centos man page for sd_id128_format_val

SD-ID128(3)							     sd-id128							       SD-ID128(3)

NAME
       sd-id128, sd_id128_t, SD_ID128_MAKE, SD_ID128_CONST_STR, SD_ID128_FORMAT_STR, SD_ID128_FORMAT_VAL, sd_id128_equal - APIs for processing
       128-bit IDs

SYNOPSIS
       #include <systemd/sd-id128.h>

       pkg-config --cflags --libs libsystemd-id128

DESCRIPTION
       sd-id128.h provides APIs to process and generate 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 fully compatible with those types of IDs.

       See sd_id128_to_string(3), sd_id128_randomize(3) and sd_id128_get_machine(3) for more information about the implemented functions.

       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 call-by-value (as opposed to call-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() may be used to denote a constant 128-bit ID in source code. A commonly used idiom is to assign a name to a 128-bit 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_CONST_STR() may be used to convert constant 128-bit IDs into constant strings for output. The following example code will output
       the string "fc2e22bc6ee647b6b90729ab34a250b1":

	   int main(int argc, char *argv[]) {
		   puts(SD_ID128_CONST_STR(SD_MESSAGE_COREDUMP));
	   }

       SD_ID128_FORMAT_STR and SD_ID128_FORMAT_VAL() may be used to format a 128-bit 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 ".
", SD_ID128_FORMAT_VAL(id));
		   return 0;
	   }

       Use sd_id128_equal() to compare 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;
	   }

       Note that new, randomized IDs may be generated with journalctl(1)'s --new-id option.

NOTES
       These APIs are implemented as a shared library, which can be compiled and linked to with the "libsystemd-id128" pkg-config(1) file.

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)

NOTES
	1. RFC 4122
	   https://tools.ietf.org/html/rfc4122

systemd 208															       SD-ID128(3)