NAME

       sd-id128, sd_id128_t, SD_ID128_MAKE, SD_ID128_MAKE_STR, SD_ID128_NULL, SD_ID128_CONST_STR,
       SD_ID128_FORMAT_STR, SD_ID128_UUID_FORMAT_STR, SD_ID128_FORMAT_VAL, sd_id128_equal,
       sd_id128_is_null - APIs for processing 128-bit IDs

SYNOPSIS

       #include <systemd/sd-id128.h>

       pkg-config --cflags --libs libsystemd

DESCRIPTION

       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[1] but use a simpler string format. 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_NULL may be used to refer to the 128bit ID consisting of only NUL bytes.

       SD_ID128_MAKE_STR() is similar to SD_ID128_MAKE(), but creates a const char* expression
       that can be conveniently used in message formats and such:

           #include <stdio.h>
           #define SD_MESSAGE_COREDUMP_STR SD_ID128_MAKE_STR(fc,2e,22,bc,6e,e6,47,b6,b9,07,29,ab,34,a2,50,b1)

           int main(int argc, char **argv) {
                   puts("Match for coredumps: MESSAGE_ID=" SD_MESSAGE_COREDUMP_STR);
           }

       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("Match for coredumps: %s", 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;
           }

       SD_ID128_UUID_FORMAT_STR() is similar to SD_ID128_FORMAT_STR() but includes separating
       hyphens to conform to the "canonical representation[2]".

       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;
           }

       Use sd_id128_is_null() to check if an 128bit ID consists of only NUL bytes:

           int main(int argc, char *argv[]) {
                   assert(sd_id128_is_null(SD_ID128_NULL));
           }

       Note that new, randomized IDs may be generated with systemd-id128(1)'s new command.

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

        2. canonical representation
           https://en.wikipedia.org/wiki/Universally_unique_identifier#Format