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.

Answer 1

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.