Run through a structure and print all the values?

Question

I have a pointer to a structure and I'd like to fetch all of its members by trial and error. I'm trying to run through the structure by incrementing the pointer by one and derefencing it. It should return a correct value from the structure (every i*sizeof(int) time) but it doesn't.

What am I doing wrong?

fn (mach_msg_header_t *InHeadP, mach_msg_header_t *OutHeadP)
{

    /*
     assume that all struct members are int types

     typedef    struct 
     {
     mach_msg_bits_t    msgh_bits;
     mach_msg_size_t    msgh_size;
     mach_port_t        msgh_remote_port;
     mach_port_t        msgh_local_port;
     mach_msg_size_t    msgh_reserved;
     mach_msg_id_t      msgh_id;
     } mach_msg_header_t;

     size of the struct is 24.
     */

    printf("ID: %d \n",InHeadP->msgh_id); //prints 'ID: 1337'
    printf("Ptr: %p\n",InHeadP);

    for (int i = 0; i <= 24; i++)
    {
        int deref = *((int*)(InHeadP+i));
        printf("InHeadP[%d]=%d\n",i,deref);
            //no sign of 1337 anywhere

    }
}

P.S. I know that I shouldn't be doing that, but this is only for testing purposes.

"It should return a correct value from the structure (every i*sizeof(int) time) but it doesn't." -- Er why should it? that's not exactly how memory gets allocated. — zellio, May 20 '11 at 15:42
Are you sure all of those fields are 32 bits? If msgh_id is 16 bits, you'll never see an `int` that is 1337. (Just saw your assume all struct types... comment. Still, could this in fact be the case?) — andrewdski, May 20 '11 at 15:43
is your machine word 4 bytes long? further, what does sizeof (mach_msg_header_t) return? — KevinDTimm, May 20 '11 at 15:45
`mach_msg_id_t` is of `int` type. Not sure about the size of it. — Kristina, May 20 '11 at 15:46

score 5 · Accepted Answer · answered May 20 '11 at 15:45

5

Because InHeadP is a mach_msg_header_t*, adding an integer to it will actually add that integer times sizeof(mach_msg_header_t), as though you were indexing an array of mach_msg_header_ts (and is actually how array indexing works). You need to cast InHeadP to an int* before performing arithmetic on it, and even then, as the struct has six fields, i should only go up to 6, not 24.

answered May 20 '11 at 15:45

jwodder

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As we all converge on the problem nearly simultaneously! I give this +1 for being first, and because he is correct about i needing to go up to 6, not 24. – andrewdski May 20 '11 at 15:51

score 3 · Answer 2 · answered May 20 '11 at 20:01

A preferred method to iterate through a structure is to create a function that returns the value or address of the field given an index. Note that if the order of the members in the struct changes, this function must change too:

int * Index_Of(unsigned int index, mach_msg_header_t * p_struct)
{
    switch (index)
    {
        case 0:  return (int *) &(p_struct->msgh_bits);
        case 1:  return (int *) &(p_struct->msgh_size);
        case 2:  return (int *) &(p_struct->msgh_remote_port);
        case 3:  return (int *) &(p_struct->msgh_local_port);
        case 4:  return (int *) &(p_struct->msgh_reserved);
        case 5:  return (int *) &(p_struct->msgh_id);
    }
    return 0;
}

Just remember that treating the structure as contiguous fields (members) is not advisable since the compiler is allowed to add padding between structure members. Thus any method to access the fields except by name, is dangerous and will lead to very difficult bugs.

BTW, treating each field as an int is also dangerous. If any of the types is changed to something else, like a double, your code will break. Compiler will not catch the error because casting tells the compiler YOU know what you are doing.

You could "turn the picture around" and implement a Visitor pattern:

struct Visitor_Void
{
    void (*fn_msgh_bits)(mach_msg_bits_t);
    void (*fn_msgh_size)(mach_msg_size_t);
    void (*fn_msgh_remote_port)(mach_port_t);
    void (*fn_msgh_local_port)(mach_port_t);
    void (*fn_msgh_reserved)(mach_msg_size_t);
    void (*fn_msgh_id)(mach_msg_id_t);
};


void Visit_Members(mach_msg_header_t * p_header,
                   struct Visitor_Void * p_visitor)
{
    (p_visitor->fn_msgh_bits)(p_header->msgh_bits);
    (p_visitor->fn_msgh_size)(p_header->msgh_size);
    (p_visitor->fn_msgh_remote_port)(p_header->msgh_remote_port);
    (p_visitor->fn_msgh_local_port)(p_header->msgh_local_port);
    (p_visitor->fn_msgh_reserved)(p_header->msgh_reserved);
    (p_visitor->fn_msgh_id)(p_header->msgh_id);
    return;
}

+1 for the warning about not making assumptions about layout, and particularly for the visitor pattern example. — Dan, May 20 '11 at 22:17
My issue is that I don't know the layout of the structure. The message header proceeds the unknown layout. — Kristina, May 21 '11 at 13:10

score 1 · Answer 3 · answered May 20 '11 at 15:46

1

No, you shouldn't be doing this!

What is particularly wrong here

for (int i = 0; i <= 24; i++)
{
    int deref = *((int*)(InHeadP+i));
    printf("InHeadP[%d]=%d\n",i,deref);
}

is that you print the first int of 24 different structs.

answered May 20 '11 at 15:46

Bo Persson

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1

@BlackBear - My solution would be to use 6 printf's and not bother with this hack. :-) – Bo Persson May 20 '11 at 15:53

score 0 · Answer 4 · answered May 20 '11 at 15:48

When you call (int*)(InHeadP+i) you are doing pointer arithmetic on mach_msg_header_t which is size 24. So your first iteration is at the beginning of the struct, and your second iteration is right after your struct. You are accessing some other memory.

Try (((int*)InHeadP)+i).

score 0 · Answer 5 · answered May 20 '11 at 15:49

0

Wait a second, this is wrong:

 int deref = *((int*)(InHeadP+i));

You want

int deref == *((int*)InHeadP+i)

You skipping 24 bytes each time through the loop, rather than 4.

answered May 20 '11 at 15:49

andrewdski

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Run through a structure and print all the values?

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