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 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 "canonical representation". They format the string based on RFC4122 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.

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)