'\" t
.TH "SD\-ID128" "3" "" "systemd 247" "sd-id128"
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.nh
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.\" * MAIN CONTENT STARTS HERE *
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.SH "NAME"
sd-id128, sd_id128_t, SD_ID128_MAKE, SD_ID128_MAKE_STR, SD_ID128_NULL, SD_ID128_CONST_STR, SD_ID128_FORMAT_STR, SD_ID128_UUID_FORMAT_STR, SD_ID128_FORMAT_VAL, sd_id128_equal, sd_id128_is_null \- APIs for processing 128\-bit IDs
.SH "SYNOPSIS"
.sp
.ft B
.nf
#include <systemd/sd\-id128\&.h>
.fi
.ft
.HP \w'\fBpkg\-config\ \-\-cflags\ \-\-libs\ libsystemd\fR\ 'u
\fBpkg\-config \-\-cflags \-\-libs libsystemd\fR
.SH "DESCRIPTION"
.PP
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
\m[blue]\fBRFC 4122\fR\m[]\&\s-2\u[1]\d\s+2
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\&.
.PP
See
\fBsd_id128_to_string\fR(3),
\fBsd_id128_randomize\fR(3)
and
\fBsd_id128_get_machine\fR(3)
for more information about the implemented functions\&.
.PP
A 128\-bit ID is implemented as the following union type:
.sp
.if n \{\
.RS 4
.\}
.nf
typedef union sd_id128 {
        uint8_t bytes[16];
        uint64_t qwords[2];
} sd_id128_t;
.fi
.if n \{\
.RE
.\}
.PP
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\&.
.PP
A couple of macros are defined to denote and decode 128\-bit IDs:
.PP
\fBSD_ID128_MAKE()\fR
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:
.sp
.if n \{\
.RS 4
.\}
.nf
#define SD_MESSAGE_COREDUMP SD_ID128_MAKE(fc,2e,22,bc,6e,e6,47,b6,b9,07,29,ab,34,a2,50,b1)
.fi
.if n \{\
.RE
.\}
.PP
\fBSD_ID128_NULL\fR
may be used to refer to the 128bit ID consisting of only
\fBNUL\fR
bytes\&.
.PP
\fBSD_ID128_MAKE_STR()\fR
is similar to
\fBSD_ID128_MAKE()\fR, but creates a
\fBconst char*\fR
expression that can be conveniently used in message formats and such:
.sp
.if n \{\
.RS 4
.\}
.nf
#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);
}
    
.fi
.if n \{\
.RE
.\}
.PP
\fBSD_ID128_CONST_STR()\fR
may be used to convert constant 128\-bit IDs into constant strings for output\&. The following example code will output the string "fc2e22bc6ee647b6b90729ab34a250b1":
.sp
.if n \{\
.RS 4
.\}
.nf
int main(int argc, char *argv[]) {
        puts("Match for coredumps: %s", SD_ID128_CONST_STR(SD_MESSAGE_COREDUMP));
}
.fi
.if n \{\
.RE
.\}
.PP
\fBSD_ID128_FORMAT_STR\fR
and
\fBSD_ID128_FORMAT_VAL()\fR
may be used to format a 128\-bit ID in a
\fBprintf\fR(3)
format string, as shown in the following example:
.sp
.if n \{\
.RS 4
.\}
.nf
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 "\&.\en", SD_ID128_FORMAT_VAL(id));
        return 0;
}
.fi
.if n \{\
.RE
.\}
.PP
\fBSD_ID128_UUID_FORMAT_STR\fR
is similar to
\fBSD_ID128_FORMAT_STR\fR
but includes separating hyphens to conform to the "\m[blue]\fBcanonical representation\fR\m[]\&\s-2\u[2]\d\s+2"\&.
.PP
Use
\fBsd_id128_equal()\fR
to compare two 128\-bit IDs:
.sp
.if n \{\
.RS 4
.\}
.nf
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;
}
.fi
.if n \{\
.RE
.\}
.PP
Use
\fBsd_id128_is_null()\fR
to check if an 128bit ID consists of only
\fBNUL\fR
bytes:
.sp
.if n \{\
.RS 4
.\}
.nf
int main(int argc, char *argv[]) {
        assert(sd_id128_is_null(SD_ID128_NULL));
}
.fi
.if n \{\
.RE
.\}
.PP
Note that new, randomized IDs may be generated with
\fBsystemd-id128\fR(1)\*(Aqs
\fBnew\fR
command\&.
.SH "NOTES"
.PP
These APIs are implemented as a shared library, which can be compiled and linked to with the
\fBlibsystemd\fR\ \&\fBpkg-config\fR(1)
file\&.
.SH "SEE ALSO"
.PP
\fBsystemd\fR(1),
\fBsd_id128_to_string\fR(3),
\fBsd_id128_randomize\fR(3),
\fBsd_id128_get_machine\fR(3),
\fBprintf\fR(3),
\fBjournalctl\fR(1),
\fBsd-journal\fR(7),
\fBpkg-config\fR(1),
\fBmachine-id\fR(5)
.SH "NOTES"
.IP " 1." 4
RFC 4122
.RS 4
\%https://tools.ietf.org/html/rfc4122
.RE
.IP " 2." 4
canonical representation
.RS 4
\%https://en.wikipedia.org/wiki/Universally_unique_identifier#Format
.RE