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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 top

#include <systemd/sd-id128.h>SD_ID128_ALLFSD_ID128_NULLSD_ID128_CONST_STR(id)SD_ID128_FORMAT_STRSD_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_STRint sd_id128_equal(sd_id128_ta, sd_id128_tb);int sd_id128_string_equal(const char *a, sd_id128_tb);int sd_id128_is_null(sd_id128_tid);int sd_id128_is_allf(sd_id128_tid);int sd_id128_in_setv(sd_id128_tid, va_listap);int sd_id128_in_set_sentinel(sd_id128_tid, ..., SD_ID128_NULL);int sd_id128_in_set(sd_id128_tid, ...);pkg-config --cflags --libs libsystemd

DESCRIPTION top

sd-id128.h is part oflibsystemd(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 byRFC 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_NULLdefines an ID consisting of onlyNULbytes (i.e. all bits off).SD_ID128_ALLFdefines an ID consisting of only0xFFbytes (i.e. all bits on).SD_ID128_MAKE_STR()is similar toSD_ID128_MAKE(), but creates aconst 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_STRandSD_ID128_FORMAT_VAL()is used to format an ID in aprintf(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_STRandSD_ID128_MAKE_UUID_STR()are similar toSD_ID128_FORMAT_STRandSD_ID128_MAKE_STR(), but include separating hyphens to conform to the "UUID canonicalrepresentation[2]". They format the string based onRFC4122[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 tosd_id128_equal(), but the first ID is formatted asconst char*. The same restrictions apply as to the first argument ofsd_id128_from_string().sd_id128_is_null()checks if an ID consists of onlyNULbytes: assert(sd_id128_is_null(SD_ID128_NULL)); Similarly,sd_id128_is_allf()checks if an ID consists of only0xFFbytes (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 anSD_ID128_NULLsentinel, 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 oversd_id128_in_set_sentinel(), adding theSD_ID128_NULLsentinel automatically. Sincesd_id128_in_set_sentinel()usesSD_ID128_NULL as the sentinel,SD_ID128_NULLcannot be otherwise placed in the argument list.sd_id128_in_setv()is similar tosd_id128_in_set_sentinel(), but takes a struct varargs argument. New randomized IDs may be generated withsystemd-id128(1)'snew command. Seesd_id128_to_string(3),sd_id128_randomize(3) andsd_id128_get_machine(3) for information about other implemented functions.

NOTES top

Functions described here are available as a shared library, which can be compiled against and linked to with thelibsystemd pkg-config(1) file. The code described here usesgetenv(3), which is declared to be not multi-thread-safe. This means that the code calling the functions described here must not callsetenv(3) from a parallel thread. It is recommended to only do calls tosetenv()from an early phase of the program when no other threads have been started.

COLOPHON top

This page is part of thesystemd (systemd system and service manager) project. Information about the project can be found at ⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a bug report for this manual page, see ⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩. This page was obtained from the project's upstream Git repository ⟨https://github.com/systemd/systemd.git⟩ on 2025-08-11. (At that time, the date of the most recent commit that was found in the repository was 2025-08-11.) If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up-to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgsystemd 258~rc2SD-ID128(3)

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