How to get the 20 least significant bits?

I have some strings that come in over a REST interface that represent some network coordinates. The string looks something like:

02:00:00:00:00:07

Bits 5 - 0 represent tier 1
bits 13 - 6 represent tier 2
bits 19 - 14 represent tier 3.

How would I parse these out in C?

I've tried looking online, but I found it quite confusing.

I don't understand your string format, but if you have an unsigned integer and you want those bits as separate values:

Code:

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#include <stdio.h>
 
int main() {
    unsigned n = 0xabcde; // 101010 11110011 011110
                          // 19-14  13-6    5-0
 
    unsigned a = (n >> 14) & 0x3f;
    unsigned b = (n >>  6) & 0xff;
    unsigned c =  n        & 0x3f;
 
    printf("%02x (%3u)\n", a, a); // 2a (  42)
    printf("%02x (%3u)\n", b, b); // f3 (  243)
    printf("%02x (%3u)\n", c, c); // 1e (  30)
 
    return 0;
}

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Assuming this "string" is a MAC address (6 bytes):

Code:

---

  union {
    unsigned int dw;
    unsigned char b[8];
  } u = { 0 };

  // str points to string...
  if ( sscanf( str, "%hhx:%hhx:%hhx:%hhx:%hhx:%hhx",
    &u.b[5], &u.b[4], &u.b[3], &u.b[2], &u.b[1], &u.b[0] ) != 6 )
  { ... error ... }
  ...

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Now you can use u.dw as john.c showd above.

Quote:

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Originally Posted by flp1969

Assuming this "string" is a MAC address (6 bytes):

Code:

---

  union {
    unsigned int dw;
    unsigned char b[8];
  } u = { 0 };

  // str points to string...
  if ( sscanf( str, "%hhx:%hhx:%hhx:%hhx:%hhx:%hhx",
    &u.b[5], &u.b[4], &u.b[3], &u.b[2], &u.b[1], &u.b[0] ) != 6 )
  { ... error ... }
  ...

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Now you can use u.dw as john.c showd above.

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Yes, that's exactly what it is, a MAC address. Thanks!