sd_id128_t (3) Linux Manual Page

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:

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;
}

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[])
{
    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;
}

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 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;
}

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.

Notes

These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.

See Also

systemd(1), sd_id128_to_string(3), sd_id128_randomize(3), sd_id128_get_machine(3), printf(3), journalctl(1), sd-journal(7), pkg-config(1), machine-id(5)

Notes

1.

RFC 4122

https://tools.ietf.org/html/rfc4122