See sd_id128_to_string(3), sd_id128_randomize(3) and sd_id128_get_machine(3) for more information about the implemented functions.

A 128-bit ID is implemented as the following union type:

typedef union sd_id128 {
  uint8_t bytes[16];
  uint64_t qwords[2];
} sd_id128_t;

This union type allows accessing the 128-bit ID as 16 separate bytes or two 64-bit words. It is generally safer to access the ID components by their 8-bit array to avoid endianness issues. This union is intended to be passed call-by-value (as opposed to call-by-reference) and may be directly manipulated by clients.

A couple of macros are defined to denote and decode 128-bit IDs:

SD_ID128_MAKE() may be used to denote a constant 128-bit ID in source code. A commonly used idiom is to assign a name to a 128-bit ID using this macro:

#define SD_MESSAGE_COREDUMP SD_ID128_MAKE(fc,2e,22,bc,6e,e6,47,b6,b9,07,29,ab,34,a2,50,b1)

SD_ID128_CONST_STR() may be used to convert constant 128-bit IDs into constant strings for output. The following example code will output the string "fc2e22bc6ee647b6b90729ab34a250b1":

int main(int argc, char *argv[]) {
  puts(SD_ID128_CONST_STR(SD_MESSAGE_COREDUMP));
}

SD_ID128_FORMAT_STR and SD_ID128_FORMAT_VAL() may be used to format a 128-bit ID in a printf(3) format string, as shown in the following example:

int main(int argc, char *argv[]) {
  sd_id128_t id;
  id = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
  printf("The ID encoded in this C file is " SD_ID128_FORMAT_STR ".\n", SD_ID128_FORMAT_VAL(id));
  return 0;
}

Use sd_id128_equal() to compare two 128-bit IDs:

int main(int argc, char *argv[]) {
  sd_id128_t a, b, c;
  a = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
  b = SD_ID128_MAKE(f2,28,88,9c,5f,09,44,15,9d,d7,04,77,58,cb,e7,3e);
  c = a;
  assert(sd_id128_equal(a, c));
  assert(!sd_id128_equal(a, b));
  return 0;
}

Note that new, randomized IDs may be generated with journalctl(1)'s --new-id option.