compare with a 6-byte long string of zeros

in my code I work with Ethernet MAC addresses and need to compare for a 00:00:00:00:00:00 address; the easiest solution I came up with is this:

#define ETH_ADDR_LEN  6
unsigned char mac[ETH_ADDR_LEN] = { 0x1, 0x2, 0x3, 0x4, 0x5, 0x6 };  /* example */
const unsigned char empty[ETH_ADDR_LEN] = { 0, };

if (memcmp(mac, empty, ETH_ADDR_LEN) == 0) {
  ....
}

Is there a more concise way to achieve my goal? Simply memcmp(mac, "", 6) won开发者_Python百科't work -- may I know why?

Thanks in advance!

There is nothing wrong with your code. Keep it as it is. The "empty" array contains 6 zeroes so it will work just fine.

The memcmp(mac, "", 6) does not work because the empty string consists of a single NUL character. Comparing 6 bytes will compare your MAC address against that NUL plus 5 bytes of potential garbage following it.

You can, however, force your string to contain six NUL characters using the following. Note that it only needs 5 explicit NULs because the string has an additional trailing NUL:

if(memcmp(mac, "\0\0\0\0\0", 6) == 0) {
  ...
}

memcmp(mac, "", 6) does not work because "" is an array of 1 character with zero value (and decays to a pointer to its single element).

Access to elements outside the array invoke Undefined Behaviour.