---
name: "sd_id128_string_equal"
section: "3"
platform: "linux"
package: "libsystemd-dev"
version: "257.13-1~deb13u1"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/sd-id128"
---

# sd\_id128\_string\_equal(3)

APIs for processing 128-bit IDs

Part of [sd-id128(3)](/sd-id128/3.md).

## NAME

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\_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

```
#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)](/libsystemd/3.md) 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)](/printf/3.md) 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)](/systemd-id128/1.md)'s **new** command.

See [**sd\_id128\_to\_string**(3)](/sd_id128_to_string/3.md), [**sd\_id128\_randomize**(3)](/sd_id128_randomize/3.md) and [**sd\_id128\_get\_machine**(3)](/sd_id128_get_machine/3.md) 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)](/pkg-config/1.md) file.

The code described here uses [**getenv**(3)](/getenv/3.md), which is declared to be not multi-thread-safe. This means that the code calling the functions described here must not call [**setenv**(3)](/setenv/3.md) 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)](/systemd/1.md), [**sd\_id128\_to\_string**(3)](/sd_id128_to_string/3.md), [**sd\_id128\_randomize**(3)](/sd_id128_randomize/3.md), [**sd\_id128\_get\_machine**(3)](/sd_id128_get_machine/3.md), [**printf**(3)](/printf/3.md), [**journalctl**(1)](/journalctl/1.md), [**sd-journal**(3)](/sd-journal/3.md), [**pkg-config**(1)](/pkg-config/1.md), [**machine-id**(5)](/machine-id/5.md)

## NOTES

- 1\.

  RFC 4122

  https://tools.ietf.org/html/rfc4122

- 2\.

  UUID canonical representation

  https://en.wikipedia.org/wiki/Universally\_unique\_identifier#Format