sd-id128(3) — Linux manual page


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

NAME         top

       sd-id128, SD_ID128_ALLF, SD_ID128_CONST_STR, SD_ID128_FORMAT_STR,
       sd_id128_equal, sd_id128_in_set, sd_id128_in_set_sentinel,
       sd_id128_in_setv, sd_id128_is_allf, sd_id128_is_null, sd_id128_t
       - 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 mostly 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 128-bit ID consisting
       of only NUL bytes (i.e. all bits off).

       SD_ID128_ALLF may be used to refer to the 128-bit ID consisting
       of only 0xFF bytes (i.e. all bits on).

       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 and SD_ID128_MAKE_UUID_STR() are similar
       to SD_ID128_FORMAT_STR and SD_ID128_MAKE_STR(), but include
       separating hyphens to conform to the "canonical
       representation[2]". They format the string based on RFC4122[1]
       Variant 1 rules, i.e. converting from Big Endian byte order. This
       matches behaviour of most other Linux userspace infrastructure.
       It's probably best to avoid UUIDs of other variants, in order to
       avoid unnecessary ambiguities. All 128-bit IDs generated by the
       sd-id128 APIs strictly conform to Variant 1 Version 4 UUIDs, as
       per RFC 4122.

       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 128-bit ID consists of only
       NUL bytes:


       Similarly, use sd_id128_is_allf() to check if an 128-bit ID
       consists of only 0xFF bytes (all bits on):


       For convenience, sd_id128_in_set() takes a list of IDs and
       returns true if any are equal to the first argument:

           int main(int argc, char *argv[]) {
             sd_id12_t a = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
             assert(sd_id128_in_set(a, a));
             assert(sd_id128_in_set(a, a, a));
             return 0;

       sd_id128_in_set() is defined as a macro over
       sd_id128_in_set_sentinel(), adding the SD_ID128_NULL sentinel.
       Since sd_id128_in_set_sentinel() uses SD_ID128_NULL as the
       sentinel, SD_ID128_NULL cannot be otherwise placed in the
       argument list.

       sd_id128_in_setv() is similar to sd_id128_in_set_sentinel(), but
       takes a struct varargs argument.

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

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

        2. canonical representation

COLOPHON         top

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systemd 249                                                  SD-ID128(3)

Pages that refer to this page: systemd-id128(1)sd_id128_get_machine(3)sd_id128_randomize(3)sd_id128_to_string(3)sd-journal(3)machine-id(5)