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


#include <systemd/sd-id128.h>

pkg-config --cflags --libs libsystemd


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[]) {

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


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)


  1. RFC 4122
  2. canonical representation

Referenced By

machine-id(5), sd_id128_get_machine(3), sd_id128_randomize(3), sd_id128_to_string(3), sd-journal(3), systemd.directives(7), systemd-id128(1), systemd.index(7).

The man pages SD_ID128_CONST_STR(3), sd_id128_equal(3), SD_ID128_FORMAT_STR(3), SD_ID128_FORMAT_VAL(3), sd_id128_is_null(3), SD_ID128_MAKE(3), SD_ID128_MAKE_STR(3), SD_ID128_NULL(3), sd_id128_t(3) and SD_ID128_UUID_FORMAT_STR(3) are aliases of sd-id128(3).

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