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_string_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>






       SD_ID128_MAKE(v0, v1, v2, v3, v4, v5, v6, v7, v8, v9, vA, vB, vC,
       vD, vE, vF)

       SD_ID128_MAKE_STR(v0, v1, v2, v3, v4, v5, v6, v7, v8, v9, vA, vB,
       vC, vD, vE, vF)

       SD_ID128_MAKE_UUID_STR(v0, v1, v2, v3, v4, v5, v6, v7, v8, v9,
       vA, vB, vC, vD, vE, vF)


       int sd_id128_equal(sd_id128_t a, sd_id128_t b);

       int sd_id128_string_equal(const char *a, sd_id128_t b);

       int sd_id128_is_null(sd_id128_t id);

       int sd_id128_is_allf(sd_id128_t id);

       int sd_id128_in_setv(sd_id128_t id, va_list ap);

       int sd_id128_in_set_sentinel(sd_id128_t id, ..., SD_ID128_NULL);

       int sd_id128_in_set(sd_id128_t id, ...);

       pkg-config --cflags --libs libsystemd

DESCRIPTION         top

       sd-id128.h is part of libsystemd(3) and provides APIs to
       generate, convert, and compare 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.

       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 by value (as opposed to
       pass-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() is used to write a constant ID in source code. A
       commonly used idiom is to assign a name to an ID using this

           #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 defines an ID consisting of only NUL bytes (i.e.
       all bits off).

       SD_ID128_ALLF defines an 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() converts constant IDs into constant strings
       for output. The following example code will output the string

           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() is used to format
       an 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;

       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 "UUID 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.

       sd_id128_equal() compares 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;

       sd_id128_string_equal() is similar to sd_id128_equal(), but the
       first ID is formatted as const char*. The same restrictions apply
       as to the first argument of sd_id128_from_string().

       sd_id128_is_null() checks if an ID consists of only NUL bytes:


       Similarly, sd_id128_is_allf() checks if an ID consists of only
       0xFF bytes (all bits on):


       sd_id128_in_set_sentinel() takes a list of IDs and returns true
       if the first argument is equal to any of the subsequent
       arguments. The argument list is terminated by an SD_ID128_NULL
       sentinel, which must be present.

       sd_id128_in_set() is a convenience function that takes a list of
       IDs and returns true if the first argument is equal to any of the
       subsequent arguments:

           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
       automatically. 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.

       New randomized IDs may be generated with systemd-id128(1)'s new

       See sd_id128_to_string(3), sd_id128_randomize(3) and
       sd_id128_get_machine(3) for information about other implemented

NOTES         top

       Functions described here are available as a shared library, which
       can be compiled against and linked to with the
       libsystemd pkg-config(1) file.

       The code described here uses getenv(3), which is declared to be
       not multi-thread-safe. This means that the code calling the
       functions described here must not call setenv(3) from a parallel
       thread. It is recommended to only do calls to setenv() from an
       early phase of the program when no other threads have been

HISTORY         top

       sd_id128_equal(), sd_id128_string_equal(), sd_id128_is_null(),
       sd_id128_is_allf(), sd_id128_in_setv(),
       sd_id128_in_set_sentinel(), and sd_id128_in_set() were added in
       version 252.

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. UUID canonical representation

COLOPHON         top

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       manager) project.  Information about the project can be found at
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       This page was obtained from the project's upstream Git repository
       ⟨⟩ on 2024-06-14.  (At that
       time, the date of the most recent commit that was found in the
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       (which is not part of the original manual page), send a mail to

systemd 257~devel                                            SD-ID128(3)

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