Hex char array 2 by 2 characters to uint8_t

Question

I'm making a program for a microcontroller (mbed LPC1768) and having a few problems to decode serial input to uint8_t.

I get a char ackBuffer[20]; filled with input from an external device. What we get is for example: F040A34B6785121\r

What I want to do is to get an uint8_t array where each number is symbolized by the hex value of 2 characters. The F is to show the message from the external device is started and the carriage return is the end of each message.

So what I want in the uint8_t array is:

0x04
0x0A
0x34
...

To make it even harder it can be the ackBuffer is a smaller size, so for example only F04004A\r (if this can be easy fixed then it's very nice, but I can work around this problem myself.

Thank you!

Answer 1

So I'm assuming that an ackMessage must start with 'F' and end with '\r'. And I'm even assuming that, since you work on a micro controller, anything that does more than required (eg. library functions) should be avoided. Given that you just have to iterate over the buffer:

uint8_t charToHex(unsigned char value)
{
  if (value >= '0' && value <= '9') return value - '0';
  else if (value >= 'A' && value <= 'F') return value - 'A' + 10;
  else if (value >= 'a' && value <= 'f') return value - 'a' + 10;
  assert(false);
}

#define BUFFER_SIZE (20)

int main(void)
{
  char ackBuffer[BUFFER_SIZE] = "F12345890AB\r";
  uint8_t result[(BUFFER_SIZE-2)/2]; // at most 9 bytes since it's 20 - 2 / 2

  char* ptr = ackBuffer;

  assert(*ptr == 'F');
  ++ptr;

  int count = 0;
  while(*ptr != '\r')
  {
    uint8_t value = charToHex(*ptr) << 4 | charToHex(*(ptr+1));
    result[count++] = value;
    ptr += 2;
  }

  int i;
  for (i = 0; i < count; ++i)
    printf("%x\n", result[i]);

  return 0;
}

Mind that you must have a charToHex consistent with the used encoding and probably you could need some more sanity checks around the code.