sd-id128(3)

NAME

   sd-id128, sd_id128_t, SD_ID128_MAKE, SD_ID128_NULL, SD_ID128_CONST_STR,
   SD_ID128_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_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;
       }

   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 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

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