sd-id128 - Man Page

APIs for processing 128-bit IDs

Synopsis

#include <systemd/sd-id128​.h>

SD_ID128_ALLF

SD_ID128_NULL

SD_ID128_CONST_STR(id)

SD_ID128_FORMAT_STR

SD_ID128_FORMAT_VAL(id)

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)

SD_ID128_UUID_FORMAT_STR

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

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 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 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 "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() is used to format an 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 "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:

assert(sd_id128_is_null(SD_ID128_NULL));

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

assert(sd_id128_is_allf(SD_ID128_ALLF));

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));
  assert(!sd_id128_in_set(a));
  assert(!sd_id128_in_set(a,
                          SD_ID128_MAKE(f2,28,88,9c,5f,09,44,15,9d,d7,04,77,58,cb,e7,3e)
                          SD_ID128_MAKE(2f,88,28,5f,9c,44,09,9d,d7,15,77,04,bc,85,7e,e3)
                          SD_ID128_ALLF));
  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 command​.

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

Notes

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 started​.

History

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

systemd(1), sd_id128_to_string(3), sd_id128_randomize(3), sd_id128_get_machine(3), printf(3), journalctl(1), sd-journal(3), pkg-config(1), machine-id(5)

Notes

  1. RFC 4122
    https://tools.ietf.org/html/rfc4122
  2. UUID canonical representation
    https://en.wikipedia.org/wiki/Universally_unique_identifier#Format

Referenced By

libsystemd(3), 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), systemd.network(5).

The man pages SD_ID128_ALLF(3), SD_ID128_CONST_STR(3), sd_id128_equal(3), SD_ID128_FORMAT_STR(3), SD_ID128_FORMAT_VAL(3), sd_id128_in_set(3), sd_id128_in_set_sentinel(3), sd_id128_in_setv(3), sd_id128_is_allf(3), sd_id128_is_null(3), SD_ID128_MAKE(3), SD_ID128_MAKE_STR(3), SD_ID128_MAKE_UUID_STR(3), SD_ID128_NULL(3), sd_id128_string_equal(3), sd_id128_t(3) and SD_ID128_UUID_FORMAT_STR(3) are aliases of sd-id128(3).

systemd 262~rc2