Help (dynamically) converting MAC address (as text) to uint32_t value

prolucid · August 20, 2015, 7:25pm

Hi there,

I'm having a heck of a time converting a MAC address to a uint32_t value. I'm new to C so please pardon my lack of understanding.

Some background. This is a datalogger. Here's the process...

arduino powers up and sends a broadcast message via xbee-api
RPi receives the xbee broadcast packet (via an asynchronous python script) and sends a unicast message back to the MAC address of the arduino
the arduino receives a packet with a bunch of data and sets global variables and goes about datalogging, etc

Let's say the datalogger receives this char array:
"1439316593:0013a20040DCCD01:5:300:60:10800:900"

I delimit by ":" and get the MAC address. No problem there.

So now I have a simple char array of "0013a20040DCCD01" and want to pass that to the serialHigh and serialLow of a function and send a unicast packet to that address.

I've removed portions of the code that are irrelevant. This works:

uint32_t sH = 0x0013a200;
uint32_t sL = 0x40DCCD01;
XBeeAddress64 addr64 = XBeeAddress64(sH, sL);
ZBTxRequest zbTx = ZBTxRequest(addr64, (uint8_t *)thisPayload, strlen(thisPayload));

However, due to the dynamic nature of the datalogger, I can't hard code a MAC address in the code - it needs to be dynamic.

So how does one go from a char of "0013a20040DCCD01" to the sH and sL uint32_t values I need?

I've tried too many things to mention without any success. I can delimit values, get values based on sub strings, etc but I'm having trouble understanding how to convert between data types.

Can anyone help guide me here? Thanks!

John

johnwasser · August 20, 2015, 7:37pm

uint32_t sH = 0;
uint32_t sL = 0;
char *MAC =  (the address of the first character of the MAC address in the string);

for (int i=0; i<16; i++) {
    unsigned char nibble = *MAC++;
    if (nibble >= '0' && nibble <= '9') {
        nibble -= '0';
    {
    else if  (nibble >= 'A' && nibble <= 'F') {
        nibble -= 'A';
        nibble += 0xA;
    }
    //  nibble now contains the binary value of the current hex characters
    if (i<8) {
       // First 8 nibbles go into sH
        sH <<= 4;
        sH |= nibble;
    } else {
        // Last 8 nibbles go into sL
        sL <<= 4;
        sL |= nibble;
    }
}

christop · August 20, 2015, 7:41pm

"0013a20040DCCD01" is not a valid MAC address. A MAC address is only 48 bits long. "0013a20040DCCD01" is 64 bits long.

guix · August 20, 2015, 7:43pm

Hello,

Split your string in two (8 chars in each) and use strtoul.

prolucid · August 20, 2015, 8:17pm

John, my goodness, you're amazing! Almost working. There was a small typo in your code (for bracket was off). I corrected the bracket so it would compile (and may have butchered your code), but something's still off. The sL is calculated correctly, just not the sH.

  for (int i = 0; i < 16; i++) {
    unsigned char nibble = *MAC++;
    if (nibble >= '0' && nibble <= '9') {
      nibble -= '0';
    }
    else if  (nibble >= 'A' && nibble <= 'F') {
      nibble -= 'A';
      nibble += 0xA;
    }
    //  nibble now contains the binary value of the current hex characters
    if (i < 8) {
      // First 8 nibbles go into sH
      sH <<= 4;
      sH |= nibble;
    } else {
      // Last 8 nibbles go into sL
      sL <<= 4;
      sL |= nibble;
    }
  }

These are the values your snippet figured out:
high: 171200
low: 40DCCD01

Again, apologies I'm not quite there yet to troubleshoot your code, but it's close. Any ideas?

christop - Yes, this is a valid MAC for xbee hardware devices. You're probably thinking of an ethernet MAC address.

guix - I'm trying your idea, too. Hang on...

prolucid · August 20, 2015, 8:26pm

guix, I tried this:

  char *high = "0013a200";
  char *low = "40DCCD01";

  sH = strtoul(high, NULL, 0);
  sL = strtoul(low, NULL, 0);

  Serial.print("high: ");
  Serial.println(sH, HEX);
  Serial.print("low: ");
  Serial.println(sL, HEX);

which yielded:

high: B
low: 28

Am I missing something here?

guix · August 20, 2015, 8:29pm

Yes, hex is base 16, not 0

sH = strtoul(high, NULL, 16);
sL = strtoul(low, NULL, 16);

prolucid · August 20, 2015, 8:33pm

John, my apologies. I had a lowercase 'a' in my MAC address. Toggled the case and it works great.

char *MAC = "0013a20040DCCD01"; // oops!
char *MAC = "0013A20040DCCD01"; // works

For others, here's the working code:

  char *MAC = "0013A20040DCCD01";

  for (int i = 0; i < 16; i++) {
    unsigned char nibble = *MAC++;
    if (nibble >= '0' && nibble <= '9') {
      nibble -= '0';
    }
    else if  (nibble >= 'A' && nibble <= 'F') {
      nibble -= 'A';
      nibble += 0xA;
    }

    Serial.print("nibble: ");
    Serial.println(nibble);
    //  nibble now contains the binary value of the current hex characters
    if (i < 8) {
      // First 8 nibbles go into sH
      sH <<= 4;
      sH |= nibble;
    } else {
      // Last 8 nibbles go into sL
      sL <<= 4;
      sL |= nibble;
    }
  }

prolucid · August 20, 2015, 8:35pm

guix, awesome, that worked. Two working examples! Learning more goodies every day. Thanks fellas!

guix · August 20, 2015, 8:42pm

NP

Johnwasser's solution is more elegant, that's what I would use if I had to.

For your lowercase problem, a simple

nibble = toupper( nibble );

would have fixed it

christop · August 20, 2015, 8:44pm

prolucid:
christop - Yes, this is a valid MAC for xbee hardware devices. You're probably thinking of an ethernet MAC address.

Hm, looks like you're right about that. I learn something new every day!

prolucid · August 20, 2015, 8:54pm

guix, thanks for the "one upper". Yea, John's code looks more efficient and I can accommodate for case issues with your one liner. Sweet!

AntonioGR · May 23, 2020, 10:48am

johnwasser:
uint32_t sH = 0;

uint32_t sL = 0;
char *MAC = (the address of the first character of the MAC address in the string);

for (int i=0; i<16; i++) {
unsigned char nibble = *MAC++;
if (nibble >= '0' && nibble <= '9') {
nibble -= '0';
{
else if (nibble >= 'A' && nibble <= 'F') {
nibble -= 'A';
nibble += 0xA;
}
// nibble now contains the binary value of the current hex characters
if (i<8) {
// First 8 nibbles go into sH
sH <<= 4;
sH |= nibble;
} else {
// Last 8 nibbles go into sL
sL <<= 4;
sL |= nibble;
}
}

Hi John.
Sorry I'm late for this post. But I need some help. How can this MAC address be saved in a char array? In the NodeDiscovery function, every Router sends back their MAC Address inside an API frame, so I'm able to read them. But every time I want to print them, I see this:
013FFFFFFA2040FFFFFFA1FFFFFFF246 instead the real Address: 0013A20040A1F246

char addr[17];
-Is this 'char addr[17]' the correct way to store the 64Bit Mac Address?

gfvalvo · May 23, 2020, 12:08pm

AntonioGR:
Sorry I'm late for this post.

Almost 5 years late, welcome to 2020! You might be better off starting a new thread that properly describes your problem.

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Help (dynamically) converting MAC address (as text) to uint32_t value

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