quick C question - typedef u_char array and its pointer

[tzushky]

u_char in c

hello everyone

i have a small C manipulation problem.

u_char is the typedef of unsigned char. Used for bytes.
I define a MAC address as

typedef u_char mac_address[6];

and later in main() i have a pointer

mac_address *mac_src;

which will be assigned a value using a pointer to some packet data. This works just fine. My problem is printing the MAC to the screen in intelligible form ( xx:xx...xx)

Why? I try and get the values of the 6 components of the mac_src in memory by mac_src[0]...etc. This gets me their addresses, which on one side I understand...but how do I get to the contents of the addresses, my MAC components?? With *mac_src[0] ?

I had honestly thought I understood the idea of arrays and pointers in C, but it's making me nuts. PLZ help!

thanks in advance

Added after 59 minutes:

nobody curious yet?

i seem to have found a very lucrative way: printf( mac_src[0][0], mac_src[0][1]...)

because it seems that mac_src[0] is my pointer value and so inside it there is the table of bytes....weird...

 

[jhbbunch]

typedef an array

mac_address is a pointer. And you declare a pointer to it. That makes your variable a pointer to a pointer. Try dereferencing it. I'm out of practice with C, but I think that is your problem.

typedef unsigned char mac_address[6];
mac_address *m;
mac_address d = {0xAE, 0x45,0xB3, 0x5A,0xE7, 0x83};
m = &d;

Now m is a pointer that points to an array. It is not an arry itself so you can't use indexing notation m[], you have to use pointer notation to access the various elements of d. The address of d is *m, the value stored at d is *d so it is also **m.
This is also the value stored at d[0]. To access the other elements use:
*(*m + 1) and so on. The compiler will use the number as the nth element of whatever value type is being accessed. In this case it does the equivalent of
*(*m + 1 * sizeof(uchar)) to calculate the address. (Actually a pentium will do it automatically with the appropriate address mode)

to format your mac address in hex 00:00:00:00:00:00 format use

char mybuffer[20];

sprintf( mybuffer, "%2X:%2X:%2X:%2X:%2X:%2X\n", **m, d[1], d[2], d[3], *(*m + 4), *(*m + 5)) ;

Added:
Now that I've thought about it a little (it's been awhile) since *m is the same as d in my example, then (*m)[5] is the same as d[5] so you can use indexing notation.