 8..15 are anded with  & 0x00 00 00 3c and then added to the address of
this record to get the next record.

First variable is a long int called "ten" and has the value 10 so the
first two bits of byte 8 hav the value 0b01 and the record length is 20
bytes the other two bit flags have the value 0b01 (0 means Not an array
and the 1 bit says this variable has been initialised).

Abit of (obviously incorrect) code follows:

    char * s = "ten";
    void * blockaddr = malloc(varspace);  // Check return value for NULL
    recptr = blockaddr;       // get address of first record
    * recptr = * recptr + 16; // insert into memory at blockaddr
                      // blockaddress + 16
    recptr += 8;          // pointer arithmetic point to byte 8

    * recptr = 1 | 20 | 0x80  // two bit flags | record length |
                                  // initialised as a byte value
    recptr++;
    *recptr++ = *s;      // recptr is a char *
    char ch;
    do {
               *recptr++ = ch = * (++ s);
    } while( ch);
    recptr +=3;               // align on four byte -
                                  // we are at the start of the next record.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://mailman.lug.org.uk/pipermail/york/attachments/20080113/30666fc1/attachment.html