[sdiy] Pointers in C

[Olivier Gillet]

int *my_ptr = my_unsigned_long_int;unsigned char FirstByte =
*my_ptr;my_ptr++;unsigned char SecondByte = *my_ptr;my_ptr++;unsigned
char ThirdByte = *my_ptr;my_ptr++;unsigned char FourthByte = *my_ptr;

This won't work because pointer arithmetic always uses the size of the
underlying type. Here we are assuming that sizeof(int) = 32

Let's say my_unsigned_long_int is at address 0x1000

> int *my_ptr = &my_unsigned_long_int;

my_ptr will store address 0x1000 (I assume you forgot the &)

> unsigned char FirstByte = *my_ptr;

FirstByte will contain the 8 MSB of the 32-bit int at address 0x1000
(ie byte at 0x1000 or byte at 0x1003 depending on the endianness of
the platform).

> my_ptr++;

my_ptr will store address 0x1004 ; because my_ptr is an int pointer ;
and pointer arithmetic always uses the size of the interlying type (



To do what you want to do, you need to use an unsigned char pointer.

unsigned char* my_ptr = (unsigned char*)(&my_unsigned_long_int).
unsigned char FirstByte = *my_ptr;
my_ptr++;
...


The union is a simpler way of accessing individual bytes or words ;
though it is still endinanness dependent. The most portable way of
accessing individual bytes from words is still shifting and masking.

Olivier