SD-ID128(3)                       sd-id128                       SD-ID128(3)

NAME         top

       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

SYNOPSIS         top

       #include <systemd/sd-id128.h>

       pkg-config --cflags --libs libsystemd

DESCRIPTION         top

       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

       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[]) {

       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

           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.

NOTES         top

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

SEE ALSO         top

       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         top

        1. RFC 4122

COLOPHON         top

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systemd 230                                                      SD-ID128(3)