# sd-id128

## 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) 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.
    


## NOTES



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


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