sd-id128, sd_id128_t, SD_ID128_MAKE, SD_ID128_CONST_STR, SD_ID128_FORMAT_STR, SD_ID128_FORMAT_VAL, sd_id128_equal — APIs for processing 128 bit IDs
#include <systemd/sd-id128.h>
pkg-config --cflags --libs libsystemd-id128 
sd-id128.h provides APIs to
                process and generate 128 bit ID values. The 128 bit ID
                values processed and generated by these APIs are a
                generalization of OSF UUIDs as defined by RFC
                4122, though use a simpler string
                formatting. These functions impose no structure on the
                used IDs, much unlike OSF UUIDs or Microsoft GUIDs,
                but are fully compatible with those types of IDs.
                
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 128bit 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.
These APIs are implemented as a shared library,
                which can be compiled and linked to with the
                libsystemd-id128
                pkg-config(1)
                file.