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


sd-id128, SD_ID128_ALLF, SD_ID128_CONST_STR, SD_ID128_FORMAT_STR, SD_ID128_FORMAT_VAL, SD_ID128_MAKE, SD_ID128_MAKE_STR, SD_ID128_MAKE_UUID_STR, SD_ID128_NULL, SD_ID128_UUID_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


#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 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.

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.


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


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)


RFC 4122
canonical representation
systemd 249